COPYRIGHT 2004 BY THE JOURNAL OF BONE AND JOINT SURGERY, INCORPORATED
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Comparison of Transcranial
Electric Motor and Somatosensory
Evoked Potential Monitoring
During Cervical Spine Surgery
BY ALAN S. HILIBRAND, MD, DANIEL M. SCHWARTZ, PHD, DABNM,
VENKAT SETHURAMAN, MD, ALEXANDER R. VACCARO, MD, AND TODD J. ALBERT, MD
Investigation performed at Surgical Monitoring Associates, Bala Cynwyd, and The Rothman Institute, Philadelphia, Pennsylvania
Background: There has been little enthusiasm for somatosensory evoked potential monitoring in cervical spine sur-
gery as a result, in part, of the increased risk of motor tract injury at this level, to which somatosensory monitoring
may be insensitive. Transcranial electric motor evoked potential monitoring allows assessment of the motor tracts;
therefore, we compared transcranial electric motor evoked potential and somatosensory evoked potential monitoring
during cervical spine surgery to determine the temporal relationship between the changes in the potentials demon-
strated by each type of monitoring and neurological sequelae and to identify patient-related and surgical factors asso-
ciated with intraoperative neurophysiological changes.
Methods: Somatosensory evoked potential and transcranial electric motor evoked potential data recorded for 427
patients undergoing anterior or posterior cervical spine surgery between January 1999 and March 2001 were ana-
lyzed. All patients who showed substantial (at least 60%) or complete unilateral or bilateral amplitude loss, for at
least ten minutes, during the transcranial electric motor evoked potential and/or somatosensory evoked potential
monitoring were identified.
Results: Twelve of the 427 patients demonstrated substantial or complete loss of amplitude of the transcranial elec-
tric motor evoked potentials. Ten of those patients had complete reversal of the loss following prompt intraoperative
intervention, whereas two awoke with a new motor deficit. Somatosensory evoked potential monitoring failed to iden-
tify any change in one of the two patients, and the change in the somatosensory evoked potentials lagged behind the
change in the transcranial electric motor evoked potentials by thirty-three minutes in the other. No patient showed
loss of amplitude of the somatosensory evoked potentials in the absence of changes in the transcranial electric mo-
tor evoked potentials. Transcranial electric motor evoked potential monitoring was 100% sensitive and 100% specific,
whereas somatosensory evoked potential monitoring was only 25% sensitive; it was, however, 100% specific.
Conclusions: Transcranial electric motor evoked potential monitoring appears to be superior to conventional soma-
tosensory evoked potential monitoring for identifying evolving motor tract injury during cervical spine surgery. Sur-
geons should strongly consider using this modality when operating on patients with cervical spondylotic myelopathy in
general and on those with ossification of the posterior longitudinal ligament in particular.
Level of Evidence: Diagnostic study, Level I-1 (testing of previously developed diagnostic criteria in series of consec-
utive patients [with universally applied reference “gold” standard]). See Instructions to Authors for a complete de-
scription of levels of evidence.
europhysiological monitoring is used during spine
surgery to assess the function of the spinal cord and
to identify any evolving iatrogenic spinal cord injury.
Studies of patients undergoing surgery for scoliosis and thora-
columbar injuries have demonstrated that continuous soma-
tosensory evoked potential (SSEP) monitoring can reduce the
risk of iatrogenic paraplegia
1-7
. Despite the success of identifi-
cation of iatrogenic injury with SSEP monitoring during op-
erations for the correction of scoliosis and other corrective
operations on the thoracic spine, SSEPs are mediated prima-
rily by the dorsal sensory spinal cord tracts. As such, the diag-
nosis of an impending motor tract injury based on changes in
SSEP amplitude and/or latency is presumptive at best. Hence,
there is the risk of a false-negative result
8-13
.
N

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THE JOURNAL OF BONE & JOINT SURGERY · JBJS.ORG
VOLUME 86-A · NUMBER 6 · JUNE 2004
TRANSCRANIAL ELECTRIC MOTOR AND SOMATOSENSORY
EVOKED POTENTIALS IN CERVICAL SPINE SURGERY
Although transcranial electric motor evoked potential
(tceMEP) monitoring has shown great promise with regard
to more rapid identification of corticospinal tract injury dur-
ing excision of spinal cord tumors or correction of scoliosis
14-20
,
there is a dearth of information related to its use to monitor
motor tract function during cervical spine surgery
21,22
. Hence,
this study was designed to determine and compare the sensi-
tivity and specificity of tceMEP and conventional SSEP moni-
toring for detection of impending spinal cord injury during
cervical spine surgery; to determine the temporal relationship
between the changes demonstrated by these two monitoring
modalities and subsequent, clinically important neurological
sequelae, and to identify patient-related and surgical risk
factors associated with intraoperative changes in these
evoked potentials.
Materials and Methods
he protocol for this study was reviewed by the institu-
tional review board of Thomas Jefferson University Hos-
pital and was granted an exempt status because it was an
anonymous retrospective chart review. Outcomes data for all
cervical spine surgical procedures performed with multimo-
dality spinal cord monitoring between January 1, 1999, and
March 31, 2001, at a single institution were reviewed. A total
of 427 procedures were performed in 242 male patients (57%)
and 185 female patients (43%) ranging in age from fifteen to
ninety-five years, with an average age of fifty years, at the time
of the index procedure. There were 324 anterior, eighty-three
posterior, and twenty combined anterior and posterior proce-
dures. Of the 427 patients, 216 (51%) presented with a preop-
erative diagnosis of cervical spondylotic myelopathy, and
twenty-two (10%) of the 216 had ossification of the posterior
longitudinal ligament. Intraoperative records were examined
in an attempt to identify the operative event that correlated
with the neurophysiologic change as well as the effect of surgi-
cal and/or anesthesia-related intervention on the changes
demonstrated by monitoring. Hospital and office charts were
also reviewed to determine the preoperative diagnoses as well
as the preoperative, immediate postoperative, and most recent
neurological data. The anesthesia protocol used during the
study period is described in detail in the Appendix.
Spinal Cord Monitoring
Spinal cord monitoring was performed continuously, from the
time of induction of anesthesia until the patient emerged from
the anesthesia, by recording both upper and lower-extremity
efferent transcranial electric motor (tceMEPs) and afferent so-
matosensory evoked potentials (SSEPs). Cortical and subcor-
tical SSEPs were elicited to a 300-µS square-wave electrical
pulse presented sequentially to the posterior tibial and ulnar
nerves at a rate of 4.7/sec. Stimulation intensity levels ranged
between 35 and 50 mA. These levels were selected as being
well within the asymptotic portion of the SSEP intensity ver-
sus amplitude plot for each patient. Cortical potentials were
recorded from gold-plated cup electrodes (Grass Instruments,
Quincy, Massachusetts) affixed to Cpz, Cp3, and Cp4 and ref-
erenced to Fpz (international 10-20 system). Subcortical cer-
vical/brainstem responses were recorded over the surface of
the C2 or C3 vertebra and also referenced to Fpz. Commer-
cially available neurophysiology instrumentation (Nicolet En-
deavor; Nicolet Biomedical, Madison, Wisconsin, or Cadwell
Sierra; Cadwell Laboratories, Kennewick, Washington) was
used for all SSEP stimulation and recording.
Transcranial electric motor evoked potentials were re-
corded over the first dorsal interosseous muscle in the upper
extremities and both tibialis anterior and abductor hallucis
muscles in the lower extremities following a brief-duration,
high-voltage (400 to 1000-V) anodal electrical stimulus train
(pulse width = 50 µS, N = 3 to 7, interpulse interval = 1 to 5
msec). The multipulse stimulus was delivered between two
corkscrew-type electrodes (A-Gram, Glenn Rock, New Jersey)
inserted over motor cortex regions at C1 and C2 (interna-
tional 10-20 system). Stimuli were delivered through a com-
mercially available transcortical stimulator (D185; Digitimer,
Welwyn Garden City, United Kingdom) with responses re-
corded on the same system used for monitoring SSEPs.
Response Interpretation
A clinically relevant neurophysiological change was defined as
an intraoperative unilateral or bilateral amplitude loss of at
least 60% with persistence over at least a ten-minute duration.
This cutoff value, equating to a greater than 2.0 standard devi-
ation criterion for a major change, was selected to reduce the
possibility of false-positive interpretation due to response
variability, as reported by York et al.
23
. If such a change in
evoked potentials was detected, anesthesia personnel were di-
rected to increase the patient’s mean arterial pressure to at
least 90 mm Hg. A failure of the response amplitude to im-
prove over the course of five to ten minutes led to the adminis-
tration of so-called spinal-cord-injury steroids consisting of
high-dose methylprednisolone (NASCIS-2 [National Acute
Spinal Cord Injury Study-2] protocol: 30 mg/kg bolus fol-
lowed by 5.4 mg/kg/hr for 23 hr)
24,25
. If the change in the
evoked potentials was temporally associated with placement
of a bone graft or internal fixation, surgical intervention in-
cluded removal of the graft or fixation as well.
Statistical Methods
The accuracy of the monitoring with regard to detecting im-
pending iatrogenic spinal cord injury was expressed by calcu-
lating sensitivity and specificity. We defined an impending
injury as any important neurophysiological change that
prompted some type of intervention (for example, increasing
the mean arterial pressure, administering steroids, or remov-
ing bone graft). A true-positive result was defined as any case
in which the changes in the evoked potentials were reversed
immediately by the intervention or in which the changes per-
sisted and the patient awoke with a new neurological deficit. A
false-positive result was defined as any case in which the changes
in the evoked potentials did not respond to intervention and
the patient awoke neurologically intact. A true-negative result
was defined as a case in which monitoring revealed no changes
T