Anne M. Padberg

Somatosensory- and motor-evoked potential monitoring without a wake-up test during idiopathic scoliosis surgery : An accepted standard of care

Study Design. This was a retrospective study of 500 patients undergoing corrective surgery between 1987 and 1997 for spinal deformity caused by idiopathic scoliosis. Objectives. To report the sensitivity and specificity of somatosensory‐evoked and neurogenic motor‐evoked potentials monitoring and the requirements for an intraoperative wake‐up test for all idiopathic scoliosis surgeries at a single institution. Summary of Background Data. Intraoperative monitoring is recommended for use during corrective spinal surgery. Accepted monitoring standards and requirements for an intraoperative wake‐up test are still debated. Methods. The study group consisted of 500 patients undergoing corrective surgery for idiopathic scoliosis between 1987 and 1997. All patients were monitored using somatosensory‐evoked and neurogenic motor‐evoked potential techniques, using a standard protocol developed at this institution. Results. The false‐positive rate (significant data change without postoperative neurologic deficit) was 0.014% (n = 7). The true‐positive rate (degradation of data that met warning criteria, with a corresponding postoperative neurologic deficit) was 0.004% (n = 2). No false‐negative results (normal data during with a postoperative neurologic deficit) were seen. The sensitivity of combined somatosensory‐evoked and neurogenic motor‐evoked potentials data in predicting neurologic status was 98.6%, and the specificity of normal data predicting normal findings in a neurologic examination was 100%. Conclusions. Combined somatosensory‐evoked and neurogenic motor‐evoked potentials monitoring during idiopathic scoliosis surgery represents a standard of care that obviates the need for an intraoperative wake‐up test when reliable data are obtained and maintained.

Triggered electromyographic threshold for accuracy of pedicle screw placement. An animal model and clinical correlation.

Study Design This study consisted of a laboratory investigation of triggered electromyographic stimulation of pedicle screws placed in a pig spine, with a correlative prospective clinical series of lumbosacral pedicle screws stimulated in a simillar fashion. Objectives To determine the threshold of stimulus intensity necessary to confirm accuracy of lumbar pedicle screw placement via a triggered electromyographic peripheral response. Summary of Background Data Documentation of lumbar pedicle screw placement is imperative to perform proper spinal instrumentation and to avoid perioperative complications. Previous electrophysiologic techniques using stimulation of a pedicle opening or pedicle screw with peripheral recording of electromyographic activity from the lower extremity muscles have been used to identify varying threshold values that indicate a break in the bony pedicle wall. Methods Six adult pigs had 107 pedicle screws placed bilaterally into the pedicles of the lumbar spine. These screws were stimulated with an ascending stimulus intensity until a peripheral triggered electromyographic response was recorded. Pedicle screws were placed in the pig either entirely in the pedicle (Group A), medial to the pedicle without direct contact to the nerve root and dura (Group B), or purposely medial to the pedicle with direct contact to the nerve root and dura (Group C). A correlative clinical series of 233 pedicle screws placed in 54 patients had a similar intraoperative neurophysiologic technique. Results In the animal model, the mean threshold differences were: Group A screws 21.9 mA, Group B screws 8.5 mA, and Group C screws 4.2 mA (P < 0.05). Ninety-three percent of the clinical Group A screws had threshold stimuli less than 8.0 mA, whereas Groups B and C screws had a mean threshold of 3.3 mA. Conclusions Triggered electromyographic stimulation is a valuable aid to determine appropriate placement of pedicle screws. We recommend the following interpretation of threshold stimulus intensity: > 8 mA- screw entirely in the pedicle; 4.0–8.0 mA-potential for pedicle wall defect; < 4.0 mA-strong likelihood of pedicle wall defect with potential for nerve root and dura contact.

Can Triggered Electromyograph Thresholds Predict Safe Thoracic Pedicle Screw Placement

Study Design. A prospective clinical study of thoracic pedicle screws monitored with triggered electromyographic testing. Objective. To evaluate the sensitivity of recording rectus abdominis triggered electromyographs to assess thoracic screw placement. Summary of Background Data. Triggered electromyographic testing from lower extremity myotomes has identified medially placed lumbar pedicle screws. Higher thresholds indicate intraosseous placement because of increased resistance to current flow. Lower thresholds correspond to compromised pedicles with potential for nerve impingement. No clinical study has correlated an identical technique with rectus muscle recordings, which are innervated from T6 to T12. Methods. A total of 677 thoracic screws were placed in 92 consecutive patients. Screws placed from T6 and T12 were evaluated using an ascending method of stimulation until a compound muscle action potential was obtained from the rectus abdominis. Threshold values were compared both in absolute terms and also in relation to other intrapatient values. Results. Screws were separated into three groups: Group A (n = 650 screws) had thresholds >6.0 mA and intraosseus placement. Group B (n = 21) had thresholds <6.0 mA but an intact medial pedicle border on reexamination and radiographic confirmation. Group C (n = 6) had thresholds <6.0 mA and medial wall perforations confirmed by tactile and/or visual inspection. Thus, 3.9% (27 of 677) of all screws had thresholds <6.0 mA. Only 22% (6 of 27) had medial perforation. Group B screws averaged a 54% decrease from the mean as compared with a 69% decrease for Group C screws (P = 0.0160). There were no postoperative neurologic deficits or radicular chest wall complaints. Conclusion. To assess thoracic pedicle screw placement, triggered electromyographic thresholds <6.0 mA, coupled with values 60–65% decreased from the mean of all other thresholds in a given patient, should alert the surgeon to suspect a medial pedicle wall breach.

Efficacy of intraoperative monitoring for pediatric patients with spinal cord pathology undergoing spinal deformity surgery

STUDY DESIGN A retrospective study of 38 pediatric patients with spinal cord pathology who underwent corrective spinal deformity surgery from January 1989 through June 1998. OBJECTIVES To report reliability and specificity in obtaining intraoperative data in this population. These data were compared with monitoring results obtained in a group of pediatric patients with idiopathic scoliosis. SUMMARY OF BACKGROUND DATA Reports in the literature suggest intraoperative monitoring for patients with spinal cord pathology may be of limited value. No optimal monitoring protocol has been suggested for this population. METHODS The study group consisted of 38 pediatric patients with a diagnosis of spinal cord pathology who underwent corrective spinal deformity surgery from January 1989 through June 1998. All patients had lower extremity function. Somatosensory and neurogenic motor evoked potentials were used to monitor neurologic status during surgery. These data were compared with data obtained in 429 pediatric patients with idiopathic scoliosis. Study patients were divided into Group I, those who had had spinal cord surgery (n = 20), and Group II, those who had not (n = 18). RESULTS Somatosensory evoked potentials were obtained in 93.2% and remained consistent with baselines in 87.2% of the study group patients. Neurogenic motor evoked potentials were obtained in 50.8% of the study subjects and remained consistent in 76.6% of those cases. The false-positive rate was 27.1% in the study group, compared with 1.4% in the group with idiopathic scoliosis. The study group had no true-positive or false-negative findings. Group I data differed from Group II data. CONCLUSIONS Intraoperative monitoring should be used in patients with spinal cord pathology who undergo surgery for spinal deformity. Monitoring should not miss a neurologic deficit but demonstrates greater variability, resulting in more frequent use of an intraoperative wake-up test.

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.

Validity and reliability of intraoperative monitoring in pediatric spinal deformity surgery: a 23-year experience of 3436 surgical cases.

Study Design. This was a 23-year retrospective study of 3436 consecutive pediatric orthopedic spinal surgery patients between 1995 and 2008. Objective. To demonstrate the effectiveness of multimodality electrophysiologic monitoring in reducing the incidence of iatrogenic neurologic deficit in a pediatric spinal surgery population. Summary of Background Data. The elective nature of many pediatric spinal surgery procedures continues to drive the need for minimizing risk to each individual patient. Electrophysiologic monitoring has been proposed as an effective means of decreasing permanent neurologic injury in this population. Methods. A total of 3436 consecutive monitored pediatric spinal procedures at a single institution between January 1985 and September 2008 were reviewed. Monitoring included somatosensory-evoked potentials, descending neurogenic-evoked potentials, transcranial electric motor-evoked potentials, and various nerve root monitoring techniques. Patients were divided into 10 diagnostic categories. True-positive and false-negative monitoring outcomes were analyzed for each category. Neurologic deficits were classified as transient or permanent. Results. Seven of 10 diagnostic groups demonstrated true positive findings resulting in surgical intervention. Seventy-four (2.2%) potential neurologic deficits were identified in 3436 pediatric surgical cases. Seven patients (0.2%) had false-negative monitoring outcomes. These patients awoke with neurologic deficits undetected by neuromonitoring. Intervention reduced permanent neurologic deficits to 6 (0.17%) patients. Monitoring data were able to detect permanent neurologic status in 99.6% of this population. The ratio of intraoperative events to total monitored cases was 1 event every 42 surgical cases and 1 permanent neurologic deficit every 573 cases. Conclusion. The combined use of somatosensory-evoked potentials, transcranial electric motor-evoked potentials, descending neurogenic-evoked potentials, and electromyography monitoring allowed accurate detection of permanent neurologic status in 99.6% of 3436 patients and reduced the total number of permanent neurologic injuries to 6.

Loss of spinal cord monitoring signals in children during thoracic kyphosis correction with spinal osteotomy: why does it occur and what should you do?

Study Design. A retrospective review of pediatric kyphosis patients undergoing a spinal cord-level osteotomy for correction. Objective. To evaluate the prevalence, etiology, timing, and intervention related to loss of spinal cord monitoring data during surgical correction of pediatric kyphosis in the spinal cord region. Summary of Background Data. Although much has been written regarding the risks inherent to scoliosis surgery, there is less literature available regarding the neurologic outcomes of pediatric kyphosis surgery. As more surgeons contemplate posterior-only kyphosis correction with spinal cord-level osteotomies, the importance of maintaining spinal cord neurologic function is paramount. Methods. Forty-two patients with pediatric kyphosis undergoing a posterior-only spinal reconstruction with a spinal cord level osteotomy or posterior-based vertebral column resection performed were reviewed. Patients were categorized by diagnosis, type and incidence of osteotomies, and loss of neurogenic mixed-evoked potential (NMEP) data. Interventions required to regain data and postoperative neurologic outcomes were also reviewed. Results. Of the 42 patients, 9 (21.4%) demonstrated a complete loss of NMEP data sometime during surgery while concomitant somatosensory sensory-evoked potentials (SSEP) remained within acceptable limits of baseline values. All 9 patients had intraoperative intervention including: blood pressure elevation (n = 1), release of corrective forces (n = 2), blood pressure elevation and correction release (n = 3), malalignment/subluxation adjustment (n = 1), further bony decompression (n = 1), or restoration of anterior column height via a titanium cage along with further posterior decompression (n = 1). In all cases, SSEPs were unchanged and NMEPs returned varying from 8 to 20 minutes after loss, with all patients having a normal wake-up test intraoperatively and a normal neurologic examination after surgery. Conclusion. Intraoperative multimodality monitoring with some form of motor tract assessment is a fundamental component of kyphosis correction surgery in the spinal cord region in order to create a safer, optimal environment and to minimize neurologic deficit. The surgeon must be able to trust the information monitoring provides and act on it accordingly.

Correlation between low triggered electromyographic thresholds and lumbar pedicle screw malposition : Analysis of 4857 screws

Study Design. A retrospective analysis of 1078 spinal surgical procedures with lumbar pedicle screw placement at a single institution. Objective. Based on previously established normative values, triggered electromyographic stimulation (TrgEMG) was re-examined to evaluate its efficacy in determining screw malposition. Summary of Background Data. Threshold values for confirmation of intraosseous placement of pedicle screws with EMG stimulation is controversial. Methods. TrgEMG threshold values for 4857 pedicle screws placed from L2 to S1 from 1996 to 2005 were analyzed. An ascending method of constant current stimulation was applied to each pedicle screw to obtain a compound muscle action potential (CMAP) from lower extremity myotomes. Previously determined threshold value normative data from a published clinical series of 233 screws were as follows: 0 to 4 mA, high likelihood of pedicle wall breach; 4 to 8 mA, possible pedicle wall breach; >8 mA, no pedicle wall defect. Results. A total of 7.74% (376 of 4857) of all screws tested had threshold values <8.0 mA. A total of 19.1% (72 of 376) of these were <4.0 mA: 54% (39 of 72) were repositioned (26) or removed (13) while the remaining 33 screws were left in place following repalpation. A total of 80.9% (304 of 376) had thresholds between 4 and 8 mA: 17.4% (53) were repositioned (38) or removed (15). Nine screws had thresholds of ≤2.8 mA and were either repositioned or removed following confirmation of a medial wall breach. A total of 74.5% (280 of 376) of all screws with thresholds <8.0 mA were verified as correctly placed by repalpation/radiography and therefore left in place. Conclusion. The probability of a medial breach pedicle screw detected by triggered EMG stimulation increases with decreasing triggered EMG thresholds: 0.31% for >8.0 mA, 17.4% for 4.0 to 8.0 mA, 54.2% for <4.0 mA, and 100% for <2.8 mA. At 2.8 mA, triggered EMG has a specificity of 100%, with sensitivity of 8.4%; at 4.0 mA, specificity of 99% and sensitivity of 36%; and at 8.0 mA, 94% specificity and 86% sensitivity. TrgEMG is an adjuncttechnique and should always be used in conjunction with palpation and radiography to optimize safe pedicle screw placement.

Somatosensory evoked potentials fail to diagnose thoracic outlet syndrome.

Somatosensory evoked potentials (SEPs) are used in the diagnosis of thoracic outlet syndrome (TOS), even as an indication for surgery. The purpose of this study was to evaluate the use of SEPs in the diagnosis of TOS. Twenty-one patients (mean age, 37 years) with TOS and 23 control subjects (mean age, 34 years) were included. Somatosensory evoked potentials of median and ulnar nerves were measured bilaterally in patients in both a relaxed and arms-elevated provocative position. A three-way analysis of variance showed no significant difference between the interpeak latencies of the TOS and control groups (p = .352). Significant differences were found in testing positions (p = .0014) and nerve tested (p = .001) in both groups. Therefore, this study suggests that SEPs are not helpful in the diagnosis of TOS.

Validity and reliability of spinal cord monitoring in neuromuscular spinal deformity surgery

Somatosensory evoked potentials (SSEPs) have become a standard of care in surgery for spinal deformity. Recent reports in the literature have suggested SSEP monitoring is not efficacious in surgeries for patients with neuromuscular disease. Electrophysiologic data were retrospectively analyzed from 74 patients with various neuromuscular disorders undergoing spinal-deformity surgery from 1989 through February 1995 at this medical center. The monitoring protocol included SSEPs recorded from multiple sites located cortically, subcortically, and peripherally. Neurogenic motor evoked potentials (NMEPs) were also employed. Anesthetic regimens were controlled for compatibility with evoked potential monitoring. Use of this intraoperative monitoring protocol resulted in reliable data for 95% of the patient population having baseline responses. Findings suggest that evoked potentials can be used effectively during surgery for neuromuscular spinal deformity. Use of a specific protocol allowed acquisition of reliable data intraoperatively, suggesting these methods are a valid means of monitoring neurologic status.