Recording of spinal somatosensory evoked potentials for intraoperative spinal cord monitoring

Intraoperative Spinal Cord Monitoring Does Not Decrease New Postoperative Neurological Deficits in Patients With Cervical Radiculopathy or Spondylotic Myelopathy Undergoing One or Two Level Anterior Cervical Discectomy And Fusion

BACKGROUND

Intraoperative neurological monitoring (IONM) is commonly used in spine surgery. However, the utility of IONM in anterior cervical decompression and fusion (ACDF) remains a topic of debate. The purpose of the study was to investigate the utility and cost of IONM (both Somatosensory evoked potentials (SSEPs) and Motor Evoked Potentials (Tc-MEPs)) in reducing postoperative neurological deficits in myelopathic and non-myelopathic patients undergoing ACDF.

METHODS

Retrospective chart review was performed to include only patients with cervical radiculopathy or myelopathy undergoing one or two level ACDF over a 7-year period at a busy academic center. SSEP and Tc-MEP tracings were reviewed for all monitored patients and significant changes and inconsistencies were noted. IONM billing codes (SSEP/Tc-MEP) were reviewed and summed to evaluate the average procedural cost. Medical records were reviewed for preoperative physical exam and for new postoperative neurological deficits on postoperative day one and again at six weeks and matched to the monitored tracings.

RESULTS

There were 249 total patients (48 Non-monitored, 201 monitored). There was no difference in gender, age, or BMI between monitored and non-monitored groups. There was no difference in new neurological deficits in monitored compared with non-monitored patients with radiculopathy (p=0.1935) or myelopathy (p=0.1977). However, when radiculopathy and myelopathy patients were combined, there was an increased incidence of new neurologic deficits in monitored patients (8.0%) versus non-monitored patients (0%) (p=0.0830). All new neurological deficits occurred in patients with normal IONM tracings. There were no new neurologic deficits in the non-monitored radiculopathy or myelopathy groups. The average IONM procedure charge was $6500.

CONCLUSION

Our results indicate that intraoperative spinal cord monitoring did not reduce new neurological deficits in our cohort of patients. The higher incidence in new neurological deficits despite no IONM changes in our monitored group suggests a lack of utility of IONM in ACDF. Furthermore, at an average of $6500 per IONM procedure, the present study underlines the importance of prudence when choosing to use IONM in the era of cost containment.Level of Evidence: III.

False-Positive and False-Negative Results of Motor Evoked Potential Monitoring During Surgery for Intramedullary Spinal Cord Tumors

BACKGROUND

Motor evoked potential (MEP) recording is used as a method to monitor integrity of the motor system during surgery for intramedullary tumors (IMTs). Reliable sensitivity of the monitoring in predicting functional deterioration has been reported. However, we observed false positives and false negatives in our experience of 250 surgeries of IMTs.

OBJECTIVE

To delineate specificity and sensitivity of MEP monitoring and to elucidate its limitations and usefulness.

METHODS

From 2008 to 2011, 58 patients underwent 62 surgeries for IMTs. MEP monitoring was performed in 59 operations using transcranial electrical stimulation. Correlation with changes in muscle strength and locomotion was analyzed. A group undergoing clipping for unruptured aneurysms was compared for elicitation of MEP.

RESULTS

Of 212 muscles monitored in the 59 operations, MEP was recorded in 150 (71%). Positive MEP warnings, defined as amplitude decrease below 20% of the initial level, occurred in 37 muscles, but 22 of these (59%) did not have postoperative weakness (false positive). Positive predictive value was limited to 0.41. Of 113 muscles with no MEP warnings, 8 muscles developed postoperative weakness (false negative, 7%). Negative predictive value was 0.93. MEP responses were not elicited in 58 muscles (27%). By contrast, during clipping for unruptured aneurysms, MEP was recorded in 216 of 222 muscles (96%).

CONCLUSION

MEP monitoring has a limitation in predicting postoperative weakness in surgery for IMTs. False-positive and false-negative indices were abundant, with sensitivity and specificity of 0.65 and 0.83 in predicting postoperative weakness.

Diagnostic Utility of Intraoperative Neurophysiological Monitoring for Intramedullary Spinal Cord Tumors: Systematic Review and Meta-Analysis

STUDY DESIGN

Systematic review and meta-analysis.

OBJECTIVE

The aim of this study was to systematically evaluate the diagnostic utility of intraoperative neurophysiological monitoring (IONM) for detecting postoperative injury in resection of intramedullary spinal cord tumors (IMSCT).

SUMMARY OF BACKGROUND DATA

Surgical management of IMSCT can involve key neurological and vascular structures. IONM aims to assess the functional integrity of susceptible elements in real time. The diagnostic value of IONM for ISMCT has not been systematically evaluated.

METHODS

We performed a systematic review of the PubMed and MEDLINE databases for studies investigating the use of IONM for IMSCT and conducted a meta-analysis of diagnostic capability.

RESULTS

Our search produced 257 citations. After application of exclusion criteria, 21 studies remained, 10 American Academy of Neurology grade III and 11 American Academy of Neurology grade IV. We found that a strong pooled mean sensitivity of 90% [95% confidence interval (CI), 84-94] and a weaker pooled mean specificity of 82% (95% CI, 70-90) for motor-evoked potential (MEP) recording changes. Somatosensory-evoked potential (SSEP) recording changes yielded pooled sensitivity of 85% (95% CI, 75-91) and pooled specificity of 72% (95% CI, 57-83). The pooled diagnostic odds ratio for MEP was 55.7 (95% CI, 26.3-119.1) and 14.3 (95% CI, 5.47-37.3) for SSEP. Bivariate analysis yielded summary receiver operative characteristic curves with area under the curve of 91.8% for MEPs and 86.3% for SSEPs.

CONCLUSIONS

MEPs and SSEPs appear to be more sensitive than specific for detection of postoperative injury. Patients with perioperative neurological deficits are 56 times more likely to have had changes in MEPs during the procedure. We observed considerable variability in alarm criteria and interventions in response to IONM changes, indicating the need for prospective studies capable of defining standardized alarm criteria and responses.

Clinical presentation and outcome of dogs treated medically or surgically for thoracolumbar intervertebral disc protrusion

To date, few studies have investigated the clinical characteristics of thoracolumbar intervertebral disc protrusion (IVDP). The aim of this retrospective study was to evaluate the presentation and outcome of dogs receiving medical or surgical treatment for thoracolumbar IVDP. Eighty-four dogs were included, with a median age of 9.4 years. German shepherd dogs and Staffordshire bull terriers were the most common breeds. Significantly more surgically treated dogs (n=53) had neurological deficits and were non-ambulatory, compared with medically treated (n=31). Outcome data were available for 27 of 31 medically managed dogs; 11 initially improved, 7 remained stable and 9 deteriorated. Of 18 dogs that initially improved or stabilised, 10 (55.6 per cent) demonstrated recurrence of clinical signs within 12 months of diagnosis. Outcome data were available for 45 of 50 surgically treated dogs that survived to hospital discharge; 34 improved, 9 remained stable and 2 deteriorated following surgery. Of 43 dogs that improved or stabilised with surgical treatment, 11 (25.6 per cent) demonstrated recurrence of clinical signs within 12 months of surgery. Overall, significantly more surgically treated dogs (71.1 per cent) had a successful outcome, consisting of sustained clinical improvement of more than 12 months duration, compared with medically treated dogs (29.6 per cent).

Individualized mini-hemilaminectomy-corpectomy (iMHC) for treatment of thoracolumbar intervertebral disc herniation in large breed dogs

OBJECTIVE

To assess the short-term, mid-term, and long-term results after an individualized mini-hemilaminectomy-corpectomy (iMHC) procedure for treatment of acute and chronic thoracolumbar intervertebral disc disease in non-chondrodystrophic dogs.

STUDY DESIGN

Prospective study.

ANIMALS

Client-owned non-chondrodystrophic large breed dogs (n = 57).

METHODS

The iMHC procedure, combining mini-hemilaminectomy (MH) and partial lateral corpectomy, was performed on non-chondrodystrophic dogs with thoracolumbar disc disease. Neurological status was evaluated before surgery, for short-term outcome on days 1 and 7 after surgery, for mid-term outcome at 6 months after surgery, and for long-term outcome at the conclusion of the study. Prognostic factors were statistically evaluated. P < .05 was considered significant.

RESULTS

iMHC was performed on 57 dogs, with minimal intraoperative and postoperative complications. Short-term neurological improvement was observed in 85.7% of dogs. Median hospitalization time after surgery was 2 days (range 0-14) and was significantly shorter for dogs with a chronic history of clinical signs (1 day, range 0-5) compared to acute onset (3 days, range 0-14) and for those that were ambulatory at initial presentation (1 day, range 0-5) compared to those that were not (3 days, range 0-14). Long-term evaluation included 53 surgeries with a mean follow-up time of 29.4 months. Outcome was excellent in 19 dogs and good in 29 dogs (90.6% success rate). Excellent mid-term and long-term results were significantly more common in the dogs with only 1 affected disc space.

CONCLUSION

The iMHC procedure resulted in a short hospitalization time, minimal postoperative deterioration, and a high success rate.

Spinal meningiomas: clinicoradiological factors predicting recurrence and functional outcome

OBJECTIVE

Spinal meningiomas are benign tumors with a wide spectrum of clinical and radiological features at presentation. The authors analyzed multiple clinicoradiological factors to predict recurrence and functional outcome in a cohort with a mean follow-up of more than 4 years. The authors also discuss the results of clinical studies regarding spinal meningiomas in the last 15 years.

METHODS

The authors retrospectively reviewed the clinical and radiological details of patients who underwent surgery for spinal tumors between 2001 and 2015 that were histopathologically confirmed as meningiomas. Demographic parameters, such as age, sex, race, and association with neurofibromatosis Type 2, were considered. Radiological parameters, such as tumor size, signal changes of spinal cord, spinal level, number of levels, location of tumor attachment, shape of tumor, and presence of dural tail/calcification, were noted. These factors were analyzed to predict recurrence and functional outcome. Furthermore, a pooled analysis was performed from 13 reports of spinal meningiomas in the last 15 years.

RESULTS

A total of 38 patients were included in this study. Male sex and tumors with radiological evidence of a dural tail were associated with an increased risk of recurrence at a mean follow-up of 51.2 months. Ventral or ventrolateral location, large tumors, T2 cord signal changes, and poor preoperative functional status were associated with poor functional outcome at 1-year follow-up.

CONCLUSIONS

Spine surgeons must be aware of the natural history and risk factors of spinal meningiomas to establish a prognosis for their patients.

D-wave recording during the surgery of a 10-month-old child

PURPOSE

The purpose of this study is to evaluate the possibility of d-wave recording in very young patients during spinal cord surgery.

METHOD

A 10-month-old patient was operated on cystic intramedullary pathology and myelon tethering at level cervical VI. to thoracal III. During the surgical detethering, we did d-wave recording for experimental purposes. After transcranial electric stimulation, we tried to detect the responses with epidural d-wave electrodes proximally and distally from the pathology.

RESULTS

We found that proximally from the pathology, we could detect reproducible d-waves.

CONCLUSION

Though earlier papers reported that due to the immature condition of the myelon d-wave recording is not possible under the age of 21 months, we proved that even in very young patients, the possibility of d-wave recording should not be excluded.

Cervical decompression and reconstruction without intraoperative neurophysiological monitoring

Object

The primary goal of this study was to review the immediate postoperative neurological function in patients surgically treated for symptomatic cervical spine disease without intraoperative neurophysiological monitoring. The secondary goal was to assess the economic impact of intraoperative monitoring (IOM) in this patient population.

Methods

This study is a retrospective review of 720 consecutively treated patients who underwent cervical spine procedures. The patients were identified and the data were collected by individuals who were not involved in their care.

Results

A total of 1534 cervical spine levels were treated in 720 patients using anterior, posterior, and combined (360°) approaches. Myelopathy was present preoperatively in 308 patients. There were 185 patients with increased signal intensity within the spinal cord on preoperative T2-weighted MR images, of whom 43 patients had no clinical evidence of myelopathy. Three patients (0.4%) exhibited a new neurological deficit postoperatively. Of these patients, 1 had a preoperative diagnosis of radiculopathy, while the other 2 were treated for myelopathy. The new postoperative deficits completely resolved in all 3 patients and did not require additional treatment. The Current Procedural Terminology (CPT) codes for IOM during cervical decompression include 95925 and 95926 for somatosensory evoked potential monitoring of the upper and lower extremities, respectively, as well as 95928 and 95929 for motor evoked potential monitoring of the upper and lower extremities. In addition to the charge for the baseline [monitoring] study, patients are charged hourly for ongoing electrophysiology testing and monitoring using the CPT code 95920. Based on these codes and assuming an average of 4 hours of monitoring time per surgical case, the savings realized in this group of patients was estimated to be $1,024,754.

Conclusions

With the continuing increase in health care costs, it is our responsibility as providers to minimize expenses when possible. This should be accomplished without compromising the quality of care to patients. This study demonstrates that decompression and reconstruction for symptomatic cervical spine disease without IOM may reduce the cost of treatment without adversely impacting patient safety.

A role for motor and somatosensory evoked potentials during anterior cervical discectomy and fusion for patients without myelopathy: Analysis of 57 consecutive cases

BACKGROUND

Although the usage of combined motor and sensory intraoperative monitoring has been shown to improve the surgical outcome of patients with cervical myelopathy, the role of transcranial electric motor evoked potentials (tceMEP) used in conjunction with somatosensory evoked potentials (SSEP) in patients presenting with radiculopathy but without myelopathy has been less clear.

METHODS

We retrospectively reviewed all patients (n = 57) with radiculopathy but without myelopathy, undergoing anterior cervical decompression and fusion at a single institution over the past 3 years, who had intraoperative monitoring with both tceMEPs and SSEPs.

RESULTS

Fifty-seven (100%) patients presented with radiculopathy, 53 (93.0%) with mechanical neck pain, 35 (61.4%) with motor dysfunction, and 29 (50.9%) with sensory deficits. Intraoperatively, 3 (5.3%) patients experienced decreases in SSEP signal amplitudes and 4 (6.9%) had tceMEP signal changes. There were three instances where a change in neuromonitoring signal required intraoperative alteration of the surgical procedure: these were deemed clinically significant events/true positives. SSEP monitoring showed two false positives and two false negatives, whereas tceMEP monitoring only had one false positive and no false negatives. Thus, tceMEP monitoring exhibited higher sensitivity (33.3% vs. 100%), specificity (95.6% vs. 98.1%), positive predictive value (33.3% vs. 75.0%), negative predictive value (97.7% vs. 100%), and efficiency (91.7% vs. 98.2%) compared to SSEP monitoring alone.

CONCLUSIONS

Here, we present a retrospective series of 57 patients where tceMEP/SSEP monitoring likely prevented irreversible neurologic damage. Though further prospective studies are needed, there may be a role for combined tceMEP/SSEP monitoring for patients undergoing anterior cervical decompression without myelopathy.

False negative and positive motor evoked potentials in one patient: is single motor evoked potential monitoring reliable method? A case report and literature review

STUDY DESIGN

A case report and literature review.

OBJECTIVE

To report a false negative and delayed positive motor-evoked potential (MEP) in 1 patient.

SUMMARY OF BACKGROUND DATA

An unreliable MEP can result in fatal outcomes because surgeons have recently begun to depend on the MEP for intraoperative decision-making.

METHODS

We report a case of a false MEP during scoliosis surgery that showed false negative and positive MEPs during a series of operations.

RESULTS

A 23-year-old man with a history of spondyloepiphyseal dysplasia presented with severe kyphoscoliosis. The initial neurologic examination did not reveal any neurologic abnormalities. Surgical correction and fusion were performed with transcranial MEP monitoring. During the entire procedure, the MEP did not reveal any signs of cord injury. However, lower limb paralysis and paresthesia was observed when the patient awakened. After 2 additional surgical procedures to recover the neurologic deficit, the MEP did not show any signs of cord injury but the patient's neurologic status had recovered slightly. At postoperative day 8, the neurologic status recovered, and a third operation was performed to fix the long rods. However, there were abnormal amplitudes in both lower limbs but the patient's neurologic status was almost normal.

CONCLUSION

From our experience of false negative and positive MEP in 1 patient, it is concluded that undesirable events can occur with use of MEP in scoliosis or other spinal surgery. Therefore, we warn the surgeons too heavily rely on the MEP monitoring, and propose a further prospective study as well as use of alternative method that can improve the reliability of single MEP.

Characteristics of mid- to long-latency spinal somatosensory evoked potentials following spinal trauma in the rat

The purpose of this study was to develop and implement a new technique for repeated monitoring of spinal mid- to long-latency somatosensory evoked potentials (SpSEPs) during sciatic nerve stimulation following recovery from spinal cord injury (SCI) in rats. Results of this study showed significant reproducibility of SpSEP components between specimens (analysis of variance [ANOVA], p > 0.05) and recording days (ANOVA, p > 0.700) using this technique. SpSEP amplitudes were significantly reduced (approximately 50% of uninjured amplitude, ANOVA, p < 0.001) following SCI and remained depressed for 10 weeks post-injury. SpSEP amplitude following high-intensity stimuli (> 1 mA) correlated with BBB locomotor score (Pearson, R > 0.353, P < 0.001). Characteristics of the mid- to long-latency SpSEPs suggest these components may reflect the integrity of the lateral pain pathway within the spinothalamic tract (STT). The technique and data presented in this study may be useful in future studies aimed at quantifying spinal cord integrity following injury and treatment using the rat model of SCI.

Electrophysiologic and clinical data support the use of dorsal root entry zone myelotomy in syringosubarachnoid shunting for syringomyelia

BACKGROUND

The objectives of this study were to correlate preoperative changes in SEPs with clinical sensory dysfunction and to establish their importance in planning the microsurgical approach, either by DM myelotomy or by DREZ myelotomy, for patients with syringomyelia.

METHODS

In addition to conducting clinical sensory examination, we evaluated the N13 potential after median nerve stimulation and CPs after tibial nerve stimulation intraoperatively before performing myelotomy on patients with syringomyelia (N = 14).

RESULTS

Eleven patients with intact DS presented with unilateral PTD, and 9 had distressing unilateral dermatomal pain. Deep sensibility was affected in 3 patients (bilaterally in 1 patient) without PTD. Patients with PTD were likely to have spontaneous pain (P = .005). A significant correlation between preoperative PTD and the absence of the N13 potential was demonstrated on the right (P = .015) and left (P = .004) sides. In patients with PTD, DREZ myelotomy on the symptomatic side is suggested as the treatment of choice, whereas DM myelotomy might be superior in patients without PTD.

CONCLUSIONS

Absence of pain or temperature sensation in patients with syringomyelia is usually accompanied by same-sided loss of the N13 potential, suggesting damage to the DH gray matter. Deep sensibility is typically normal, and DREZ myelotomy with preservation of DCs is proposed as the treatment of choice. Conducted potentials are usually distorted in patients with normal pain or temperature sensation and affected vibration and posture sensation, suggesting damage to DCs and making DM myelotomy the treatment of choice. Electrophysiologic and clinical data support the use of DREZ myelotomy in syringosubarachnoid shunting for syringomyelia in patients whose DCs have an intact function.

Outcome of surgery for intramedullary spinal ependymoma

BACKGROUND AND OBJECTIVES

Ependymoma is the most frequently encountered intramedullary tumor. Total surgical resection is the therapeutic modality of choice whenever possible, but carries a significant risk of morbidity. This study was designed to define prognostic factors that affect clinical outcome after surgical resection of spinal intramedullary ependymoma.

PATIENTS AND METHODS

The medical records, radiological and pathological studies of all patients with intramedullary spinal ependymomas treated surgically in one institution were reviewed retrospectively. Spinal myxopapillary ependymomas were excluded. In a multivariable regression analysis, possible prognostic factors were correlated with the 6-month postoperative neurological status using McCormick's grading scale.

RESULTS

Surgery was performed on 17 patients (14 males, 3 females, mean age of 42+/-15 years) with spinal ependymoma. The cervical spine was the most common tumor location (71%). Total surgical resection of the tumor was achieved in 11 cases (65%). Intraoperative neurophysiological monitoring was used in 8 cases (47%). Postoperatively, 11 patients (65%) either improved or had no change from their preoperative neurological status. None of the 11 totally resected tumors has shown evidence of recurrence in a follow-up period range from 8-120 months (median, 33 months). Of several possible prognostic factors, the only statistically significant correlation identified with the 6-month postoperative neurological status was the preoperative McCormick grading score.

CONCLUSIONS

Preoperative neurological status was the only statistically significant factor in determining the postoperative neurological outcome of patients with spinal intramedullary ependymomas. Early diagnosis and referral for surgery to specialized centers are recommended as controllable factors in improving outcome.

Protective effect of sildenafil (Viagra) in transient spinal cord ischemia

Prospective study of the neuroprotective activity of sildenafil in a rat spinal ischemia model. The present study involved 21 male Sprague-Dawley rats. The animals were divided into 3 groups. Physiological serum was administered intraperitoneally to the 8 rats in the control group at the beginning of reperfusion for a period of 20 min after abdominal aortal occlusion. Sildenafil (Viagra) was administered as a single 10-mg/kg/day intraperitoneal dose to the 8 rats in the sildenafil group at the beginning of reperfusion after 20 min of abdominal aortal occlusion. No occlusion was performed and no agent was administered to the 5 rats in the sham group, but the abdominal aorta was reached by means of surgical intervention. Before the animals were sacrificed, several physiological and biochemical parameters were investigated, preoperative and postoperative motor functions were also assessed, and somatosensory evoked potential (SEP) monitoring and histopathological examinations were carried out. No differences were found between the physiological and biochemical parameters in each of the 3 groups. Neurological scoring performed after reperfusion demonstrated a significant improvement in the neurological results relative to those of the control group over 48 h in subjects that received sildenafil. These animals also showed better 24-hour SEP results, measured in terms of extended latency and decreased amplitude, than the control animals. A histopathological study showed reduced ischemic symptoms in rats that received sildenafil compared with those in the control group. However, no anomalies were observed in the sham group with respect to the histopathological and neurological findings. These results indicate that neurological damage due to spinal-cord ischemia-reperfusion injury can be reduced by sildenafil.

Management of spinal meningiomas: surgical results and a review of the literature

OBJECT

In this report, the authors describe their experience in the surgical management of spinal meningiomas at two neurosurgical centers. The results of a literature review are also presented.

METHODS

Eighty consecutive patients (22 men and 58 women) with spinal meningiomas who had undergone an operation at two specific neurosurgical centers were included in this study. Functional outcomes were evaluated using univariate and multivariate analyses. A review of the literature yielded an additional 651 patients with spinal meningiomas from 9 large studies.

RESULTS

On multivariate analysis, the variable of a poor preoperative neurological state (p < 0.02, odds ratio [OR] 13.6, 95% confidence interval [CI] 2.6-71.4) and invasion of the arachnoid/pia mater (p < 0.03, OR 15.2, 95% CI 2.5-90.4) were independent predictors of a poor outcome, whereas invasion of the arachnoid/pia (p < 0.02, OR 8.9, 95% CI 2.2-35) and duration of symptoms (p < 0.001, OR 1.12/month, 95% CI 1.05-1.2) predicted no improvement (stable or deteriorated condition). The Cox proportional hazards regression analysis showed three significant predictor variables for recurrence: invasion of the arachnoid/pia (p < 0.05; hazard ratio [HR] 1.8, 95% CI 1.2-3.6), Simpson resection grade (p < 0.012, HR 6.8, 95% CI 1.5-3.0), and histological tumor grade (Grade I; p < 0.001, HR 0.001-0.17).

CONCLUSIONS

Because of the excellent outcome of surgery for benign spinal meningiomas and the association between duration of symptoms and neurological compromise with a poor functional outcome, early operation is the treatment of choice. In cases of malignant transformation, adjuvant therapies must be considered.

Intraoperative somatosensory evoked potential monitoring during anterior cervical discectomy and fusion in nonmyelopathic patients--a review of 1,039 cases

BACKGROUND CONTEXT

Intraoperative somatosensory evoked potential (SSEP) monitoring has been shown to reduce the incidence of new postoperative neurological deficits in scoliosis surgery. However, its usefulness during cervical spine surgery remains a subject of debate.

PURPOSE

To determine the utility of intraoperative SSEP monitoring in a specific patient population (those with cervical radiculopathy in the absence of myelopathy) who underwent anterior cervical discectomy and fusion (ACDF) surgery.

STUDY DESIGN

Retrospective review.

PATIENT SAMPLE

A total of 1,039 nonmyelopathic patients who underwent single or multilevel ACDF surgery. The control group (462 patients) did not have intraoperative SSEP monitoring, whereas the monitored group (577 patients) had continuous intraoperative SSEP monitoring performed.

OUTCOME MEASURE

A new postoperative neurological deficit.

METHODS

SSEP tracings were reviewed for all 577 patients in the monitored group and all significant signal changes were noted. Medical records were reviewed for all 1,039 patients to determine if any new neurological deficits developed in the immediate postoperative period.

RESULTS

None of the patients in the control group had any new postoperative neurological deficits. In the monitored group there were six instances of transient SSEP changes (1 due to suspected carotid artery compression; 5 thought to be due to transient hypotension) which resolved with the appropriate intraoperative intervention (repositioning of retractors; raising the arterial blood pressure). Upon waking up from anesthesia, one patient in the monitored group had a new neurological deficit (partial central cord syndrome) despite normal intraoperative SSEP signals.

CONCLUSIONS

ACDF appears to be a safe surgical procedure with a low incidence of iatrogenic neurological injury. Transient SSEP signal changes, which improved with intraoperative interventions, were not associated with new postoperative neurological deficits. An intraoperative neurological deficit is possible despite normal SSEP signals.

Motor Evoked Potential Monitoring Improves Outcome after Surgery for Intramedullary Spinal Cord Tumors: A Historical Control Study

OBJECTIVE:

The value of intraoperative neurophysiological monitoring (INM) during intramedullary spinal cord tumor surgery remains debated. This historical control study tests the hypothesis that INM monitoring improves neurological outcome.

METHODS:

In 50 patients operated on after September 2000, we monitored somatosensory evoked potentials and transcranially elicited epidural (D-wave) and muscle motor evoked potentials (INM group). The historical control group consisted of 50 patients selected from among 301 patients who underwent intramedullary spinal cord tumor surgery, previously operated on by the same team without INM. Matching by preoperative neurological status (McCormick scale), histological findings, tumor location, and extent of removal were blind to outcome. A more than 50% somatosensory evoked potential amplitude decrement influenced only myelotomy. Muscle motor evoked potential disappearance modified surgery, but more than 50% D-wave amplitude decrement was the major indication to stop surgery. The postoperative to preoperative McCormick grade variation at discharge and at a follow-up of at least 3 months was compared between the two groups (Student's t tests).

RESULTS:

Follow-up McCormick grade variation in the INM group (mean, +0.28) was significantly better (P = 0.0016) than that of the historical control group (mean, –0.16). At discharge, there was a trend (P = 0.1224) toward better McCormick grade variation in the INM group (mean, –0.26) than in the historical control group (mean, –0.5).

CONCLUSION:

The applied motor evoked potential methods seem to improve long-term motor outcome significantly. Early motor outcome is similar because of transient motor deficits in the INM group, which can be predicted at the end of surgery by the neurophysiological profile of patients.

Neuromonitoring during surgery for metastatic tumors to the spine: intraoperative interpretation and management strategies

Resection of metastatic tumors of the spine poses great technical challenges, with the potential of creating severe neurologic deficits. Several modalities of electrophysiologic monitoring, including SSEPs and MEPs, have evolved to aid in resection of these tumors. This review has presented additional techniques-such as mapping of the dorsal columns with antidromic-elicited SSEPs to plan the myelotomy and direct intra-medullary stimulation-that help to identify the extent of the tumor margin at its interface with functional tracts. Neuromonitoring can potentially minimize the sensory and motor damage that can occur during resection of metastatic tumors of the spine. Further experience with these techniques should allow improved results follow-ing surgical procedures in functionally eloquent are as of the spinal cord during the surgical management of metastatic tumors.

Thoracolumbar Lateral Corpectomy for Treatment of Chronic Disk Herniation: Technique Description and Use in 15 Dogs

OBJECTIVE

To describe a technique for thoracolumbar lateral corpectomy and to evaluate its use for treatment of chronic thoracolumbar disk disease in dogs.

STUDY DESIGN

Retrospective study.

ANIMALS

Fifteen dogs with signs of chronic thoracolumbar disk herniation.

METHODS

After a dorsal or lateral approach to the spine, a lateral slot was created in 2 adjacent vertebral bodies on either side of the herniated disk and extruded/protruded material was removed. Data collected included history, duration of clinical signs, presurgical assessment of neurologic status, postsurgical neurologic status, complications, and outcome.

RESULTS

Ambulatory capacity was maintained or regained, and neurologic status improved by 1 grade (3 dogs), 2 grades (8), 3 grades (2), or 4 grades (2). Eleven dogs were considered free of disease. A seroma in 1 dog was the sole complication observed.

CONCLUSIONS

Lateral corpectomy permits relatively easy removal of protruded-extruded disk material from within the vertebral canal in chronic disk disease without further iatrogenic injury to the spinal cord.

CLINICAL RELEVANCE

Lateral corpectomy is an alternative to dorsal decompression for treatment of ventral and lateroventral thoracolumbar chronic disk disease in dogs.

Spinal meningiomas: surgical management and outcome

Advances in imaging and surgical technique have improved the treatment of spinal meningiomas; these include magnetic resonance imaging, intraoperative ultrasonography, neuromonitoring, the operative microscope, and ultrasonic cavitation aspirators. This study is a retrospective review of all patients treated at a single institution and with a pathologically confirmed diagnosis of spinal meningioma. Additionally the authors analyze data obtained in 556 patients reported in six large series in the literature, evaluating surgical techniques, results, and functional outcomes. Overall, surgical treatment of spinal meningiomas is associated with favorable outcomes. Spinal meningiomas can be completely resected, are associated with postoperative functional improvement, and the rate of recurrence is low.

Impact of somatosensory evoked potential monitoring on cervical surgery

Controversy still exists about the necessity of somatosensory evoked potential (SSEP) monitoring during cervical surgery. The purpose of this prospective study is to determine the impact of SSEP monitoring on anterior cervical surgery. Intraoperative SSEP monitoring was performed in 100 patients treated by an anterior cervical approach. The patients were divided into three groups according to their preoperative clinical condition. Somatosensory evoked potential monitoring was performed during five stages of the procedure: M1, after the induction of anesthesia; M2, during positioning; M3, during distraction of the intervertebral space; M4, throughout decompression; and M5, during graft placement. Normal SSEPs were obtained during M1 from all the patients in group 2. Pathologic SSEPs were recorded at M1 in 45 patients from group 1. No SSEPs were recorded at M1 in six patients in group 3. A deterioration of the SSEPs was observed in 35 patients during M2. Deteriorated SSEPs were observed during M3 in 14 patients. No deterioration of the SSEPs was recorded during M4. Intraoperative SSEP monitoring is easy to perform and helps to increase safety during anterior cervical surgery. Critical phases of the surgical procedure were identified and the surgical strategy was modified as a result of this study.

Spinal cord mapping as an adjunct for resection of intramedullary tumors: surgical technique with case illustrations

OBJECTIVE

Resection of intramedullary spinal cord tumors may result in transient or permanent neurological deficits. Intraoperative somatosensory evoked potentials (SSEPs) and motor evoked potentials are commonly used to limit complications. We used both antidromically elicited SSEPs for planning the myelotomy site and direct mapping of spinal cord tracts during tumor resection to reduce the risk of neurological deficits and increase the extent of tumor resection.

METHODS

In two patients, 3 and 12 years of age, with tumors of the thoracic and cervical spinal cord, respectively, antidromically elicited SSEPs were evoked by stimulation of the dorsal columns and were recorded with subdermal electrodes placed at the medial malleoli bilaterally. Intramedullary spinal cord mapping was performed by stimulating the resection cavity with a handheld Ojemann stimulator (Radionics, Burlington, MA). In addition to visual observation, subdermal needle electrodes inserted into the abductor pollicis brevis-flexor digiti minimi manus, tibialis anterior-gastrocnemius, and abductor halluces-abductor digiti minimi pedis muscles bilaterally recorded responses that identified motor pathways.

RESULTS

The midline of the spinal cord was anatomically identified by visualizing branches of the dorsal medullary vein penetrating the median sulcus. Antidromic responses were obtained by stimulation at 1-mm intervals on either side of the midline, and the region where no response was elicited was selected for the myelotomy. The anatomic and electrical midlines did not precisely overlap. Stimulation of abnormal tissue within the tumor did not elicit electromyographic activity. Approaching the periphery of the tumor, stimulation at 1 mA elicited an electromyographic response before normal spinal cord was visualized. Restimulation at lower currents by use of 0.25-mA increments identified the descending motor tracts adjacent to the tumor. After tumor resection, the tracts were restimulated to confirm functional integrity. Both patients were discharged within 2 weeks of surgery with minimal neurological deficits.

CONCLUSION

Antidromically elicited SSEPs were important in determining the midline of a distorted cord for placement of the myelotomy incision. Mapping spinal cord motor tracts with direct spinal cord stimulation and electromyographic recording facilitated the extent of surgical resection.

Mechanisms of signal change during intraoperative somatosensory evoked potential monitoring of the spinal cord

In scoliosis surgery, intraoperative somatosensory evoked potential (SSEP) monitoring has reduced the incidence of postoperative neurologic deficits. Many factors affect the amplitude and latency of SSEP waveforms during surgery. Somatosensory evoked potential amplitude decreases with ischemia and anoxia because of temporal dispersion of the afferent volley and conduction block in damaged axons. In conjunction with surgical manipulations, minor drops in blood pressure may result in substantial SSEP changes that reverse when perfusion pressure is increased. Irreversible anoxic injury to central nervous system white matter with loss of SSEP waveforms is dependent on calcium influx into the intracellular space. Somatosensory evoked potential monitoring may be less sensitive for detecting acute insults in the presence of preexisting white matter lesions. Increased extracellular potassium from acute baro-trauma can block axonal conduction transiently even when there is no axonal disruption. Marked temperature-related drops in SSEP amplitude may occur after exposure of the spine but before instrumentation and deformity correction. Hypothermia may increase false-negative outcomes. Short-interval double-pulse stimulation may improve the sensitivity of the SSEP in detecting early ischemic changes. For neurosurgical procedures on the spinal cord the use of SSEP monitoring in improving postoperative outcome is less well established.

Attenuation of somatosensory evoked potentials during positioning in a patient undergoing suboccipital craniectomy for Chiari I malformation with syringomyelia

Intraoperative electrophysiologic monitoring can diminish the risk of neurologic injury by enabling the detection of injury at a time when it can be reversed or minimized. This report describes a 14-year-old girl with a Chiari's malformation type I and syringomyelia who underwent a suboccipital decompression and dural patch grafting with concurrent somatosensory evoked potentials. When the patient was turned into the prone position and the neck was flexed, the left-sided somatosensory evoked potential deteriorated. After the patient's neck was repositioned, the left median nerve potential improved but did not return to baseline. Postoperatively, the patient had decreased proprioception of her left arm, which completely resolved at 2-week follow-up. This single case report does not establish the need for routine somatosensory evoked potential monitoring. Nevertheless, deterioration of the potential in this case led directly to a change in the surgical positioning, which may have significantly reduced the chances of a permanent neurologic injury.

Spinal cord and nerve root monitoring during surgical treatment of lumbar stenosis

The author describes application of intraoperative neurophysiologic monitoring to surgical treatment of lumbar stenosis. Benefits of somatosensory and motor evoked potential studies during surgical correction of spinal deformity are well known and documented. Free-running and evoked electromyographic studies during pedicle screw implantation is an accepted practice at many institutions. However, the functional integrity of spinal cord, cauda equina, and nerve roots should be monitored throughout every stage of surgery including exposure and decompression. Somatosensory evoked potentials monitor overall spinal cord function. Intraoperative electromyography provides continuous assessment of motor root function in response to direct and indirect surgical manipulation. Electromyographic activities observed during exposure and decompression of the lumbosacral spine included complex patterns of bursting and neurotonic discharge. In addition, electromyographic activities at distal musculature were elicited by impacting a surgical instrument or graft plug against bony elements of the spine. All electromyographic events provided direct feedback to the surgical team and were regarded as a cause for concern. Simultaneously monitored evoked potential and electromyographic studies protect spinal cord and nerve roots during seemingly low-risk phases of a surgical procedure when neurologic injury may occur and the patient is placed at risk for postoperative myelopathy or radiculopathy.

Prolonged somatosensory evoked potential depression following a brief exposure to low concentrations of inhalation anaesthetic in a 3-year-old child

A 3-year-old child was brought to the operating room for removal of a brainstem juvenile pilocytic astrocytoma. Following inhalation induction and intubation, he was maintained on 0.5% isoflurane. Somatosensory evoked potentials (SSEPs) were recorded but unobtainable initially and up to 90 min after all inhalation agents were discontinued. The operation was cancelled and the patient was transported to the paediatric intensive care unit (PICU). Subsequent PICU testing revealed a depression of amplitude with propofol and absence of potentials with 0.5% isoflurane. He returned to the operating room, was induced with propofol, and maintained with a propofol: nitrous oxide:fentanyl technique. This anaesthetic technique allowed adequate tumour resection with appropriate monitoring of SSEPs. These findings suggest that a total intravenous anaesthetic technique may be preferable for resection of spinal cord tumours where SSEPs are monitored.

Somatosensory evoked potential monitoring in anterior thoracic vertebrectomy

OBJECT

Spine surgeons have used intraoperative cortical and subcortical somatosensory evoked potential (SSEP) monitoring to detect changes in spinal cord function when intraoperative procedures can be performed to prevent neurological deterioration. However, the reliability of SSEP monitoring as applied to anterior thoracic vertebral body resections has not been rigorously assessed.

METHODS

The authors retrospectively reviewed hospital charts and operating room records obtained between August 1993 and December 1998 and found that SSEP monitoring was used in 44 surgical procedures involving an anterior approach for thoracic vertebral body resections. There were no patients in whom SSEP changes did not return to baseline during the surgical procedure. Patients in four cases, despite their stable SSEP recordings throughout the procedure, were noted immediately postoperatively to have experienced significant neurological deterioration. The false-negative rate in SSEP monitoring was 9%. Sensitivity was determined to be 0%.

CONCLUSIONS

It is important to recognize high false-negative rates and low sensitivity of SSEP monitoring when it is used to record spinal cord function during anterior approaches for thoracic vertebrectomies. The insensitivity of SSEPs for motor deterioration during anterior thoracic vertebrectomies is likely due to the limitation of SSEPs, which monitor only posterior column function whereas motor paths are conveyed in the anterior and anterolateral spinal cord. The authors believe that SSEPs can not be relied on to detect reversible spinal damage during anterior thoracic vertebrectomies.

Spinal cord monitoring

Over the past two decades, intraoperative spinal cord monitoring has matured into a widely used clinical tool. It is used when the spinal cord is at risk for damage during a surgical procedure. This includes orthopedic, neurosurgical, and certain cardiothoracic procedures. Both somatosensory evoked potential (SEP) and direct motor pathway stimulation techniques are available. The SEP techniques are used most widely, are generally accepted, and have been shown to reduce surgical morbidity. A large multicenter study has shown that SEP monitoring reduces postoperative paraplegia by more than 50-60%. Techniques and literature on clinical applications are reviewed in this report.

The role of motor evoked potentials during surgery for intramedullary spinal cord tumors

OBJECTIVE

This is a prospective study of the methodology and clinical applications of motor evoked potentials (MEPs) during surgery for intramedullary spinal cord tumors.

METHODS

Transcranial electrical stimulation was used to activate corticospinal motoneurons, and the traveling waves of the spinal cord were recorded through catheter-electrodes placed epi- or subdurally. Intraoperative MEP monitoring was performed in 32 consecutive patients (age range, 1-50 yr) undergoing resection of intramedullary spinal cord tumors. In 19 patients, MEPs were present before myelotomy (monitorable group), and in 10 patients, MEPs were absent before myelotomy (unmonitorable group). Placement of an epidural electrode was not possible in two patients, and technical problems prevented recording in one.

RESULTS

MEP amplitudes decreased intraoperatively by more than 50% of baseline in three patients, all of whom had postoperative paraplegia. Two of these patients recovered within 1 week after surgery, and one remained paraplegic. None of the patients with preserved MEP amplitude (> 50%) sustained immediate significant postoperative deterioration. Motor function was significantly deteriorated 1 week after surgery in one patient in the monitorable group and in five patients in the unmonitorable group. MEP monitorability was significantly associated with good surgical outcome for adult patients (P < 0.05), although not for pediatric patients (P > 0.6). Preoperative motor status and surgical outcome were not significantly associated for the adult (P = 0.13) or pediatric groups (P > 0.4).

CONCLUSION

MEP monitorability was a better predictor of functional outcome than the patient's preoperative motor status for the adult group. Significant predictors of MEP monitorability in the adult group were preoperative motor function (P < 0.01), history of no previous treatment (surgery or irradiation) (P < 0.01), and small tumor size (P < 0.05). Weak associations with monitorable MEPs existed for low-grade tumors (P = 0.09), the presence of baseline somatosensory evoked potentials (P = 0.10), and tumor pathological abnormalities (ependymoma) (P = 0.13). No associations were determined for sex (P > 0.4), associated syrinx (P > 0.3), or tumor location (P > 0.5). In the pediatric group, none of the examined factors were associated with MEP monitorability (P > 0.3). A decline of more than 50% in MEP amplitude during tumor removal should serve as a serious warning sign to the surgeon.

Spine and scalp recordings as a function of intensity. A model for changes during spinal cord monitoring

STUDY DESIGN

Spinal cord monitoring has used both spine and scalp recordings as indicators of spinal cord integrity. The relative merits of spine or scalp recordings to predict the quality of the afferent volley in the somatosensory pathway were addressed in this study by using various stimulus intensities as a way to model alterations of the size of the afferent volley.

OBJECTIVES

The results were analyzed to determine the correlation of central recordings taken at the spine or scalp with peripheral recordings.

SUMMARY OF BACKGROUND DATA

Spinal cord monitoring with somatosensory evoked potentials has been achieved with recordings of signals generated by either the spinal cord or the somatosensory cortex. Spine recordings are thought to be more stable, yet little evidence exists to document this statement.

METHODS

Seven patients were studied in the course of standard intraoperative spinal cord monitoring. Responses were recorded at the popliteal fossa, thoracic epidural, cervical spine, and scalp to tibial nerve stimulation at intensities varying from 0.5 to 2.0 times muscle twitch threshold.

RESULTS

Normalized amplitudes of the response at the popliteal fossa were used to reflect the magnitude of the afferent volley. The amplitudes of the popliteal fossa response showed a high correlation (r = 0.90) with normalized amplitudes of epidural and cervical spine responses and moderate correlation (r = 0.49) with normalized amplitudes of scalp responses. The width of the 95% confidence limits for the inverse prediction of the afferent volley from epidural and cervical responses was nearly a third narrower than that from scalp responses. At low stimulus intensities, scalp responses were consistently observed when spine responses were absent, and scalp responses had lower response thresholds than did spine responses. The latencies of the popliteal fossa responses were not well correlated with latencies of either the epidural or cervical responses.

CONCLUSIONS

These correlation and inverse prediction data suggest that the size of an afferent volley may be predicted more accurately by spine responses than by scalp responses. The presence of scalp responses at intensities too low to elicit detectable spinal-level responses suggests that scalp responses may be considered a sensitive indicator of a minimal afferent volley.

The effects of epidural and intravenous lidocaine on somatosensory evoked potentials after stimulation of the posterior tibial nerve

The measurement of somatosensory evoked potentials (SEPs) after stimulation of the posterior tibial nerve (PTN-SEPs) has been proposed as an objective indicator of the quality of lumbar epidural block. It is unclear whether peak latency increases after epidural application of local anesthetics may be due in part to systemic effects of the drug absorbed from the epidural space. In this clinical study, we compared PTN-SEPs after intravenous and epidural administration of lidocaine to those of a control group who did not receive lidocaine. Plasma concentrations of lidocaine remained within expected ranges for epidural and intravenous administration. No subjects developed signs for overdose or toxicity. After epidural application of 2% lidocaine, mean latencies of peaks P1, N1, and P2 increased significantly in comparison to baseline values. In 3 of 10 patients, latency changes were not observed. Intravenous lidocaine did not produce statistically significant changes in latencies, although a trend toward increasing latencies appeared to be present. In the control group without lidocaine, no statistically significant changes occurred during the 1-h study period. No correlation was found between peak latency changes and plasma concentrations of lidocaine. We conclude that latency increases observed after epidural application of lidocaine are due primarily to local, not systemic, effects of the local anesthetic.

Spinal cord evoked potential monitoring after spinal cord stimulation during surgery of spinal cord tumors

Spinal cord evoked potentials (SCEPs) after spinal cord stimulation were used as a method of spinal cord monitoring during surgery of 6 extramedullary and 14 intramedullary spinal cord tumors. SCEPs were recorded from an epidural electrode placed rostral to the level of the tumor. Electrical stimulation was applied on the dorsal spinal cord from a caudally placed epidural electrode. The wave forms of SCEPs consisted of a sharp negative peak (N1) in 15 cases and two negative peaks (N1 and N2) in 5 cases. The N2 wave was markedly attenuated by posterior midline myelotomy, whereas the N1 activity showed less-remarkable changes by myelotomy. An increase in N1 amplitude was observed after the removal of the tumor in four extramedullary and three intramedullary cases. Of six patients that showed decreased N1 amplitude after the removal of the tumor, five patients developed postoperative motor deficits. However, there were four false-negative cases and one false-positive case in regard to changes of N1 amplitude and postoperative motor deficits. Four false results occurred in intramedullary cases. In two of them, postoperative symptoms indicated intraoperative unilateral damage to the spinal cord. The position of the stimulating electrode, the difference in thresholds of the axons for electrical stimulation between the right and left side of the spinal cord, or the change of the distance between the electrode and the spinal cord surface may account for these false results. Thus, our analysis of the changes of SCEP wave forms and early postoperative symptoms indicates that the sensitivity of this monitoring method to detect intraoperative insults to the spinal cord is unsatisfactory in spite of the reproducible wave forms. We conclude that SCEP monitoring can be used as an alternative method or in combination with other types of evoked potentials in patients with severe spinal cord lesions who show abnormal somatosensory evoked potentials preoperatively.

Loss of somatosensory evoked potentials during intramedullary spinal cord surgery predicts postoperative neurologic deficits in motor function [corrected]

STUDY OBJECTIVES

To estimate the sensitivity and specificity of somatosensory evoked potentials (SSEPs) for predicting new postoperative motor neurologic deficits during intramedullary spinal cord surgery; to establish whether SSEPs more accurately predicted postoperative deficits in position and vibration sense than in strength.

DESIGN

Prospective open and retrospective study.

SETTING

University-affiliated hospital.

PATIENTS

20 patients with intramedullary spinal cord tumors scheduled for surgery with intraoperative SSEPs.

INTERVENTIONS

Median, ulnar, and tibial nerve cortical and subcortical SSEPs were recorded continuously.

MEASUREMENTS AND MAIN RESULTS

Conventional intraoperative SSEP criteria considered indicative of neurologic injury were modified and defined as either the complete and permanent loss of the SSEP or the simultaneous amplitude reduction of 50% or greater in the nearest recording electrode rostral to the surgical site and 0.5 millisecond increase in the central latency. Our definition required confirmation of both amplitude and latency changes on a repeated average. All patients had 1 or more SSEPs, which were reproducible and sufficiently stable for analysis throughout the operation. Six patients developed new postoperative neurologic deficits. One had new motor deficits in an extremity from which no baseline SSEPs could be elicited. In each of the other 5 patients, significant SSEP changes preceded the postoperative motor deficits in the extremity or extremities monitored. In no patient without a new postoperative motor deficit was there a significant change in the SSEP. In only 2 of these 5 patients was there a documented postoperative loss or diminution in vibration or position sense.

CONCLUSIONS

Intraoperative SSEP changes during intramedullary spinal cord surgery are a sensitive predictor of new postoperative motor deficits, but such changes may not correlate reliably with postoperative deficits in position or vibration sense. In this setting SSEP monitoring serves primarily to reassure the operating team that, when the SSEPs remain constant, the surgery has not caused additional injury.

Direct spinal versus peripheral nerve stimulation as monitoring techniques in epidurally recorded spinal cord potentials

We recorded spinal cord evoked potentials (SCEPs) and spinal somatosensory evoked potentials (spinal SEPs) in 30 operations following stimulation of the epidural spinal cord and the peripheral nerve, respectively, to compare their feasibility as an intraoperative technique for spinal cord monitoring. SCEPs produced quicker responses and had larger amplitudes with simpler waveforms. SCEPs could reflect residual function of the pathological spinal cord and predict the postoperative clinical outcome, findings which are not observed with spinal SEPs. Moreover, SCEPs had a much higher sensitivity to spinal cord insult. Therefore, we conclude that the SCEPs were more appropriate indicator than the spinal SEPs as an intra-operative monitoring method for spinal cord function.

Origin and distribution of brain-stem somatosensory evoked potentials in humans

The distribution of somatosensory evoked potentials (SEPs) recorded from the brain-stem surface was studied to investigate their generator sources in 14 patients during surgical exploration of the posterior fossa. Two distinct SEPs of different morphologies and electrical orientation were obtained by median nerve stimulation. A small positive-large negative-late prolonged positive wave was recorded from the cuneate nucleus and its vicinity. There was a phase-reversal between the cuneate nucleus and the ventral surface of the medulla, depicting a dipole for dorso-ventral organization. From the pons and midbrain, triphasic waves with predominant negativity were obtained. This type of SEP had identical wave forms between the dorsal, lateral and ventral surface of the pons and midbrain. It showed an increase in negative peak latency as the recording sites moved rostrally, suggesting an ascending axial orientation. In a patient with pontine hemorrhage, the killed end potential, a large monophasic positive potential was obtained from the lesion. This potential occurs when an impulse approaches but never passes beyond the recording electrode. Therefore, the triphasic SEP from the pons and midbrain reflects an axonal potential generated in the medial lemniscal pathway.

Conduction properties of epidurally recorded spinal cord potentials following lower limb stimulation in man

Spinal somatosensory evoked potentials were recorded in 35 neurologically normal patients undergoing surgery for scoliosis. During posterior procedures the recording electrodes were placed in the dorsal epidural space and during anterior operations in the intervertebral discs. Stimulation was of the tibial nerve in the popliteal fossa and the posterior tibial and sural nerves at the ankle. At thoracic levels the response consisted of at least 3 components with different peripheral excitation thresholds and spinal conduction velocities (range 35-85 m/sec). All components were conducted mainly in tracts ipsilateral to the stimulus, component 1 being most laterally located. At low stimulus intensity only the fastest activity was recorded but this was markedly delayed over low thoracic segments and was recorded as a repetitive discharge rostrally. Higher intensities elicited additional components which were conducted at a slower but relatively uniform velocity; consequently they might overlap with or even overtake the fast activity at mid-to-low thoracic levels. Component 1 was much less prominent when the posterior tibial nerve was stimulated at the ankle and absent from the (cutaneous) sural nerve response; remaining potentials were conducted at velocities similar to those of components 2 and 3 following tibial nerve stimulation at the knee. Small 'stationary' potentials were recorded at all thoracic levels, probably due to the change in conductivity as the volley entered the spinal cord. Efferent activity was recorded at and below the thoraco-lumbar junction, possibly related to the H-reflex or F-wave. Similar, although smaller, afferent potentials were recorded from the anterior side of the vertebral column. Component 1 is likely to be due to the stimulation of group 1 muscle afferents which terminate in the dorsal horn and activate second order neurones, many of whose axons go to form the ipsilateral dorsal spinocerebellar tract. Components 2 and 3 are believed to be largely cutaneous in origin and to be conducted mainly in the dorsal columns.

Subpial spinal evoked potentials in patients undergoing junctional dorsal root entry zone coagulation for pain relief

Seven patients with complete avulsion of the brachial plexus underwent junctional coagulation lesions of the dorsal root entry zone (DREZ) for relief of intractable pain in the paralyzed arm. Intra-operative monitoring by recording spinal cord somatosensory evoked potentials (SEP) resulting from tibial nerve stimulation was done using subpial recording electrodes situated dorsal to the posterior median sulcus at the C4 and T2 segment. SEP on the normal side showed an initial positive wave and two negative waves followed by a group of high frequency waves of relatively high amplitude which continued into high frequency, low amplitude potentials. The conduction velocity of the fastest spinal evoked potential components were, on average, 86 m/s. Recordings from the side of avulsion revealed a steep positive potential of high amplitude which appeared in five patients prior to the creation of the DREZ lesion. This effect was assumed to be secondary to spinal cord damage caused by avulsion. During the DREZ coagulation the SEP from the unaffected side did not change. On the side of DREZ coagulation the velocity of the fastest fibres decreased. Four patients reported sensory deficits after the operation, which were transient in three. In one of these patients, the first two negative potentials disappeared. In the fourth patient, who had permanent sensory deficits, the positive steep potential appeared after generation of the lesion. Our results point to the usefulness of the subpial SEPs monitoring during microneuro-surgical procedures on the spinal cord to provide further insight into evoked electrical activity of the normal and injured spinal cord, and to minimize post-operative neurological morbidity.

Intraoperative cortical somatosensory evoked potentials for detection of sciatic neuropathy during total hip arthroplasty

Approximately 3% of patients undergoing hip arthroplasty develop postoperative sciatic neuropathy. The factors associated with changes in somatosensory evoked potentials (SSEP) and sciatic neuropathy were examined in patients undergoing hip arthroplasty, to evaluate whether the use of intraoperative SSEP could help reduce the incidence of postoperative sciatic neuropathy. Eighty-eight patients were assigned to either monitored or unmonitored groups. SSEP were recorded following peroneal nerve stimulation, using contralateral stimulation to detect systemic influences on SSEP. Amplitude reduction of less than 50% of control and/or latency increase of greater than 10% of control was considered significant, and surgical intervention was attempted to restore SSEP. Previous surgery and a lateral incision approach tended to be associated with sciatic neuropathy (p less than 0.053). The incidence of sciatic neuropathy in the monitored group (4.3%) was not different from the unmonitored group (2.4%). Isolated reduction in amplitude or prolongation in latency of the SSEP was not predictive of postoperative neurologic function of the sciatic nerve. Six patients, two of whom developed sciatic neuropathy, demonstrated complete flattening of the SSEP. Both of these patients had flattened SSEP for two or more surgical events (p less than 0.01) and flattened SSEP were present at the end of the surgical procedure. There were no false-negative SSEP changes. Simultaneous amplitude and latency changes appear to be predictive of sciatic nerve function following hip arthroplasty.