Pathogenesis and Pathophysiology of Bacterial Meningitis · ism to nasopharyngeal epithelial cells (Fig. 2) (143, 145). Meningococci, like gonococci, possess fimbriae that differ

CLINICAL MICROBIOLOGY REVIEWS, Apr. 1993, p. 118-136 Vol. 6, No. 20893-8512/93/020118-19$02.00/0Copyright © 1993, American Society for Microbiology

Pathogenesis and Pathophysiology of Bacterial MeningitisALLAN R. TUNKELl* AND W. MICHAEL SCHELD2

Department of Internal Medicine (Infectious Diseases), Medical College of Pennsylvania, Philadelphia,Pennsylvania 19129,1 and Departments of Internal Medicine (Infectious Diseases) and Neurosurgety,

University of Virginia Health Sciences Center, Charlottesville, Virginia 229082

INTRODUCTION ...................................................... 118Epidemiology ...................................................... 118Animal Models ...................................................... 119

MUCOSAL COLONIZATION AND SYSTEMIC INVASION...................................................... 119Fimbriae ...................................................... 119Polysaccharide Capsule ...................................................... 120Antibodies ...................................................... 120Other Bacterial Components...................................................... 121

BACTEREMIA ...................................................... 121Polysaccharide Capsule ...................................................... 121Host Defense Mechanisms ...................................................... 121

MENINGEAL INVASION ...................................................... 121Site of Invasion...................................................... 121Fimbriae ...................................................... 122Monocytes ...................................................... 122Other Bacterial Components...................................................... 122Secondary Bacteremia ...................................................... 122

ALTERATIONS OF THE BBB...................................................... 122LPS...................................................... 123Cytokines ...................................................... 123Localization of BBB Injury ...................................................... 123In Vitro Model of the BBB...................................................... 124

BACTERIAL SURVIVAL WITHIN THE SUBARACHNOID SPACE ..............................................124CSF Complement ...................................................... 124CSF Antibody ...................................................... 125CSF Leukocytes ...................................................... 125

INDUCTION OF SUBARACHNOID SPACE INFLAMMATION ....................................................125Cell Wall..................................................... 126LPS..................................................... 126Inflammatory Mediators ..................................................... 126

INCREASED INTRACRANIA L PRESSURE ..................................................... 127CEREBRAL VASCULITIS ..................................................... 128ALTERATIONS IN CEREBRAL BLOOD FLOW ..................................................... 128ADJUNCTIVE THERAPEUTIC STRATEGIES ..................................................... 129

Experimental Studies ...................................................... 129Clinical Trials ..................................................... 131

REFERENCES ..................................................... 132

INTRODUCTION during 1986 were lower (e.g., 19% for meningitis due to S.pneumoniae) (178), suggesting that improvements in early

Epidemiology detection and antibiotic treatment may have occurred in the1980s. Bacterial meningitis also remains a significant prob-

Bacterial meningitis remains a relatively common and lem in other parts of the world. A recent review of all casesdevastating disease with an overall annual attack rate in the of bacterial meningitis admitted to an isolation-fever hospitalUnited States of -3.0 cases per 100,000 population. Mortal- in Salvador, Brazil, for the decade 1973 through 1982 re-ity rates associated with the three most common causative vealed an approximate annual incidence of 45.8 cases peragents of bacterial meningitis, Haemophilus influenzae, 100,000 population and an overall mortality rate of 33% (24).Neisseria meningitidis, and Streptococcus pneumoniae, The three common meningeal pathogens (H. influenzae, N.were 6.0, 10.3, and 26.3%, respectively, in the UnitedStatesS. pneumonae) accounted for 72% of allfrom 1978 through 1981 (136). Case fatality rates in a mengtdsadS.p uoie)conedfr7% falfrom1hofhfe s s Can se fanglt esCunty

acases and 70% of the deaths. In addition to this unacceptablesubsequent study of five states and Los Angeles County mortality, there is a high rate of neurologic sequelae inchildren and adults who survive their episodes of bacterial

* Corresponding author. meningitis (17, 33, 107, 109, 156). This stable but unsatisfac-

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PATHOGENESIS OF BACTERIAL MENINGITIS 119

tory situation indicates the ongoing need to study the patho-genesis and pathophysiology of bacterial meningitis in anattempt to improve the response to conventional antimicro-bial therapy (126, 148, 165, 166).

Animal Models

Experimental animal models have been employed exten-sively during the past 2 decades to increase our understandingof the pathogenesis and pathophysiology of bacterial menin-gitis (163). In the most commonly employed infant rat model,animals developed meningitis following intranasal challengewith H. influenzae type b (84). This model most closelysimulates the presumed pathogenesis of H. influenzae menin-gitis in humans, since there was an initial nasopharyngealfocus followed by bacteremia and an age-dependent suscep-tibility to meningeal invasion (83). The incidence of bacterialmeningitis, irrespective of rat host age, was directly related tothe intensity of bacteremia. Infant rats also developed men-ingitis after orogastric challenge with Eschenchia coli (82).The infant rat model of H. influenzae meningitis has beenused primarily to study the early pathogenic events of bacte-rial meningitis. These include the determinants of nasopha-ryngeal colonization and translocation into the bloodstream,intravascular survival of the organism, bacteremia, and themechanisms of central nervous system (CNS) invasion. Theanimal's small size is a disadvantage of the infant rat modelbecause only small samples (7 to 25 RI) of cerebrospinal fluid(CSF) are obtainable from 5- to 10-day-old rats, precludingfrequent sampling of CSF. Therefore, this model is lesssuitable for study of the pathophysiologic consequences ofbacterial meningitis, since frequent sampling of CSF is usu-ally required for studying these events. Infant primates havealso been used in the study of the pathogenesis of bacterialmeningitis. In one study (130), bacteremia and meningitisdeveloped in 89 and 94% of animals, respectively, followingthe atraumatic intranasal inoculation of H. influenzae type b.Although this model closely simulates the spectrum of inva-sive H. influenzae disease seen in humans, it is employedinfrequently because of expense.

Experimental models of meningitis in adult rabbits or ratsrely on the direct intracisternal inoculation of bacteria forinitiation of infection (30, 110). These animals reliably developlethal infections with a predictable time course, although thenatural bacteremia-meningitis sequence is bypassed, therebycreating an artificial pathogenesis. However, these modelshave been extremely useful for the study of the pathophysi-ologic consequences of bacterial meningitis after organismshave reached the subarachnoid space. CSF samples fromthese adult animals may be obtained often.The following sections briefly review the pathogenesis and

pathophysiology of bacterial meningitis, as depicted in Fig.1, emphasizing the relationships between specific bacterialvirulence factors and host defense mechanisms that areresponsible for clinical expression of disease (126, 165, 166).New areas of investigation, as suggested by several of thesteps depicted in Fig. 1, may lead to improvements in therefractory mortality and unacceptable morbidity in patientswith bacterial meningitis.

MUCOSAL COLONIZATION AND SYSTEMICINVASION

Initiation of most cases of bacterial meningitis begins withhost acquisition of a new organism by nasopharyngealcolonization. Following infection of human nasopharyngeal

NASOPHARYNGEAL COLONIZATION

LOCAL INVASION

BACTEREMIA

Z ENDOTHEUAL CELL INJURY

INCREASED BBBPERMEABIUTY MENINGEAL INVASION

SUBARACHNOID SPACE INFLAMMATION

INCREASED CSF OUTFLOW RESISTANCE

HYDROCEPHALUS

VASOGENIC EDEMA INTERSTITIAL EDEMA CYTC

INCREASED INTRACRANIAL PRESSURE

- CEREBRALM VASCULJTIS

DTOXIC EDEMbJA |

CEREBRALINFARCTION

I/DECREASED CEREBRAL BLOOD FLOW

FIG. 1. Scheme depicting the pathogenesis and pathophysiologyof bacterial meningitis.

cells in organ culture in vitro with meningococci or H.influenzae type b, several events are observed (143): (i)association with mucus independent of capsular polysaccha-ride, fimbriae, or immunoglobulin A subclass 1 (IgAl) pro-tease production; (ii) cytotoxicity characterized by break-down of epithelial-cell tight junctions, sloughing of ciliatedcells, and ciliostasis; (iii) selective attachment to nonciliatedepithelial cells; (iv) multiplication and formation of micro-colonies on the epithelial surface; (v) invasion of the epithe-lium by intracellular or intercellular routes; and (vi) passageof organisms to the submucosa. These events require viableorganisms and are not completed by commensal species. Inaddition, meningococci and H. influenzae type b appear toinvade nasopharyngeal mucosa by different mechanisms:meningococci utilize parasite-directed endocytosis, and H.influenzae type b adheres to cells, causing a breakdown intight junctions between epithelial cells and leading to inva-sion by an intercellular mechanism. The bacterial virulencefactors and the host defense mechanisms responsible forthese events are discussed in detail below.

Fimbriae

Many of the major meningeal pathogens possess surfacecharacteristics that enhance mucosal colonization. Fim-briae, or pili, are specific organelles found on many bacteriaand often mediate adhesion of bacteria to host cells (10). Thefimbriae of N. meningitidis mediate adherence of the organ-ism to nasopharyngeal epithelial cells (Fig. 2) (143, 145).Meningococci, like gonococci, possess fimbriae that differ intheir morphologic, antigenic, and binding properties (51).Fimbriae appear morphologically as aggregated bundles orsingle filaments. The aggregated bundles are found primarilyamong disease isolates and exhibit a low degree of adherenceto human buccal epithelial cells, whereas the single filamentsare found predominantly among colonizing isolates withmedium to high adherence characteristics. These fimbriatedstrains account for 80% of primary meningococcal isolatesfrom nasopharyngeal carriers and from the CSF of patients

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120 TUNKEL AND SCHELD

as-- mm vwFIG. 2. Scanning electron micrograph of nasopharyngeal organ

cultures 6 h after infection with encapsulated N. meningitidis 34NP,showing attachment of meningococci to nonciliated epithelial cellsand formation of a microcolony on the epithelial surface (magnifi-cation, x8,775). Reproduced from reference 143 with permission ofThe University of Chicago Press.

with meningitis (32), although all fimbriae were lost on serialsubculture in the laboratory. Following attachment via aspecific cell surface receptor, meningococci are transportedwithin a phagocytic vacuole across nonciliated nasopharyn-geal columnar epithelial cells (74, 144); this series of eventsappears to be essential for development of invasive menin-gococcal disease. Fimbriae have also been implicated in theadherence of H. influenzae to upper respiratory tract epithe-lial cells (72, 146), and lack of fimbrial expression impairs theability of H. influenzae type b to colonize the nasopharynx(177). However, fimbriae have not been found on H. influ-enzae type b isolated from the CSF or blood of patients withinvasive disease (72, 106), suggesting that although fimbriaeplay a role in initial adherence within the nasopharynx, theirpresence is not necessary for the organism to cause menin-gitis. Acquisition and colonization of H. influenzae type bmay also be promoted following viral infection by a varietyof respiratory viruses, including influenza A Victoria andrespiratory syncytial virus (140). The precise role of a

preceding upper respiratory viral infection in the enhance-ment of nasopharyngeal colonization and the subsequentdevelopment of meningitis is controversial, however, andrequires further study.

Polysaccharide Capsule

Bacterial encapsulation may be important for nasopharyn-geal colonization and systemic invasion of meningeal patho-gens. Strains of H. influenzae colonize the nasopharynges ofmost children by the age of 3 months, although most of thesestrains are unencapsulated (140). Among the six encapsu-

lated types of H. influenzae (a through f) type b strainsconstitute less than 5% of nasopharyngeal isolates, althoughmore than 95% of meningeal and systemic infections arecaused by type b strains. Experimental studies with infantrats and laboratory transformants selected by capsular typehave shown that, while all encapsulated strains of H. influ-enzae have the potential for systemic invasion after intrap-eritoneal inoculation, type b strains are the most virulent andare the only capsular types capable of systemic invasionfollowing intranasal inoculation (85, 120). Indeed, the pres-ence of serum antibodies to polyribosyl-ribitol phosphate,the capsular polysaccharide of type b isolates, is protectiveagainst invasive disease (2). Antibodies to type b capsule arealmost uniformly detectable in humans by the age of 4 years,even in the absence of known exposure to H. influenzae typeb. The presence of antibodies to type b capsule may berelated to the abilities of other encapsulated strains of H.influenzae to produce some type b capsular material. It hasalso been suggested that the source of bacterial capsule maybe secondary to acquisition of new DNA from organismscolonizing the gastrointestinal tract (81, 138). For example,E. coli K100 possesses a capsule that is immunologicallyrelated to the type b capsule of H. influenzae and maystimulate the production of cross-reacting anticapsular anti-bodies by the host, suggesting that the protection againstinfection with H. influenzae type b is due to priming of serumanticapsular antibodies.

Polysaccharide capsule may also be an important viru-lence factor for invasive disease caused by S. pneumoniae.Of the 84 pneumococcal serotypes known, 18 are responsi-ble for 82% of cases of bacteremic pneumococcal disease (9,39), and there is a close correlation between bacteremicsubtypes and those implicated in meningitis (20, 44, 50).However, low degrees of adherence to human pharyngealepithelial cells have been observed among pneumococcalstrains isolated from patients with serious infections such assepticemia and meningitis (4), suggesting that adherencemay be less important in pneumococcal pathogenicity. Astudy of S. pneumoniae adherence to nasopharyngeal cellsdemonstrated that these organisms were more often foundon desquamated cells than on cells taken from intact epithe-lium (71). Nasopharyngeal mucus may provide a protectednidus from which pneumococci can spread (45), although thevarious factors responsible for invasiveness among certainpneumococcal serotypes remain unknown.

Antibodies

Natural antibodies such as IgA, found predominantly inmucosal secretions, may inhibit the adherence of microor-ganisms to mucosal surfaces. Formation of these antibodiesis stimulated by colonization of organisms that share cross-reactive antigens with pathogenic strains (138). However,the presence of high concentrations of circulating IgA anti-bodies to N. meningitidis may paradoxically permit thedevelopment or exacerbate the progression of invasive dis-ease by preferentially binding to the organism, therebyblocking the beneficial effects of IgG and IgM antibodies (53,54, 64). In addition, many pathogenic Neisseria, Haemoph-ilus, and Streptococcus species produce IgAl proteases thatcleave IgA in the hinge region of the immunoglobulin mole-cule (108); these enzymes may have a pathogenic role byfacilitating adherence of bacterial strains to mucosal surfacesthrough local destruction of IgA (86). The exact role of IgAprotease production in the pathogenic sequelae of bacterialmeningitis remains unclear, however.

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Other Bacterial Components

Other studies have examined various surface components(e.g., lipopolysaccharide [LPS] and outer membrane pro-teins) of H. influenzae to determine their roles in the entrystages of pathogenesis. Antibodies against these componentsconfer protection against repeated challenge with the organ-ism (3, 55, 56, 65). More recently, hemocin, the bacteriocinproduced by H. influenzae, has been shown to be stronglyassociated with type b encapsulated strains and may play arole in host nasopharyngeal colonization and/or systemicinvasion by this organism. After intranasal inoculation ofinfant rats with an equal mixture of a non-hemocin-produc-ing strain and its hemocin-producing transformant, organ-isms capable of hemocin production predominated in iso-lates from both nasopharyngeal and blood cultures (70).However, the precise roles of capsular polysaccharide, LPS,hemocin, outer membrane proteins, and other surface com-ponents in mucosal translocation and bloodstream invasionof H. influenzae and other major meningeal pathogens re-main incompletely defined.

BACTEREMIA

Polysaccharide Capsule

Once the mucosal barrier is crossed, bacteria gain accessto the bloodstream and must then overcome host defensemechanisms to survive and invade the CNS. Surface encap-sulation is the most important virulence factor in this regard.Bacterial capsule, by effectively inhibiting neutrophil phago-cytosis and resisting classic complement-mediated bacteri-cidal activity, may enhance bloodstream survival of theorganism and facilitate intravascular replication (166). In-deed, the most common meningeal pathogens (H. influen-zae, N. meningitidis, S. pneumoniae, E. coli, and Strepto-coccus agalactiae) are all encapsulated. Furthermore,certain specific capsular types among the great many re-quired are associated disproportionately with the develop-ment of bacterial meningitis. For example, about 84% ofcases of neonatal meningitis due to E. coli are caused bystrains bearing the Kl antigen, which is antigenically relatedto the capsular material of serogroup B meningococci andtype III group B streptococci (119). In the absence ofKl-specific host antibody, these organisms are profoundlyresistant to phagocytosis (28). Human monoclonal antibod-ies with specific reactivity for epitopes on the Kl capsule ofE. coli and/or the group B polysaccharide of N. meningitidismay be useful for prevention and/or treatment of blood-stream infections caused by these organisms (114); however,this concept remains conjectural for humans.

Host Defense Mechanisms

Several host defense mechanisms can counteract theantiphagocytic effects of bacterial capsule. For example, thecapsular polysaccharides of S. pneumoniae activate thealternative complement pathway, resulting in cleavage of C3and subsequent attachment of C3b to the bacterial surface,thereby facilitating opsonization, phagocytosis, and intra-vascular clearance of the organism (38). Although antipneu-mococcal cell wall antibody and antipneumococcal capsularantibody promote the efficient deposition of C3b on thepneumococcal surface, C3b deposited on the surface ofpneumococcal capsule is a more efficient opsonin in vitroand in vivo than C3b activated by anti-cell wall antibody (22,

23). Impairment of the alternative complement pathway, asin patients with sickle cell disease (103) and in patients whohave undergone splenectomy, predisposes to the develop-ment of pneumococcal meningitis. The complement cascadeis also activated by H. influenzae type b (113). C3-depletedrats show a greater incidence and magnitude of bacteremiaafter either intravenous or intraperitoneal challenge with H.influenzae of various serotypes (a, b, c, or d) than do normalrats (27, 29, 187). Although the incidence of bacteremia dueto type b organisms increases from 63 to 95% in comple-ment-depleted rats, the incidence and severity of meningitisare unaffected.

Activation of the complement system is an essential hostdefense mechanism in protection against invasive disease byN. meningitidis. Patients with deficiencies in the terminalcomplement components (C5, C6, C7, C8, and perhaps C9),the so-called membrane attack complex, are particularlyprone to infection with neisserial species, including N.meningitidis, though usually with a favorable outcome forthe patient when appropriate treatment is instituted (122).The reasons for the decreased mortality rate in the comple-ment-deficient patients are not clear. It has been suggestedthat the presence of complement-activating products (i.e.,the membrane attack complex), in concert with other medi-ators, may contribute to the development of multiorganfailure and death. A qualitative relationship exists among thelevel of circulating meningococcal LPS, a fatal outcome, andthe degree of complement activation (19), indicating thatprognosis is worse for patients with intact complementsystems.

MENINGEAL INVASION

The mechanisms by which bacterial pathogens gain accessto the CNS are largely unknown. One factor may relate tothe concentration of organisms in the blood (84). In theexperimental infant rat model, the intranasal inoculation ofH. influenzae type b initially produced a low-grade bactere-mia (about 102 CFU/ml) and no organisms were present inthe CSF (141). Culture-positive meningitis was observedonly after an intense bacteremia (>10W CFU/ml) had beenpresent for at least 6 h (100). Meningitis was also induced inan age-dependent manner, with a higher incidence in 5-day-old than in 20-day-old rats (83). In the animals that ultimatelydeveloped meningitis, sustained bacteremia, as opposed totransient bacteremia, was documented. However, sustainedbacteremia is not the only factor responsible for meningealinvasion, because many other organisms (e.g., viridansstreptococci) that produce continuous bacteremia duringinfective endocarditis rarely produce bacterial meningitis.

Site of Invasion

The exact sites of CNS invasion by meningeal pathogensare unclear. Early studies with the experimental rat modelsuggested that the route of invasion from the bloodstream tothe CSF was through the dural venous sinus system (141).However, subsequent experiments with the same animalmodel suggested that during the ensuing bacteremia a non-specific, sterile, focal inflammation above the cribriformplate facilitated invasion of the CNS at that site. Furtherstudies of infant rats and primates demonstrated that bacte-ria may enter the CSF via the choroid plexus, which has anexceptionally high rate of blood flow (-200 ml/g/min), per-haps because more bacterial organisms are delivered to thissite than to other anatomic locations in the CNS per unit of

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time. Sampling of CSF compartments early during bacterialmeningitis has demonstrated higher bacterial densities in thelateral ventricles than in the cisterna magna, lumbar sub-arachnoid space, or supracortical subarachnoid space. Al-though with time equilibrium is reached in these otherlocations, the data suggest initial bacterial entry into CSF inthe lateral ventricles, presumably through the choroid plexi.

Fimbriae

Recent experimental studies have suggested that receptorsfor some meningeal pathogens are present on cells in thechoroid plexus and/or cerebral capillaries, which may facil-itate movement of these pathogens into the subarachnoidspace. In cryostat sections of infant rat brain cortical slices,strains of E. coli possessing S fimbriae are seen to bind to theluminal surfaces of the vascular endothelium and the epithe-lium lining the choroid plexus and brain ventricles (101).Pretreatment of the brain sections with neuraminidase or thetrisaccharide receptor analog of S fimbriae abolished thisbinding. Subsequent experiments demonstrated that 1 h afterintraperitoneal challenge with the S-fimbriated strain of E.coli, about 50% of organisms in the CSF were S fimbriatedand 50% were nonfimbriated (128), suggesting that phasevariation to the nonfimbriated form may be necessary forthese bacteria to invade the CNS. Nevertheless, the specificadherence of meningeal pathogens to sites within the CNS isone hypothesis raised to explain bacterial neurotropism andcurrently is an area of intense investigation.

Monocytes

Additional studies to determine the pathogenic mecha-nisms responsible for meningeal invasion utilized histologicand scanning microscopic techniques to examine theneuraxes of pigs inoculated intravenously with a pathogenicstrain of Streptococcus suis type 2 (180). The animals weresacrificed 17 to 47 h after intravenous challenge, and the onlypathologic lesions detected were associated with the choroidplexus, manifested as disruption of the plexus brush border,decrease in the number of Kolmer cells, and exudation offibrin and inflammatory cells into the ventricles. Intracellularbacteria were demonstrated in the parenchyma of the cho-roid plexus, in the ventricular monocytes, and within periph-eral blood monocytes. Circulating monocytes, which werealso found to contain phagocytized bacterium-sized parti-cles, migrated into the CSF via the choroid plexus, suggest-ing that bacteria may gain access to the CSF in associationwith monocytes migrating along normal pathways (the so-called Trojan horse hypothesis for CNS invasion).

Other Bacterial Components

Other bacterial virulence factors have been studied todetermine their possible roles in meningeal invasion. Theliberation of LPS from N. meningitidis may contribute to thepathogenicity of this organism in invasive infections (1).Meningococci vary in their ability to liberate endotoxin, withincreased amounts liberated from patients with invasivedisease. For example, serogroup B meningococcal strainsreleased slightly more free endotoxin when isolated fromblood or CSF than when isolated from the nasopharynges ofpresumably healthy persons. Outer membrane proteins mayalso be important. One report has suggested that H. influen-zae strains with outer membrane protein subtype lc causemore episodes of meningitis and fewer episodes of epiglot-

titis than do strains of subtype 1 (151), perhaps because ofthe ability of each subtype to release LPS under appropriatecircumstances. However, the precise role of outer mem-brane protein subtypes in meningeal invasion is unclear.

Secondary Bacteremia

Following bacterial invasion of the subarachnoid space, asecondary bacteremia may result from the local CNS sup-purative process, allowing the meningeal pathogen to con-tinuously enter and leave the CSF compartment under quitedynamic circumstances. In an experimental canine model ofpneumococcal meningitis, the early transport of bacteriafrom CSF to blood was presumed to be transendothelialthrough arachnoid villi containing pores large enough toaccommodate bacteria, which would then enter the superiorsagittal sinus and return to the central venous blood (135).This transport occurred only following active bacterial mul-tiplication in CSF and before the height of the febrileresponse or CSF pleocytosis. This phenomenon was alsoobserved with H. influenzae type b, inoculation of whichinto the cisterna magna of experimental animals produced analmost instantaneous bacteremia (140).

ALTERATIONS OF THE BBB

Bacterial meningitis, like many other disease states, in-creases the permeability of the blood-brain barrier (BBB).The major sites of the BBB are the arachnoid membrane,choroid plexus epithelium, and cerebral microvascular en-dothelium. Previous extensive morphologic studies havedemonstrated intact arachnoid membranes in animals withbacterial meningitis (176). Therefore, the increased BBBpermeability seen in this disorder must occur at the level ofthe choroid plexus epithelium, the cerebral microvascularendothelium, or both; the cerebral microvascular endothe-lium has been the site of intensive study in recent years as aresult of techniques for isolation of cerebral microvessels orendothelial cells or both. The features that distinguish cere-bral capillaries from other capillaries throughout the bodyare (i) adjacent endothelial cells fused together by pentalam-inar tight junctions (zonulae occludens) that prevent inter-cellular transport; (ii) rare or absent pinocytotic vesicles;and (iii) abundant mitochondria (18, 49). Therefore, theincreased BBB permeability that occurs during bacterialmeningitis at the level of the cerebral capillary endothelialcell may result from separation of intercellular tight junc-tions, from increased pinocytosis, from both alterations, orby processes as yet unknown.An adult rat model of bacterial meningitis was used to

investigate the propensity for bacterial meningitis to inducefunctional and morphologic alterations of the BBB (110).Following the intracisternal inoculation of either E. coli, S.pneumoniae, or H. influenzae into rats, a uniform hostresponse to all three encapsulated pathogens was observedat the level of the cerebral capillary endothelial cell, charac-terized morphologically by an early and sustained increase inpinocytotic vesicle formation and a progressive increase inseparation of intercellular tight junctions from 4 to 18 hpostinoculation (Table 1). These morphologic changes cor-related with the functional penetration of albumin across theBBB, with the highest values of albumin entry occurring 18h after intracisternal inoculation, when both morphologicchanges were evident. Following intracisternal inoculationof an unencapsulated strain of H. influenzae (Rd strain),there was an increase in pinocytotic vesicle formation, but



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PATHOGENESIS OF BACT7ERIAL MENINGITIS 123

TABLE 1. Correlation of morphologic and functionalalterations of the BBB in an experimental ratmodel of H. influenzae-caused meningitisa

Time after % Penetrationtreatment Inoculum (n) of "-albuminCBBB mor-(h) ~~~~~~~~(mean± SE) phooy

4 Saline (3) 0.26 ± 0.08H. influenzae Rd (3) 4.12 + 1.25c t PVH. influenzae type b (4) 3.80 + 0.75c t PV

18 Saline (4) 1.25 ± 0.27H. influenzae Rd (4) 4.64 ± 0.80c t PVH. influenzae type b (4) 8.10 ± 1.20cd T PV + t SJ

a Reproduced from reference 110 with permission of the American Societyfor Clinical Investigation.

b PV, pinocytotic vesicles; SJ, separated junctions; T, increase.c P < 0.05 compared with control.d p < 0.05 compared with H. influenzae type b at 4 h and H. influenzae Rd

at 18 h.

separation of intercellular tight junctions did not occur. Thisdiscrepancy between encapsulated and unencapsulatedstrains of H. influenzae likely occurred secondary to theremoval of unencapsulated organisms from the CSF by hostdefense mechanisms, whereas deficient opsonic mechanismsin the CSF (see below) permitted sustained concentrations ofthe encapsulated strain. Therefore, encapsulation of H.influenzae was not essential for BBB injury but facilitatedthe progression of such injury by avoidance of host defensemechanisms.The effect of the host leukocyte response on altered BBB

permeability was subsequently examined in the experimen-tal rat model by first rendering the animals leukopenic 4 daysfollowing intraperitoneal injection of cyclophosphamide(68). Functional increases in BBB permeability, assessed bypenetration of radioactive albumin from blood to CSF, wereobserved in both normal and leukopenic rats at 18 h follow-ing inoculation of either encapsulated or unencapsulatedstrains of H. influenzae, but permeability was greater afterchallenge with the encapsulated strain. Significant increasesin BBB permeability occurred in the near absence of leuko-cytes in CSF late in the disease process, although thepresence of leukocytes augmented changes in permeability.At 18 h following inoculation, alterations of BBB permeabil-ity correlated with concentrations of bacteria in the CSF.

LPSSince bacterial capsule was not essential for BBB injury in

the experimental rat model of bacterial meningitis and sinceit was recognized that pneumococcal capsule did not induceinflammation within the subarachnoid space (see below),BBB permeability was examined following intracisternalinoculation of purified H. influenzae type b LPS. Afterintracisternal inoculation of LPS into rats, the followingresults were observed (183): (i) dose-dependent increases inBBB permeability from 2 pg to 20 ng, with attenuation inpeak response after challenge with 500 ng or 1 ,ug; (ii)time-dependent increases in BBB permeability, with maxi-mal alteration at 4 h and complete reversal at 18 h (Fig. 3);(iii) greater increases in permeability after challenge withLPS than after challenge with the live parent strain despiteidentical LPS concentrations; and (iv) close correlationbetween BBB permeability and CSF pleocytosis 4 h afterintracisternal inoculation. Preincubation of LPS with poly-myxin B (a cationic antibiotic that binds to the lipid A region

at

LL Wen cnC. +1

Cco

50000 -

40000 -

30000 -

20000 -

10000 -

v-108

8 I}Bi

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TIME (H)FIG. 3. Kinetics of changes in concentration of leukocytes

(WBC) in CSF and in BBB permeability (BBBP) after inoculationwith LPS in an experimental rat model. Reproduced from reference183 with permission of the American Society for Clinical Investiga-tion. SE, standard error of the mean.

of LPS) and neutrophil acyloxyacyl hydrolase (which re-moves nonhydroxylated fatty acids from the lipid A region ofthe LPS molecule) inhibited the effect of LPS on BBBpermeability, strongly implicating the lipid A region of LPSin the observed effects in the CNS. Monoclonal antibodiesdirected against the oligosaccharide portion of LPS did notdecrease permeability. No change in BBB permeability wasobserved following intracisternal inoculation of LPS intoleukopenic rats. Similar results in rats were obtained follow-ing intracistemal inoculation of H. influenzae type b outermembrane vesicles (182), which may represent a relevant,nonreplicating vehicle for the delivery of the toxic moietiesof LPS to host cells.

CytokinesBecause the increased BBB permeability induced by LPS

in the experimental rat model was not maximal until 4 hfollowing intracisternal inoculation, it was suggested that acommon host mediator(s) was responsible. Therefore, it wasnext determined whether specific inflammatory cytokines,which mediate many of the deleterious effects of LPS, alsoincreased permeability (112). Intracisternal inoculation ofhuman recombinant interleukin-lp (IL-1ip) into rats led to apeak increase in BBB permeability about 3 h after inocula-tion (Fig. 4), which is earlier than the peak response ob-served with LPS (at 4 h). This effect was significantlyattenuated by preincubation of the cytokine with a mono-clonal antibody to IL-1l3 and was totally abolished in leuko-penic animals. Preincubation of IL-1lB with polymyxin B didnot alter IL-1,B activity. No permeability changes wereobserved following intracisternal inoculation of human re-combinant tumor necrosis factor alpha (TNF-a) into rats,although rabbit TNF-ot clearly elicits subarachnoid spaceinflammation following intracistemal inoculation of the ho-mologous species. All available evidence suggests that bothcytokines are important and that they act synergistically,since inoculation with submaximal doses of IL-lp plusTNF-a at concentrations that produced no changes individ-ually enhanced BBB permeability.

Localization of BBB InjuryThe precise localization of BBB injury in bacterial menin-

gitis was subsequently examined by in situ tracer perfusion

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Time Post Inoculation (hr)FIG. 4. Kinetics of changes in CSF traversal of systemically

administered 1251-BSA in an experimental rat model after intracis-ternal inoculation with recombinant IL-1i, recombinant TNF-a,and controls. *, P < 0.05. BBBP, BBB permeability; SE, standarderror of the mean. Reproduced from reference 112 with permissionof the American Society for Clinical Investigation.

and immunolabeling procedures to identify the topographyand microvascular exit pathways of bovine serum albumin(BSA) (111). Two tracers were used: colloidal gold, which isuseful in ultrastructural definition of albumin exit pathways,and monomeric BSA, which has a faster rate of transcytosisthan colloidal gold. After intracisternal challenge of rats withE. coli (O111:B4) LPS, an inducible increase in immunode-tectable monomeric BSA binding to the luminal membranesof all microvascular segments in the pia-arachnoid andsuperficial brain cortex was observed by transmission elec-tron microscopy. Uptake of colloidal gold and monomericBSA by plasmalemmal vesicles was similar, but there was nodetectable transcytosis to the abluminal side of the endothe-lium. Exit of both perfused colloidal gold-BSA and immuno-detectable BSA was through open intercellular junctions ofvenules in the pia-arachnoid, specifically and topographi-cally localizing the BBB injury in bacterial meningitis to themeningeal venules.

In Vitro Model of the BBB

Further studies were performed to delineate the preciseeffect of H. influenzae type b LPS on the cerebral microvas-culature. Purified preparations of cerebral microvascularendothelial cells (identified by positive immunofluorescentstaining for factor VIII-related antigen and by the ability totake up acetylated low-density lipoproteins) from rat cere-

bral cortices were grown on a semipermeable support tocreate an in vitro model of the BBB (162, 164). Isolatedcerebral microvascular endothelium has been shown toretain many of the characteristics of an intact BBB, includ-ing the ability to form intercellular tight junctions in vitro.Treatment of intact cerebral microvascular endothelial cellmonolayers with LPS (0.1 ,ug/ml) in serum-free media led toa statistically significant increase in the percentage of radio-active albumin able to permeate the monolayer (2.2 to 5.6%;P < 0.001) in the absence of host inflammatory cells. Thisincrease in permeability was not associated with cytotoxicityas assessed by release of lactate dehydrogenase into themedia. To determine the putative intracellular regulatory

mechanisms responsible for this increased ability to perme-ate monolayers in the in vitro model, the effects of LPS onthe formation of various second messenger systems in thecerebral microvascular endothelial cells in response to LPSstimulation were examined (161). Production of both cyclicAMP and cyclic GMP was increased in response to LPS, butproduction of cyclic AMP only was evident prior to theincreased permeation of the cell monolayer. This observa-tion suggests that the increased BBB permeability within thecerebral microvascular endothelium during bacterial menin-gitis occurs via a cyclic AMP-dependent process.

In contrast, other investigators, utilizing primary culturesof bovine brain microvascular endothelial cells (102), foundthat H. influenzae type b or purified type b LPS causedmarked cytotoxicity of these cells in culture, an effect thatcould be completely blocked by polymyxin B. The presenceof serum was essential for the LPS-induced cytotoxic effect,and a monoclonal antibody against CD14, a receptor in-volved in mediating the actions of LPS in monocytes,completely blocked the cytotoxic effect. Further studies arenecessary, however, to precisely define the cellular mecha-nisms of altered BBB permeability during bacterial menin-gitis.

BACTERIAL SURVIVAL WITHIN THESUBARACHNOID SPACE

CSF ComplementOnce meningeal pathogens penetrate the subarachnoid

space, host defense mechanisms are inadequate to controlthe infection (131). Complement components in CSF areusually absent or present in only minimal concentrations (25,115, 139, 157). Meningeal inflammation leads to increased,but low, concentrations of complement in CSF. The impor-tance of this relative complement deficiency in normal andinfected CSF may be critical, since specific antibody and/orcomplement is essential for opsonization of encapsulatedmeningeal pathogens, efficient phagocytosis, and removal ofpathogens by the spleen (21). Opsonic and bactericidalactivities are absent or barely detectable in patients withvarious forms of meningitis; similar observations have beenmade in experimental animal models of bacterial meningitis.A serum-sensitive E. coli strain was not opsonized in vitrowhen the strain was incubated with CSF from rabbitschallenged intracisternally with E. coli Kl, although CSFfrom animals with Staphylococcus aureus-caused meningitiswas opsonically active in vitro against E. coli (13). Theabsence of opsonization after E. coli challenge may havebeen due to the absorption of a specific opsonin by E. coliearly in the course of infection. Alternatively, more serumprotein, and therefore opsonins, may have penetrated intoCSF in staphylococcal rather than E. coli meningitis. Al-though there are low functional and bactericidal activities inpurulent CSF, the presence of some measurable opsonicactivity may correlate with a favorable outcome. For exam-ple, outcome was better in patients whose concentrated CSFsamples demonstrated opsonic activity (15 of 27 patients[186]), suggesting that complement-mediated opsonic activ-ity can appear in the CSF during bacterial meningitis,particularly in patients who recover completely.

Several explanations of the low concentrations of comple-ment in CSF during meningitis have been advanced: (i)insufficient traversal across the BBB; (ii) variable subarach-noid space inflammation; (iii) enhanced clearance or removalfrom the subarachnoid space; (iv) low production rates in the



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CNS; and (v) degradation at the site of infection (131). Thislast possibility has been investigated in patients and inanimal models of meningitis. Leukocyte proteases have beenshown to degrade functional complement components (e.g.,C3b) in CSF from patients with meningococcal meningitis,with the formation of nonopsonic products (e.g., C3d) (179).In the experimental rabbit model of pneumococcal meningi-tis, the intracisternal inoculation of phenylmethylsulfonylfluoride, a nonspecific protease inhibitor, led to a decline inpneumococcal concentrations in CSF compared with thoseof saline-inoculated controls (131). Therefore, during bacte-rial meningitis, complement components crossing the BBBmay be degraded by leukocyte proteases, resulting in ineffi-cient opsonic activity at the site of infection similar tomechanisms postulated to occur in the pleural space duringthe development of an empyema.

CSF Antibody

Immunoglobulin concentrations are also low in normalCSF, with an average blood/CSF ratio of IgG of about 800:1.Immunoglobulin concentrations in CSF increase during bac-terial meningitis but remain low compared with simultaneousconcentrations in serum, and IgG does not appear in the CSFuntil late in the course of disease (142, 179). An experimentalrabbit model of pneumococcal meningitis has been utilizedto study the importance of antibodies in host defense againstbacterial meningitis. Intracisternal inoculation of type-spe-cific antibodies against S. pneumoniae decreased pneumo-coccal concentrations in CSF, but at a much slower rate thandid treatment with appropriate bactericidal antibiotics (131).The systemic administration of type-specific monoclonalantibodies has been examined in the experimental rabbitmodel. Intravenous injection of a bactericidal monoclonalantibody against the polyribosyl-ribitol phosphate of H.influenzae type b produced high concentrations of antibodyin serum, but there was poor BBB penetration (c5.5%),even in the presence of meningeal inflammation (47). Theseresults suggested that systemic administration of type-spe-cific antibodies alone is suboptimal. The pioneering work ofFlexner (42), who demonstrated that systemic and intrathe-cal administration of antimeningococcal antiserum raised inhorses was effective in reducing the mortality rate in menin-gococcal meningitis from approximately 80 to 30%, suggeststhat combined sites of administration may be useful. Itremains to be determined, however, whether the intrathecaladministration of antibodies may be useful in the adjunctivetreatment of bacterial meningitis.

CSF Leukocytes

One of the hallmarks of bacterial meningitis is the devel-opment of a neutrophilic pleocytosis within the CSF, al-though the precise mechanisms of leukocyte traversal acrossthe BBB remain to be defined (52, 184). In an experimentalanimal model of pneumococcal meningitis, the complementcomponent C5a has been suggested as one chemotacticsubstance in CSF (35, 96, 158), with chemotactic activityappearing 2 to 4 h before neutrophil influx into the CSF. Theprecise role of C5a as a chemotactic substance has recentlybeen examined in the experimental rabbit model, in whichthe intracistemal inoculation of C5a caused a rapid earlyinflux of leukocytes into CSF 1 h after inoculation (60). Thisresponse was attenuated by coadministration of CSa withprostaglandin E2 (PGE2) in a dose-dependent manner, sug-gesting a direct anti-inflammatory action of PGE2 on CSa-

generated CSF pleocytosis during bacterial meningitis.However, despite the entry of leukocytes, host defensemechanisms in CSF remain suboptimal because of the rela-tive lack of functional opsonic and bactericidal activity.Therefore, phagocytosis is inefficient at this protected site,leading to huge concentrations of bacteria in the CSF duringmeningitis (36, 37).There is controversy over the precise role of neutrophils in

host defense within the CNS during bacterial meningitis. Inexperimental animal models, low concentrations of leuko-cytes in CSF have generally been associated with increasedmortality rates (46, 134). Although a study of dogs withpneumococcal meningitis revealed that leukopenic animalshad a higher survival rate than animals with normal periph-eral leukocyte counts (62 versus 47 h) (104), the smallnumber of animals studied precluded statistical analysis. Inan experimental rabbit model of pneumococcal meningitis,the parameters of bacterial growth rate; final concentrationsof bacteria in CSF; and concentrations of protein, glucose,and lactate in CSF were no different in animals renderedleukopenic by the prior intravenous inoculation of nitrogenmustard than in nonleukopenic animals. However, the re-sultant bacteremia was about 100-fold greater in leukopenicanimals (34), suggesting that neutrophils either preventedtraversal of pneumococci from CSF to blood or enhancedneutrophil-mediated bacterial elimination from the blood-stream at extraneural sites.The precise pathway by which neutrophils enter the

subarachnoid space remains unknown. Adherence of neu-trophils to vascular endothelial cells is a likely prerequisitefor traversal into the CSF. Pretreatment of endothelial cellswith cytokines induces formation of specific adhesion mol-ecules such as endothelial leukocyte adhesion molecule 1(16). Neutrophil adherence to vascular endothelial cells isenhanced by pretreatment of the endothelial cells with LPS(137) or with inflammatory cytokines such as IL-1 and TNF(15, 137, 165). Similar mechanisms may also enhance neu-trophil binding to cerebral vascular endothelium (11). Arecent study of an experimental rabbit model has demon-strated that the intravenous inoculation of a monoclonalantibody (IB4) against the CD18 family of receptors onleukocytes blocked the accumulation of leukocytes in thesubarachnoid space despite intracisternal challenge with H.influenzae type b, N. meningitidis, pneumococcal cell wall,or LPS (170). The monoclonal antibody also attenuated aparameter of BBB permeability (i.e., increased protein con-centrations in the CSF). Furthermore, development of cere-bral edema and death were prevented in monoclonal anti-body-treated animals challenged with lethal doses of S.pneumoniae. Penetration of antibiotics into CSF, bacterialconcentrations in CSF, and bactericidal response to ampi-cillin therapy were not affected by monoclonal antibodyadministration, although the animals exhibited a delay in theonset of bacteremia and there was an attenuated CSFinflammatory response after ampicillin-induced bacterialkilling. These results suggest that systemic inoculation ofmonoclonal antibodies directed at leukocyte-endotheliuminteractions can block leukocyte-mediated damage withinthe CNS during bacterial meningitis.

INDUCTION OF SUBARACHNOID SPACEINFLAMMATION

The ability of meningeal pathogens to induce a markedsubarachnoid space inflammatory response contributes tomany of the pathophysiologic consequences of bacterial

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meningitis (e.g., cerebral edema and increased intracranialpressure). For example, this inflammatory response may bea crucial factor in the development of sensorineural deafnessin patients with bacterial meningitis. Utilizing an infant ratmodel of pneumococcal meningitis, multiple intraperitonealinocula (1 x 108 to 10 x 108 CFU every 24 h for 3 days) ofS. pneumoniae in 5-day-old rats led to bacteremia andmeningitis in 50 and 46% of animals, respectively (121).Hematoxylin-eosin-stained sections of brain tissue from ratswith positive CSF cultures revealed inflammation in themeninges and scala tympani but not in the scala media. Inaddition, perilymphatic inflammation occurred significantly(P < 0.001) more than did endolymphatic inflammation,suggesting that bacteria reached the cochlea via the cochlearduct, thereby providing a connection between perilymphaticfluid and CSF. Recent studies in other animal model systemshave focused on the bacterial virulence factors that areresponsible for this inflammatory response, providing manynew, exciting areas of investigation.

Cell Wall

Despite the fact that bacterial capsule is largely responsi-ble for intravascular and subarachnoid space survival ofmeningeal pathogens, capsular polysaccharides are remark-ably noninflammatory when injected in purified form into thesubarachnoid space. In contrast, cell walls of gram-positivebacteria are potent inducers of inflammation, and S. pneu-moniae cell walls activate the alternative complement path-way (181). Various surface components of S. pneumoniaehave been examined to determine their roles in subarachnoidspace inflammation. A CSF inflammatory response was

invoked in an experimental rabbit model by intracisternalinoculation of live encapsulated and unencapsulated S.pneumoniae cells, heat-killed unencapsulated pneumococci,and pneumococcal cell walls (169); no inflammation was

induced by intracisternal inoculation of either heat-killedencapsulated strains or isolated capsular polysaccharide.Pneumococcal cell walls inoculated intracisternally were themost potent inducers of CSF inflammation among thesesubstances (168). In addition, independent intracisternalinjection of the major components of the pneumococcal cellwall, teichoic acid and peptidoglycan, also induced CSFinflammation. Teichoic acid had the highest specific activityof the cell wall fractions tested, and activity was markedlyreduced if either component (teichoic acid or peptidoglycan)was extensively degraded. These findings suggest that pneu-mococcal cell wall lytic products released during antibiotic-induced autolysis during treatment of bacterial meningitiscontribute to the host inflammatory response in the sub-arachnoid space.

LPS

Intracisternal inoculation of purified H. influenzae type bLPS (in contrast to purified capsular polysaccharide) alsoinduces subarachnoid space inflammation. In an experimen-tal rabbit model, the intracisternal inoculation of purifiedLPS (2 fg to 200 ng) produced a dose-dependent increase inconcentrations of leukocytes in CSF, whereas no inflamma-tion was induced by intracisternal inoculation of purified H.influenzae type b capsular polysaccharide (149). This inflam-matory response was blocked by polymyxin B and neutro-phil acyloxyacyl hydrolase, a fact that supports the impor-tance of the lipid A region of the LPS in the induction of CSFinflammation. Pretreatment of the LPS with a monoclonal


antibody directed against epitopes on the oligosaccharideportion of the LPS molecule did not reduce the inflammatorypotential of the LPS. Similar results were observed in anexperimental rat model following the intracisternal inocula-tion of purified LPS, with the maximal amount of inflamma-tion observed 8 h after inoculation of a 20-ng dose (Fig. 3)(183). In addition, the intracisternal inoculation of outermembrane vesicles from H. influenzae type b induced men-ingeal inflammation in both rabbits and rats in a dose- andtime-dependent manner (92, 182); this response was blockedby polymyxin B but not by two monoclonal antibodiesdirected against surface epitopes of the oligosaccharide sidechains of LPS within outer membrane vesicles. This obser-vation supports the concept that the delivery of LPS viaouter membrane vesicles induces subarachnoid space in-flammation.

Inflammatory Mediators

Recently, evidence that supports the hypothesis thatpneumococcal cell wall or H. influenzae type b LPS inducessubarachnoid space inflammation through the local CNSrelease of inflammatory mediators such as IL-1, TNF (14,80, 159), and/or prostaglandins (77, 95) has been accumulat-ing. Intact pneumococci, pneumococcal cell wall, or lipotei-choic acid induced IL-1 production by human peripheralmonocytes in vitro (118). IL-1 production by monocytes wasdramatically reduced by chemical alteration of phosphoryl-choline, the major determinant of teichoic acid, or bypretreatment with a phosphorylcholine antibody. TNF pro-duction was not induced in monocytes incubated with pneu-mococcal cell wall in vitro.

In an experimental rat model, the intracisternal inocula-tion of H. influenzae type b LPS led to elevated concentra-tions of IL-1 and TNF in CSF within 30 to 120 min (166).Elevated concentrations of TNF in CSF have also beenobserved in the experimental rabbit model following intra-cisternal inoculation of LPS (90). TNF activity in CSF wasfirst detected 45 min after intracisternal challenge with LPS,with peak activity at 2 h and persistence for about 5 h.Following intracisternal challenge with live H. influenzaetype b, peak TNF activity in CSF was comparable to thatafter LPS challenge, although activity persisted for -14 h.TNF activity and meningeal inflammation (i.e., CSF pleocy-tosis) in CSF were significantly reduced by therapy withdexamethasone or by the intracisternal administration ofgoat anti-human TNF-ao antibodies. Interestingly, simulta-neous analysis of serum samples revealed no detectableTNF activity, indicating that TNF was principally producedwithin the CNS. This local production of TNF-ot has alsobeen observed in patients with bacterial meningitis (79). Inaddition, the finding of increased CSF concentrations ofTNF-a may be specific for bacterial meningitis, as wassuggested in a recent study. TNF-ot concentrations, mea-sured in mice or humans with either bacterial or viralmeningitis, were elevated in CSF only during bacterialmeningitis (67); however, only a few patients were analyzed.These results were confirmed in a subsequent prospectivestudy of patients with infectious meningitis. The presence ofTNF-a in CSF was indicative of a bacterial etiology, al-though the absence of TNF-ao did not exclude the diagnosisof bacterial meningitis (93). In addition, elevated concentra-tions of PGE2, prostacyclin, IL-11, and TNF in CSF werefound in the majority of infants and children with bacterialmeningitis (91). Systemic dexamethasone administration sig-


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TABLE 2. Concentrations of endotoxin in CSF and bacterial titers before and after antibiotic therapy in eight patientsa

Time (l CFU/ml)TotaTiter Loglo ng of endotoxin/mi (mean + SE)(log10CFU/ml) ~Total Bound Free (%

Before therapy 6.69 + 0.71 2.06 + 0.27 2.04 ± 0.28 0.75 + 0.21 (8.5 ± 4.2)After therapy 5.24 + 0.85 1.53 + 0.29 1.06 ± 0.33 1.29 + 0.23 (63.5 + 11.5)P valueb <0.005 <0.02 <0.01 <0.01 (<0.005)

a Data from reference 5 with permission of The University of Chicago Press.b Significance of paired t test (two tailed).

nificantly reduced concentrations of PGE2 and IL-lp in CSFand improved several indices of meningeal inflammation.

Inoculation of purified rabbit TNF-a or human recombi-nant IL-1B into the CSF of rabbits also produced significantCSF inflammation (117). Simultaneous administration oflower doses of each cytokine resulted in an apparent syner-gistic inflammatory response that was manifested by a morerapid and significantly increased influx of leukocytes intoCSF than occurred with administration of each cytokinealone. In contrast, in an experimental rabbit model ofpneumococcal meningitis, CSF leukocytosis, BBB perme-ability, and brain edema were induced by intracisternalinoculation of human recombinant TNF-a, macrophage in-flammatory proteins 1 and 2, and IL-la but not by intracis-ternal inoculation with IL-lp (127). Antibodies homologousto each mediator inhibited leukocytosis and brain edema. Inrabbits treated with a monoclonal antibody to CD18 torender neutrophil-endothelial cell interactions dysfunctional,each cytokine lost the ability to cause leukocytosis and brainedema. Therefore, these cytokines have multiple complexand interrelated activities in the CNS that contribute totissue damage during pneumococcal meningitis.These findings have implications with regard to outcomes

in patients with bacterial meningitis. Outcome from menin-gitis due to gram-negative bacilli has been correlated withpersistence of organisms and higher concentrations of endo-toxin in CSF (as detected by the Limulus lysate assay) (73).A study of children with H. influenzae meningitis docu-mented that treatment with ceftriaxone induced a markedincrease in concentrations of free LPS in CSF within 2 to 6h (Table 2) that correlated with the Herson-Todd severityscore and the number of febrile hospital days (5). These datasuggest that early release of "free" LPS (i.e., LPS notpresent in outer membranes of viable organisms) after anti-microbial therapy enhances the host subarachnoid spaceinflammatory response. The degree of elevated concentra-tions of IL-1lB in CSF also correlated significantly withoutcome from neonatal gram-negative bacillary meningitis inchildren (88). In another study of infants and childrenpredominantly with H. influenzae type b meningitis, patientswith IL-113 concentrations in CSF of .500 pg/ml were morelikely to develop neurologic sequelae (87). Although ele-vated concentrations of TNF in CSF were observed in 50 to75% of patients with bacterial meningitis, there was nocorrelation between concentrations of TNF in CSF andoutcome. The roles of other inflammatory cytokines in theinduction of subarachnoid space inflammation are less clear.Elevated concentrations of platelet-activating factor in CSFhave been demonstrated in children with H. influenzaemeningitis (6) and correlated with bacterial density and withLPS and TNF-a concentrations in CSF. These increasedconcentrations of TNF-a and platelet-activating factor wereassociated with severity of disease. Increased concentra-tions of IL-6, occurring after release of TNF-ot and before

neutrophilic infiltration into CSF (174), have also beenobserved in patients with bacterial meningitis (58, 123, 175).These experimental and clinical studies strongly suggest thatrelease of inflammatory mediators into the CSF duringbacterial meningitis is responsible for induction of a markedsubarachnoid space inflammatory response and may possi-bly correlate with morbidity and mortality from this disor-der.The source of these inflammatory cytokines within the

CSF of patients with bacterial meningitis is unclear. LPSstimulation of astrocytes and microglia in vitro leads torelease of various cytokines (43), and vascular endothelialcells in culture produce IL-1 in response to stimulation witheither LPS or TNF (69, 78, 94). One study utilizing purifiedpreparations of cerebral microvascular endothelial cells fromrats has demonstrated that these cells release IL-6 in vitro inresponse to LPS stimulation (160). Further studies, how-ever, are warranted to address these issues as they pertain tothe patient with bacterial meningitis.

INCREASED INTRACRANIAL PRESSURE

The major element that contributes to an increase inintracranial pressure during bacterial meningitis is the devel-opment of cerebral edema, which may be vasogenic, cyto-toxic, and/or interstitial in origin and may result in life-threatening cerebral herniation and other complications (26,40, 41, 57, 98). Vasogenic cerebral edema is principally aconsequence of increased BBB permeability (see above).Cytotoxic cerebral edema results from swelling of the cellu-lar elements of the brain, most likely due, in bacterialmeningitis, to release of toxic factors from neutrophils orbacteria or both. Secretion of antidiuretic hormone alsocontributes to the pathogenesis of cytotoxic edema, withresultant hypotonicity of extracellular fluid and increasedpermeability of the brain to water (62). Interstitial edemareflects obstruction of flow in normal CSF pathways (e.g.,from the subarachnoid space to blood), as in hydrocephalus.In an experimental rabbit model of pneumococcal or E.coli-caused meningitis, the CSF outflow resistance, defined(and quantified) as factors that inhibit the flow of CSF fromthe subarachnoid space to the major dural sinuses, wasmarkedly elevated (133); these alterations remained for aslong as 2 weeks despite rapid CSF sterilization with penicil-lin therapy. An increase in the outflow resistance to CSFmovement may cause interstitial brain edema and/or theresultant hydrocephalus during bacterial meningitis.Subsequent studies have examined these concepts in more

detail by measuring the water content of the brain (indicativeof cerebral edema if elevated), concentrations of lactate inCSF, and CSF pressure in animals with pneumococcalmeningitis (154). All three parameters were elevated ininfected animals. Treatment with ampicillin sterilized theCSF rapidly and normalized the water content of the brain

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and intracranial pressure within 24 h, but the concentrationof lactate in the CSF remained elevated. The bacterial cellcomponents responsible for the production of brain edemawere studied in an experimental model of E. coli-causedmeningitis (155). Treatment with cefotaxime but not chlor-amphenicol induced a marked rise in concentrations ofendotoxin in the CSF that were associated with an increasein the water content of the brain. These effects were neu-tralized by either polymyxin B or a monoclonal antibodyagainst lipid A, indicating that increased concentrations ofendotoxin in CSF may be associated with brain edema. Theputative role of the leukocyte in these processes was re-cently examined in an experimental meningitis model (152).In sterile meningitis induced by the intracisternal inoculationof N-formylmethionylleucylphenylalanine (fMLP), a chemo-tactic peptide, both high (10-' M) and low (10-5 M) dosesinduced a CSF pleocytosis, although only high doses pro-duced an increase in the water content of the brain; intra-cranial pressure and the concentrations of lactate and pro-tein in the CSF were unaltered. No increase in the watercontent of the brain was observed in neutropenic animals.When high doses of N-formylmethionylleucylphenylalaninewere injected during the course of pneumococcal meningitisin rabbits, the results were similar, suggesting that neutro-phils contributed to cerebral edema if adequately stimulated.The parameters of increased intracranial pressure and in-creased concentrations of lactate and protein in the CSFseemed unrelated to the presence of neutrophils. This arearemains controversial, however, because neutrophils arerequired for the increased BBB permeability seen in re-sponse to intracisternal injection of bacterial products andinflammatory mediators (112, 183). Additional studies areneeded to more precisely define the role of the neutrophilwithin the CNS in the pathophysiology of bacterial menin-gitis.

Variability among bacterial strains may also be an impor-tant determinant in the production of brain edema. A recentstudy found that intracisternal injection of three differentpneumococcal isolates resulted in pronounced differences inthe pathophysiologic profiles 24 h after challenge (153).Following intracisternal inoculation of pneumococcal cellwall fragments, the chemical composition of the fragments,specifically the degree of teichoication, was found to influ-ence the induction of brain edema during bacterial meningi-tis. Spontaneous release of cell wall is reduced for certainpneumococcal strains and may thus limit their inflammatorypotential. Further work is needed, however, to determinewhether these differences affect the clinical expression ofdisease.

CEREBRAL VASCULITIS

Bacterial meningitis exerts profound effects on bloodvessels coursing through the subarachnoid space (116). Theresulting vasculitis leads to narrowing and/or thrombosis ofcerebral blood vessels and the propensity for ischemiaand/or infarction of underlying brain. Arteriography in chil-dren with bacterial meningitis uniformly demonstrates leak-age of contrast material or other vascular abnormalities inthe subarachnoid space, although these changes reverted tonormal following successful antibiotic therapy. Severe neu-rologic complications (e.g., hemiparesis and quadriparesis)with permanent sequelae may result from involvement of thelarge arteries at the base of the brain (59). Vasospasm mayalso occur secondary to release of humoral factors elabo-rated within the CSF or blood vessel wall and may subse-

quently lead to vasodilatation or organic stenosis or bothlater in the course of disease (185). Phlebitis of the majorcortical draining vessels or dural sinuses or both may resultin thrombosis with secondary brain infarction, focal neuro-logic deficits, and prominent seizure activity.

ALTERATIONS IN CEREBRAL BLOOD FLOW

In combination with increased intracranial pressure, cere-bral vasculitis may result in altered cerebral blood flow inpatients with bacterial meningitis. In the infant rhesus mon-key model of H. influenzae meningitis, cerebral blood flow(measured by an autoradiographic technique utilizing[14C]antipyrine) was lower in certain areas of the cortex(postcentral, temporal, and occipital areas) than in thehypothalamus and midbrain while the brain stem was hyper-perfused, suggesting that one of the initial physiologicchanges in H. influenzae meningitis is cerebral corticalhypoperfusion with resultant relative cerebral anoxia (141).A recent report has demonstrated that cerebrovascular au-toregulation is lost in experimental bacterial meningitis(171). Cerebral blood flow was increased when systemicblood pressure was raised and decreased when blood pres-sure was lowered, indicating that flow was pressure passive.Similar changes were observed in intracranial pressure;increased blood flow led to increased intracranial pressure.Furthermore, in an experimental rat model of meningitis(105), an increase in cerebral blood flow was observed withinthe first few hours of intracisternal inoculation of either livepneumococci or pneumococcal cell wall fragments. Theseresults suggested that maintenance of adequate intravascularvolume status and minimization of stimuli that increasesystemic blood pressure may be important in the treatmentof bacterial meningitis and of potentially practical clinicalrelevance. Measurement of cerebral blood flow (by thexenon-133 intra-arterial injection method) in an earlier studyof patients with bacterial meningitis revealed a 30 to 40%reduction in average total blood flow in five patients withpneumococcal meningitis (mean age, 54 years) but not in fivepatients with meningococcal meningitis (mean age, 20 years)(165). An inverse relationship between cerebral blood flowvelocity and intracranial pressure has been seen in infantswith bacterial meningitis (76). Among eight patients, thesealterations were detected only in the four older infants (ages,3 to 10 months) and not in the four neonates (ages, 5 to 30days), in whom no changes in cerebral blood flow velocitywere observed.A subsequent study that measured total and regional

cerebral blood flow by stable xenon computed tomographyin 20 children seriously ill with bacterial meningitis revealeda global decrease in cerebral blood flow and even moreregional variability (7). Autoregulation of cerebral blood flowwas preserved in the patients studied, although hyperventi-lation reduced cerebral blood flow below the ischemicthreshold, raising important concerns about the routine useof hyperventilation in the management of increased intracra-nial pressure in patients with bacterial meningitis. Someauthors have suggested that in infants and children withbacterial meningitis and initially normal computed tomogra-phy or magnetic resonance imaging scans, the chance thatcerebral blood flow would be reduced to ischemic levels isunlikely, and in this situation hyperventilation may safelyreduce elevated intracranial pressure for the first 24 to 48 hbefore its effect diminishes (8). However, in children withcerebral edema revealed by neuroimaging studies, cerebralblood flow is more likely to be normal or reduced, so that



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TABLE 3. Effect of cyclooxygenase and lipoxygenase inhibitors on CSF leukocytosis in rabbits after intracisternalinjection of 30 p.g of pneumococcal cell walls'

Compound anDose (mg/k Times (h)C CSF leukocytosisd after:androute" ~~~~~~~ ~~~5h 7 h 24 h

Control 677 34 1,545 ±90 870 ± 92Methylprednisolone 30, i.m. -1 50 ± 20 198 ± 30e 54 ± 20Diclofenac sodium 5, i.v. -1, +2 110 ± 40 433 ± 210e 1,183 ± 150Indomethacin 5, p.o. -1, +2, +5 155 ± 60 300 ± 50 320 ± 50Nordihydroguaiaretic acid 5, p.o. -1, +2, +5 1,093 - 500 870 ± 240Oxindanac 5, p.o. -1, +2, +5 118 ± 57 59 ± 14e 500 ± 325

a Reproduced from reference 167 with permission of The University of Chicago Press.bi.m., intramuscular; i.v., intravenous; p.o., oral.c Time of administration in reference to time in hours of cell wall challenge dose.d Mean number of leukocytes per microliter of CSF + standard deviation. Value at 0 h, 28 + 14 cells per p.I.e p < 0.01 compared with control at 7 h.

hyperventilation may decrease the intracranial pressure atthe expense of a significant reduction of cerebral blood flow,possibly approaching ischemic thresholds. These patientsmay benefit from early use of diuretics (e.g., furosemide) orosmotically dehydrating agents such as mannitol, providingthat intravascular volume is protected. Corticosteroids mayalso be useful in this situation. However, it must be stressedthat controlled clinical trials examining these issues have yetto be performed. Although the definitive changes in cerebralblood flow during bacterial meningitis are controversial andmay vary with the stage of disease, these blood flow alter-ations may lead to regional hypoxia, increased concentra-tions of lactate in the brain secondary to utilization ofglucose by anaerobic glycolysis, and CSF acidosis, whichmay be a precursor to encephalopathy. This phenomenonhas recently been examined in an experimental rabbit modelof pneumococcal meningitis (173) in which animals given alower fluid regimen (50 ml/kg per 24 h) of normal saline hadlower mean arterial blood pressure, lower cerebral bloodflow, and higher concentrations of lactate in the CSF com-pared with animals that received a higher fluid regimen (150ml/kg per 24 h). These results suggest that intravascularvolume status may be a critical factor in determination ofcerebral blood flow and, therefore, the degree of cerebralischemia in meningitis.

ADJUNCTIVE THERAPEUTIC STRATEGIES

Experimental Studies

Because of the information supporting subarachnoid spaceinflammation as a major factor contributing to morbidity andmortality from bacterial meningitis, several studies in exper-imental animal models have examined whether attenuationof this inflammatory response might be beneficial. As statedabove, the generation of pneumococcal cell wall componentsafter treatment with bacteriolytic antibiotics in the experi-mental rabbit model may contribute to an increased inflam-matory response within the subarachnoid space (168, 169).This CSF inflammatory response was reduced by agentsknown to exert their effects by inhibition of the cyclooxyge-nase pathway of arachidonic acid metabolism (Table 3)(167). Treatment with methylprednisolone or oxindanac inaddition to antimicrobial agents was particularly effective indecreasing pneumococcal cell wall-induced inflammation,whereas another inhibitor, diclofenac sodium, was effectiveonly when administered 5 and 7 h after inoculation ofbacteria into CSF, but did not produce an inhibitory re-

sponse 24 h after challenge with cell wall. An inhibitor of thelipoxygenase pathway, nordihydroguaiaretic acid, was inef-fective in preventing cell wall-induced inflammation. Similarresults were observed with these adjunctive agents afterintracisternal challenge with live pneumococci. There was acorrelation between the concentrations of the arachidonicacid metabolite PGE2 and of leukocytes in the CSF afterintracisternal challenge with either live pneumococci orpneumococcal cell walls, and inhibition of the cyclooxyge-nase pathway reduced CSF inflammation and the concentra-tion of PGE2 in the CSF. Administration of the nonsteroidalanti-inflammatory agent indomethacin also led to a decreasein both the water content of the brain and concentrations ofPGE2 during experimental rabbit pneumococcal meningitis,although there was no reduction in intracranial pressure(172). In addition, an anti-inflammatory agent (dexametha-sone or oxindanac) lessened the massive influx of serumalbumin and other proteins of high and low molecular massesinto the CSF during the early phases of experimental pneu-mococcal meningitis (61). Ampicillin given alone or in com-bination with indomethacin was ineffective in preventing thisinflux, and the abnormal protein profile in the CSF persistedfor up to 30 days after the initiation of the experimentalinfection.

Several corticosteroid agents have been evaluated in ex-perimental animal models of bacterial meningitis. Earlystudies revealed a significant reduction in the mass ofleukocytes within the meninges of rabbits with pneumococ-cal meningitis following methylprednisolone administrationcompared with concentrations in infected controls (97);chemotactic activity, chemotactic response, and phagocyto-sis in CSF were, however, not altered by methylpred-nisolone treatment, although there was an attenuation ofrabbit neutrophil adherence to a nylon fiber column. CSFoutflow resistance (defined as factors that inhibit the flow ofCSF from the subarachnoid space to the major dural sinusesand quantified following infusion of mock CSF into thesubarachnoid space) was also reduced by methylpred-nisolone therapy and to a greater extent than in untreated orpenicillin-treated rabbits with pneumococcal meningitis(133). The reduction in resistance was apparent within 4 h ofthe second injection of methylprednisolone (at a dose of 30mg/kg of body weight intramuscularly 16 and 20 h followingintracistemal inoculation); a rebound increase in CSF out-flow resistance was not seen after corticosteroid therapy wasdiscontinued.The effects of methylprednisolone or dexamethasone on

water content of the brain, CSF pressure, and lactate con-

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TABLE 4. Influence of methylprednisolone and dexamethasone on manifestations of a 24-h infection with pneumococci in rabbitsa

Treatment Bacterial titer Leukocyte count Concn of lactate Change in CSF Water content (g(log10o CFU/ml) (103/mm3) in CSF (mg/dl) pressure (mm Hg) [dry wt]) of brain

No infection ND <0.01 14.0 ± 2.6 +0.8 ± 1.4 396 ± 1424-h infectionNo treatment 7.0 ± 1.2 2.5 ± 6.6 75.3 ± 25.6 +7.5 ± 6.5 410 ± llbMethylprednisolone 7.6 ± 0.7c 3.9 ± 4.9 64.3 ± 33.1 +7.8 ± 5.4 395 ± 9Dexamethasone 6.7 ± 0.6 2.1 ± 2.2 43.8 ± 12.3 +1.8 ± 2.7d 399 ± 12

a Reproduced from reference 154 with permission of The University of Chicago Press. All values are expressed as means + standard deviations. ND, notdetermined.

b p < 0.02 in comparison with other groups.c P < 0.05 in comparison with dexamethasone-treated group.d p < 0.02 in comparison with other groups infected for 24 h.

centrations in the CSF of rabbits with pneumococcal men-ingitis have also been examined (Table 4) (154). Administra-tion of both agents completely reversed the development ofbrain edema at the times studied, but only dexamethasonereduced the increases in CSF pressure and lactate; methyl-prednisolone, but not dexamethasone, was associated withan increase in concentrations of bacteria in CSF. However,neither agent was superior to therapy with ampicillin alone inreducing cerebral edema or intracranial pressure. No com-parison between ampicillin alone and ampicillin plus corti-costeroids, a comparison that would be relevant to thepotential clinical efficacy of adjunctive corticosteroids inhumans, was made. In another study, treatment with ceftri-axone was compared with treatment with ceftriaxone plusdexamethasone in an experimental rabbit model of H. influ-enzae meningitis (150). Combination therapy consistentlyreduced the water content of the brain, CSF pressure, andthe lactate concentration in the CSF to a greater degree thanceftriaxone alone, although the differences were not statis-tically significant. By 29 h after inoculation (a well-estab-lished disease in this model), the values were comparablewhether the animals received antibiotic alone, dexametha-sone alone, or the combination. The authors suggested thatdexamethasone might be more beneficial if administeredearly during antibiotic therapy after (or even before) antibi-otic-induced lysis and release of microbial products. In asubsequent analysis utilizing the rabbit model of H. influen-zae type b meningitis (89), a significant increase in concen-trations of endotoxin in CSF was documented 2 h followingceftriaxone administration; this increase was followed by arise in TNF concentrations in CSF (Fig. 5). Simultaneousadministration of dexamethasone and ceftriaxone did notaffect the appearance of endotoxin in CSF, but there was amarked attenuation in concentrations of TNF in CSF mea-sured 8 h later. Adjunctive dexamethasone therapy alsoresulted in a significant decrease in the resultant CSF leuko-cytosis and a trend towards earlier improvement in concen-trations of glucose, lactate, and protein in CSF.

Therefore, adjunctive dexamethasone therapy in concertwith antimicrobial agents appeared to improve a number ofparameters of subarachnoid space inflammation in experi-mental animal models of bacterial meningitis. These param-eters improved without any apparent decrease in the rate ofbacterial killing within the CSF in vivo, although otherexperimental studies have shown that administration ofmethylprednisolone decreased the entry of ampicillin andgentamicin into CSF (132).The subarachnoid space inflammatory response may con-

tribute to the pathogenesis of hearing loss in bacterial

meningitis, which is due in part to the development oflabyrinthritis following spread of infection to the inner ear.The infant rat model has been utilized to determine theinfluence of corticosteroid administration on the inflamma-tory reaction in the cochleas of infected animals (63). Infantrats were inoculated intraperitoneally with H. influenzaetype b, and 24 h later they were treated with ampicillin orampicillin plus dexamethasone. At 48 h, concentrations ofleukocytes in CSF were significantly lower in the dexameth-asone group than in the ampicillin-alone group, although nohistologic differences in the degree of cochlear inflammationwere noted between groups. The extent of cochlear inflam-mation was minimal only in the ampicillin group, however.

Pentoxifylline, a phosphodiesterase inhibitor that de-creases endotoxin-induced TNF-a production and attenu-ates the inflammatory action of IL-1 and TNF on leukocytefunction (147), has also been examined in the experimentalrabbit model of H. influenzae type b meningitis (125).Administration of pentoxifylline 20 min before intracisternalchallenge with H. influenzae type b LPS significantly re-duced concentrations of leukocytes, protein, and lactate inCSF. Peak concentrations of TNF in CSF were reduced bymore than one-third in pentoxifylline-treated animals, al-though this reduction was not statistically significant andwas unlikely to be solely responsible for the marked modu-lation of meningeal inflammation. Dexamethasone was su-perior to pentoxifylline in modulation of these inflammatorychanges in CSF, and no appreciable synergism was observedwhen dexamethasone and pentoxifylline were used together.

Recent studies have examined the effects of a monoclonalantibody (IB4) directed against the CD18 family of receptorson leukocytes on reduction of subarachnoid space inflam-mation. In one study, intravenous inoculation of IB4 blockedthe accumulation of leukocytes in the subarachnoid spacedespite intracisternal challenge with H. influenzae type b, N.meningitidis, pneumococcal cell wall, or LPS (170). Inaddition, the parameters of BBB permeability, developmentof cerebral edema, and death were prevented in the animalschallenged with lethal doses of S. pneumoniae that alsoreceived the monoclonal antibody. In a second study, IB4and dexamethasone were administered together in a rabbitmodel of H. influenzae type b meningitis. The result was amarked attenuation of all indices of meningeal inflammationand a reduction in water accumulation in the brain comparedwith the results obtained when each agent was given aloneand when infected animals were left untreated (124). Despitethis profound amelioration of meningeal inflammation, clear-ance rates of bacteria from CSF to blood and from vascularcompartments were unaffected. These results indicate that



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Time (hours)FIG. 5. (Upper) Comparative changes in concentrations of

TNF-at in CSF in rabbits that received ceftriaxone (CTX) alone at 6h (*, K; n = 5), CTX plus dexamethasone (DXM) simultaneously at6 h (0, 0; n = 6), CTX at 6 h and DXM at 7 h (A, A; n = 4) or no

treatment (controls) (U, El; n = 5). (Lower) Counts of leukocytes(WBC) in CSF for the same four groups of rabbits as in the upper

panel. Meningitis was induced at time zero. Reproduced fromreference 89 with permission of The University of Chicago Press.

dual therapy with agents directed against the CSF produc-tion of cytokines and recruitment of leukocytes into thesubarachnoid space may be associated with improved out-come in bacterial meningitis.

Clinical Trials

Taken together, the experimental studies described aboveindicate that adjunctive dexamethasone therapy diminishesthe CSF inflammatory response and the subsequent patho-physiologic consequences of this inflammation, specifically,elevated water content in the brain and elevated CSF pres-

sure. On the basis of these observations, several trials wereundertaken to examine the effects of adjunctive corticoster-oids on outcomes in patients with bacterial meningitis. Earlyclinical trials performed in the 1960s failed to show any

benefit for adjunctive corticosteroids (either methylpred-nisolone or dexamethasone) (12, 31). However, these trialswere performed before much of the recent information on

pathogenesis and pathophysiology of bacterial meningitishad been elucidated, and the studies also suffered fromseveral flaws (e.g., inadequate corticosteroid dose and/orcontrol group). Therefore, we concentrate on evaluation ofseveral recent and relevant clinical studies published in theliterature.One study was a double-blind, placebo-controlled trial of

adjunctive dexamethasone therapy in infants and childrenwith bacterial meningitis (66). The patients received anantibiotic (cefuroxime or ceftriaxone) with either dexameth-asone or placebo. The patients who received an antibioticplus dexamethasone became afebrile sooner, had more rapidnormalization of CSF parameters (concentrations of glu-cose, protein, and lactate), and were significantly less likelyto acquire moderate to severe bilateral sensorineural hearingloss (15.5 versus 3.3%). In addition, concentrations of IL-1,Bbut not TNF-cx in CSF were significantly lower 18 to 36 hlater in patients given adjunctive dexamethasone. However,these findings were significant only in patients with menin-gitis caused by H. influenzae type b, and the benefits interms of morbidity (sensorineural hearing loss) were statis-tically significant only in patients receiving cefuroxime andnot in those receiving ceftriaxone. The latter point is impor-tant, because cefuroxime has recently been shown to beinferior to ceftriaxone in a randomized prospective study ofthe therapy of childhood bacterial meningitis (129). In addi-tion, four patients who received adjunctive dexamethasonedeveloped gastrointestinal hemorrhage, and two of thesepatients required blood transfusions.A second study, from Egypt, of children and adults with

bacterial meningitis demonstrated a significant reduction inmortality rate and overall neurologic sequelae in patientswith pneumococcal meningitis who received adjunctivedexamethasone therapy concomitant with antibiotics (ampi-cillin plus chloramphenicol) (48). However, no significantdifferences between groups in time to afebrility or improve-ment in CSF parameters were observed; furthermore, theantibiotics were given intramuscularly, there was no docu-mentation of possible adverse effects, and an extraordinarilyhigh percentage of patients presented in a comatose state. Infact, most patients (370 of 429) had received inadequatetherapy for the 3 to 5 days prior to hospitalization. Therewere no differences between the two groups in mortality rateor rate of hearing impairment for patients with meningococ-cal or H. influenzae meningitis, but children with H. influ-enzae meningitis were too young to be tested audiometri-cally.

In a third recently published trial centered in Costa Rica(99), infants and children with bacterial meningitis wererandomized in a placebo-controlled, double-blind fashion toreceive cefotaxime with or without adjunctive dexametha-sone therapy; in this study, the dexamethasone was admin-istered 15 to 20 min before the first dose of cefotaxime in aneffort to attenuate the CSF inflammatory response associ-ated with administration of bacteriolytic antibiotics. Twelvehours after treatment was begun, meningeal inflammationand concentrations of TNF-a and platelet-activating factorin CSF had decreased more rapidly in dexamethasone-treated patients. In addition, by 24 h, the clinical conditionsand mean prognostic scores were significantly better amongpatients receiving adjunctive dexamethasone therapy. Whenthe patients were monitored for a mean of 15 months, thosewho had received adjunctive dexamethasone had a signifi-cantly decreased incidence of one or more neurologic se-quelae, although reduction of audiologic impairment was

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only a trend. Overall mortality was not reduced in thedexamethasone group.

Despite the studies described above, controversy regard-ing the routine use of adjunctive dexamethasone therapy inall patients with bacterial meningitis remains (75, 165). Thedata support the use of adjunctive dexamethasone in infantsand children with bacterial meningitis caused by H. influen-zae type b. Routine use of dexamethasone therapy foradults, however, cannot be presently recommended, pend-ing results of ongoing studies. If dexamethasone is used, thetiming of administration is crucial. Administration before orwith antibiotics is optimal for attenuating the subarachnoidspace inflammatory response. Patients receiving such ther-apy also need careful monitoring (in addition to the usualroutine for critically ill patients) for the possibility of gas-trointestinal hemorrhage. Future studies of the pathogenesisand pathophysiology of bacterial meningitis should lead tothe development of other adjunctive treatment strategiesthat may improve the outcome of this disorder.

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Pathogenesis and Pathophysiology of Bacterial Meningitis · ism to nasopharyngeal epithelial cells (Fig. 2) (143, 145). Meningococci, like gonococci, possess fimbriae that differ