Changes In Somatosensory Evoked Potentials Research Articles

The prevention of neural complications in the surgical treatment of scoliosis: the role of the neurophysiological intraoperative monitoring

Iatrogenic spinal cord injury is the most feared complication of scoliosis surgery. The importance of combined somatosensory evoked potentials (SEP) and motor evoked potentials (MEP) monitoring during spine surgery is well known. The current authors retrospectively evaluated the results of neurophysiological intraoperative monitoring (IOM) in a large population of patients who underwent surgical treatment for spinal deformity. Intraoperative monitoring of SEPs and transcranial electrical stimulation MEPs (TES-MEP) was performed in 172 successive patients who underwent surgical treatment of idiopathic (128 pts), congenital (15 pts) or syndromic (29 pts) scoliosis. The first 106 patients (Group 1) underwent only SEP monitoring, while the other 66 patients (Group 2) underwent combined SEP and TES-MEP monitoring, when the technique was introduced in the current authors' institution. Halogenate anaesthesia (Sevoflurane, MAC 0.6-1.2) was performed in Group 1 cases, total intravenous anaesthesia (Propofol infusion, 6-10 mg/kg/h) in Group 2 patients. A neurophysiological "alert" was defined as a reduction in amplitude (unilateral or bilateral) of at least 50% for SEPs and of 65% for TES-MEPs compared with baseline. In Group 1, two patients (1.9%) developed postoperative neurologic deficits following surgical correction of spinal deformity, consisting of permanent paraparesis in one case and transient paraparesis secondary to spinal cord ischaemia in the other. Twelve patients presented intraoperative significant changes of neurophysiological parameters that improved following corrective actions by surgeons and anaesthesiologists, and did not show any postoperative neurologic deficits. In ten cases the alert was apparently unrelated to surgical manoeuvres or to pharmacological interventions and no postoperative neurologic deficits were noted. Considering the patients of Group 2, two patients (3.0%) presented transient postoperative neurologic deficits preceded by significant intraoperative changes in SEPs and TES-MEPs. In five cases a transient reduction in the amplitudes of SEPs (1 patient) and/or TES-MEPs (5 patients) was recorded intraoperatively with no postoperative neurologic deficits. In conclusion, in the current series of 172 patients the overall prevalence of postoperative neurologic deficit was 2.3% (4 patients). When combined SEP and TES-MEP monitoring was performed, the sensitivity and specificity of IOM for sensory-motor impairment was 100 and 98%, respectively. Combined SEP and TES-MEP monitoring must be regarded as the neurophysiological standard for intraoperative detection of emerging spinal cord injury during corrective spinal deformity surgery. Early detection affords the surgical team an opportunity to perform rapid intervention to prevent injury progression or possibly to reverse impending neurologic sequelae.

Somatosensory Evoked Potential Monitoring During Endoscopic Endonasal Approach to Skull Base Surgery: Analysis of Observed Changes

Intraoperative neurophysiological monitoring, including upper- and lower-extremity somatosensory evoked potentials (SSEPs), has been used to identify and prevent injury to neurovascular structures during conventional skull base surgery. The expanded endonasal approach (EEA) is a novel minimally invasive approach to skull base surgery. However, it carries the risk of injury to neurovascular structures, including the internal carotid artery, anterior cerebral artery, and cranial nerves. To evaluate the value of SSEP monitoring to predict and/or prevent neurovascular deficits during EEA to skull base surgery. We retrospectively identified 999 consecutive patients who had intraoperative neurophysiological monitoring during EEA skull base surgery at our institution. A total of 976 patients had SSEP monitoring and a documented postoperative neurological examination. The incidence of changes in SSEP during the procedure was 20 of 976 (2%). The incidence of new postoperative neurological deficits was 5 of 976 (0.5%). The positive and negative predictive values of SSEPs during EEA to predict neurovascular deficits were 80.00% and 99.79%, respectively. Intraoperative SSEP monitoring was able to identify impending risk to neurovascular structures to prevent permanent postoperative neurological deficits. We advocate a comprehensive approach to neurophysiological monitoring during EEAs, including SSEPs, spontaneous and triggered electromyography of the cranial nerves III through XII, brainstem auditory evoked potentials, and electroencephalogram, depending on the surgical approach and location of the neural structures at risk.

Monitoring mild impact iatrogenic spinal cord injury by using somatosensory-evoked potential: an experimental study

：Objective To investigatethe changes of somatosensory-evoked potential(SEP)during an mild impact spinal cord injuryin rats 80 as to evaluate its potential value in prevention of such iatrogenic damage.Methods Twenty-four SD rats weighing(340±28)g were randomly divided into twogroups,ie,sham control group(only exposure without impact at C6)and injury group(mildimpact spihal cord injury at C6).SEP wss recorded in both groups.The changes of SEP inwaveform,amplitude and latency were observed and compared between groups and betweenoperations.The gross dissection and histologic analysis were performed after surgery forcomparative study. Results SEP waveforms,amplitude and latency showed no significantchange in the sham control group.In contrast,the SEP waveform and amplitude animals showedsignificant changes in the injury group after impact spinal cord injury and the amplitudewas decreased from pre-injury(1.3±0.7)μV to post-injury(0.5±0.4)μV(P 0.05).However,this abnormal change appeared in a temporary period at(5.7±3.2)minutesafter impact and lasted for about(7.1±3.3)minutes.Diffused hemorrhagic nidus could beseen in the injured cord,which was not found in the sham control group. Condusions Mildimpact spinal cord injury may induce transient abnormalities of SEP in waveform andamplitude,which requires careful monitoring in clinical practice.The sudden change in SEPmay be associated with impact and vibration damage to the spinal cord,suggesting timelyuse of protection measures for spinal cord. Key words: Spinal cord injuries; Evokedpotentials,somatosensory; Rats,Sprague-Dawley

Dynamic monitors of brain function: a new target in neurointensive care unit

IntroductionSomatosensory evoked potential (SEP) recordings and continuous electroencephalography (EEG) are important tools with which to predict Glasgow Outcome Scale (GOS) scores. Their combined use may potentially allow for early detection of neurological impairment and more effective treatment of clinical deterioration.MethodsWe followed up 68 selected comatose patients between 2007 and 2009 who had been admitted to the Neurosurgical Intensive Care Unit of Treviso Hospital after being diagnosed with subarachnoid haemorrhage (51 cases) or intracerebral haemorrhage (17 cases). Quantitative brain function monitoring was carried out using a remote EEG-SEP recording system connected to a small amplification head box with 28 channels and a multimodal stimulator (NEMO; EBNeuro, Italy NeMus 2; EBNeuro S.p.A., Via P. Fanfani 97/A - 50127 Firenze, Italy). For statistical analysis, we fit a binary logistic regression model to estimate the effect of brain function monitoring on the probability of GOS scores equal to 1. We also designed a proportional odds model for GOS scores, depending on amplitude and changes in both SEPs and EEG as well as on the joint effect of other related variables. Both families of models, logistic regression analysis and proportional odds ratios, were fit by using a maximum likelihood test and the partial effect of each variable was assessed by using a likelihood ratio test.ResultsUsing the logistic regression model, we observed that progressive deterioration on the basis of EEG was associated with an increased risk of dying by almost 24% compared to patients whose condition did not worsen according to EEG. SEP decreases were also significant; for patients with worsening SEPs, the odds of dying increased to approximately 32%. In the proportional odds model, only modifications of Modified Glasgow Coma Scale scores and SEPs during hospitalisation statistically significantly predicted GOS scores. Patients whose SEPs worsened during the last time interval had an approximately 17 times greater probability of a poor GOS score compared to the other patients.ConclusionsThe combined use of SEPs and continuous EEG monitoring is a unique example of dynamic brain monitoring. The temporal variation of these two parameters evaluated by continuous monitoring can establish whether the treatments used for patients receiving neurocritical care are properly tailored to the neurological changes induced by the lesions responsible for secondary damage.

Application of combined monitoring of motor evoked potentials and somatosensory evoked potentials during intramedullary spinal cord tumor operation

Objective To investigate the reliability,superiority and value of combined monitoring of motor evoked potentials(MEP) and somatosensory evoked potentials(SEP) during intramedullary spinal cord tumor surgery under total intravenous anesthesia.Method 72 patients with intramedullary spinal cord tumor were monitored somatosensory evoked potentials and muscle motor evoked potentials during operation.McCormick scale was adopted to evaluate the patients' clinical nerve function.The postoperative to preoperative McCormick grade variation and combimed monitoring of SEP and MEP were compared in this group.Result 14 patients' nerve function was improved.The findings of monitoring were evaluated in 18 patients with depressed McCormick scores.There was significant difference in the latency and amplitude of waves between pretreatment and post-treatment (P＜0.05).The changes of SEP and MEP were correlative with the changes of spinal cord function.Conclusion Combined monitoring of SEP and MEP could help to improve postoperative McCormick scale outcome significantly for patients with intramedullary spinal cord tumor.The waveform of either SEP or MEP might be stable and reliabie in monitoring patients under total intravenous anesthesia.It might be helpful in avoiding 'false negative and/or false positive results'as well as the postoperative neurological sequelae. Key words: Spinal cord; Somatosensory evoked potentials; Motor evoked potentials

Spinal cord integrity monitoring by adaptive coherence measurement

Objective Injury during routine spinal cord procedures could result in devastating consequences for the surgical patient. Spinal cord monitoring through somatosensory evoked potentials (SEPs) remains a viable method for prevention of serious injury. Methods The adaptive coherence estimation (ACE) is a method to iteratively calculate signal match quality through successive filter entrainment. Here we compare the speed of detection with ACE to conventional amplitude measurements. Both absolute magnitude of ACE and amplitude as well as slope change detector algorithm (Farley–Hinich) was run as well to determine the earliest time when a significant change occurred. Results The standard error for the ACE algorithm is close to one tenth of the amplitude measure, Since the ACE algorithm achieved low variance during baseline measurement, we were able to achieve rapid detection of injury. For absolute magnitude detection ACE was faster than amplitude for the 20 g injury weight class. It took an average of 10 epochs to detect the injury with adaptive coherence and nearly 19 with standard amplitude metrics using absolute magnitude changes. Abrupt change detection methods using slope change show that ACE provides more favorable detection capabilities comparable to amplitude. Additionally, there was a significant increase in the ROC curve between ACE and amplitude alone ( p < 0.05). Conclusions Because of its excellent detection capabilities, the adaptive coherence method provides an excellent supplement to traditional amplitude for capturing injury-related changes in SEPs. Significance Adaptive coherence remains a viable method for rapidly and accurately detecting spinal injury.

Bilateral Femoral Artery Ischemia Detected by Multimodality Neuromonitoring During Posterior Scoliosis Surgery

Case report. To report a case of a bilateral femoral artery ischemia detected by neuromonitoring during posterior scoliosis surgery and to review relevant literature regarding this rare complication. Lower extremity ischemia is a potentially devastating risk of posterior spinal surgery. Ischemia can be a result of thrombotic occlusion or vascular compression during patient positioning. Multimodality neuromonitoring, increasingly used to prevent neurologic injury, can also detect hypoperfusion to the extremities. To date, there have been no reports of bilateral lower extremity ischemia detected by multimodality neuromonitoring during posterior spine surgery. A 15-year-old boy with adolescent idiopathic scoliosis underwent posterior spinal fusion with instrumentation. Intraoperative changes in somatosensory-evoked potentials and motor-evoked potentials were noted 1 hour into the case, before instrumentation or the reduction maneuver. After trouble shooting methods did not localize a technical cause for the changes, the patient's lower extremities were noted to be hypoperfused and pulseless. The patients was repositioned and lower extremity perfused improved. Palpable distal pulses were detected. Neuromonitoring signals returned to baseline and the surgery completed. The patient had no postoperative neurologic or vascular deficits. Lower extremity ischemia secondary to prone positioning is a rare risk of posterior spinal surgery. This is the first case report of this potentially devastating, but preventable complication detected by multimodality neuromonitoring.

Somatosensory evoked potentials and high frequency oscillations are differently modulated by theta burst stimulation over primary somatosensory cortex in humans

Objective The effects of theta burst stimulation (TBS) have been extensively investigated in primary motor cortex, where it leads to long-lasting LTP/LTD-like effects on synaptic plasticity. This study aimed to extend these observations to sensory cortex. Methods Fourteen healthy subjects participated in the study. Conditioning 600-pulse intermittent TBS (iTBS) and continuous TBS (cTBS) were delivered to left somatosensory cortex (S1) with an intensity of 80% active motor threshold. Somatosensory evoked potentials (SEPs) were evoked by median nerve electrical stimulation at right wrist. High frequency oscillations (HFOs) were obtained by digital filtering of original SEPs and divided into early and late subcomponents, relative to N20 peak latency. Results Repeated-measures ANOVA showed that iTBS facilitated N20 onset–N20 peak at 15 min and N20 peak–P25 at 15 and 30 min after conditioning, whereas cTBS did not. iTBS left the early and late HFOs unchanged. Conversely, cTBS facilitated the early HFOs, whereas it inhibited the late HFOs at 15 min after conditioning. Conclusions S1-iTBS facilitated SEPs without changes in HFOs whereas cTBS modulated early and late HFOs without changes in SEPs. Significance S1-TBS produces lasting changes in the excitability of intracortical circuits generating SEPs and HFOs differentially through mechanisms of LTP/LTD-like synaptic plasticity.

Localization of cervical and cervicomedullary stimulation leads for pain treatment using median nerve somatosensory evoked potential collision testing

Spinal cord stimulation (SCS) is being currently used to treat medically refractory pain syndromes involving the face, trunk, and extremities. Unlike thoracic SCS surgery, during which patients can be awakened from conscious sedation to confirm good lead placement, safe placement of paddle leads in the cervical spine has required general anesthesia. Using intraoperative neurophysiological monitoring, which is routinely performed during these cases at the authors' institution, the authors developed an electrophysiological technique to intraoperatively lateralize lead placement in the cervical epidural space. Data from 44 patients undergoing median and tibial nerve somatosensory evoked potential (SSEP) monitoring during cervical laminectomy or hemilaminectomy for placement or replacement of dorsal column stimulators were retrospectively reviewed. Paddle leads were positioned laterally or just off midline and parallel to the axis of the cervical spinal cord to effectively treat what was most commonly a predominant unilateral pain syndrome. During SSEP recording, the spinal cord stimulator was activated at 1.0 V and increased in increments of 1.0 V to a maximum of 6.0 V. A unilateral reduction or abolishment of SSEP amplitude was regarded as an indicator of lateralized placement of the stimulator. A bilateral diminutive effect on SSEPs was interpreted as a midline or near midline lead placement. Epidural stimulation abolished or significantly reduced SSEP amplitudes in all patients undergoing placement for a unilateral pain syndrome. In 15 patients, electrodes were repositioned intraoperatively to achieve the most robust SSEP amplitude reduction or abolishment using the lowest epidural stimulation intensity. In all cases in which a significant unilateral reduction in SSEP was observed, the patient reported postoperative sensory alterations in target locations predicted by intraoperative SSEP changes. Placement of cervical spinal cord stimulators for bilateral pain syndromes often resulted in bilateral but asymmetrical SSEP changes. In no cases were significant SSEP changes, other than those induced using the device to directly stimulate the dorsal surface of the spinal cord, observed. No case of new postoperative neurological deficit was observed. Somatosensory evoked potentials can be used safely and successfully for predicting the lateralization of cervical spinal cord stimulator placement. Moreover, they can also intraoperatively alert the surgical team to inadvertent displacement of a lead during anchoring. Further studies are needed to determine whether apart from assisting with proper lateralization, SSEP collision testing may help to optimize electrode positioning and improve pain control outcomes.

The Incidence, Volume, Absorption, and Timing of Supratentorial Pneumocephalus During Posterior Fossa Neurosurgery Conducted in the Sitting Position

Supratentorial pneumocephalus (STP) is a known complication of neurosurgical procedures of the posterior fossa when conducted in the sitting position. Few studies have examined STP as differentiated from pneumocephalus in the operative field. Ninety-five of 106 consecutive patients had postoperative radiographic studies and median nerve somatosensory evoked potential (SSEP) recording during surgery. STP was identified on postoperative skull films or computerized tomography (CT). STP volume was measured on CT scans. SSEP changes were identified in the monitoring records. STP was identified in 40 patients (42.1%). STP volume on CT scans within 4 hours of surgery ranged from 6 to 280 cm3 (cubic centimeters). An extraventricular drain or ventriculo-peritoneal shunt did not increase the incidence of STP (P=0.85). The absorption of STP in 5 patients with multiple CT scans showed an immediate reduction of 24% followed by exponential decay with an half life of 1.5 days. Four patients with an extraventricular drain or ventriculo-peritoneal shunt had excessive volume of STP when measured at 1 day or later. Six patients with significant SSEP amplitude reductions attributed to STP had volumes exceeding 90 cm3 on a CT scan within 4 hours of surgery. The onset of these changes occurred at various times from dural opening to closing. STP is common after posterior fossa neurosurgical procedures conducted in the sitting position. Further studies are needed to fully characterize the absorption of air and the timing of the entry.

Effects of Remote Ischemic Preconditioning on Biochemical Markers and Neurologic Outcomes in Patients Undergoing Elective Cervical Decompression Surgery

Remote ischemic preconditioning (RIPC) may protect the spinal cord from ischemic injury. This randomized clinical trial was designed to assess whether a large clinical trial testing the effect of RIPC on neurologic outcome in patients undergoing spine surgery is warranted. This trial was registered with ClinicalTrials.gov, number NCT00778323. Forty adult cervical spondylotic myelopathy patients undergoing elective decompression surgery were randomly assigned to either the RIPC group (n=20) or the control group (n=20). Limb RIPC consisted of three 5-minutes cycles of upper right limb ischemia with intervening 5-minute periods of reperfusion. Neuron-specific enolase and S-100B levels were measured in serum at set time points. Median nerve somatosensory-evoked potentials (SEPs) were also recorded. Neurologic recovery rate was evaluated using a Japanese Orthopaedic Association scale. RIPC significantly reduced serum S-100B release at 6 hours and 1 day after surgery, and reduced neuron-specific enolase release at 6 hours, and then at 1, 3, and 5 days after surgery. No differences were observed in SEP measurements or the incidence of SEP changes during surgery between the control and RIPC groups. Recovery rate at 7 days, and at 1 and 3 months after surgery was higher in the RIPC group than in the control group (P<0.05). Our results for markers of neuronal ischemic injury and rate of recovery suggest that a clinical trial with sufficient statistical power to detect an effect of RIPC on the incidence of neurologic complications (paresis, palsy, etc) due to spinal cord ischemia-reperfusion injury after spine surgery is warranted [corrected].

A Comparison of Three Methods of Pain Control for Posterior Spinal Fusions in Adolescent Idiopathic Scoliosis

Retrospective cohort study. To compare the efficacy of patient-controlled analgesia (PCA) with morphine alone, a single preoperative intrathecal morphine injection and PCA (IT/PCA), and epidural catheter infusion without PCA (EPI) for postoperative pain control after posterior spinal fusion (PSF) and segmental spinal instrumentation (SSI) in adolescent idiopathic scoliosis (AIS). Postoperative pain control after PSF and SSI in AIS can be managed in different ways. EPI provides for a longer period of pain relief but is reliant on the correct placement and maintenance of the catheter in the epidural space. A single preoperative intrathecal morphine injection also provides for long acting analgesia. No direct comparison of these 3 methods of postoperative pain control has been reported. An IRB-approved retrospective chart review was performed at 2 institutions from 1997 to 2005. The medical record was reviewed to determine pain scores after surgery at multiple time periods. The 3 groups were compared using Student t test and chi and significance was defined as P < 0.05. There was no statistical difference in the gender, age, magnitude of curve, or number of levels fused in the IT/PCA (N = 42), PCA (N = 41), or EPI (N = 55) groups. Postoperative pain scores were lowest in the IT/PCA group in the first 8 hours (P < 0.05) but the pain scores in the EPI group were then lower through 24 hours (P < 0.05). Total morphine use (mg/kg) was lower in the IT/PCA group compared with the PCA group at 12 hours and 24 hours (P = 0.0001). Return to solid food ingestion was quickest in the EPI group (2.0 days) followed by the IT/PCA (2.6 days) and PCA alone (3.2 days) (P < 0.002). Respiratory depression and transient neurologic change occurred most frequently in the EPI group (EPI 11/55 pts vs. 1/42 IT/PCA vs. 0/41 PCA P < 0.001). Pruritus was greatest in the epidural group (11/55 P < 0.05). There were no intraoperative somatosensory-evoked potential changes or permanent neurologic injury recorded in any group. An EPI controls postoperative pain for the longest period of time and allows for a quicker return to consumption of solid foods. However, a single preoperative intrathecal morphine injection controls the pain equally for the first 24 hours with less pruritus and with less adverse events thus requiring less nursing and physician intervention after PSF and SSI in AIS. All methods were safe with no neurologic injury recorded.

The alert applying value of somatosensory evoked potential examination in total vertebral osteotomy.

Objective To investigate risk factors related to changes in somatosensory evoked poten-tials (SEPs) in total vertebral osteotomy. Methods From April 2002 to December 2007, 118 patients with severe kyphoscoliosis deformity were treated with posterior total vertebral osteotomy. There were 68 males and 50 females with the mean age of 29 years (range, 4-73years). The pre-operative scoliosis Cobb angle was 70° (40°-130°) and kyphosis Cobb angle was 78° (32°-147°). The preoperative and intraoperative SEPs were recorded. The highest, lowest and mean arterial pressure (MAP) were recorded at the beginning, ending, and during the procedure. More than 50% decrease in SEPs amplitudes or more than 10% increase in latency was considered to be the warning threshold. Results The average blood loss volumn was 2000ml (800-6500 ml). The MAPs were less than 60 mm Hg in 14 patients, and 8 of those patients had false-posi-tive readings. There were 100 patients (84.6%) without monitoring changes and no postoperative neurologicdeficit, 1 patient (0.8%) with false-negative reading, 8 patients (6.8%) with true-positive readings, 9 patients (7.6%) had false-positive readings. Nine patients had a postoperative motor or sensory deficit. Higher false-positive rates were seen in patients with greater lability in MAP. In 12 patients, N50 latency increased and the latter two peaks amalgamated to form one slower wave. Conclusion False-positive readings are asso-ciated with high variability in MAP. The concussion, translation, rotation and distraction of the spinal cord and transient ischemia may cause the restrain of the SEPs. N50 latency increased and the latter two peaks a-malgamated to form one slower wave is the alert value possibly. Key words: Spine; Osteotomy; Evoked potentials, somatosensory; Monitoring, intraoperative

Combined motor- and somatosensory-evoked potential monitoring for spine and spinal cord surgery: correlation of clinical and neurophysiological data in 85 consecutive procedures

Prospective study. The primary objective of neurophysiological monitoring during surgery is to prevent permanent neurological sequelae. To avoid neurological injury, we applied somatosensory-evoked potentials (SEPs) and/or motor-evoked potentials (MEPs). We evaluated whether the combination of SEP and MEP for spinal surgery may be beneficial. Asian Medical Center, University of Ulsan College of Medicine, Seoul, Korea. Combined SEP/MEP monitoring was attempted in 100 consecutive procedures for spinal operations. Trains of transcranial electrical stimulation over the motor cortex were used to elicit MEPs from the muscles of the upper/lower limbs. The tibial and median nerves were stimulated to record SEP. Combined SEP/MEP recording was successfully achieved in 85 of 100 operations. In 61 of 85 operations (71%), SEP and MEP were stable, and all patients remained neurologically intact after surgery. Significant MEP changes were recorded in 20 operations, either combined with (n=4) or without (n=16) SEP changes. In 7 of these 20 operations, MEP recovered to some extent after surgical intervention, and these patients showed no neurological changes. In the remaining 13 operations, MEP did not recover and the patients had a transient (n=4) or a permanent (n=3) motor deficit. Significant SEP changes with stable MEP were observed in four operations, all of which were not related to postoperative motor deficit. Combined SEP/MEP monitoring provided higher sensitivity and higher positive/negative predictive value than single-modality monitoring techniques. Detection of MEP changes and adjustment of surgical strategy may prevent irreversible pyramidal tract damage.

Continuous EEG-SEP monitoring in severe brain injury

To monitor acute brain injury in the neurological intensive care unit (NICU), we used EEG and somatosensory evoked potentials (SEP) in combination to achieve more accuracy in detecting brain function deterioration. Sixty-eight patients (head trauma and intracranial hemorrhage; GCS<9) were monitored with continuous EEG-SEP and intracranial pressure monitoring (ICP). Fifty-five patients were considered "stable" or improving, considering the GCS and CT scan: in this group, SEP didn't show significant changes. Thirteen patients showed neurological deteriorations and, in all patients, cortical SEP showed significant alterations (amplitude decrease>50% often till complete disappearance). SEP deterioration anticipated ICP increase in 30%, was contemporary in 38%, and followed ICP increase in 23%. Considering SEP and ICP in relation to clinical course, all patients but one with ICP less than 20 mmHg were stable, while the three patients with ICP greater than 40 mmHg all died. Among the 26 patients with ICP of 20-40 mmHg, 17 were stable, while nine showed clinical and neurophysiological deterioration. Thus, there is a range of ICP values (20-40 mmHg) were ICP is scarcely indicative of clinical deterioration, rather it is the SEP changes that identify brain function deterioration. Therefore, SEP have a twofold interest with respect to ICP: their changes can precede an ICP increase and they can constitute a complementary tool to interpret ICP trends. It has been very important to associate SEP and EEG: about 60% of our patients were deeply sedated and, because of their relative insensitivity to anesthetics, only SEP allowed us to monitor brain damage evolution when EEG was scarcely valuable. We observed 3% of nonconvulsive status epilepticus compared to 18% of neurological deterioration. If the aim of neurophysiological monitoring is to "detect and protect", it may not be limited to detecting seizures, rather it should be able to identify brain deterioration, so we propose the combined monitoring of EEG with SEP.

Analysis of motor and somatosensory evoked potentials during thoracic and thoracoabdominal aortic aneurysm repair

Use of motor evoked potentials (MEP) and somatosensory evoked potentials (SSEP) monitoring during thoracic and thoracoabdominal aortic surgery is controversial. This study evaluated the intraoperative use of SSEP and MEP during thoracoabdominal repair and assessed their role in decreasing the risk of spinal cord ischemia and paralysis. We conducted paired SSEP and MEP monitoring to assess agreement between the methods and their ability to predict neurologic outcome in 233 patients. Changes in SSEP and MEP monitoring were classified as no change, reversible change, or irreversible change during the intraoperative period and by the conclusion of surgery. Agreement between the methods was computed using the Cohen kappa statistic. Sensitivity, specificity, and positive and negative predictive values were computed for each method on the immediate and delayed neurologic deficit. Immediate neurologic deficit, determined immediately upon awakening from anesthesia and confirmed by a neurologist, occurred in eight of 233 (3.4%) patients. For any change (reversible plus irreversible), agreement between MEP and SSEP was relatively low (kappa = 0.53), despite being highly statistically significant (P < .001). MEP tended to overestimate SSEP for immediate neurologic deficit, demonstrating a 53% false-positive rate, compared with a 33% false-positive rate for SSEP (specificity ratio, 1.42; P < .0001). With irreversible change, agreement between the methods was 90% (kappa = 0.896, P < .0001). Only irreversible change was significantly associated with neurologic outcome (odds ratio, 21.9; P < .00001 for SSEP; 60.8, P < .0001 for MEP), but sensitivity and positive predictive values were low (37% and 33% for SSEP; 22% and 45% for MEP, respectively). Reversible changes in neurophysiologic monitoring were not significantly associated with immediate neurologic deficit. Negative predictive values for all negative evoked potential findings were >98% for immediate deficit. No evoked potential variables were associated with delayed deficit. SSEP and MEP monitoring were highly correlated only when intraoperative changes were irreversible. Reversible changes were not significantly associated with immediate neurologic deficit. Irreversible changes were significantly associated with immediate neurologic deficit, and the findings were identical for SSEP and MEP in this variable, indicating that the more complex MEP measures do not add further information to that obtained from SSEP. Normal SSEP and MEP findings had a strong negative predictive value, indicating that patients without signal loss are unlikely to awake with neurologic deficit.

Upper-limb somatosensory evoked potential monitoring in lumbosacral spine surgery: a prognostic marker for position-related ulnar nerve injury

Background context Somatosensory evoked potential (SSEP) is used to monitor integrity of the brain, spinal cord, and nerve roots during spinal surgery. It records the electrical potentials from the scalp after electrical stimulation of the peripheral nerves of the upper or lower limbs. The standard monitoring modality in lumbosacral spine surgery includes lower-limb SSEP and electromyography (EMG). Upper-limb SSEP monitoring has also been used to detect and prevent brachial plexopathy and peripheral nerve injury in thoracic and lumbosacral spine surgeries. We routinely monitor lower-limb SSEP and EMG in lumbosacral spine procedures at our institution. However, a few patients experienced postoperative numbness and/or pain in their ulnar distribution with uneventful lower-limb SSEP and EMG. Purpose We hypothesized that the postoperative upper extremity paresis in lumbosacral surgeries may result from compression and/or stretch of the brachial plexus and/or ulnar nerve while the patients were in prone position. Using upper-limb SSEP, we investigated whether we observe any significant change in the SSEP, and if so, whether we can prevent or reduce frequency of postoperative upper extremity deficits. Study design/setting In this prospective study, we monitored upper-limb SSEP, in addition to lower-limb SSEP and EMG, in 230 elective, posterior lumbosacral spinal procedures. All operations were performed by a group of four neurosurgeons. Patient sample One hundred and thirty-one female and 99 male with an age range of 28 to 86 years between January 2004 and December 2005 were studied. Outcome measures Amplitude and latency of upper-limb or ulnar SSEP were continuously compared with those of the baseline. A greater than or equal to 50% decrease in SSEPs amplitude and/or a greater than or equal to 10% increase in latency were considered to be significant. Methods After intubation, patients were positioned prone on Jackson or Andrews spinal table. Anesthesia was maintained with inhalant gas (desflurane or sevoflurane) and propofol infusion with and without minimal infusion of narcotics (fentanyl, sufentanyl, or remifentanil). Intraoperative neurophysiologic monitoring of upper-limb or ulnar SSEP was achieved by continuously recording cortical and subcortical responses after alternate stimulation of the ulnar nerve at the wrist. In our institutional protocol, a greater than or equal to 50% decrease in SSEPs amplitude and/or a greater than or equal to 10% increase in latency were considered to be significant to alert the operating surgeons. When significant changes occurred, the surgeon was immediately notified. Also, reevaluation of vital signs, depth of anesthesia, and patient's position, and technical troubleshootings were subsequently followed. Results We observed a greater than or equal to 50% decrease in amplitude of ulnar SSEP in 10 patients without significant changes in lower-limb SSEP (peroneal or posterior tibial nerve SSEP) or EMG during surgery. Eight patients had changes in unilateral limbs, and two patients had changes in bilateral limbs. Two patients with significant changes in unilateral limbs showed changes twice. The mean SSEP amplitude for the 14 changes was 29.2±3.1% (mean±SEM, standard error of mean) of the baseline value at the average surgical time of 60±1.5 minutes. With repositioning of the arms, the amplitudes were immediately restored with the average of 70.2±7.1% (n=14) of the baseline value. The mean amplitude of upper-limb SSEP was 73.4±8.7% (n=12) of the baseline at wound closure. The average surgical time was 154±29.2 minutes per case for the 10 patients. There was no documented postoperative upper extremity paresis in all 230 patients. Conclusions The present study demonstrates that upper-limb SSEP monitoring could detect position-related ulnar neuropathy in 5.2% of the patients undergoing lumbosacral spine surgery.

32. The changes of somatosensory-evoked potentials preceded by contralateral median nerve stimulation in sleep

32. The changes of somatosensory-evoked potentials preceded by contralateral median nerve stimulation in sleep

Predictive value of intraoperative neurophysiological monitoring during cervical spine surgery: a prospective analysis of 1055 consecutive patients

Despite the growing use of multimodal intraoperative monitoring (IOM) in cervical spinal surgery, limited data exist regarding the sensitivity, specificity, and predictive values of such a technique in detecting new neurological deficits in this setting. The authors sought to define the incidence of significant intraoperative electrophysiological changes and new postoperative neurological deficits in a cohort of patients undergoing cervical surgery. The authors conducted a prospective analysis of a consecutive series of patients who had undergone cervical surgery during a 5-year period at a university-based neurosurgical unit, in which multimodal IOM was recorded. Sensitivity, specificity, positive predictive values (PPVs), and negative predictive values (NPVs) were determined using standard Bayesian techniques. The study population included 1055 patients (614 male and 441 female) with a mean age of 55 years. The IOM modalities performed included somatosensory evoked potential (SSEP) recording in 1055 patients, motor evoked potential (MEP) recording in 26, and electromyography (EMG) in 427. Twenty-six patients (2.5%) had significant SSEP changes. Electromyographic activity was transient in 212 patients (49.6%), and 115 patients (26.9%) had sustained burst or train activity. New postoperative neurological deficits occurred in 34 patients (3.2%): 6 had combined sensory and motor deficits, 7 had new sensory deficits, 9 had increased motor weakness, and 12 had new root deficits. Of these 34 patients, 12 had spinal tumors, of which 7 were intramedullary. Overall, of the 34 new postoperative deficits, 21 completely resolved, 9 partially resolved, and 4 had no improvement. The deficits that completely resolved did so on average 3.3 months after surgery. Patients with deficits that did not fully resolve (partial or no improvement) were followed up for an average of 1.8 years after surgery. Somatosensory evoked potentials had a sensitivity of 52%, a specificity of 100%, a PPV of 100%, and an NPV of 97%. Motor evoked potential sensitivity was 100%, specificity 96%, PPV 96%, and NPV 100%. Electromyography had a sensitivity of 46%, specificity of 73%, PPV of 3%, and an NPV of 97%. Combined neurophysiological IOM with EMG and SSEP recording and the selective use of MEPs is helpful for predicting and possibly preventing neurological injury during cervical spine surgery.

Changes in somatosensory-evoked potentials and high-frequency oscillations after paired-associative stimulation

Paired-associative stimulation (PAS), combining electrical median nerve stimulation with transcranial magnetic stimulation (TMS) with a variable delay, causes long-term potentiation or depression (LTP/LTD)-like cortical plasticity. In the present study, we examined how PAS over the motor cortex affected a distant site, the somatosensory cortex. Furthermore, the influences of PAS on high-frequency oscillations (HFOs) were investigated to clarify the origin of HFOs. Interstimulus intervals between median nerve stimulation and TMS were 25 ms (PAS(25)) and 10 ms (PAS(10)). PAS was performed over the motor and somatosensory cortices. SEPs following median nerve stimulation were recorded before and after PAS. HFOs were isolated by 400-800 Hz band-pass filtering. PAS(25) over the motor cortex increased the N20-P25 and P25-N33 amplitudes and the HFOs significantly. The enhancement of the P25-N33 amplitude and the late HFOs lasted more than 60 min. After PAS(10) over the motor cortex, the N20-P25 and P25-N33 amplitudes decreased for 40 min, and the HFOs decreased for 60 min. Frontal SEPs were not affected after PAS over the motor cortex. PAS(25/10) over the somatosensory cortex did not affect SEPs and HFOs. PAS(25/10) over the motor cortex caused the LTP/LTD-like phenomena in a distant site, the somatosensory cortex. The PAS paradigms over the motor cortex can modify both the neural generators of SEPs and HFOs. HFOs may reflect the activation of GABAergic inhibitory interneurons regulating pyramidal neurons in the somatosensory cortex.