Background: Despite the many reports attesting to the efficacy of
intraoperative somatosensory evoked potential monitoring in reducing the
prevalence of iatrogenic spinal cord injury during corrective scoliosis
surgery, these afferent neurophysiological signals can provide only indirect
evidence of injury to the motor tracts since they monitor posterior column
function. Early reports on the use of transcranial electric motor evoked
potentials to monitor the corticospinal motor tracts directly suggested that
the method holds great promise for improving detection of emerging spinal cord
injury. We sought to compare the efficacy of these two methods of monitoring
to detect impending iatrogenic neural injury during scoliosis surgery.
Methods: We reviewed the intraoperative neurophysiological
monitoring records of 1121 consecutive patients (834 female and 287 male) with
adolescent idiopathic scoliosis (mean age, 13.9 years) treated between 2000
and 2004 at four pediatric spine centers. The same group of experienced
surgical neurophysiologists monitored spinal cord function in all patients
with use of a standardized multimodality technique with the patient under
total intravenous anesthesia. A relevant neurophysiological change (an alert)
was defined as a reduction in amplitude (unilateral or bilateral) of at least
50% for somatosensory evoked potentials and at least 65% for transcranial
electric motor evoked potentials compared with baseline.
Results: Thirty-eight (3.4%) of the 1121 patients had recordings
that met the criteria for a relevant signal change (i.e., an alert). Of those
thirty-eight patients, seventeen showed suppression of the amplitude of
transcranial electric motor evoked potentials in excess of 65% without any
evidence of changes in somatosensory evoked potentials. In nine of the
thirty-eight patients, the signal change was related to hypotension and was
corrected with augmentation of the blood pressure. The remaining twenty-nine
patients had an alert that was related directly to a surgical maneuver. Three
alerts occurred following segmental vessel clamping, and the remaining
twenty-six were related to posterior instrumentation and correction. Nine
(35%) of these twenty-six patients with an instrumentation-related alert, or
0.8% of the cohort, awoke with a transient motor and/or sensory deficit. Seven
of these nine patients presented solely with a motor deficit, which was
detected by intraoperative monitoring of transcranial electric motor evoked
potentials in all cases, and two patients had only sensory symptoms.
Somatosensory evoked potential monitoring failed to identify a motor deficit
in four of the seven patients with a confirmed motor deficit. Furthermore,
when changes in somatosensory evoked potentials occurred, they lagged behind
the changes in transcranial electric motor evoked potentials by an average of
approximately five minutes. With an appropriate response to the alert, the
motor or sensory deficit resolved in all nine patients within one to ninety
days.
Conclusions: This study underscores the advantage of monitoring the
spinal cord motor tracts directly by recording transcranial electric motor
evoked potentials in addition to somatosensory evoked potentials. Transcranial
electric motor evoked potentials are exquisitely sensitive to altered spinal
cord blood flow due to either hypotension or a vascular insult. Moreover,
changes in transcranial electric motor evoked potentials are detected earlier
than are changes in somatosensory evoked potentials, thereby facilitating more
rapid identification of impending spinal cord injury.
Level of Evidence: Diagnostic Level I. See Instructions
to Authors for a complete description of levels of evidence.