Intraoperative Somatosensory Evoked Potential Changes Research Articles

Intraoperative neurophysiological monitoring during urgent surgical extracranial internal carotid artery recanalization

ObjectiveThe clinical outcome of surgical extracranial internal carotid artery (eICA) recanalization may be adversely affected by intraoperative ischemia. Median nerve somatosensory evoked potential (SEP) amplitude correlates well with cerebral blood flow.Our study presents the value of intraoperative SEP and selective shunting in the prevention of intraoperative ischemia development during urgent eICA recanalization. MethodsProspective recruitment of patients with acute unilateral eICA occlusion. All underwent surgical recanalization with intraoperative monitoring of scalp median SEPs. Preoperative clinical findings, cerebral collaterals, and 3 month functional outcome were evaluated. ResultsThe cohort consisted of 33 patients. Intraoperative SEP amplitude decreased significantly in 6 (18.2%). An intraluminal shunt was inserted twice (6.1%), surgical complications occurred in 6 (18.2%), intracerebral hemorrhage was not found.Favorable outcome 3 months after surgery according to the modified Rankin scale (mRS 0–2) was achieved in 28 (84.8%), 3 patients died (9.1%). ConclusionsIntraoperative SEP during urgent eICA recanalization seems to be beneficial.Thanks to the effective measure based on the intraoperative SEP changes, the clinical outcome in four(12.1%) could be positively affected. SignificanceThe results suggest that selective shunting based on intraoperative median SEPs may prevent intraoperative ischemia and may improve overall outcome of urgent eICA recanalization.

Arterial collateral anatomy predicts the risk for intra-operative changes in somatosensory evoked potentials in patients undergoing carotid endarterectomy: a prospective cohort study

BackgroundDuring carotid endarterectomy (CEA), significant amplitude decrement of somatosensory evoked potentials (SEPs) is associated with post-operative neurological deficits.ObjectiveTo investigate the association between an incomplete circle of Willis and/or contralateral ICA occlusion and subsequent changes in intra-operatively monitored SEPs.MethodsWe performed a retrospective analysis of a single center, prospective cohort of consecutive patients undergoing CEA over a 42-month period after reviewing the collateral arterial anatomy on pre-operative radiological imaging. The primary endpoint was an intra-operative decline in SEPs > 50% compared to the baseline value during arterial cross-clamping. Univariate and multivariate logistic regression analyses were performed to investigate a potential association between contralateral ICA occlusion, incomplete circle of Willis, and subsequent alteration in SEPs.ResultsA total of 140 consecutive patients were included, of which 116 patients (82.9%) had symptomatic carotid stenosis of at least 50% according to the classification used in the North American Carotid Surgery Trial (NASCET) (Stroke 22:711–720, 1991). Six patients (4.3%) showed contralateral ICA occlusion, 22 patients (16%) a missing/hypoplastic anterior communicating artery (Acom) or A1 segment, and 79 patients (56%) a missing ipsilateral posterior communicating artery (Pcom) or P1 segment. ICA occlusion and missing segments of the anterior circulation (missing A1 and/or missing Acom) were associated with the primary endpoint (p = 0.003 and p = 0.022, respectively).ConclusionContralateral ICA occlusion and missing anterior collaterals of the circle of Willis increase the risk of intra-operative SEP changes during CEA. Pre-operative assessment of collateral arterial anatomy might help identifying patients with an increased intra-operative risk.

Peripheral Nerve Injury in Thoracic Surgery Detected by Automated Somatosensory Evoked Potential Monitoring.

Study Objective. Our objectives were to estimate the incidence of symptoms of peripheral nerve injury (sPNI) in thoracic surgical patients undergoing video-assisted thoracic surgery or open thoracotomy and to determine whether intraoperative somatosensory evoked potentials (SSEPs) waveform changes correlate with postoperative peripheral neuropathic symptoms. Methods. We conducted a prospective observational study in the operating room of a tertiary hospital. We measured SSEPs intraoperatively and assessed patients for sPNI postoperatively. Results. Forty-four patients consented. Six were excluded from analysis. We found that 42% (95% confidence interval [CI] = 26% to 57%) of patients undergoing thoracic surgery had significant changes in SSEP amplitude and latency. Furthermore, 16% (95% CI = 4% to 28%) of patients had new postoperative symptoms of sensory or motor deficits in an upper extremity. We calculated a sensitivity of 66.7% (95% CI = 29.0% to 100%) and a specificity of 50% (95% CI = 33% to 67.3%) for the identification of sPNI based on automated intraoperative SSEP changes. Conclusions. We identified the incidence of SSEP changes in thoracic surgery (42%) and the incidence of postoperative sPNI after thoracic surgery (16%). We identified a positive correlation between intraoperative SSEP changes and postoperative sPNI, which after multivariate analysis was not significant given the small sample size of the study. By the time sensory and/or motor changes are detected postoperatively, it may be too late to reverse the nerve damage. Future versions of the EPAD device could provide anesthesiologists a way to monitor for the development of sPNI, and make changes before a potential injury becomes permanent.

Somatosensory Evoked Potentials as a Stand-Alone Tool During Spine Surgery: An Egyptian Preliminary Report.

Monitoring of somatosensory evoked potentials (SSEPs) serves as an early warning system to detect spinal cord injury and is correlated with postoperative sensory findings. It is an indirect indicator of motor function. This study aimed to evaluate the usefulness of intraoperative SSEPs monitoring as a stand-alone tool during spinal surgeries when motor evoked potentials are not available, to prevent and predict new postoperative neurologic deficits. Motor evoked potentials were not used as the equipment needed to record them was not available at the time of this study. This study included 50 patients, aged 14 to 67 years, undergoing extramedullary manipulations, decompression of an epidural abscess or neoplasm, removal of intramedullary tumor, or arteriovenous malformation or spine correction procedures. Somatosensory evoked potentials were analyzed for latency and peak-to-peak amplitude. Critical SSEP changes were defined as a 50% decrease in amplitude or a 10% increase in latency. Somatosensory evoked potentials had an overall sensitivity of 81.8%, a specificity of 100%, a positive predictive value of 100%, and a negative predictive value of 91.3%. Intraoperative SSEPs have proved to be highly sensitive and specific for iatrogenic injury, mechanical stress caused by cord traction/compression, dural traction, lowered systemic blood pressure, and cord hypothermia. The reversibility of intraoperative SSEP changes showed a highly significant relation to the number of cases with new postoperative deficits as well as type and site of pathologic study (P = 0.00, P = 0.01, and P = 0.00, respectively) but not with the level of pathologic study (P = 0.49).

Perioperative stroke after cerebral aneurysm clipping: Risk factors and postoperative impact

Stroke is a devastating complication after intracranial aneurysm clipping. Understanding the risk factors that prognosticate perioperative stroke may help to identify patients that would benefit from neuroprotective therapy. This study assesses patient-specific independent predictors of perioperative stroke in relation to surgical aneurysm clipping. Additionally, this study evaluates the postoperative complications of stroke. We performed a retrospective chart review of 437 patients with ruptured and unruptured intracranial aneurysms, which underwent surgical clipping from 2006 to 2013. Multivariate logistical regression was utilized to assess the effect of age, race, gender, subarachnoid hemorrhage, Hunt and Hess (H/H) grade, aneurysm location, and intraoperative somatosensory evoked potentials (SSEPs) changes on the frequency of perioperative stroke. Thirty-five (8.0%) patients developed a stroke within 24h postoperatively. Patients with significant intraoperative SSEP changes were 7.33 (95% confidence interval [CI]: 3.51–15.31) times more likely to develop perioperative strokes. In patients who presented with H/H grade 5, the odds ratio for developing perioperative stroke was 9.21 (95% CI: 1.28–66.13) respectively. In the absence of aneurysm rupture, patients presenting with new-onset stroke were more likely to suffer postoperative complications, stay in the intensive care unit longer, and be discharged to in-patient rehabilitation compared to patients without new-onset stroke. This study suggests that severity of subarachnoid hemorrhage based on the patient’s clinical condition increases the risk of perioperative stroke in patients with surgical aneurysm clipping. SSEP changes and high-grade H/H scores can serve as independent predictors of perioperative stroke, with the latter having the greatest predictive value.

Somatosensory Evoked Potentials and Electroencephalography during Carotid Endarterectomy Predict Late Stroke but not Death

Late stroke and death rates are anticipated to be higher in patients undergoing carotid endarterectomy (CEA) compared with healthy counterparts. However, little is known regarding predictors other than the baseline comorbidities. We have recently shown that dual intraoperative somatosensory evoked potentials (SSEPs) and electroencephalography (EEG) monitoring improves the ability to predict perioperative strokes. We seek to determine if dual intraoperative monitoring (IOM) can further predict long-term strokes and death. Consecutive patients who underwent CEA under dual SSEP and EEG IOM between January 1, 2000 and December 31, 2010 were analyzed. Patients were divided in 2 groups, those with and those without IOM changes. IOM changes were classified as either occurring during carotid cross-clamp placement or at any time during the operation. End points were time to stroke and death. Log-rank tests and Cox regression analysis were used to identify predictors of postoperative stroke and death. A total of 853 CEAs (mean age 70.6±9.5years, 58.7% male, 38.9% symptomatic) were performed during the study period with a mean clinical follow-up of 48±38months. One hundred seven patients (13.6%) had significant SSEP or EEG changes at the time of clamping, while considerably more patients (217, 25.4%) had SSEP and/or EEG changes recorded at any point during the procedure, including clamping. Baseline characteristics including rates of bilateral disease, statin use, and antiplatelet use were similar between groups. Female gender, symptomatic disease, and significant contralateral carotid stenosis were more frequent in the group with IOM changes. The overall stroke-free survival rate at 5years was significantly higher in patients without IOM changes (94.7% vs. 88.2%, P<0.05) and at 10years (86.1% vs. 78.0%, P<0.05). Despite differences in stroke-free survival, overall survival at 10years was not different between groups (44.0% in patients with IOM changes vs. 42.8% in patients without, P=0.7). Renal insufficiency (hazards ratio [HR] 2.13, P=0.03), diabetes (HR 1.84, P=005), and age > 80 at the time of operation (HR 3.24, P=0.001) were significant predictors of late stroke, while statins were significantly protective (HR 0.55, P=0.05). Controlling for these factors, IOM changes (HR 2.5, P=0.004) were a strong predictor of long-term risk of stroke after CEA. Intraoperative SSEP and/or EEG changes are predictive of late stroke but not death following CEA indicating a need for further elucidation and management of the underlying risk factors driving the elevated stroke risk in this subset of CEA patients.

Causal factors for position-related SSEP changes in spinal surgery.

Somatosensory evoked potentials (SSEPs) are effective in detecting upper extremity positional injuries; however, causal factors for which patient population is most at risk are not well established. To review causal factors for intraoperative SSEP changes due to patient positioning. A case series with retrospective chart analysis was performed. 398 patient charts and intraoperative neurophysiological monitoring data from patients who underwent thoracolumbar and lumbosacral spine surgery were reviewed in a consecutive sequence from 2012 to 2013. Adverse events (AE) with the upper extremity SSEP recordings were compared to the independent variables, sex, positioning, length of procedure, and body habitus. Thoracolumbar and lumbosacral spine surgeries using contemporaneous ulnar and median nerve SSEPs were reviewed. The one-way analysis of variance (ANOVA) test, Chi-square, and independent samples t test were used to determine statistical significance in having an upper extremity SSEP AE to the aforementioned independent variables. The sample consisted of 209 males (52.5%) and 189 females (47.5%) (n=398). AE to the upper extremity SSEP was seen in 44 patients. Sex was found to be statistically significant for isolated ulnar nerve AE (P≤0.001) with males being most at risk (87.5%). AE for isolated median nerve SSEP was statistically significant for supine and prone positions (P=0.043). Length of procedure was statically significant for isolated ulnar nerve SSEP AE (P=0.039). BMI was statistically significant for generalized upper extremity SSEP AE (P=0.016), as well as isolated ulnar SSEP AE (P=0.006), isolated median SSEP AE (P≤0.001) and contemporaneous median and ulnar SSEP AE of the same limb (P≤0.001). Sex, patient positioning, length of procedure, and BMI are determinants for upper extremity neural compromise during thoracolumbar and lumbosacral spine surgeries.

Impact of different perioperative intraluminal shunt insertion methods on final patient outcomes after carotid endarterectomy in a sample of 250 patients

Background and purposeCarotid endarterectomy (CEA) is a common and effective surgical method of stroke prevention. The procedure is performed under general anesthesia and is usually accompanied by simultaneous intraoperative somatosensory evoked potential (SEP) monitoring. If a more than 50% decrease in N20/P25 SEP wave amplitude in 3 or more recordings occurs during surgery, a shunt is inserted. Shunt surgery is associated with higher risk of vessel wall injury and possible central embolization. In an effort to minimize the number of shunted patients, we modified shunt insertion timing criteria according to intraoperative SEP changes and reviewed a sample of patients for whom this modified approach was utilized. Methods250 patients (171 males, 79 females, mean age=67.00±8.55 SD, max. 86, min. 45) indicated for CEA were retrospectively enrolled in the study. Shunting criteria included long-term loss of SEP that was not affected by full anesthesia with elevated mean arterial pressure and increased sedation. Neurological complications (measured as changes in NIHSS) were recorded and compared. ResultsThe overall incidence of perioperative adverse events (i.e. stroke/death) following CEA was 2.8% (2.0 and 0.8% in 5 and 2 patients, respectively). A drop in SEP was observed in 68 cases (27.2%). Early persistent declines in cortical response amplitude that developed into complete persistent SEP amplitude loss resulted in shunt placement in 5 cases (2.0%). Perioperative neurological complications were observed in all patients and independently of intraoperative SEP response development (2.9% in patients with SEP loss vs. 2.7% in the remainder of the sample, p=0.79). ConclusionSurgery with modified shunt insertion timing demonstrated standard results. Due to the potential for vessel wall injury and embolization it is crucial to pay attention to shunt insertion timing in accordance with the individual course of surgery and intraoperative SEP development.

Diagnostic value of somatosensory evoked potential changes during carotid endarterectomy: a systematic review and meta-analysis.

Perioperative stroke is a persistent complication of carotid endarterectomy (CEA) for patients with symptomatic carotid stenosis (CS). To evaluate whether changes in somatosensory evoked potential (SSEP) during CEA are diagnostic of perioperative stroke in patients with symptomatic CS. We searched PubMed and the World Science Database for reference lists of retrieved studies and/or experiments on SSEP use in postoperative outcomes following CEA in patients with symptomatic CS from January 1, 1950, through January 1, 2013. We independently screened all titles and abstracts to identify studies that met the inclusion criteria and extracted relevant articles in a uniform manner. Inclusion criteria included randomized clinical trials, prospective studies, or retrospective cohort reviews; population of symptomatic CS; use of intraoperative SSEP monitoring during CEA; immediate postoperative assessment and/or as long as a 3-month follow-up; a total sample size of 50 or more patients; studies with adult humans 18 years or older; and studies published in English. Whether intraoperative SSEP changes were diagnostic of perioperative stroke indicated by postoperative neurological examination. Four-hundred sixty-four articles were retrieved, and 15 prospective and retrospective cohort studies were included in the data analysis. A 4557-patient cohort composed the total sample population for all the studies, 3899 of whom had symptomatic CS. A change in SSEP exhibited a strong pooled mean specificity of 91% (95% CI, 86-94) but a weaker pooled mean sensitivity of 58% (95% CI, 49-68). A pooled diagnostic odds ratio for individual studies of patients with neurological deficit with changes in SSEPs was 14.39 (95% CI, 8.34-24.82), indicating that the odds of observing an SSEP change among those with neurologic deficits were 14 times higher than in individuals without neurologic deficit. Intraoperative SSEP is a highly specific test in predicting neurological outcome following CEA. Patients with perioperative neurological deficits are 14 times more likely to have had changes in SSEPs during the procedure. The use of SSEPs to design prevention strategies is valuable in reducing perioperative cerebral infarctions during CEA.

Intraoperative Neurophysiological Monitoring in Anterior Lumbar Interbody Fusion Surgery

Somatosensory evoked potential (SSEP) and motor evoked potentials (MEP) are frequently fused to monitor neurological function during spinal deformity surgery. However, there are few studies regarding the utilization of intraoperative neuromonitoring during anterior lumbar interbody fusion (ALIF). This study presents the authors' experience with intraoperative neuromonitoring in ALIF. A retrospective review of all patients undergoing ALIF with intraoperative neuromonitoring from November 2008 to July 2013 was performed. Factors including gender, operative time, blood loss, and number and levels of interbody fusions were analyzed as risk factors for interoperational alerts. A total of 189 consecutive patients who underwent ALIFs were studied. All 189 patients had SSEP, and 131 patients had MEP as part of the intraoperative neuromonitoring in addition. The remaining 58 patients did not have MEP due to neuromuscular blockade requested by the exposure surgeon. There were no isolated intraoperative MEP changes. A total of 15 (7.9%) patients experienced intraoperative alerts. Thirteen (6.8%) of them were in SSEP. Two (1.1%) had MEP and SSEP changes together. None of these patients had new neurologic deficits postoperatively because of the surgeon's responses to the intraoperative alert. Increased risk of SSEP changes was seen in patients undergoing fusion of both L4/5 and L5/S1 (P = 0.024) and longer surgical duration (P = 0.036). No correlation was found between age and positive SSEP changes (P > 0.05). Somatosensory evoked potential changes occur relatively, frequently, and intraoperatively during ALIF. No patients with positive intraoperative SSEP changes demonstrated new postoperational deficits. Concurrent fusion of both the L4/5 and L5/S1 levels was significant risk factors for SSEP changes leading to intraoperative alerts. Operative duration and increased blood loss during surgery trended toward but did not reach statistical significance.

Impact of Changes in Intraoperative Somatosensory Evoked Potentials on Stroke Rates After Clipping of Intracranial Aneurysms

Somatosensory evoked potential (SSEP) monitoring is used during intracranial aneurysm surgery to track the effects of anesthesia, surgical manipulation, and temporary clipping. To present the outcomes of 663 consecutive patients (691 cases) treated surgically for intracranial aneurysms who underwent intraoperative SSEP monitoring and to analyze the sensitivity and specificity of significant SSEP changes in predicting postoperative stroke. Of 691 surgeries analyzed, 403 (391 anterior circulation, 12 posterior circulation) were unruptured aneurysms and 288 (277 anterior, 11 posterior) were ruptured. Postoperatively, symptomatic patients underwent computed tomography imaging. Positive predictive value, negative predictive value, sensitivity, and specificity were calculated with a Fisher exact test (2-tailed P value). Changes in SSEP occurred in 45 of 691 cases (6.5%): 16 of 403 (4.0%) in unruptured aneurysms and 29 of 288 (10%) in ruptured aneurysms. In unruptured aneurysms, reversible SSEP changes were associated with a 20% stroke rate, but irreversible changes were associated with an 80% stroke rate. In ruptured aneurysms, however, reversible changes were associated with a 12% stroke rate, and irreversible changes were associated with a 42% stroke rate. The overall accuracy of SSEP changes in predicting postoperative stroke was as follows: positive predictive value, 30%; negative predictive value, 94%; sensitivity, 25%; and specificity, 95%. Intraoperative SSEP changes are more reliable in unruptured aneurysm cases than in ruptured cases. Whereas irreversible changes in unruptured cases were associated with an 80% stroke rate, such changes in ruptured cases did not have any adverse ischemic sequelae in 58% of patients. This information is helpful during the intraoperative assessment of reported SSEP changes.

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.

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.

Retrospective study of somatosensory evoked potential monitoring in deep hypothermic circulatory arrest

Background. We evaluated the efficiency of median-nerve somatosensory evoked potentials (SEPs) monitoring in determining the optimal level of hypothermia in 62 consecutive patients operated on under deep hypothermic circulatory arrest (CA). Methods. CA was started at 1°C below the temperature at which both brainstem and cortical SEP components disappear. No additional method of cerebral protection was used. Results. New neurological complications were observed in 15 patients: long-lasting in 11 and transient in 4. A retrospective analysis of SEP monitoring identified the origin of the complications in 12 cases: early CA with incomplete cooling due to emergency (3 cases); inefficient retrograde perfusion through the femoral artery during cooling due to the dissection flap effect (4 cases); embolism during rewarming (2 cases); delayed embolism (2 cases); hemorrhagic shock (1 case). In 2 cases, neurological sequelae involved the lower limbs (extracerebral origin). One case without intraoperative SEP changes was neurologically abnormal preoperatively and did not change postoperatively. There were no cases with sequelae due to excessive CA duration. Conclusions. The use of SEP monitoring to determine the optimal level of hypothermia efficiently prevents neurological sequelae of CA. It helps in monitoring the degree of cerebral protection during cooling (flap effect), and rewarming.

Intraoperative Somatosensory Evoked Potential Findings in Acute and Chronic Spinal Canal Compromise

Early and long-term postoperative outcome were analyzed by retrospective record review and compared with results of intraoperative somatosensory evoked potential (SEP) findings in 70 patients with chronic lumbar stenosis. Adverse SEP changes occur in up to 2% of patients during scoliosis surgery and may reverse with intraoperative intervention. Little is known about the short- or long-term results of intraoperative intervention based on adverse SEP changes in patients with chronic lumbar stenosis during lumbar decompression and fusion. To compare intraoperative SEP changes with immediate and long-term neurologic outcome in patients undergoing lumbar decompression, instrumentation and fusion for chronic lumbar stenosis and to determine whether the early correlation between intraoperative SEP deterioration and clinical outcome persisted. Monitoring SEPs using an alternating arm and leg stimulation paradigm allowed rapid identification of intraoperative changes. Retrospective record review was conducted without knowledge of intraoperative SEP findings. Clinical and SEP findings were then compared, to determine whether the strong association between intraoperative SEP results and early clinical outcome persisted. Nine of 12 patients who had unilateral intraoperative SEP deterioration that resolved with intervention had no adverse sequelae; the remaining three had new ipsilateral weakness that persisted during a 9-24-month follow-up. Intraoperative SEPs deteriorated in 15% of patients with normal and abnormal baseline SEPs. Intraoperative SEP deterioration could not be predicted by preoperative radicular pain, focal symptomatology or baseline SEP findings. 80% of patients with normal SEPs but only 54% with abnormal SEPs had immediate and sustained pain relief. SEP deterioration that reversed with surgical intervention or high-dose steroids resulted in no adverse neurologic outcome. Acute, unilateral, unresolved intraoperative SEP deterioration was associated with long-term ipsilateral weakness not predicted by clinical or neurologic findings before surgery. Clinical improvement persisted in 92% of patients, 4% were unchanged, and 4% had persistent neurologic changes during an average 12-month follow-up period. The findings underscore the need for monitoring SEPs during surgery in all patients undergoing invasive lumbar surgery and for rapid identification and intervention should a unilateral SEP change persist.

Effects of hypothermia and sternal retractors on median nerve somatosensory evoked potentials.

Somatosensory evoked potentials (SEPs) are altered by hypothermia, which is often used during cardiopulmonary bypass (CPB). However, the effect of hypothermia on SHP amplitudes is unclear. Also, the sternal retractors used during open heart surgery are reported to cause brachial plexus distension and SEP changes. Median nerve SEPs under hypothermic CPB were studied in 29 elective patients scheduled for open heart surgery. In 23 patients who underwent left internal mammary artery (IMA) dissection, the effects of sternal retractors on cortical SEP before the initiation of CPB were investigated. A latency shift of all SEP components was detected when nasopharyngeal temperature decreased from 35.7(SD 0.4) degrees C to 27.8(SD 0.25) degrees C. The mean cortical N20 latency was increased by 39% (P < 0.0001), cervical N13 by 33% (P < 0.0001), and peripheral N9 by 27% (P < 0.0001). The latency changes were reversible when normothermia was restored. The effect of hypothermia on SEP amplitudes was more complex. The mean amplitude of N20 decreased from 2.7 microV to 2.2 microV (P < 0.05) and the amplitude of N13 from 2.5 microV to 2.0 microV (P < 0.0001). In contrast, the N9 component showed an increase from 1.4 microV to 2.1 microV (P < 0.0001) during hypothermia. The sternal retractors did not cause significant cortical SEP amplitude changes during IMA dissection or sternotomy. Also, the latency changes were small, although significant (P < 0.05). Despite the moderate amplitude changes produced by hypothermia, SEPs can be successfully monitored during hypothermia. Theoretically, the different behaviour of amplitude in peripheral and cranial components of SEP during hypothermia is interesting. Hypothermia has a more profound effect on synaptic transmission, represented by the cortical N20 latency, than on the peripheral nerve conduction velocity. Intraoperative monitoring of temperature is essential whenever SEPs are recorded. The sternal retractors were not responsible for the intraoperative SEP changes.

M51 Use of laser evoked potentials in the assessment of the somatosensory system

Stroke is a devastating complication after intracranial aneurysm clipping. Understanding the risk factors that prognosticate perioperative stroke may help to identify patients that would benefit from neuroprotective therapy. This study assesses patient-specific independent predictors of perioperative stroke in relation to surgical aneurysm clipping. Additionally, this study evaluates the postoperative complications of stroke. We performed a retrospective chart review of 437 patients with ruptured and unruptured intracranial aneurysms, which underwent surgical clipping from 2006 to 2013. Multivariate logistical regression was utilized to assess the effect of age, race, gender, subarachnoid hemorrhage, Hunt and Hess (H/H) grade, aneurysm location, and intraoperative somatosensory evoked potentials (SSEPs) changes on the frequency of perioperative stroke. Thirty-five (8.0%) patients developed a stroke within 24 h postoperatively. Patients with significant intraoperative SSEP changes were 7.33 (95% confidence interval [CI]: 3.51–15.31) times more likely to develop perioperative strokes. In patients who presented with H/H grade 5, the odds ratio for developing perioperative stroke was 9.21 (95% CI: 1.28–66.13) respectively. In the absence of aneurysm rupture, patients presenting with new-onset stroke were more likely to suffer postoperative complications, stay in the intensive care unit longer, and be discharged to in-patient rehabilitation compared to patients without new-onset stroke. This study suggests that severity of subarachnoid hemorrhage based on the patient’s clinical condition increases the risk of perioperative stroke in patients with surgical aneurysm clipping. SSEP changes and high-grade H/H scores can serve as independent predictors of perioperative stroke, with the latter having the greatest predictive value.

Evoked Potential Monitoring of the Upper Extremities During Thoracic and Lumbar Spinal Deformity Surgery

We prospectively investigated the usefulness of somatosensory evoked potential (SEP) monitoring to detect clinically significant peripheral nerve or brachial plexus compression in the upper extremities during spinal deformity surgery. All patients had bilateral median and ulnar nerve SEPs evaluated as a baseline, both intermittently throughout the surgical procedure and at termination of surgery. This information was correlated with pre- and postoperative upper extremity neurologic examinations. Twenty-five upper extremities in 21 patients had intraoperative SEP changes consisting of at least a 60% decline in amplitude or a 10% increase in latency. On many occasions, repositioning of the upper extremities improved the SEP responses to acceptable baselines. Overall, SEP monitoring of the upper extremities was 78% sensitive for detecting upper extremity sensory deficits, 100% sensitive for detecting combined sensory and motor deficits, and 98.5% specific for predicting normal postoperative upper extremity function. We recommend the simultaneous electrophysiologic monitoring of the upper and lower extremities during spinal surgery to allow identification of brachial plexus and/or peripheral nerve stretch/compression that may be amenable to intraoperative correction.

The efficacy of femoral nerve intraoperative somatosensory evoked potentials during surgical treatment of thoracolumbar fractures.

Somatosensory evoked potential (SEP) monitoring of thoracolumbar procedures typically includes posterior tibial and peroneal nerve recordings. The addition of femoral nerve SEP monitoring, however, should better predict the evolution of postoperative neurologic deficits affecting the midlumbar roots. To assess the value of intraoperative femoral SEPs, 26 cases of traumatic injury to the thoracolumbar spine between T12 and L4 were prospectively assessed. Twenty-four had clear femoral nerve responses bilaterally recorded from an epidural electrode. Five had significant intraoperative SEP changes: one had isolated femoral nerve changes, two had only peroneal or tibial nerve changes, and two had concomitant changes in both femoral and peroneal or tibial nerves. Loss of the femoral nerve response in one patient was correlated with marked postoperative knee extensor weakness, in spite of immediate action taken by the surgeon. The authors conclude that femoral nerve SEPs provide an effective tool to monitor the midlumbar roots intraoperatively.

Intraoperative Somatosensory Evoked Potentials Monitoring of Pelvic and Acetabular Fractures

The efficacy of intraoperative somatosensory evoked potential (SSEP) monitoring was evaluated in the surgical management of 82 patients with pelvic and acetabular fractures. The injuries consisted of 45 acetabular fractures, 30 pelvic ring disruptions, and seven combined injuries. Preoperative neurological deficits were recorded in 34% of the study group (29% of those with an acetabular fracture and 47% of those with a pelvic ring injury). Three patients sustained an iatrogenic sciatic nerve injury during the study period (all of which were documented in the first 40 cases). Two patients sustained an exacerbation of an existing sciatic nerve injury. In the group of pelvic fractures, hazardous parts of the exposure, reduction, and fixation were identified by the SSEP monitoring. Removal of the provocative stimulus by the surgeon led to reversal of the SSEP abnormalities, and none of this group of patients sustained an iatrogenic injury. When the intraoperative SSEP changes were noted during an acetabular fracture fixation, immediate attempts were made to relieve the excessive tension on the sciatic nerve by replacing or removing a retractor, flexing the knee, extending the hip, or dividing the femoral insertion of the gluteus maximus. None of the SSEP changes were associated with the lacerative injury to the sciatic nerve. For the method to be clinically effective in reducing the incidence of neurological deficit, even subtle changes in the SSEP tracing must be recognized immediately by the neurophysiologist so that a corresponding corrective measure can be rapidly undertaken by the surgeon to remove the offending stimulus.