Efficacy of Anal Needle Electrodes for Intraoperative Spinal Cord Monitoring with Transcranial Muscle Action Potentials

Identifying Suspected Volume Conduction Contamination of External Anal Sphincter Motor Evoked Potentials in Lumbosacral Spine Surgery

PURPOSE

Iatrogenic injury to sacral nerve roots poses significant quality of life issues for patients. Motor evoked potential (MEP) monitoring can be used for intraoperative surveillance of these important structures. We hypothesized that volume conducted depolarizations from gluteus maximus (GM) may contaminate external anal sphincter (EAS) MEP results during lumbosacral spine surgery.

METHODS

Motor evoked potential from the EAS and medial GM in 40 patients were prospectively assessed for inter-muscle volume conduction during lumbosacral spine surgeries. Peak latency matching between the EAS and GM MEP recordings conditionally identified volume conduction (VC+) or no volume conduction (VC-). Linear regression and power spectral density analysis of EAS and medial GM MEP amplitudes were performed from VC+ and VC- data pairs to confirm intermuscle electrical cross-talk.

RESULTS

Motor evoked potential peak latency matching identified putative VC+ in 9 of 40 patients (22.5%). Mean regression coefficients (r2) from peak-to-peak EAS and medial GM MEP amplitude plots were 0.83 ± 0.04 for VC+ and 0.34 ± 0.06 for VC- MEP (P < 0.001). Power spectral density analysis identified the major frequency component in the MEP responses. The mean frequency difference between VC+ EAS and medial GM MEP responses were 0.4 ± 0.2 Hz compared with 3.5 ± 0.6 Hz for VC- MEP (P < 0.001).

CONCLUSIONS

Our data support using peak latency matching between EAS and GM MEP to identify spurious MEP results because of intermuscle volume conduction. Neuromonitorists should be aware of this possible cross-muscle conflict to avoid interpretation errors during lumbosacral procedures using EAS MEP.

Characteristics of Tc-MEP Waveforms in Spine Surgery for Patients with Severe Obesity

STUDY DESIGN

Prospective multicenter study.

OBJECTIVE

The aim of this study was to evaluate transcranial motor evoked potential (Tc-MEP) waveform monitoring in spinal surgery for patients with severe obesity.

SUMMARY OF BACKGROUND DATA

Spine surgeries in obese patients are associated with increased morbidity and mortality. Intraoperative Tc-MEP monitoring can identify neurologic deterioration during surgery, but has not been examined for obese patients.

METHODS

The subjects were 3560 patients who underwent Tc-MEP monitoring during spine surgery at 16 centers. Tc-MEPs were recorded from multiple muscles via needle or disc electrodes. A decrease in Tc-MEP amplitude of ≥70% from baseline was used as an alarm during surgery. Preoperative muscle weakness with manual muscle test (MMT) grade ≤4 was defined as a motor deficit, and a reduction of one or more MMT grade postoperatively was defined as deterioration.

RESULTS

The 3560 patients (1698 males, 47.7%) had a mean age of 60.0 ± 20.3 years. Patients with body mass index >35 kg/m2 (n = 60, 1.7%) were defined as severely obese. Compared with all other patients (controls), the rates of preoperative motor deficit (41.0% vs. 29.6%, P < 0.05) and undetectable baseline waveforms in all muscles were significantly higher in the severely obese group (20.0% vs. 1.7%, P < 0.01). Postoperative motor deterioration did not differ significantly between the groups. The sensitivity and specificity of the alarm criterion for prediction of postoperative neurologic complications were 75.0% and 83.9% in severely obese patients and 76.4% and 89.6% in controls, with no significant difference between the groups.

CONCLUSION

Tc-MEPs can be used in spine surgery for severely obese cases to predict postoperative motor deficits, but the rate of undetectable waveforms is significantly higher in such cases. Use of a multichannel waveform approach or multiple modalities may facilitate safe completion of surgery. Waveforms should be carefully evaluated and an appropriate rescue procedure is required if the alarm criterion occurs.Level of Evidence: 3.

Evaluation of the Association between Neck Pain and the Trapezius Muscles in Patients with Cervical Myelopathy Using Motor Evoked Potential: A Retrospective Study

STUDY DESIGN

Retrospective study.

PURPOSE

We aimed to use motor evoked potentials (MEPs) to examine the association of electrophysiological assessment of the trapezius muscle with neck pain.

OVERVIEW OF LITERATURE

Previous reports on the association of neck pain with the trapezius muscle have focused on surface electromyograms and muscle oxygenation; however, to our knowledge, none of these studies included detailed data on MEPs.

METHODS

The study included 100 patients with cervical myelopathy who underwent surgery at the National Center for Geriatrics and Gerontology in Obu, Japan from June 2010 to March 2013. Before the surgery, neck pain was evaluated using a Visual Analog Scale (a score ≥50 indicated neck pain and a score <50 indicated no neck pain). The preoperative cross-sectional areas of the trapezius muscles were measured with cervical magnetic resonance imaging sagittal T2-weighted images. Cranial stimulation under general anesthesia was used to derive the MEPs, enabling the measurement of latency and amplitude, using preoperative MEPs of the trapezius muscles.

RESULTS

The MEP of the trapezius muscle in patients with neck pain had significantly shorter latencies than those in patients who did not have neck pain. However, there was no significant difference in the amplitude between patients with and without neck pain. However, this tended to be greater in patients with neck pain as compared to that in those without neck pain. The cross-sectional area of the trapezius muscle in patients with neck pain was significantly smaller than that in those who did not have neck pain.

CONCLUSIONS

MEPs revealed electrophysiological abnormalities of the trapezius muscles in patients with neck pain, supporting a relationship of neck pain with the trapezius muscles.

Effects of Preoperative Motor Status on Intraoperative Motor-evoked Potential Monitoring for High-risk Spinal Surgery: A Prospective Multicenter Study

STUDY DESIGN

Prospective multicenter observational study.

OBJECTIVE

To evaluate transcranial motor-evoked potentials (Tc-MEPs) baseline characteristics of lower limb muscles and to determine the accuracy of Tc-MEPs monitoring based on preoperative motor status in surgery for high-risk spinal disease.

SUMMARY OF BACKGROUND DATA

Neurological complications are potentially serious side effects in surgery for high-risk spine disease. Intraoperative spinal neuromonitoring (IONM) using Tc-MEPs waveforms can be used to identify neurologic deterioration, but cases with preoperative motor deficit tend to have poor waveform derivation.

METHODS

IONM was performed using Tc-MEPs for 949 patients in high-risk spinal surgery. A total of 4454 muscles in the lower extremities were chosen for monitoring. The baseline Tc-MEPs was recorded immediately after exposure of the spine. The derivation rate was defined as muscles detected/muscles prepared for monitoring. A preoperative neurological grade was assigned using the manual muscle test (MMT) score.

RESULTS

The 949 patients (mean age 52.5 ± 23.3 yrs, 409 males [43%]) had cervical, thoracic, thoracolumbar, and lumbar lesions at rates of 32%, 40%, 26%, and 13%, respectively. Preoperative severe motor deficit (MMT ≤3) was present in 105 patients (11%), and thoracic ossification of the posterior longitudinal ligament (OPLL) was the most common disease in these patients. There were 32 patients (3%) with no detectable waveform in any muscles, and these cases had mostly thoracic lesions. Baseline Tc-MEPs responses were obtained from 3653/4454 muscles (82%). Specificity was significantly lower in the severe motor deficit group. Distal muscles had a higher waveform derivation rate, and the abductor hallucis (AH) muscle had the highest derivation rate, including in cases with preoperative severe motor deficit.

CONCLUSION

In high-risk spinal surgery, Tc-MEPs collected with multi-channel monitoring had significantly lower specificity in cases with preoperative severe motor deficit. Distal muscles had a higher waveform derivation rate and the AH muscle had the highest rate, regardless of the severity of motor deficit preoperatively.Level of Evidence: 3.

Larger muscle mass of the upper limb correlates with lower amplitudes of deltoid MEPs following transcranial stimulation

To perform spinal surgery safely, it is important to understand the risk factors, including factors that negatively influence intraoperative neuromonitoring (IONM). Transcranial motor evoked potentials (TcMEPs) are important in IONM. Therefore, we aimed to investigate whether muscle mass affects the waveforms of TcMEPs to understand the risk factors influencing TcMEPs. We enrolled 48 patients with thoracolumbar spinal diseases who underwent surgery at our facility between April 2015 and March 2018. Before surgery, the body composition, including muscle mass and fat mass, of all patients was measured using bioelectrical impedance analysis (BIA). During surgery, cranial stimulation under general anesthesia was used to derive TcMEPs, enabling us to measure the amplitude, using the control wave of the TcMEPs of the deltoid muscles and the abductor digiti minimi (ADM) muscles. We found a negative correlation between the amplitude of deltoid-muscle TcMEPs and muscle mass of the upper limb. The amplitude of deltoid-muscle TcMEPs did not correlate with the skeletal muscle index (SMI), muscle mass of the lower limb, or body fat mass. The amplitude of ADM-muscle TcMEPs did not correlate with SMI, muscle mass of any limb, or body fat mass. In conclusion, a larger muscle mass of the upper limb correlated with a lower amplitude of deltoid-muscle TcMEPs. By contrast, there was no correlation between the muscle mass of the upper limb and the amplitude of ADM-muscle TcMEPs. These findings suggest that TcMEPs of the ADM are less influenced by muscle mass and are more stable than those of the deltoid.

Optimal stimulation intensity for Br(E)-MsEP waveform derivation at baseline in pediatric spinal surgery

OBJECTIVES

Br(E)-MsEP monitoring is widely used in spinal surgery for detection of spinal cord injury. However, Br(E)-MsEP waveform derivation requires high-intensity stimulation, and this raises a concern of adverse effects due to the immature corticospinal tract in pediatric patients. The purpose of this study is to determine the optimal stimulation intensity required for derivation of Br(E)-MsEP waveforms at baseline in pediatric spinal surgery.

PATIENTS AND METHODS

The subjects were 85 pediatric patients (4-15 years old, mean age at surgery: 11.1 years old) who were treated with spinal surgery using a posterior only approach under Br(E)-MsEP monitoring. The main diagnoses were adolescent idiopathic scoliosis (n = 44), syndromic and neuromuscular scoliosis (n = 23), and congenital scoliosis (n = 12). A total of 1513 muscles in the lower extremities were chosen for monitoring.

RESULTS

A baseline waveform was obtained in all 85 cases and baseline Br(E)-MsEP responses were obtained from 1437/1513 muscles (95%). The mean stimulation intensity for baseline waveform derivation was 156.4 mA (range: 100-200 mA), and the stimulation intensity was significantly correlated with age (p < 0.05). The mean stimulation intensities were 129 ± 12, 138 ± 20, and 167 ± 25 mA for children <5, 6 to 10, and 11 to 15 years old, respectively.

CONCLUSION

There are no criteria for derivation of Br(E)-MsEP waveforms in pediatric patients undergoing spinal surgery. The stimulation intensity increased with age, and starting at a lower stimulation strength than that used in adults is appropriate for younger children.