intraoperative neurophysiologic monitoring: focus on cervical

Intraoperative neurophysiologic monitoring: focus on cervicalmyelopathy and related issues

Vincent J. Devlin, MDa,*, Paul A. Anderson, MDb, Daniel M. Schwartz, PhD, DABNMc,Robin Vaughan, PhD, DABNMd

aGeisinger Medical Center, Department of Orthopedic Surgery, M.C. 21-30, 100 North Academy Avenue, Danville, PA 17822, USAbUniversity of Wisconsin Hospitals, 600 Highland Avenue, Suite K4-738, Madison, WI 53792, USA

cSurgical Monitoring Associates, 25 Bala Avenue, Suite 105, Bala Cynwyd, PA 19004, USAdNeurophysiology Incorporated, 5395 Ruffin Road, #102, San Diego, CA 92123, USA

Abstract BACKGROUND CONTEXT: The use of neurophysiologic monitoring during surgical proce-dures for cervical spondylotic myelopathy (CSM) is controversial.PURPOSE: The aim of this article is to review the literature regarding various monitoring tech-niques as applied to the patient with CSM.STUDY DESIGN/METHODS: A systematic literature review.CONCLUSIONS: Neurophysiologic monitoring is a diagnostic tool for assessment of neurologicfunction during cervical spine surgery. Recording of somatosensory evoked potentials (SSEPs),transcranial electrical motor evoked potentials (tceMEPs), and electromyograms (EMGs) may beuseful as these monitoring modalities provide complementary information. ! 2006 ElsevierInc. All rights reserved.

Keywords: Neurophysiologic monitoring; Cervical spondylotic myelopathy

Introduction

Recent advances in the field of spinal monitoring haveprovided a wide array of techniques to assess the functionalintegrity of the nervous system. These methods have beenused both as an adjunct to clinical evaluation and for neu-rologic surveillance during surgical procedures which placethe spinal cord or nerve roots at risk of injury. Use of intra-operative neurophysiologic monitoring (IONM) has be-come commonplace in during spine surgery. The purposeof modern IONM is to provide feedback to surgeons andanesthesiologists regarding changes in neural function be-fore the development of irreversible neural injury, therebypermitting intervention to prevent or minimize postopera-tive neurologic deficit [1].

Historically, the Stagnara wake-up test was the firstwidely used method for spinal monitoring [2]. Although

this test provides assessment of gross integrity of motorfunction during and at the conclusion of a spine procedure,it cannot be administered in a continuous fashion duringsurgery and is unable to provide information regarding spi-nal cord sensory tract function or individual nerve rootfunction. Perhaps the greatest shortcoming of the wake-uptest is its performance at a single point in time during sur-gery; namely following correction. This presumes, there-fore, that spinal cord injury cannot occur at any othertime during the case. This temporal delay between the timeof insult to detection by a wake-up test not only preventsidentification of the specific surgical maneuver responsiblefor injury, but also delays timely intervention to prevent orminimize neurologic deficit. In addition, clinical manifesta-tion of spinal cord injury does not always present itself ina time-locked manner. It is entirely possible for paralysisto present long after a wake-up test would have beenperformed.

It is now possible to assess the functional integrity of thedorsal sensory and ventral motor spinal tracts and nerveroots continuously and in essentially real-time, from the on-set of anesthesia induction through emergence usingsomatosensory evoked potential monitoring (SSEP), trans-cranial electrical motor evoked potential monitoring

FDA device/drug status: not applicable.Nothing of value received from a commercial entity related to this

manuscript.* Corresponding author. Geisinger Medical Center, Department of

Orthopedic Surgery, M.C. 21-30, 100 North Academy Avenue, Danville,PA 17822. Tel.: (570) 271-6541; (570) 271-5872.

E-mail address: [email protected] (V.J. Devlin)

1529-9430/06/$ – see front matter ! 2006 Elsevier Inc. All rights reserved.doi:10.1016/j.spinee.2006.04.022

The Spine Journal 6 (2006) 212S–224S

mailto:[email protected]

(tceMEPs), and intraoperative electromyography (EMG)[3–5]. Despite the routine use of IONM during instru-mented thoracic and lumbar spine procedures in many cen-ters, its application to cervical spinal surgery has not beennearly as widespread. Potential reasons attributed to thisdiscrepancy include: lack of proven efficacy, cost consider-ations, inadequate understanding regarding contemporarytechniques which utilize multiple monitoring modalitiescombined into an appropriate surgical plan, lack of consen-sus as to the appropriate indications for utilization ofIONM, and shortage of highly qualified and experiencedpersonnel to provide monitoring services [6]. This articleprovides an overview and update regarding IONM witha special focus on issues related to the patient with cervicalmyelopathy.

Overview of spinal monitoring techniques

Monitoring of spinal cord function

Somatosensory evoked potentialsPrinciples. Somatosensory evoked potentials (SSEPs) arecortical or subcortical responses to repetitive electricalstimulation of a mixed peripheral nerve. Typical stimula-tion sites include the posterior tibial nerve (ankle), the pe-roneal nerve (fibular head), and the ulnar or median nerves(wrist). The ulnar nerve is the preferred stimulation site forupper extremity SSEPs because the lower spinal nerveentry between C7 and T1 permits assessment of the entirecervical neural axis. Electrical stimulation applied to a pe-ripheral nerve creates an afferent volley which enters thespinal cord through dorsal nerve roots at several segmentallevels, and may ascend the spinal cord via multiple path-ways. The general consensus is that the dorsal or posteriorcolumn spinal pathways are the site of primary mediationfor SSEPs [7]. Other pathways such as the dorsal spinocer-ebellar tracts and anterolateral tracts may also contributeto early SSEP responses that are used for monitoring spi-nal cord function. Upon ascending the spinal cord, theneural signal enters the medullary nuclei in the brainstem.Because there are no synapses between the peripheralnerve and the brainstem nuclei, subcortical SSEPs are pre-dominantly a reflection of the integrity of spinal cordwhite matter. The importance of this fact is that an SSEPrecorded up to the level of the lower brainstem, is affected on-ly minimally by general anesthetics. However, subcorticalSSEPs can be contaminated easily by myogenic artifact inthe unrelaxed patient. After synapsing in the medullary nu-clei, the neural signal crosses the brainstem and ascends inthe medial lemniscal pathways. It synapses once again inthe thalamic nuclei and then projects up to the sensorimotorcortex where additional synaptic interaction may occur. Be-cause these cortical synapses are the sites of action for inha-lational anesthetic agents, the selection of an anesthetictechnique that will optimize cortical SSEP responses is cru-cial, as will be discussed in a later section.

Parameters of interest. Data including signal amplitude(power) and latency (velocity) are recorded continuouslyduring surgery and compared with baseline and recently ac-quired data. Of these two parameters, amplitude is most rel-evant. It would be extremely unlikely to sustain a spinalcord injury without amplitude changes. However, changesin latency are quite common and are less significant. SSEPdata should be constantly updated to control for anestheticand metabolic changes. Baselines may be altered during thesurgical monitoring to reflect the above changes and usedcomparatively immediately before surgical changes thatmight affect the neurologic function of the patient (eg, im-mediately prior to distraction, graft placement). Criteria forsurgeon notification vary from center to center but in gen-eral include an intraoperative unilateral or bilateral ampli-tude loss of at least 50–60%.

Limitations. SSEPs assess directly spinal cord sensorytracts but provide only indirect information about motortracts. Damage to the spinal cord motor tracts can occurwithout a concomitant change in SSEPs. In addition, SSEPsmay be poorly defined or unrecordable in patients with se-vere myelopathy, spinal cord tumor, obesity, or peripheralneuropathy either alone or in combination.

Anesthesia considerations. SSEP recordings are influencedheavily by inhalational anesthetic agents including nitrousoxide. Subcortical SSEPs are most optimal when the patientis chemically paralyzed as muscle artifact is diminished inthe recording thus enhancing the quality of the response.Myogenic interference is much less problematic whenrecording cortical SSEPs. To address these concerns, manyspine centers have switched to a total intravenous anestheticregimen which will be discussed further in a later section.

Predictive value. Despite the high negative predictivevalue of SSEPs for ruling out motor deficit during surgicalcorrection for scoliosis, SSEPs have been less helpful inmonitoring other spinal pathologies. In patients undergoingcervical surgery, May et al. [8] reported that SSEPs were99% sensitive but only 27% specific in identifying neuro-logic deterioration. In addition there remains a small butdefinite risk of false-negative findings when monitoring pa-tients with preexisting spinal cord compromise, such as my-elopathy or acute spinal cord injury. In such patients, thevascular supply to both the anterior and lateral aspects ofthe spinal cord supplied by the anterior spinal artery is vul-nerable to hypotension-induced ischemic injury that maynot be detectable with SSEP monitoring at all or duringthe critical time needed to initiate intervention to preventor minimize neural injury [9].

Transcranial electric motor-evoked potentials

Principles. Motor evoked potentials (MEPs) are neuroel-ectric impulses elicited from descending motor pathways

213SV.J. Devlin et al. / The Spine Journal 6 (2006) 212S–224S

including the corticospinal tract (CST), spinal cord inter-neurons, anterior horn cells, peripheral nerves, and skeletalmuscles innervated by excited alpha motor neurons follow-ing the transcranial application of a high-voltage electricalstimulus. A low-output impedance electrical stimulator isused to generate a high-volume, short-duration stimulusor pulse train via a series of electrodes placed over variousregions of the scalp to excite a selected area of the motorcortex. This results in stimulation of CST axons whichcourse from the cortex through the internal capsule tothe caudal medulla. Here, the fibers cross over in the lowerlateral brainstem and descend into the lateral and anteriorfuniculi of the spinal cord. In contrast to white matter me-diated SSEPs, CST axons that originate in the premotorand motor cortex enter the spinal cord gray matter wherethey interact with spinal interneurons. The axons go onto synapse with alpha motor neurons which innervateperipheral muscle. Lateral CST fibers that synapse in thecervical segment of the spinal cord are arranged mediallyfollowed laterally by fibers that synapse in the thoracic,lumbar, and sacral regions, respectively. MEPs can berecorded either from the spinal cord (I and D waves) ordirectly from muscle (compound muscle action potential[CMAP]). Although MEPs can also be elicited by transcra-nial magnetic stimulation, the technical challenges forrecording these signals in the operating room are too greatto warrant use, particularly given the simplicity of electricstimulation [10–12].

Parameters of interest. Transcranial electric motor-evokedpotentials (tceMEPs) from the CST can be recorded fromthe spinal epidural or subdural space via a catheter-typeelectrode or from peripheral musculature. Responses re-corded from the epidural space consist of what is knownas a ‘‘D-wave’’ so-called because it represents direct activa-tion of the CST cells. In awake or lightly anesthetized pa-tients the ‘‘D-wave’’ is followed by a series of ‘‘I-waves’’generated indirectly by cortical synapses. These descendingcortical volleys then summate to excite anterior horn cellsand spinal alpha motor neurons thereby inducing a com-pound muscle action potential. D waves are advantageousto record intraoperatively during excision of intramedullaryspinal cord tumors. However, the requirement of electrodeplacement either percutaneously or through a laminotomyprecludes routine use in most centers, particularly for mon-itoring most common cervical procedures. In addition,D-waves reflect global CST function which is problematicfor monitoring the cervical spinal cord because a selectiveinjury to the cervical cord motor fibers that spares the lowerextremity fibers may not be detected. Except in extenuatingcircumstance, therefore, it is both easier and preferable torecord myogenic motor responses (CMAP) from upper(control) and lower extremity peripheral muscle. CMAPsmay be recorded either from surface electrodes or subder-mal needle electrodes placed over key peripheral muscles.The warning criterion is typically a 75% or more decrease

in CMAP amplitude but must be individualized based ona variety of patient-specific factors.

Limitations. Transcranial electric motor evoked potentialsare compromised in the presence of neuromuscular relaxa-tion. Muscle relaxants should be avoided during criticalparts of the procedure and their use limited to low-risk por-tions of the procedure such as during spinal exposure.

Anesthesia considerations. Motor evoked potentials areinfluenced heavily by inhalational anesthetics and requiretotal intravenous anesthesia for reliable recording. Chemi-cal paralysis will prevent elicitation of MEPs.

Predictive value. Transcranial electric motor evoked po-tentials are exquisitely sensitive and specific for the diagno-sis of intraoperative cervical spinal cord injury [13].

Additional techniques for monitoring of spinalcord function in special situations

H-reflex and F-response. An intraoperatively recorded H-reflex or F-response can be used to augment transcranialmotor evoked potentials for rapid detection of acute spinalshock [14]. Both the H-reflex and F-response aid in the in-traoperative assessment of spinal cord systems responsiblefor the control of complex motor behavior. As such, theyprovide a model for understanding the mechanisms of spi-nal cord pathophysiology. Severe acute spinal cord injuryleading to spinal shock results in suppression of H-reflexesand F-responses due to hyperpolarization of caudal motorneurons presenting within seconds after spinal cord injury.Despite many advantages over tceMEPs, including requir-ing less anesthetic restrictions, the H-reflex and F-responsestend to be highly variable, and appear to be recordable inonly approximately 60–70% of pediatric cases and proba-bly less than 40% of adult cases. Yet, these monosynapticresponses serve as an excellent crosscheck and back-up totceMEPs, when they can be obtained.

Neurogenic spinal evoked potentials (NSEP). Among themost controversial techniques used to monitor spinal cordfunction is the neurogenic descending evoked potential.This presumed orthodromic response is elicited to transoss-eous (spinous process, lamina) or epidural electro-spinalstimulation, and recorded over lower extremity peripheralnerves (eg, popliteal fossae). Initially, it was thought tobe mediated within spinal motor tracts. Because of its tech-nical simplicity both for stimulation and recording, as wellas minimal negative effects from most anesthetic agents, in-cluding neuromuscular blockade, the so-called neurogenic‘‘motor’’ evoked potential became highly popular for morethan a decade. Unfortunately, myriad research studies havenow clarified that both the neurogenic evoked response andSSEPs are actually mediated through common spinal cordpathways [15–18]. A neurogenic response is not a motor

214S V.J. Devlin et al. / The Spine Journal 6 (2006) 212S–224S

evoked potential, but rather, represents antidromic spinalcord somatosensory activity. This has been confirmed inclinical practice by the finding that NSEPs have been suc-cessfully recorded from paraplegic patients demonstratingthat this response does not depend on functionally intactmotor tracts. Despite the disappointing evidence that theNSEP is a sensory versus motor evoked potential, its useshould not be dismissed entirely. There are very rare occa-sions where it is not possible to record an SSEP, tceMEP, orH-reflex, and yet, epidural stimulation at the rostral thoraciclevel elicits a descending sensory potential that is record-able over the popliteal fossae.

Monitoring of nerve root function

ElectromyographyPrinciples. SSEPs are neither sensitive nor specific foridentification of injury to a specific spinal nerve root owingto their multiple nerve root mediation. Electromyographic(EMG) techniques overcome this limitation and can beclassified into two categories based on method of elicita-tion: mechanical and electrical. Mechanically elicitedEMG, also called spontaneous EMG (spEMG), may be use-ful during the dynamic phases of surgery (during implantplacement, nerve root manipulation). Electrically elicitedEMG, also called stimulus-evoked EMG (stEMG) or trig-gered EMG (trEMG) may be useful during static phasesof surgery. Together these EMG techniques encourage earlydetection of excessive nerve-root traction, mechanical in-jury, or cortical breach. The stEMG principle for identify-ing cortical breach resulting from placement of pediclescrews is based on the fact that cortical bone has a high re-sistivity (low conductivity) to electrical current flowwhereas soft tissue has a low electrical resistivity [19].The tip of a monopolar probe is touched to the screw shankor hexagonal port, and the electrical current output is in-creased via an electrical triggering device. If there is a cor-tical perforation, the normally high resistance of the intactbony wall will be reduced and the flow of electrical currentfrom the cathode to the anode will take the path of least re-sistance, namely through the breach to the root. As a result,the nerve root will depolarize at a much lower current(!7.0 mA) compared with an intact pedicle (10–12 mA).Subsequently the root will fire and the peripherally inner-vated muscle will contract, and this will be recorded asa compound muscle action potential (CMAP). This screwstimulation technique has been most widely used in thelumbar region [20,21] and has subsequently been adaptedto the thoracic region [22–24] and cervical region [25].Use of spontaneous EMG (spEMG) techniques to detect in-traoperative cervical nerve root mechanical injury or exces-sive traction has been reported [26].

Parameters of interest. Microtrauma to a spinal nerve rootprovokes ion depolarization, and the resultant muscle ormotor unit potential can be recorded from a muscle

innervated by that specific nerve root. Abrupt traction ofa spinal nerve root or mechanical contact by a surgical in-strument will elicit intermittent EMG ‘‘burst’’ or sustained‘‘train’’ activity. Gradual traction may elicit a smaller re-sponse or even no response. Simple ‘‘burst’’ activity reflectsmechanical contact with a nerve root and is diagnosticallymeaningless. Train EMG reflects a state of traction, me-chanical irritation, or thermal change (e.g., secondary tocool irrigation). While the occurrence of long-term unre-solved ‘‘trains’’ suggests that the root is highly irritated, itdoes not infer that a neurologic deficit will result.

Limitations. Chronically compressed motor nerve rootshave an elevated threshold [27]. Therefore, these chroni-cally compressed roots will not fire spontaneously or withstEMG techniques resulting in false-positive tests. A quietspontaneous EMG of a chronically compressed nerve rootdoes not mean the root is not undergoing injury by tractionor mechanical contact. Also, thresholds for stimulation ofnormal nerve roots do not apply to chronically compressednerve roots. Chronically compressed roots must serve astheir own control to establish a safe triggered EMGthreshold.

Anesthesia considerations. All depolarizing and nondepo-larizing paralytic agent must be avoided as they block theneuromuscular junction and preclude muscle contraction,thereby producing a false-negative stEMG test.

Predictive value. The predictive value of intensity ofscrew stimulation needed to elicit myogenic responsesand the risk for neurologic injury is considered to exceed95% based on support from multiple studies in the lumbarspine. It is recognized that physiologic factors can contrib-ute to false-negative results in the setting of chronicallycompressed nerve roots and metabolic conditions such asdiabetes. Direct stimulation of nerve roots at risk is amethod which can overcome this problem when clinicallyfeasible. It is also recognized that while low thresholdreadings from screw stimulation suggest that the path ofleast electrical resistance is located near a nerve root, itcannot be distinguished whether the cause is a crackedpedicle, thin wall of osteoporotic bone, or an exposedpedicle screw.

Transcranial electric motor-evoked potentials(tceMEPs) for assessment of root function

Postoperative C5 nerve root palsy may occur after ante-rior or posterior cervical procedures. The prevalence of C5nerve root injury is reported as high as 12.9% after lami-nectomy [28] and 14.9% after laminoplasty [29]. In manycases paralysis develops 24 hours or more after surgery.Routine intraoperative neurophysiologic monitoring(IONM) of upper extremity SSEPs, dermatomal evoked po-tentials (DEPs), and tceMEPs recorded from hand musclesgenerally has failed to detect this serious complication. In


an effort to reduce postoperative C5 nerve root injury, intra-operative deltoid and biceps tceMEPs in conjunction withspontaneous electromyography monitoring have beenutilized [26]. These neurophysiologic methods may playcomplementary roles for early detection (spEMG) andfunctional assessment (tceMEPs) of C5 nerve root injuryduring posterior cervical decompressive procedures.

Dermatomal evoked potentialsDermatomal evoked potentials (DEPs) are a monitoring

modality whose use has not withstood the test of time andhas been replaced by EMG and MEP techniques for moni-toring nerve roots. DEPs were initially advocated as a mo-dality for assessing adequacy of nerve root decompression.The functional integrity of an individual nerve root was as-sessed by stimulation of a dermatomal field and recordingan afferent evoked potential over the scalp similar to thatdescribed for mixed nerve SSEPs. DEPs are limited tomonitoring of a sensory nerve root and are most useful innonmyelopathic patients with acute radiculopathy of lessthan 3–6 months duration. They are not particularly sensi-tive for instantaneous recognition of sharp root injury as oc-curs with placement of bone screws. In addition, DEPs arehighly contaminated in the unrelaxed patient. BecauseMEPs are much more important for guarding the spinalcord and require avoidance of muscle relaxation, and giventhe highly questionable reliability and validity of DEPs,there is little place for this modality in current practice [30].

Brachial plexus monitoringA tangential benefit of IONM is the ability of SSEPs or

tceMEPs to identify impending brachial plexopathy or ul-nar nerve neuropathy secondary to malpositioning. Inter-mittent monitoring of ulnar nerve SSEPs recorded eitherdirectly from the brachial plexus (Erb’s point) or cervicalspine, coupled with tceMEPs recorded over deltoid, exten-sor carpi radialis, and intrinsic first dorsal interosseousmuscles, are highly effective for identifying emerging bra-chial plexopathy or ulnar neuropathy. Numerous reports at-test to the efficacy of IONM for this purpose [31–33].

Pathophysiology of spinal monitoring changes

In spinal surgery, neurologic complications usually aresecondary to contusion (eg, mechanical trauma), direct dis-tortion of neural elements (such as during deformity reduc-tion), or ischemic insult. By convention, all evokedpotentials are evaluated in terms of measured amplitude(voltage), latency (time), and morphology (shape). If injuryoccurs, from any cause, a cascade of changes involving so-dium, potassium, and calcium channels occurs [34]. Thiscascade causes blockage of axonal transmission whichleads ultimately to an uncoupling of oxidative phosphoryla-tion, thereby precluding adenosine triphosphate production.The net result is loss of cellular function and structural

integrity which manifests as a voltage drop in evoked po-tential amplitude, not a prolongation of latency. Other thanlatency shifts associated with increased concentration of in-halational or intravenous agents, lowering of core body orlimb temperature or perhaps hypercarbia, evoked potentiallatency rarely changes in the absence of an amplitude loss.Therefore, reliance on the 10% latency prolongation rulecommonly used to define a significant SSEP change willcreate an excessive number of false-positive alerts.

Spinal cord contusion typically results in a transient spi-nal cord conduction block resulting in marked amplitudesuppression (50–75%) of SSEPs or tceMEPs which shouldtypically resolve within 15–20 minutes. Such changes areusually aided by increasing mean arterial blood pressureto promote improved spinal cord perfusion, as well as tem-porary cessation of further surgical maneuvers. More seri-ous concussive injury, such as those caused by anuncontrolled surgical instrument or pedicle screw impinge-ment upon the spinal cord, will obliterate both sensory andmotor evoked potentials entirely.

Spinal cord ischemia may result from: 1) stretching ofcritical spinal cord vascular supply during correctional ma-neuvers or placement of a cervical strut graft, 2) prolongedhypotension or 3) after ligation of anterior segmental ar-teries. Speed of corrective maneuvers also seems to playa role in developing myelopathy in this setting and is par-ticularly important relative to hemodynamic management.Maintenance of mean arterial blood pressure near normalpromotes tissue accommodation to elongation while com-pensating for changes in spinal cord perfusion pressure.Prolonged hypotension, whether deliberate or systemic,can result in spinal cord vascular injury. TceMEPs are par-ticularly sensitive to blood pressure changes and can beused quite effectively to titrate how much of a hypotensivestate the spinal cord will withstand. Because the motor andsensory components of the spinal cord are separated, andbecause the spinal cord blood supply is heterogeneic (ie,anterior and posterior spinal arteries), monitoring a singleevoked potential modality may not reflect the global statusof spinal cord function. The blood supply nourishing theposterior column sensory pathways which mediate SSEPsare the posterior spinal arteries. It is entirely possible tohave selective loss of SSEPs with complete sparing of mo-tor function [35]. Conversely, selective ischemia of the an-terior spinal cord region, as in the case of anterior spinalartery syndrome, may manifest as a loss of MEP amplitudein the absence of concurrent change in SSEPs [36].

Based on the pathophysiology of spinal cord injury,combined monitoring techniques including somatosensoryand transcranial electric motor evoked potentials, aug-mented by H-reflexes and F-responses, when possible,could facilitate rapid identification of either concussive orischemic changes leading to spinal shock. The pathophysi-ology of spinal shock is thought to involve neuronal mech-anisms of inhibition and reorganization after acute spinalcord injury. Although not entirely clear, it may be mediated


by synaptic changes in the segments of the cord below thesite of injury. These changes seem to relate to heightenedpresynaptic inhibition and hyperpolarization of the spinalmotor neurons. The H-reflex and F-response, respectively,seem to be exquisitely sensitive to these changes. As a re-sult, they may actually obliterate before a loss of tceMEPs,thereby serving as an early harbinger of neurological disas-ter. Yet, unreliable intraoperative recording of the H-re-flexes or F-responses, particularly in adults who oftenpresent with complex medical issues, precludes their rou-tine use and hence, lessens their overall value.

Spinal nerve roots are also susceptible to injury due tomechanical or ischemic insult. Anatomically, the dorsaland ventral roots split into rootlets and mini-rootlets prox-imally. This division site is the location where the nerveroot is susceptible to mechanical injury as the axons, en-closed by a thin root sheath, cerebrospinal fluid, and menin-ges, lack the protective covering of epineurium andperineurium that is present in peripheral nerves. There alsoseems to be an area of hypovascularity between the proxi-mal and middle third of the dorsal and ventral roots wherethe nerve is susceptible both to mechanical and ischemic in-sult [37]. If there is microtrauma leading to mechanical ormetabolic nerve root irritation, the nerve root will depolar-ize, resulting in an action potential that leads to the releaseof acetylcholine and depolarization of the motor end platefrom innervated muscle fibers. These mechanically pro-duced nerve action potentials can be identified, both visu-ally and acoustically, by monitoring electromyography aswas discussed previously.

Effects of anesthetics on neurophysiological signals

The success of intraoperative monitoring is highly de-pendent on appropriate anesthetic management [38–41].Paramount is the understanding that essentially all anes-thetic agents depress synaptic function both in brain andspinal cord gray matter. In spinal cord monitoring, the mar-gin for interpretation error is narrow because the signal am-plitudes are inherently quite small (ie, microvolt range).Any anesthetic that depresses signal amplitude will poten-tially lead to increased variability and interpretive ambigu-ity. When signal amplitude is artificially depressed andhighly fluctuant as a result of anesthesia, it creates a situa-tion where signal change must be interpreted in the pres-ence of extreme clinical uncertainty.

In general, all inhalational agents (isoflurane, desflurane,sevoflurane) produce a dose-related increase in latency andreduction in amplitude of the cortical SSEP. While the ex-act sites of action for these potent agents remain unclear,these gases appear to dissolve in the neuronal blood plasmamembrane interfering with electrical function. The resultingeffect is inhibition of ion channel function with significantalteration in synaptic and axonal transmission. As a result,neurophysiological signals that rely on synaptic function

will be influenced to a far greater extent than signals thatare not synaptically dependent. Even at steady-state, lowend-tidal concentrations (0.25–0.5 mean alveolar concentra-tion (MAC)), response amplitude not only can becomehighly unstable and variable, but in many instances, eithertoo small to detect a real change or completely obliterated.

The effect of potent anesthetics is much less on the sub-cortical SSEP recorded over the cervical spine, comparedwith its cortical counterpart, and is minimal on spinal epi-dural or peripheral responses. In the past, when the onlymodality monitored was the SSEP, it was possible to over-come the adverse effects of inhalational anesthesia simplyby recording a subcortical potential in the presence of mus-cle relaxation. The debilitating effect of volatile anestheticson the excitability of cortical axons needed to elicit and re-cord transcranial motor evoked potentials is even more crit-ical today, given that the use of tceMEPs is increasing. Theneural mechanism of tceMEP amplitude depression in thepresence of a volatile anesthetic is due to the blockage ofsynaptic transmission both at the cortical and spinal ante-rior horn cells levels. There seems to be general consensusthat nitrous oxide reduces cortical SSEP and tceMEP am-plitudes to a sufficiently great extent that it should beavoided entirely. This is particularly important if usinga volatile agent because the introduction of nitrous oxidelowers the MAC, thereby having an additive effect onevoked potential amplitudes [42–46].

From the foregoing discussion it can be seen that whengeneration of a cortical signal, either ascending (SSEP) ordescending (tceMEP) is necessary, as with contemporaryspinal cord monitoring, it is best to avoid inhalationalagents soon after induction and intubation in order to en-sure optimal amplitudes and unambiguous interpretation.On the basis of the unpredictable amplitude variabilityand depression associated both with volatile agents andwith nitrous oxide, it has become routine in many high-vol-ume pediatric and adult spine surgery centers to use a totalintravenous anesthetic regimen [47]. If total intravenous an-esthetic is precluded, one should begin surgery with a com-bined low level (eg, 0.3 MAC) volatile agent, augmented bycombination intravenous drugs.

Before the advent of multimodality techniques for mon-itoring of spinal cord and nerve root function, it was com-monplace to keep the patient fully relaxed. Neuromuscularrelaxants have no adverse effect on SSEPs. In fact, a com-pletely relaxed patient reduces contaminating myogenic in-terference and fosters better SSEP waveform resolution,particularly for the subcortical response recorded over thecervical spine. Patient relaxation also eliminates any ‘‘shak-ing’’ of the upper and lower limbs secondary to peripheralnerve electrical stimulation. In contrast, neuromuscularblockade will compromise tceMEP, H-reflex, F-response,and EMG recordings, thereby introducing one more vari-able to cause interpretive ambiguity. While their has beensome suggestion that myogenic tceMEP and EMG monitor-ing is possible in the presence of partial muscle relaxation,


there are myriad factors that can lead to false-negative re-sults that were not considered in these limited sample stud-ies. For example, a definitive differential intramuscular andintra-side sensitivity to neuromuscular blockade has beennoted. Thus, partial muscle relaxation may cause completeobliteration of one muscle group (eg, foot muscle), whilesparing, at least to some degree, that of another (eg, intrinsichand muscle). If, therefore, partial neuromuscular blockadewas based on train-of-four testing from face or hand mus-cles, it may preclude tceMEP recordings over the lower ex-tremities and may result in a false-positive interpretation.Finally, one should not dismiss the interactive effects thatvolatile or intravenous anesthetics have on the pharmacody-namics and pharmacokinetics of nondepolarizing relaxants;that is, most of the anesthetic agents used will potentiateneuromuscular blockade. To this end, the use of partialneuromuscular blockade is simply too uncontrollable tojustify use.

Surprisingly little is known or understood about the in-fluence of anesthesia on spinal cord perfusion and metabo-lism as compared with that of the brain. During spinesurgery, extrinsic pressure on the spinal cord such asstretching of the spinal cord vascular supply, mechanicalcompression of the spinal cord from tumor tissue, herniateddisc, bone displacement, and the like, may potentially com-promise spinal cord perfusion pressure, thereby reducingspinal cord blood flow. When coupled with the somewhatunknown influences of anesthesia on spinal cord bloodflow, it becomes critical to be vigilant to the patient’s blood

pressure. There is experimental evidence to suggest thatmoderate hypotension can produce irreversible paralysisin a partially compressed, albeit functionally intact spinalcord, whereas mild hypotension can decrease spinal cordblood flow and electrical transmission. IONM data provideexcellent clinical material to demonstrate the relationshipbetween mean arterial pressure and spinal cord perfusion.It would appear that unlike the brain, spinal cord blood flowmay not be as auto-regulated and thus, may be predisposedto ischemic injury when stressed during hypotension.

State-of-the-art approach to neurophysiologicmonitoring during surgery for cervical myelopathy

Intraoperative neurophysiological monitoring plan

Successful neurophysiologic monitoring requires thecollaborative efforts of surgical, anesthesia, and moni-toring personnel. Patient-specific factors (eg, preoperativeneurologic status, diabetes), procedure-related factors, andpossible interventional measures that can be initiated to re-verse impending neurologic injury should be reviewed pre-operatively. Monitoring of ventral motor spinal cord tracts(tceMEPs), dorsal spinal cord sensory tracts (SSEPs), andmotor nerve roots (EMG, tceMEPs) is performed fromthe time of anesthesia induction through emergence[48,49]. The surgical levels determine the neural structuresat risk and guide the monitoring plan (Fig. 1). In upper cer-vical procedures (e.g., C1–C2), in addition to the risk of

CervicalMonitoringModalities

AboveC4?

YES

Risk ofVertebral

ArteryInjury?

YES

MonitorBrainstemFunction

BAER

UpperSSEP

MonitorBrachial Plexus

TCeMEP

LowerSSEP

TCeMEP

stEMG

PedicleScrews?

YES NO

spEMG

MonitorNerveRoots

MonitorLower

CervicalSpinalCord

Monitor UpperCervical Spinal Cord

NO

NO

Fig. 1. Decision matrix to define what neural structures are at risk and which monitoring modalities should be used during cervical spine surgery. (FromSchwartz and Sestokas [48].)


spinal cord injury there exists risk of brainstem infarct sec-ondary to vertebral artery injury. SSEPs can be supple-mented with brainstem auditory evoked potentials toenhance monitoring of brainstem perfusion. In proceduresabove the level of C4, upper extremity SSEPs are generallyadequate and lower extremity SSEPs are not required, be-cause both the median and ulnar nerves enter the spinalcord at spinal root levels below C4. Thus, these ascendingpotentials have no opportunity to bypass a site of injury viaanother spinal nerve root entry level. The combination ofupper extremity SSEPs and tceMEPs recorded from upperand lower extremities provides excellent coverage of ven-tral and dorsal spinal cord tracts and can also be used tomonitor for positional brachial plexopathy. In proceduresbelow C4, the vertebral artery is not generally at high riskand emphasis shifts to monitoring for potential injury tocervical nerve roots, especially C5, using spEMG and tce-MEP recorded from the deltoid as well as hand muscles. Ifpedicle screws are used, stEMG is appropriate to assesspedicle wall integrity. Monitoring of cord function is per-formed with tceMEPs and SSEPs recorded from both upperand lower extremities. There exists a subset of patients withcervical myelopathy for whom SSEPs are poorly definedand unmonitorable. In many of these cases, tceMEPs canstill be recorded. In a recent analysis [50] of spine surgerypatients with unobtainable evoked potential data despitefunctional neural integrity, the incidence of absent datawas extremely low in the population with degenerative spi-nal disease (0.38%) and highest in patients with neuromus-cular disease (6.8%).

Intraoperative neurophysiologic monitoring sequence

We initiate IONM immediately after induction in orderto prevent exacerbation of spinal cord compression as a re-sult of neck extension [51,52]. Postinduction preintubationbaseline tceMEPs are obtained. Then the anesthesiologistmay begin intubation while monitoring is continued. Afterintubation, tceMEPs are obtained upon airway access be-fore taping the endotracheal tube. SSEPs are obtained aswell during this time. SSEPs and tceMEPs are repeatedafter presurgical maneuvers including traction weightplacement, shoulder taping, and position changes. If moni-toring changes are noted with weight placement or taping,the forces applied to the neck and upper extremities are de-creased. Improvement in monitoring potentials typicallyoccurs within a few minutes of these corrective maneuvers.At our institution, if monitoring changes occur after turningthe patient from supine to prone, the mean arterial bloodpressure is raised to at least 90 mm in order to ensure ade-quate spinal cord perfusion. If tceMEPs and SSEPs do notimprove (e.g., 30% improvement in tceMEP amplitude)within a short time (e.g., 15 minutes), then a spinal cord in-jury dose of methylprednisolone is considered and the pa-tient is turned supine while monitoring continues duringemergence from anesthesia.

After the patient is positioned and stable baseline neu-rophysiologic potentials are documented, cervical surgerycan proceed. During anterior procedures, tceMEPs aredocumented during critical portions of the procedureincluding intervertebral distraction, implant or graft place-ment, neck extension, or application of additional tractionweight. Posterior cervical procedures are associated withincreased risk for neurologic injury to both the spinal cordand spinal nerve roots due to either mechanical or vascularetiology. Monitoring of tceMEPs is an option during lam-inectomy/foraminotomy, posterior implant placement, andother posterior surgical maneuvers. Monitoring of SSEPsis useful both as a back-up to tceMEPs for detection ofcompressive injury as well as for detection of compro-mised brainstem and spinal cord blood flow. Inadequateblood pressure may leave the spinal cord with insufficientreserve to withstand surgical manipulations and may bedetected by careful intraoperative neurophysiologic moni-toring. Surveillance of the status of spinal nerve roots us-ing EMG and tceMEPs recorded from the deltoid andbiceps is considered when the C5 and C6 nerve rootsare at risk.

Current state of the medical literature regardingneurophysiologic monitoring during surgery forcervical myelopathy and related disorders

A computerized search of the database of the NationalLibrary of Medicine from 1996 through 2005 was con-ducted using the search terms ‘‘electrophysiology andspine/surgery’’ or ‘‘electromyography and spine/surgery’’or ‘‘evoked potentials and spine surgery’’ yielding a totalof 287 citations. Restricting the search to ‘‘electrophysiol-ogy and cervical spine/surgery’’ or ‘‘electromyographyand cervical spine/surgery’’ or ‘‘evoked potentials andcervical spine surgery’’ yielded a total of 72 citations. Ref-erences relevant to the use of intraoperative neurophysio-logic monitoring during cervical procedures were broadlycategorized according to whether they reported use of a sin-gle monitoring modality or multiple modalities for intrao-perative assessment of spinal cord function. Referencesrelevant to the use of monitoring for assessment of cervicalnerve roots were also analyzed. Studies reporting the use ofneurophysiologic monitoring techniques in relation tothe detection of miscellaneous related intraoperative neuro-logic or vascular problems during spine surgery in generaland cervical surgery in particular were reviewed separately.

Spinal cord monitoring

Somatosensory evoked potentials (SSEPs)

Epstein et al. [53] compared the morbidity and mortalityof 100 consecutive SSEP monitored cervical procedureswith a historical control population of 218 patients who


underwent unmonitored cervical procedures for myelopa-thy and radiculopathy. SSEPs were determined to be valu-able in improving patient outcome after cervical surgery inthe monitored group. In the unmonitored group, 3.7% be-came quadriplegic and 0.5% died whereas no instances ofquadriplegia or death were noted in the monitored group.The reduction in neurologic deficit was attributed to earlydetection of vascular or mechanical compromise of the spi-nal cord or nerve roots thereby permitting alteration of an-esthetic or surgical technique including reversal ofhypotension, adjustment of operative position, release ofdistraction, and cessation of manipulation. However, thehistorical cohort design of the paper limits the conclusionswhich can be made regarding efficacy of intraoperativeSSEP monitoring. May et al. [8] reported a series of 191 pa-tients undergoing cervical surgery for diverse diagnoses.Upper limb SSEP responses were recorded reliably in182 patients with a sensitivity of 99% and specificity of27% in 10 patients who developed neurologic signs postop-eratively. Potential risk factors for electrophysiologic andneurologic deterioration were determined as: 1) preopera-tive myelopathy; 2) long segmental extent of surgery; 3)upper cervical surgery; 4) use of instrumentation; and 5)application of corrective forces to the neck. Myelopathyalone was such a strong risk factor in this series that it over-shadowed the effect of the other recognized factors. It wasargued that the false-positives in this series may have in-cluded a number of patients in whom neurologic deteriora-tion was successfully prevented as a consequence of thesurgeon’s response to the report of SSEP amplitude loss.Kombos et al. [54] prospectively evaluated SSEP monitor-ing in 100 patients (Group 1dcervical myelopathy; Group2dradiculopathy or mild hyperreflexia; Group 3dacuteneurologic deficit) treated with anterior cervical decom-pression and fusion. SSEPs were performed during fivestages of the procedure: M1, after induction of anesthesia;M2, during positioning; M3, during distraction of the inter-vertebral space; M4, throughout decompression; and M5,during graft placement. No SSEP changes were identifiedin any patients during induction (M1). Deterioration ofSSEPs was seen across all groups (35% of all patients;41% in Group 1, 23% in Group 2, 47% in Group 3) duringpositioning (M2) and 5 minutes afterward. Deterioration ofSSEPs was reported with distraction of the intervertebralspace (M3) in Group 1 (17%) and Group 3 (40%). No SSEPchanges were noted with decompression of the spinal cord(M4) in any group. Acute deterioration of SSEPs was re-corded in one Group 2 patient during graft placement(M5). In this study, intraoperative SSEP monitoring duringanterior cervical spine surgery permitted modification ofsurgical strategy to reduce the SSEP deterioration. Themost common changes occurred during patient positioningand were more frequent in patients with myelopathy.

Jones et al. [55] reported two cases of quadriparesis fol-lowing routine anterior cervical discectomy which were notdetected with SSEP monitoring. They concluded that

although the neurological risks associated with anteriorcervical discectomy are generally considered to be low(0.1–0.4.6%), they are not negligible. Irrespective of thecause and level of neurologic deficit, they advocated com-bined monitoring of SSEPs and MEPs during anterior cer-vical discectomy. Sebastien et al. [56] reported SSEP dataobtained during cervical procedures in 210 patients. SSEPchanges were noted in 84 patients (40%) and were attrib-uted to mechanical stress (13 patients), regional ischemia(17 patients), and manipulation or placement of instrumen-tation (44 patients). No false-negatives were reported.Sloan et al. [57] reported three cases of SSEP loss duringanterior cervical surgery attributed to retractor placementwhich was believed to have caused occlusion of the carotidartery. It was reported that intervention to reverse thesemonitoring changes prevented cerebral ischemia and conse-quent cortical damage. In contrast to the above studies,Taunt et al. [58] reported a series of 175 patients in which163 patients were monitored with SSEPs from the medianand posterior tibial nerves during anterior cervical surgery.Patient groups included radiculopathy (132 patients), mye-lopathy (30 patients), unstable cervical fractures (11 pa-tients), and cervical pain (2 patients). 96.3% of patientshad no SSEP changes and no neurologic deterioration aftersurgery. Monitoring data were reported as showing threefalse-positives (1.8%) and one false-negative (0.6%). Thesingle false-negative case was a right deltoid palsy notdetected with median nerve stimulated SSEPs. It can bequestioned whether this finding is accurately termed a‘‘false-negative’’ as SSEPs cannot be expected to detectsingle nerve root deficits. Anterior cervical decompressionand fusion was considered by the author to be a safe proce-dure with a low rate of complications for which intraoper-ative SSEP monitoring was not helpful.

Transcranial electric motor-evoked potentials(tceMEPs)

In 1989 Kitagawa et al. [59] reported the use of intrao-perative monitoring during upper cervical spine surgery in20 patients. MEPs were produced by transcranial electricalstimulation and recorded from an epidural electrode. Fivepatients had transient attenuation of approximately 50%but experienced complete recovery after intraoperativeadjustments by the surgeon, and none of these patientsdeveloped neurologic deficits postoperatively. One patientwho developed complete loss of MEPs during surgery be-came a respiratory quadriplegic. There were no false-nega-tives. MEPs were sensitive to the operative procedure andresponded within a few seconds to intraoperative adjust-ments. Gokaslan et al. [60] demonstrated that motor path-ways could be successfully monitored during anteriorcervical surgery via MEPs obtained with a transcutaneousepidural electrode. The electrode placement was success-fully achieved in 15 patients but was unsuccessful in a sin-gle patient with cerebral palsy. No significant changes in


MEPs occurred during surgery, and all patients had motorfunction at or above baseline after surgery. Although thereremains lack of consensus about the optimal method forelicitation and recording of MEPs to electrical stimulationof the motor cortex, there is a growing consensus as tothe value of this monitoring modality for both brain andspine surgery [61].

Nerve root monitoring

Beatty et al. [62] described the use of continuous intra-operative EMG recording during spinal surgery. In a seriesof 150 cases, 30 cervical cases were monitored and EMGrecording was reported to yield valuable information indi-cating when undue retraction was exerted on a nerve rootor when a nerve root was adequately decompressed. How-ever, there was a 20–25% false-negative rate in which nofiring was obtained with nerve root retraction. Jellishet al. [63] described intraoperative EMG monitoring ofthe posterior pharynx as a surrogate for monitoring recur-rent laryngeal nerve function. Recurrent laryngeal nervefunction has also been monitored by use of a special elec-trified endotracheal tube (Medtronic Xomed, Jacksonville,FL) which serves as the recording device [64].

Postoperative C5 nerve root palsy remains problematicin patients who undergo cervical decompressive proce-dures. The average reported incidence of postoperativeC5 palsy is 5.6% in cervical spondylotic myelopathy pa-tients, 8.3% in patients with ossification of the posteriorlongitudinal ligament, and literature review demonstratesthat the incidence of this complication does not vary signif-icantly according to whether an anterior or posterior surgi-cal approach is used [65]. Postoperative C5 nerve root palsyremains incompletely understood and is considered to havemultiple potential etiologies. In some reports, traumaticsurgical technique or a tethering effect induced by exces-sive migration of the spinal cord after decompression areconsidered causative factors. Upper trunk brachial plexusinjury resulting from positioning or intraoperative tractionis an additional etiology. Impairment of autoregulation inthe spinal cord gray matter has been suggested to play a rolein cases where the onset of paralysis is delayed and occursin the postoperative period [66]. Sasai et al. [67] reportedthat preoperative electromyography was a sensitive predic-tor of postoperative C5 palsy after laminoplasty. Treatmentof preexistent subclinical C5 root compression with selec-tive foraminotomy in addition to posterior central canal de-compression could potentially avoid this complication inselect patients. Routine intraoperative neurophysiologicmonitoring (IONM) of upper extremity SSEPs, DEPs, andtceMEPs recorded from hand muscles generally has failedto detect this serious complication. In an effort to reducethe incidence of postoperative C5 nerve root injury, intrao-perative deltoid and biceps tceMEP and spontaneous EMGmonitoring has been reported [26]. These two neurophysi-ologic methods are considered to play complementary roles

for early detection (spEMG) and functional assessment(tceMEPs) of C5 nerve root injury during posterior cervicaldecompressive procedures. Recently, Jimenez et al. [68]reported the value of continuous C5 EMG monitoring inidentifying and preventing postoperative C5 palsies duringcervical surgery. A prospective cohort of 161 patientsmonitored with both spontaneous and triggered EMG tech-niques, SSEPs, and tceMEPs were compared with a histori-cal control group of 55 patients monitored without use ofEMG techniques. With comparison to the control group,the incidence of postoperative C5 palsy was decreased from7.3% to 0.9% with monitoring. Unfortunately, the historicalcohort study design precludes any firm conclusions regard-ing the role of monitoring in this observed decrease. Nerveroot monitoring may potentially play a role in recognitionof intraoperative C5 root impairment caused by directtrauma or positioning but is not currently helpful in predict-ing delayed onset root impairment which develops duringthe postoperative period.

Multimodality IONM

Hilibrand et al. [13] reported a series of 427 consecutivecervical spine cases (324 anterior procedures, 83 posteriorprocedures, 20 combined anterior and posterior procedures)in which multimodality IONM was performed. Fifty-onepercent (216 patients) underwent surgery for cervical spon-dylotic myelopathy and 10% (22 patients) had ossificationof the posterior longitudinal ligament. SSEPs and tceMEPsrecorded from hand muscles were performed in all patientsduring surgery. Twelve patients developed substantial orcomplete loss of amplitude of the tceMEPs. Ten of thesepatients had complete reversal of amplitude loss withprompt intervention (increasing mean arterial pressure, re-moval of anterior bone graft) while two patients awoke witha new motor deficit after surgery. SSEPs failed to identifyany changes in one of these two patients; SSEP changeslagged behind tceMEP changes by 33 minutes in the otherpatient who awoke with neurologic deficit. In patients whohad major potential changes detected by both SSEPs andtceMEPs, the SSEP changes lagged behind the tceMEPchanges by an average of 16 minutes. This delay would re-duce the window of opportunity for intervention and couldtheoretically prevent or compromise reversal of spinal cordinjury if patients were only monitored with SSEPs. In thisstudy, tceMEPs were 100% sensitive and 100% specific,whereas SSEPs were only 25% sensitive but 100% specific.In addition, all except one of the 12 patients who developedtceMEP changes underwent surgery for cervical spondy-lotic myelopathy and four of these patients also presentedwith ossification of the posterior longitudinal ligament.Based on this study, the authors strongly recommend useof both tceMEPs and SSEPs when operating on patientswith cervical spondylotic myelopathy in general, especiallyif they have ossification of the posterior longitudinalligament.


Bose et al. [69] reported a series of 119 patients treatedwith instrumented anterior cervical fusion primarily forradiculopathy who underwent IONM with SSEPs and tce-MEPs. Six neurophysiologic alerts prompted surgeon or an-esthesiologist intervention. Two alerts were related tosudden hypotension, three alerts were the result of neck hy-perextension, and one was the result of arm positioningduring surgery. Three patients awoke with new motorweakness after surgery. One was correctly predicted bymonitoring, but deficits were not detected in two patients.One patient developed additional postoperative weaknessof the C5–C6 nerve roots which could not be detected dur-ing surgery because of absent baseline tceMEPs from theaffected muscles. One patient developed quadriparesiswhich could not be detected with monitoring owing to ex-cessive use of neuromuscular blockade during surgery. Theauthors suggested that intraoperative monitoring providedvaluable information during patient positioning especiallywith respect to the degree of neck extension. In addition,they reported that conventional criteria for assessing ade-quacy of blood pressure do not address the question of spi-nal cord perfusion directly as they do not take into accountpreexisting vascular compromise of the spinal cord. Fanet al. [26] described refinement of the standard multimodal-ity monitoring protocol (tceMEPs, SSEPs, spEMG) to in-clude tceMEPs recorded from deltoid and biceps ina series of patients undergoing cervical laminectomy formyelopathy in an effort to identify impending C5 nerveroot injury.

Conclusions

In patients with cervical myelopathy, the spinal cord isalready compromised to a point at which there is little re-serve for surgical maneuvers and the slightest adverse ac-tion can result in dramatic consequences. The changesinduced by cervical spondylotic myelopathy are mainly is-chemic in origin. The same ischemic mechanism likely ac-counts for the deterioration noted in patients in whomcorrective forces are applied in the course of surgical treat-ment. These forces, rather than direct cord injury, are prob-ably responsible for the observed electrical changes whenthe head and neck are manipulated during positioning orduring intraoperative maneuvers that involve graft/cageplacement, correction of kyphosis, or alteration of spinalalignment via spinal instrumentation. These maneuversare associated with vascular stretching which may ulti-mately lead to compromise of spinal cord function. Hypo-tension is an additional factor which may lead toirreversible neurologic deficit in a partially compressedbut functionally intact spinal cord. IONM data potentiallyprovide useful data regarding such changes in the surgicalpatient.

Medical evidence exists to support the validity of neuro-physiologic monitoring as a diagnostic tool for assessment

of neurologic function during cervical spine surgery. Whensuch information is desired, recording of both SSEPs andtceMEPs should be performed as they provide complemen-tary information and monitor different spinal cord tracts.According to the tenets of evidence-based medicine, inthe absence of a randomized controlled trial regarding useof IONM during cervical surgery, statements regarding itsefficacy in improving neurologic outcomes after surgeryfor cervical myelopathy are not conclusive, and recommen-dations regarding use of IONM only represent options anddo not reflect any consensus guideline or standard of care.A randomized prospective study comparing clinical and ra-diographic outcomes in similar groups of patients undergo-ing surgery for cervical myelopathy either with or withoutintraoperative neurophysiologic monitoring would providehigh-quality evidence supporting or refuting the hypothesisthat the added expense associated with its use is justified bya clinical benefit.

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intraoperative neurophysiologic monitoring: focus on cervical

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