Subdural Injection

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ANESTH ANALG 175

YXS.67.17i9

Inadvertent Subdural Injection:

A

Complication

of

an Epidural Block

Timothy Lubenow,

MD,

Elisa Keh-Wong, MD, Kathy Kristof,

BSN

Olga Ivankovich, MD,

and Anthony

D.

Ivankovich,

MD

LUBENOW

T,

KEH-WONG E, KRlSTOF K

IVANKOVICH 0 , IVANKOVICH AD. Inadvertent

subdural injection: a complication

of

an epidural block

Anesth Analg 1988;67:175-9.

Tzilenty-one h i d r e d eighty two consecutizle lzirribar epi

dural injections

zcwe

studied to determine

the

incidence

of

iriaiiziertent

subdural

hlock retrcispectiuel /. sitbtliiral

block is defined s an estensiz1e

neurd

block in

the

absence

of

S I ~ J U Y ~ C ~ ~ J O ~ ~

zriicturc,

that

is

out

of

proportion

to the

amount of local anesthetic injected. Siibdural injection s

conr;ilicntioii of epidural block that probabl y occiirs t l ow

frequently than previously recognized.

n

earlier report ins

esf i innted the incidence of siibdlird block

to

be

0 . 7

5 . This

study, liozuezlcr, reports n incidence of 0.82% froni a

s m p l e size of 2182 patients.

Cndarwic

dissection

700s

also

~~erfor ir ied,iirtlier

clarifying tlic

presence a n d riiztcnnic

position

of

the siibdurnl

spare.

Key

Words:

ANESTHETIC

T E C H N I Q U E S -

epidural.

It is generally accepted that the subdural space exists

in the cerebral meninges. The potential extension of

this subdural space, however, down into the spinal

segment of the meninges has not been well appreci-

ated. This subdural space can have clinical signifi-

cance when local anesthetics are inadvertently depos-

ited there, causing unexpected sensory, sympathetic,

and motor blocks.

Clinically the extraarachnoid space has been dem-

onstrated during myelograms with an incidence re-

ported between 1and 13%(1-3). This extraarachnoid

subdural space lies between the dura and arachnoid

membranes. It contains a small amount of serous

fluid to moisten the surfaces of the opposing mem-

branes. While not communicating with the subarach-

noid space, the extra-arachnoid subdural space does

continue for a short distance along the cranial and

spinal nerves (4). It is larger in the cervical than in

lumbar region, and is widest in its lateral and dorsal

aspects (5). Here, there is free communication with

the lymphatic vessels of the spinal nerves. Moreover,

there are isolated connective-tissue trabeculae, espe-

cially on the posterior aspect, which contact the

Presented a t the 61st Congress of the Interna tional Anesthes ia

Research Society, March 1418, 1987, Orlando, Florida.

Received from the Department

of

Anesthesiology, Rush Prys-

byterian St.

Lukes

Medical Center, Chicago, Illinois. Accepted for

publication

on

Oct.

1

1987.

inside surface of the dura and the outside surface of

the arachnoid.

Accidental subdural injections were first described

by de Saram

(6)

and Dawkins

7) ,

but no large series

have examined its occurrence. There has been several

case reports of accidental subdural catheterizations

that have been radiographically confirmed (&lo) .

Dawkins description of a massive epidural fits the

clinical presentation of an inadvertent subdural injec-

tion. He describes an unexpected widespread nerve

block occurring after a negative aspiration test asso-

ciated with symptoms such as pupillary dilation,

consistent with a high sympathetic block. In addition,

the patients experienced a 20-minute delay in the

onset of symptoms. This is in contrast to an acciden-

tal subarachnoid injection in which symptoms char-

acteristically develop in

1-2

minutes. The purpose of

this study is to retrospectively evaluate

a

large series

of epidural injections to determine the incidence of

inadvertent subdural block.

Methods

During the 30-month study period (March

1984-

September

1986),

2182 lumbar epidural steroid injec-

tions were performed at the Pain Center for various

forms of low back pathology. During this period any

patient whu exhibited any untoward or unpleasant


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176

A N E S l H

ANALG

1988:67

759

L UB E NOW

ET

AL.

side effects from the injection (e .g ., headache, hypo-

tension, nausea, motor or extensive sensory block)

was identified for follow-up. The patients ranged in

age from 17 to 86 years and each received a single

epidural injection via a lumbar interspace, between

L1

and L5. The blocks were performed by an attend-

ing anesthesiologist or

a

supervised resident using

bupivacaine, 4-6 cc of 0.25% or 6-8 cc of 0.125%, in

combination with methylprednisolone acetate 80-120

mg (Depo Medrol, Upjohn Company, Kalamazoo,

Michigan). The epidural space was identified by the

loss-of-resistance technique. After a careful negative

aspiration test, injections were performed with dis-

posable 17- or 18-gauge Touhy point needles. Aspi-

ration was routinely done before, during, and after

each injection. After the injections, the patients were

observed for approximately 1 hour before discharge

from the center.

Records were evaluated in the following manner

for the presence or absence of clinical findings con-

sistent with subdural injection. I n any patient exhib-

iting a complication as mentioned above, a detailed

description of the complication and clinical findings

was obtained and recorded in the patients chart at

the time

of

occurrence. Clinical findings were classi-

fied into two levels of criteria, major and minor.

Findings considered major criteria were:

1)

a negative

aspiration test, or 2) an unexpected widespread sen-

sory block after epidural injection. The three minor

criteria were:

1)

a delayed onset of

10

minutes or more

of

a

sensory or motor nerve block, 2)

a

variable motor

blockade occurring, despite use of low doses of

bupivacaine, or

3 )

sympatholysis out of proportion to

the administered dose of local anesthetic. A positive

subdural injection was judged to have occurred in

both of the major criteria and at least one minor

criteria were present. With the criterion of negative

aspiration test we excluded any patient who had a

wet tap before the apparent successful epidural injec-

tion. All of these records of morbid events were then

retrospectively evaluated by one reviewer (TL)

to

determine if criteria for a subdural block were

present. From 38 potential subdural injections, 18

were judged by an additional investigator (ADI) as

having met the criteria for

a

subdural injection.

Results

Eighteen patients met the criteria for

a

subdural

block, establishing an incidence of 0 .82%. One pa-

tient exhibited all three minor criteria, while an

additional seven patients displayed two of the minor

criteria (Table

1 .

All 18 patients developed sensory levels much

higher than would be expected from the amount of

local anesthetic administered. One patient had a

sensory level of C4 after injection of 6 cc of 0.25%

bupivacaine. In none of the 18 patients was CSF

aspirated. Ten of the 18 patients developed motor

block. Delayed onset times of greater than 10 minutes

were noted in

11

patients (61%)with the longest time

to onset of symptoms being

30

minutes. Hypoten-

sion, defined as a drop in systolic pressure of at least

30 from baseline, occurred in 11 patients. Eight of

the 11 patients had moderate to severe hypotension

with a drop in pressure greater than 40 of the

baseline. Six of these patients had severe decreases in

blood pressures. In all cases, hypotension responded

to fluids or ephedrine

( 5 1 5

mg).

Five of the 18 patients (2870) had had previous

back surgery. These five patients represent

a

higher

percentage of patients than what is seen in our

overall patient population (12%)).Six of the 18 re-

ceived 0.25% bupivacaine, while 12 received 0.125%

bupivacaine.

Further studies were also performed on cadavers

to provide additional information on the subdural

space. The existence

of

the subdural space was con-

firmed by cadaveric dissection. A lumbar laminec-

tomy was performed and the spinal cord and me-

ninges were exposed from the S1 to the L 1 levels.

Dissected dura mater

was

found to have two layers:

an outer, thicker, opaque layer and an inner, more

translucent layer. Deep to these layers there existed a

potential space easily identified after reflecting the

dura mater. The arachnoid mater was noted to be

a

translucent membrane separating the subdural space

from the subarachnoid space. Deep to the arachnoid

mater the spinal nerves and subarachnoid space were

identified. Our depiction of the anatomy is similar to

the description made 23 years ago

by

Sechzer

(11).

Discussion

Epidural nerve blocks occasionally exhibit an atypical

pattern of spread. This may be caused by relative

overdose or accidental injection into the subdural or

subarachnoid spaces . Several investigators have

demonstrated radiological confirmation of catheters

present in the subdural space, especially in cases of

massive epidurals (8,12). A recent report describes

the ease of intentional subdural puncture and further

suggests that accidental subdural punc ture may occur

in attempted epidural block even in experienced

hands

(13).

Consequently, it appears that accidental

subdural injection probably occurs more frequently

than previouslv recognized.


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INADVERTENT SUBDURAL INlECTlON OF EPIDURAL BLOCK

ANE S T H ANAL G 177

1988,67 175-9

Table

1.

Summary

of Patient

Data

Bupivacaine Onset Previous Major Minor

Patient concentrat ion Aspiration Level Sensory Motor Degree of time Recovery back criteria criteria

no.

( )

Vol. test injected level block hypotens ion (min) time (hr) surgery met met

1

2

3

5

6

7

8

9

10

11

12

13

4

15

16

17

18

0 .25

0.25

0.25

0.125

0.125

0.125

0.125

0.125

0.125

0.125

0.125

0 .25

0.125

0.125

0.125

0.125

0.25

0.25

6

4

6

8

8

8

8

8

8

8

8

6

8

8

8

8

6

6

T12-Ll T4

L4-5 L2

L 3 4 T4

L 3 4

T2

L 3 4 T10

L 3 4 T I2

L 3 4 T6

L 3 4 T I2

L 4 5 T8

L2-3 T10

L M L10

L 3 4

T10

L1-2 T6

L4-5 T10

L2-3 T4

L 3 4 T9

L 4 5 c 4

L3-4 T2

Dense,

LE

bilateral

Dense, LE

bilateral

Moderate,

LE

bilateral

Moderate,

LE bilateral

None

Mild LE

bilateral

Dense,

LE

bilateral

None

None

Mild,

LE

bilateral

None

None

None

Dense, Le

bilateral

Moderate,

LE

bilateral

Dense, LE

bilateral

None

None

40 )

None

50

50

None

None

None

30

50

None

None

30%

4 0 4

None

50

None

50

50

1 0 3 . 5

10 4.0

10 3 .0

20 6.0

5 3.0

5 2.0

5 3.0

30 2.0

30 2.0

10 4.0

10 2 .0

5 1 .5

20 3.0

5

3 .0

5 3.0

5 2 .0

10 3 .0

15 3 .5

Yes, fusion

L 4 5 , 5 -s 1

No

NO

No

No

No

No

No

Yes, LAM

x 2

Yes, LAM

Yes, LAM

N o

Yes, LAM

No

No

N

No

No

2 2

- 1

2 2

-

3

1

2 1

2

1

2

2 2

2 1

-

1

2 1

2 2

2

I

2 2

2 1

- 1

2 2

7

7

7

3

L AM , laminectorny, LE,

lower

extrcmity

Intentional neurolytic subdural puncture has been

previously described (14). This technique involves

identification of the epidural space using the loss-of-

resistance technique. The needle is then rotated

through an arc of 180 with applied gentle pressure.

In order to avoid accidental subdural puncture, the

authors believe that a properly placed epidural nee-

dle should never be rotated to point the bevel in a

superior or inferior position. If one rotates the needle

to produce an intentional subdural puncture, this

same practice, if repeated for an epidural block, may

produce an accidental subdural puncture.

The three most common features noted in this

study were: 1) an unexpectedly high sensory block, 2)

exaggerated hypotension, and 3) unexpected motor

block. An interesting characteristic

of

subdural blocks

in the study is the variability in onset time. The

fastest onset time was between 5 and 10 minutes,

while other patients did not notice symptoms or

exhibit signs until 30 minutes after injection. These

findings do not differ significantly from other studies.

Case reports have documented the onset of symp-

toms to be as long as 30 minutes. Other descriptions

of accidental subdural injections have reported onsets

to be

as

short as

7

minutes (15). We believe that

subdural blocks do exhibit

a

variability in onset time.

This is dependent, perhaps, upon the relative

amount of local anesthetic deposited in the subdural

space, and may

also

be responsible for the wide-

spread sensory block and exaggerated hypotension.

Another explanation for the unexpected high sensory

and sympathetic blocks may be that previous back

surgery produced scarring and cicatrization, thereby

partly obliterating the epidural space in the lower

lumbar area. This partial obliteration of the epidural

space may cause marked cephalad spread. There are,

however, many exceptions to this hypothesis. Only

5

of the 19 patients had had previous back surgery. The

patients who had the most dramatic symptoms (pa-

tients 3, 4, and 17)had had no back surgery. Three of

the six patients who had previous back surgery

(Patients 10, 11, and 13) were among those who

exhibited the mildest symptoms. The presence of

previous back surgery with deformity of the epidural

space does not explain all of the observed events.

However, it appears that patients who have had back


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178 ANESTH ANALG

1988.67:17'59

LUBENOW ET A1

surgery are more prone to accidental subdural injec-

tion. This is likely because the anatomy may be

altered secondary to scarring and retraction, produc-

ing

a

thin epidural and wide subdural space.

Epidural blocks seem more likely to produce acci-

dental subdural injection than do spinal blocks. This

may be due to differences in technique and the type

of needle used. Epidural injections use a large, blunt-

tipped, long-bevel needle that is introduced very

slowly, sometimes

a

millimeter at

a

time. In contrast,

for a subarachnoid puncture,

a

thinner, sharper nee-

dle is introduced, usually at

a

much faster rate.

I t

is

more likely that the blunt needle tip will pierce the

dura without piercing the arachnoid. The large open-

ing of the epidural needle may straddle the subdural

and epidural, allowing part of the local anesthetic to be

injected into the subdural space while some of

it

could

be deposited in the epidural space (Fig.

1).

This

partitioning of anesthetic may explain the difference in

degree of symptoms. Patients experiencing profound

sensory and motor block obviously would have had

more anesthetic deposited in the subdural space.

Another explanation regarding the difference in

symptomatology may relate in part to the anatomic

distribution of sensory, sympathetic, and motor

nerve fibers. The anterior nerve roots carry motor and

sympatlietic nerve fibers, while sensory fibers are

within posterior nerve roots. Because the subdural

space has more potential capacity posteriorly and

laterally, one should expect a sensory block. Mean-

while,

a

motor or sympathetic block would be present

only if local anesthetic traveled anteriorly within this

Figure 1. This i l lustration depicts rel-

ative position

of

intrathecal, epidural,

and subdural needle placement . Note

that if the needle is in the subdural

space, w i th the dura s t raddl ing the

bevel, some

of

the local anesthetic

may be deposited in the subdural

space while so me will be placed in the

epidural space.

subdural space (Fig. 2). Therefore, positioning of the

patient after the block would influence the type of

block to

a

large extent. Moreover, a motor and

sympathetic block would occur more readily if a

patient were in the lateral position, whereas sensory

block would predominate

if

the patient were supine

after the injection.

The absence of significant hypotension, in con-

junction with

a

profound motor block as demon-

strated by some of our patients, may reflect hydration

status more than anything else. The hypotension

seen in our patients was dramatic in certain cases, but

was easily treated in

all

cases with relatively

small

amounts of fluid (250-500 cc) and small doses of

ephedrine. Only one patient required

15

mg ephed-

rine. All others with hypotension responded to 5 or

10 mg ephedrine. Hypotension, which is easily

treated, has been a feature of all previously confirmed

subdural injections (16). This contrasts accidental

subarachnoid injection where hypotension is charac-

teristically more profound and difficult to correct.

The cadaveric dissection was perforined to further

exemplify the presence and anatomic proportion of

the subdural space. It

is

well accepted that the sub-

dural space exists in the cerebral meninges, and that

certain clinical entities are seen when pathology is

present in the subdural space (e.g., subdural hema-

toma). However, the extension of the subdural space

down into the spinal segment of th e meninges has

been previously regarded b y some authors as having

questionable clinical significance

(4).

Our dissection

support s the presence of the subdural space within


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INADVERTENT SUBDURAL INJECTION

OF

EPIDURAL BLOCK

ANESTH ANALG

19SS;h7

175-9

179

Figure

2.

This illustration shows the

anatomic relationship

of

dura and

arachnoid. The subdural space exists

as

a

potential space encircling the

arachnoid membrane and contained

within the dura.

the spinal cord segment of the meninges. A previous

study on autopsy subjects has also portrayed the

subdural space as a readily identifiable potential

space. In our dissection, the potential subdural space

and its relationship to the dura a nd arachnoid mem-

branes was found to be similar to its portrayal by

other authors (4,11,13). A s depicted in Figure

1,

a

needle may pierce the dura but not the arachnoid and

be contained within the subdural space. Local anes-

thetics, if deposited here, can travel cephalad and

caudad in this narrow potential space, producing the

unexpected extensive sensory, sympathetic, and mo-

tor blocks encountered in this series of therapeutic

epidural drug depositions.

In conclusion, after subdural deposition of a local

anesthetic, the development of an extensive sensory

and motor block, with or without hypotension, may

occur up to 30 minutes after the injection. The differ-

ential diagnosis

of

a possible subdural injection

should be entertained as readily as one would sus-

pect a subarachnoid injection. subdural block

should be considered when there has been extensive

sensory or motor blockade after a negative CSF

aspiration test when small volumes and dilute con-

centrations of local anesthetics are utilized. We rec-

ommend that outpatients receiving epidural injec-

tions of any amount of local anesthetics be observed

for at least 1 hour before discharge because

of

poten-

tial for a subdural injection.

References

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

14.

15.

16.

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the epidural space. Anaes-

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Williams PL, Warwick

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Goldstein M. Migration

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Subdural Injection