Jeffrey H. Owen

Sensitivity and specificity of somatosensory and neurogenic-motor evoked potentials in animals and humans.

The purpose of this study was to report the effects of spinal cord compression, ischemia, and distraction on clinical status, and somatosensory (SEP) and neurogenic-motor evoked potentials (NMEPs) in animals. The authors also reported their clinical experience with NMEPs elicited from humans undergoing surgery for spinal deformities. Results from the animal studies Indicate that NMEPs are more sensitive and specific to the effects from spinal cord compression, ischemia, and distraction than SEPs. In every situation, NMEPs always correlated with the animals post-surgical clinical status, while SEPs demonstrated an unacceptable false positive and false negative rate. In the 111 clinical cases in which NMEPs were administered, reliable NMEPs were easily elicited in more than 90% of the cases. In the remaining cases, no reliable NMEPs could be recorded because of procedural errors, which have been resolved. The results from this study suggest that the use of NMEPs should be considered as an adjunct to SEPs when monitoring spinal cord function during surgery.

Direct noninvasive monitoring of spinal cord motor function during thoracic aortic occlusion: Use of motor evoked potentials

Spinal cord monitoring during thoracic aneurysmectomy by somatosensory evoked potentials has been criticized for its failure to measure anterior (motor) spinal cord function. We have developed a clinically applicable, noninvasive technique for intraoperative monitoring of motor evoked potentials (MEP), which allows direct functional assessment of spinal cord motor tracts during thoracic aortic occlusion. Twelve dogs underwent continuous intraoperative monitoring of MEP before, during, and after thoracic aortic cross-clamping. Motor tract response to noninvasive cord stimulation (5 to 10 mA, 0.02 msec, 4.3 H2) was recorded by subcutaneous electrodes placed along the length of the spine (T-10, L-1, and L-4). Six animals (group I) subjected to aortic cross-clamping alone demonstrated a characteristic time- and level-dependent deterioration and loss of MEP. Ischemic cord dysfunction (as determined by time from clamping to loss of MEP) progressed from the distal to the proximal cord (L-4 = 11.3 +/- 1.5 minutes; L-1 = 14.9 +/- 2.3 minutes; T-10 = 16.9 +/- 2.3 minutes; p less than 0.05 between all levels). Reperfusion of the distal aorta 20 minutes after clamping resulted in MEP return that progressed from the proximal (T-10) to distal (L-1 and L-4) cord. In another six animals (group II), distal perfusion (mean blood pressure = 95 mm Hg) was maintained for 1 hour after cross-clamping by left atrial-femoral artery bypass. Normal configuration and amplitude of MEP was maintained throughout the cross-clamping period. These data suggest that distinctive changes in MEP indicative of reversible ischemia of spinal cord motor tracts occur after aortic cross-clamping. Such ischemia begins in the most distal cord, exhibits upward progression with time, and can be prevented by maintenance of adequate distal aortic perfusion. Clinical use of MEP monitoring during thoracic aneurysmectomy may provide a method for intraoperative assessment of the adequacy of motor tract perfusion.

The clinical application of neurogenic motor evoked potentials to monitor spinal cord function during surgery.

The purpose of this study was to report results from 300 cases (177 children, 123 adults) administered somatosensory and neurogenic motor evoked potentials during surgery. Of these 300 cases, there were 16 cases of spinal fractures, 16 neurosurgical cases, 28 vascular cases, and 240 cases of elective posterior spinal deformity requiring instrumentation. Results indicated that somatosensory evoked potentials, especially cortical components, demonstrated greater variability than neurogenic motor evoked potentials. Variability was attributed to anesthesia and unknown factors. Neurogenic motor evoked potentials proved to be a more valid indicator of postoperative motor status than somatosensory evoked potentials. Based on their anatomic substrates and results from this study, it was recommended that somatosensory evoked potentials and neurogenic motor evoked potentials be used to monitor spinal cord function during surgery that would place that structure at risk.

Complications of spinal fusion for scheuermann kyphosis: a report of the scoliosis research society morbidity and mortality committee.

Study Design. Retrospective review of a prospectively collected, multicentered database from the Scoliosis Research Society. Objectives. To evaluate incidences of complications in a series of spinal fusions for Scheuermann kyphosis (SK) and to assess whether the incidence of complications is associated with patient age and surgical approach. Summary of Background Data. Although there is some evidence that adolescents have lower complication rates for spinal deformity surgery, this has not been well-documented for SK. Moreover, there is a lack of consensus on surgical approach for the management of SK. Methods. The Scoliosis Research Society morbidity and mortality database was queried to identify cases of SK from 2001 to 2004. Complications rates were analyzed based on patient age and surgical approach. Pediatric and adult patients were defined as ≤19 and >19 year old, respectively. Results. A total of 683 procedures involving spinal fusion for SK were identified. Mean patient age was 21 years (range: 5–75 years), with the majority (73%) of patients ≤19 years old. Procedures included 338 (49%) posterior spinal fusions (PSF), 73 (11%) anterior spinal fusions (ASF), and 272 (40%) same-day ASF and PSF. Ninety-nine complications were reported (14%). The most common complication was wound infection (3.8%). The acute neurologic complication rate was 1.9%, including 4 spinal cord injuries (0.6%). The mortality rate was 0.6%. Complications were more common among adult (22%) compared with pediatric patients (12%) (P = 0.002). The overall incidence of complications did not differ significantly between the PSF (14.8%) and same-day ASF/PSF (16.9%) procedures (P = 0.5). Conclusion. The incidence of complications associated with spinal fusion for SK in adults is significantly greater than in pediatric patients. There were no significant differences in complication rates between PSF and same-day ASF/PSF procedures. These data may be used to counsel patients regarding complications associated with spinal fusion for SK in the hands of experienced spinal deformity surgeons.

The use of mechanically elicited electromyograms to protect nerve roots during surgery for spinal degeneration.

Study Design. This study investigated the use of mechanically elicited electromyograms during the placement of pedicle screws in 89 patients undergoing surgery for spinal stenosis. Summary of Background Data. Several methods for monitoring nerve roots are available. However, mechanically elicited electromyograms may be more sensitive to mechanical irritation of nerve roots by pedicle screws than by other methods. Methods. Mechanically elicited electromyograms were recorded in muscle groups innervated by cervical or lumbar nerve roots. Confirmation of surgical activity with the level of the electromyogram was correlated. Results. Results indicated that mechanically elicited electromyograms are extremely sensitive to nerve root irritation. Compared to other methods, electromyograms are a viable alternative. Conclusions. Results from this study indicated that mechanically elicited electromyograms are sensitive and specific to nerve root firings and should be considered for use during the dynamic phases of surgery.

Evaluation of pedicle screw insertion monitored by intraoperative evoked electromyography.

The insertion of pedicle screws monitored by evoked electromyography (EMG) was prospectively evaluated in the 132 consecutive patients. The technique involved constant-voltage stimulation and was statistically evaluated at both the arbitrary 20- and 40-V settings. The patients were postoperatively evaluated clinically and radiographically. Computed tomography (CT) scanning was performed for new neurologic deficits. Results were divided into three groups: type 1, a negative EMG response; type 2, a positive EMG response, but no corrective action taken; and type 3, a positive EMG response and corrective action undertaken. Nonparametric statistics were used to evaluate the results at both the 20- and 40-V settings. In the type 3 group, in which corrective action was undertaken, there were no neurologic injuries or screw removals, a statistically significant result. Looking at the two intensity levels, at 20- and 40-V settings, there were no statistically significant differences in the three classifications at either intensity level. We concluded the evoked EMG for monitoring pedicle screw insertion is an efficacious adjunct. A positive response at < 20 V with the constant-voltage technique warrants corrective action.

Effects of spinal cord lesioning on somatosensory and neurogenic-motor evoked potentials.

Somatosensory (SEPs) and neurogenic-motor evoked potentials (NMEPs) were elicited from 16 hogs and two humans before, during, and after spinal cordotomy, dorsal, or ventral root rhizotomy. Results indicated that SEPs appear to be insensitive to the effects of motor tract lesioning in hogs and humans. In every case of motor paraplegia, SEPs remained unchanged in the presence of abnormal ischiatic/sciatic NMEPs. These results suggest that SEPs are not adequately sensitive to the functional status of the motor system in hogs and humans. Ischiatic/sciatic NMEPs remained unchanged after sensory tract lesioning, suggesting that these NMEPs are insensitive to the functional status of the sensory system. These results suggest that SEPs and NMEPs should be used in combination when monitoring spinal cord function during surgeries that place that structure at risk.

Use of sciatic neurogenic motor evoked potentials versus spinal potentials to predict early-onset neurologic deficits when intervention is still possible during overdistraction

Spinal evoked potentials, sciatic neurogenic motor evoked potentials, and somatosensory evoked potentials were recorded before and after overdistraction of the spinal cord, and compared with the clinical status of 14 pigs. The sciatic neurogenic motor evoked potential consisted of two components: fast and slow. The fast component was more sensitive and associated to a greater degree with motor function in wake-up tests than the slow component somatosensory evoked potential and spinal evoked potential. Furthermore, the loss of only the fast component in the initial status allowed the possibility of improvement of motor activity in the final wake-up test. The peripheral neurogenic motor evoked potentials recording yielded more information about spinal cord function: motor and sensory. The current study suggests that a peripheral response is a better index to the onset of overdistraction and to the efficiency of intervention, when the neurologic deficit after overdistraction of the spine is reversible.

Relationship between duration of spinal cord ischemia and postoperative neurologic deficits in animals

Twenty hogs were administered the following procedures before, during, and after overdistraction of the spinal column at T5–T6: somatosensory (SEP) and neurogenic-motor evoked potentials (NMEPs), hydrogen clearance procedures, Stagnara wake-up tests, and aortic-injection of silastic plastic. To ensure that overdistraction was possible, a nonosseous, circumferential osteotomy was made at T5–T6 and distraction applied in one-ratchet increments using Harrington instrumentation. Overdistraction was maintained for 3, 5, 6,10,15, 20, 25, or 30 minutes. Results indicated that the duration of overdistraction, as represented by lost NMEPs, was always correlated with the animals clinical status on wake-up test. If overdistraction was maintained more than 6 minutes, 100% of the animals demonstrated positive wake-up results; if maintained between 5 and 6 minutes, 75% demonstrated positive wake-up results; and if maintained less than 5 minutes, only 25% demonstrated positive wake-up results. Time-to-loss of the NMEPs and SEPs, after onset of overdistraction, fell within two groups: slow and fast. In the slow group, it required slightly more than 20 minutes (mean = 20.6) for the potentials to be lost, while in the fast-loss group data were lost in slightly less than 4 minutes (mean = 3.6). Blood flow studies and inspection of the spinal cord revealed that the mechanism of action for the slow group appeared to be ischemia of the spinal cord that extended several centimeters above and below the site of maximum distraction. In the fast-loss group, it appeared that gross structural damage, with some very localized ischemia, were the mechanisms of actions influencing the integrity of the spinal cord. Comparatively, NMEPs continue to demonstrate a greater sensitivity than SEPs to the effects of overdistraction on spinal cord function. Additionally, NMEPs demonstrated a greater correlation with clinical status than SEPs. In conclusion, the joint administration of NMEPs and SEPs appear to provide the surgeon with important and continuous information regarding the gross motor and sensory tracts of the spinal cord. Additionally, the onset and elapsed time of overdistraction can be measured, which may be helpful for monitoring and improving the efficacy of intervention techniques.

Effects of Distraction on Physiologic Integrity of the Spinal Cord, Spinal Cord Blood Flow, and Clinical Status

The authors determined the effects of distraction of the spine on physiologic integrity of the spinal cord using neurogenic motor evoked potentials (NMEPs), somatosensory evoked potentials (SEPs), spinal cord blood flow measurements, and clinical status in nine hogs. Spinal cord blood flow was measured after each level of distraction using the hydrogen washout technique. The results indicated that blood flow of at least 65% of baseline was required to maintain physiologic integrity of the spinal cord, and that a decrease of blood flow to 12% of baseline was associated with paraplegia. Neurogenic motor evoked potentials always correlated with the animals postsurgical clinical status, whereas the SEP was falsely negative in one animal.