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Anatomy and Pathophysiology of Spinal Cord Injury ... · PDF fileintroduced caudad to the conus medullaris, thereby ... tional contact with the easily moveable cauda equina may bear

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    natomy and Pathophysiology of Spinal Cordnjury Associated With Regional Anesthesiand Pain Medicine

    oseph M. Neal, M.D.

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    he American Society of Regional Anesthesia andPain Medicine (ASRA) convened a panel in April

    005 to create a Practice Advisory on the Neurologicomplications of Regional Anesthesia and Pain Med-

    cine. This review deals with the pathophysiology ofpinal cord injury. The Practice Advisory recommen-ations are based on extensive review of existing an-mal and human studies, case reports, pathophysiol-gy, and expert opinion.The pathophysiology of spinal cord injury associ-

    ted with anesthesia techniques is reviewed inepth, including mechanical trauma from directeedle injury or mass lesions, vascular injury fromirect needle trauma or spinal cord infarction, andeurotoxicity from local anesthetics and adjuvants.ight specific recommendations are offered thatay reduce the likelihood of spinal cord injury

    ssociated with regional anesthetic or pain medi-ine techniques. Spinal cord injuries associated withegional anesthesia and pain medicine are exceed-ngly rare. The Practice Advisorys recommenda-ions may, in selected cases, reduce the likelihood ofnjury, but the vast majority of these injuries areeither predictable nor preventable.Injury to the neuraxis as a consequence of regional

    nesthesia or pain medicine procedures is ultimatelyinked to anatomic and/or physiologic damage to thepinal cord, the spinal nerve roots, or their bloodupply. Mechanisms of injury are sometimes identifi-ble, as in the case of epidural hematoma, but can also

    From the Department of Anesthesiology, Virginia Mason Med-cal Center, Seattle, WA.

    Accepted for publication October 3, 2006. Updated July 3,008.James P. Rathmell, M.D. served as acting Editor-in-Chief for

    his manuscript.Presented as part of the American Society of Regional Anesthesia

    nd Pain Medicines Practice Advisory on Neurological Complica-ions of Regional Anesthesia and Pain Medicine, Toronto, ON,anada, April 23, 2005.Reprint requests: Joseph M. Neal, M.D., Department of Anes-

    hesiology, Virginia Mason Medical Center, 1100 Ninth AvenueB2-AN), Seattle, WA 98101. E-mail: [email protected]

    2008 by the American Society of Regional Anesthesia andain Medicine.

    r1098-7339/08/3305-0001$34.00/0doi:10.1016/j.rapm.2006.10.014

    Regional Anesthesia and Pain Medicine, Vol 33, No

    e exceedingly difficult to pinpoint, as exemplified byost cases of presumed spinal vascular injury. This

    rticle will review the pathophysiology of spinal cordnjury, including mechanical, vascular, and neuro-oxic etiologies. Its goal is to provide an anatomic andathophysiologic basis from which to build an under-tanding of neuraxial complications associated withegional anesthesia and pain medicine.

    echanical Injury

    Many neuraxial anesthetic complications are sec-ndary to mechanical injury of the spinal cord,pinal nerve roots, or the spinal nerves as they exithe intervertebral foramina. Injury to these struc-ures may involve the vertebral column, space-oc-upying lesions within the vertebral canal, or directrauma. These various mechanisms ultimately leado loss of anatomic and/or physiologic neural integ-ity and often result in permanent injury.1

    irect Needle Trauma

    The vertebral column acts as a protective bar-ier to the sensitive neural structures containedithin. The anesthesiologist desires to gain access

    o these underlying spaces in a controlled, preciseanner. Deposition of anesthetic agents into the

    ubarachnoid space presumes that the needle isntroduced caudad to the conus medullaris, therebyvoiding contact with the spinal cord. Case reportsnd medicolegal review suggest that direct spinal cordrauma has been associated with excessively caudadermination of the spinal cord and/or inaccurate de-ermination of bony landmarks that overlie where theonus medullaris ends.1,2 The spinal cords termina-ion typically coincides with the L1-2 vertebral in-erspace, but wide variation exists, with the terminusotentially occurring as high as T12 or as low as L4.3

    line drawn between the iliac crests (Tuffiers line)sually corresponds to the L4-5 interspace or the L4pinous process, but may instead cross the L3-4 or5-S1 interspaces.3 Furthermore, a practitioners

    dentification of a vertebral interspace is often inaccu-

    ate by 1 level cephalad or caudad, and up to 4 levels

    5 (SeptemberOctober), 2008: pp 423434 423

    mailto:[email protected]

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    424 Regional Anesthesia and Pain Medicine Vol. 33 No. 5 SeptemberOctober 2008

    n patients whose surface landmarks are difficult toalpate.2,4 These anatomic variations potentially leado needle placement more cephalad than intended,xposing the spinal cord to direct trauma.Two other anatomic occurrences contribute to

    nintentionally placing a needle too close to thepinal cord. Accurate placement of an epidural nee-le relies on the ligamentum flavum to signal prox-mity to the epidural space and to indicate entrynto it when loss of resistance occurs. However, theigamentum flavum does not always fuse in the

    idline,5 potentially permitting needle passage di-ectly into the epidural or subarachnoid space with-ut benefit of the customary firmness followed byoss of resistance (Fig 1). This anatomic anomalyccurs throughout the neuraxis, but is particularlyrevalent in the upper thoracic and cervical regions.5,6

    imilar failure to contact identifiable landmarks dur-ng needle passage can arise with congenital dysra-hisms, such as spina bifida occulta. Second, the po-ential to unintentionally penetrate the meningesncreases substantially as one moves cephalad alonghe neuraxis, because the posterior-to-anterior di-ensions of the epidural space decrease from 5 mm

    o 8 mm in the lumbar spine to 1 mm to 2 mm inhe upper thoracic and cervical spine.5 Once a nee-le enters the spinal cord, damage occurs as a resultf physical disruption of neural elements with ac-ompanying edema or hematoma,7,8 central syrinxreation from injected local anesthetic solution,9,10

    ocal anesthetic or adjuvant toxicity, or a combina-ion of these mechanisms.11 Permanent damage isore likely to accompany the injection of solutions

    nto the spinal cord; the simple passage of a needlento the spinal cord or nerve roots without subse-uent injection may not necessarily cause injury.Trauma to spinal nerve roots or spinal nerves

    ig 1. Cryomicrotome axial section of the C7-T1 spine.ote that the ligamentum flavum has failed to fuse in

    he midline (arrow), thereby permitting needle entrynto the epidural space without the customary loss ofesistance. Cryomicrotome from Quinn H. Hogan, M.D.eprinted from Hogan.5

    epresents another cause of mechanical injury. b

    idline or paramedian approaches to the neuraxishould easily avoid contact with spinal nerves,hich are partially protected by the vertebral lam-

    nae and transverse processes, and are sufficientlyateral to avoid contact with medially directed nee-les. Needles that unintentionally deviate lateralan contact the spinal nerve or the anterior or pos-erior ramus outside the foramen; or if medial to theacet within the lateral recess, can contact the dorsalerve roots. Spinal nerves are also vulnerable toeedle injury during perispinal techniques such asaravertebral block or from too medially directedeedles during psoas compartment block (Fig 2). In

    ig 2. Midline or paramedian approaches to the thoraciceuraxis (needles A and B) are unlikely to encounterpinal nerves or major feeding arteries. However, unin-entionally lateral approaches (needle C) are most likelyo contact the spinal nerve or the anterior or posteriorrimary ramus outside of the foramen. A transforaminalpproach (needle D) has the potential to come in closeroximity to the spinal nerve or spinal artery branch. Notehat transforaminal approaches are typically at the cervicalr lumbar levels, not the T6 level as illustrated. Illustration

    y Gary J. Nelson. Reprinted from Neal and Rathmell.12

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    Spinal Cord Injury Neal 425

    ain medicine, spinal nerves and nerve roots arespecially vulnerable to needles directed towardshe intervertebral foramen, as with cervical or lum-ar transforaminal approaches (Fig 3).13 A rareathway to spinal cord injury can occur when aeedle enters a peripheral nerve and subsequently

    njected substances travel retrograde along the per-neurium to the spinal cord.14

    Innervation of the meninges and spinal cord is anmportant component of neuraxial pathophysiol-gy and the patients recognition of needle trauma.common misperception is that injury to the spinal

    ord is always heralded by intense pain or pares-hesia, yet the spinal cord is devoid of sensory in-ervation. Needles or catheters can enter the spinalord without warning.2,15-18 Conversely, the actualnjection of substances into the spinal cord is moreommonly associated with intense sensation,2,19,20

    hich has been postulated to result from rapidlyncreasing intramedullary pressure leading to the

    assive discharge of afferent neurons. Meningealnnervation is poorly understood.21 Sensory neu-ons are variably present in meningeal tissue, asvidenced by the inconsistent awareness of pres-ure, pain, or paresthesia when needles puncturehe meninges.7,15,18,18a Epidural local anesthetics doessen the awareness of meningeal puncture,22

    hich provides indirect evidence of clinically rele-ant senso

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