Somatosensory evoked potentials in acute spinal cord injury: prognostic value

The somatosensory evoked potential is absent in patients with complete motor and sensory loss below the level of spinal cord injury. When spinal cord injury is incomplete, these alterations in potential may be elicited from stimulation of a nerve entering the cord below the level of injury. The presence of such potentials soon after injury, or their early return, and progressive normalization of the wave form are sensitive early indications of favorable prognosis. Indeed, recovery of the somatosensory evoked potentials frequently precedes major clinical improvement and may occur in advance of clinical recovery or posterior column function.     

Somatosensory evoked potentials in carotid surgery

During carotid surgery a monitoring device that will identify patients with inadequate cerebral perfusion and impending cerebral damage after carotid clamping is desirable. Such patients may benefit from cerebral protective measures, which should be applied selectively as their use can also lead to complications. METHODS. In order to evaluate the reliability of somatosensory evoked responses as a means of detecting patients with insufficient collateral perfusion after carotid cross clamping, a prospective study involving 482 operations for reconstruction of supraaortic vessels was performed. Somatosensory evoked potentials (SEPs) were recorded from a cervical (C2-Fz) and a parietal (C3'/C4'-Fz) electrode above the ipsilateral hemisphere following stimulation of the contralateral median nerve. RESULTS. In 22 procedures (4.6%) complete flattening of the cortical SEP occurred after carotid cross clamping. In 7 of 9 cases in which no indwelling shunt was used despite electrical silence neurological deficits were found postoperatively. The SEP amplitude was restored in 12 of the remaining 13 patients with complete loss of the SEP after shunt insertion. Only 3 of these patients demonstrated neurological impairment. During 460 operations evoked potentials were always present. Nevertheless, 5 neurological sequelae were noticed despite unchanged SEP after carotid artery clamping. All deficits, however, were caused by embolization and were unrelated to reduced blood flow after carotid cross clamping. CONCLUSIONS. Our results confirm the reliability of SEP monitoring for the detection of significant cerebral ischemia after carotid clamping. In absence of the cortical SEP immediate shunt placement is necessary to avoid neurological deficits. On the other hand, the risks attendant on indiscriminate cerebral support (embolism after shunt placement, cardiac ischemia due to induced hypertension) can be avoided in the presence of cortical potentials. This allows protection of the heart and the brain by anesthetic management and enables the surgeon to perform endarterectomy with no hurry, to avoid technical failure. SEP data may also be helpful in decision making on reoperation to look for sources of embolization. In conclusion, advanced monitoring by somatosensory evoked responses may help to improve the outcome of carotid surgery.     

Somatosensory evoked potentials in hysterical paraplegia

Somatosensory evoked potentials were determined in three patients with hysterical neurologic deficits after minor trauma. In each case the patient denied any sensation of the stimulus in the affected extremity; however, normal evoked potentials were recorded. Objective evidence of the hysterical nature of the neurologic deficit was, therefore, provided.     

Somatosensory influence on corticomotor evoked potentials

The clinical utility of corticomotor evoked potentials (CMEPs) as a method of evaluating and monitoring patients with spinal cord disorders is being intensively studied. Relatively few neuronal mechanisms responsible for waveform production are clearly known. Although CMEP components are dependent upon activity carried in descending motor pathways, somatosensory information can influence the basic waveform structure. By stimulating peripheral afferent fibers at varying frequencies, intensities, and trains, two CMEP component groups were identified based on latency. The configuration of the short-latency waveforms was influenced primarily by large-diameter afferents. Long-latency waveforms were altered primarily by small-diameter afferents. The present investigation describes both segmental and suprasegmental modification of CMEP characteristics based on afferent fiber group stimulation. If both motor and sensory systems can be accurately assessed, the clinical applications of CMEPs are considerably enhanced.     

Somatosensory evoked potentials during isoflurane anaesthesia

Short latency somatosensory evoked potentials (SEPs) to median nerve stimulation during isoflurane anaesthesia were recorded in 12 elective–surgery patients. The effect of isoflurane on the shape, amplitude and latency of SEPs was evaluated. SEPs were recorded at awake, 1 MAC, 1.5 MAC, at electroencephalogram (EEG) burst suppression and at continuous suppression levels. Finally, SEPs were recorded when anaesthesia was lightened back to 1 MAC. The peak latency and amplitude of the first cortical N₂₀ wave were measured. The latencies increased with increasing isoflurane concentrations. At high concentrations only an almost monophasic N₂₀ wave was recorded, reduced in shape and amplitude. No specific changes could be correlated with the burst suppression or suppression patterns. This suggests that EEG and SEP generators are differently affected with increasing isoflurane concentration. The results indicate that SEPs can also be recorded in drug–induced EEG suppression.     

Somatosensory evoked potentials in surgery of the spine

The authors report on their experience using cortical somatosensorially evoked potentials in 23 studies on 21 patients. The method is used intraoperatively when the spinal cord is at risk, for early identification of functional damage to the cord. The potentials found are classified according type of manifestation and compared with the results obtained by other authors.     

Somatosensory evoked potentials under thiopental and etomidate

The somatosensory evoked potential in response to median nerve stimulation was recorded in 42 patients during infusion of either 15 mg/kgbw thiopentone (TH) or 1 mg/kgbw etomidate (E) within 15 min and before and after injection of 0.3 mg/kgbw etomidate bolus. Cortical and cervical responses were analysed simultaneously and central conduction time (CCT) was calculated. Marked alterations of waveforms and an increase in latency of the primary cortical SEP and of CCT were observed in all patients. Infusion of TH or E was followed by a diminution of middle and long latency components. Amplitude of the cortical N20 was found to be unchanged during and after TH and to be increased after infusion or injection of E, indicating the synchronizing properties of this drug. The cervical SEP (N14) remained entirely unchanged in response to both agents. During hypnotic drug administration a pronounced increase in latencies and CCT as well as a decrease in the number of identifiable peaks has to be considered when SEP monitoring is performed intraoperatively or in intensive care treatment.     

Somatosensory evoked potentials for orthopaedic spine trauma

Monitoring somatosensory evoked potentials (SSEPs) for intraoperative assessment of spinal cord activity provides a reliable and valid measure of sensory function during manipulation of structures placing cord function at risk. We describe a multichannel technique with artifact reductions that has proved successful in 415 spine cases including 146 posttraumatic injuries. Accurate prediction of sensory function in near or at 100% of cases is possible. No patient has recovered with less than the predicted sensory function. Statistics and cases are presented. A professional-level consultant role for SSEP monitoring is suggested as necessary for valid use of the technique.     

Somatosensory evoked potentials in cerebral aneurysm surgery.

Monitoring of median nerve somatosensory evoked potentials (SSEP) during surgery for a basilar artery aneurysm under moderate hypothermia revealed an unexpected loss of the first cortical peak. This was due to compression of the middle cerebral artery under the retractor during the surgical approach to the aneurysm and would have continued unnoticed for some time in the absence of monitoring, possibly resulting in infarction in the territory of the middle cerebral artery. When the surgeon was alerted the artery was released and the evoked potential returned within about 24 min. There was no new focal neurological deficit postoperatively.     

Somatosensory evoked potentials and neurological grades as predictors of outcome in acute spinal cord injury

An analysis of Motor Index score, pinprick sensory score, joint position sense score, somatosensory evoked potential (SSEP) grade in the ulnar (SSEPu) and posterior tibial (SSEPt) regions, and overall SSEP grade (mean SSEPu + t) was conducted in 36 patients with cervical spinal cord injuries to determine the relationship of these scores, both individually and in combination, to functional outcome (as determined using the Barthel Index) at 6 months after injury. The clinical and electrophysiological data were obtained on the same day within 2 weeks after injury. Nineteen patients underwent two SSEP tests 1 week apart within the first 3 weeks following injury in an attempt to identify mean SSEPu + t improvement. Somatosensory evoked potential grading was based on the presence or absence of the cortical evoked potential, the amplitude of the early cortically generated waveform (P22 or P37), and the interpeak latency across the lesion site. Mean SSEPu + t had the strongest individual relationship with outcome (R-square 0.75, p less than 0.0001) and mean SSEPu + t improvement over a 1-week interval during the first 3 weeks after injury was associated with Motor Index score improvement over a 6 month period. Joint position sense score was the best clinical predictor of outcome (R-square 0.64, p less than 0.0001). Mean SSEPu + t correlated with outcome more closely than the combination of Motor Index score and pinprick sensory score. Mean SSEPu + t in combination with all three clinical indicators produced the strongest correlation with outcome (R-square 0.87, p less than 0.0001). This study confirms the prognostic value of quantitative SSEP analysis for patients with acute spinal cord injuries.     

Somatosensory evoked potentials aiding the diagnosis of brain death

SEP were recorded in 14 patients, who fulfilled the clinical and electroencephalographic criteria of brain death. The results are compared with the respective ones in healthy subjects. Beside the absence of cortical N 20 in each brain dead patient, reduction of amplitude or absence of near field negativity (N 13b) from upper neck regardless of the position of the reference electrode represents the predominant result. The near field potential from the lower neck (N 13a) was unaffected. The counterpart in the far field potential recorded from F z was amplitude reduction of P 13. These results suggest that the dissociation of N 13a and N 13b can confirm the diagnosis of brain death. Moreover these results support the view of two independent generators of N 13a and N 13b despite their identical amplitude and latency.     

Somatosensory evoked potentials during hypoxia and hypocapnia in conscious humans

Purpose

The objective of the study was to evaluate the effects of moderate hypoxia and hypocapnia on the latency and amplitude of cortical somatosensory evoked potentials (SSEPs) in conscious human subjects.

Methods

In ten volunteers the amplitude and latency of the cortical somatosensory evoked potentials were recorded during stimulation of the left posterior tibial nerve. Measurements of SSEPs and respiratory variables were made breathing ambient air, air containing a reduced oxygen percentage (17% O₂, 14% O₂(n = 6) or 11% O₂ (n = 10)), and again during voluntary hyperventilation breathing ambient air (PEtCO₂ = 20 mmHg, n = 10).

Results

Hypoxia (11% O₂) caused mild stimulation of ventilation (P < 0.05) but had no effects on the latency or amplitude of the SSEP. Lesser degrees of hypoxia had no effects. Hyperventilation caused a small (2–4%) decrease) in the latency of the SSEP and an increase in the amplitude of the SSEP (P< 0.05).

Conclusions

These findings in conscious subjects were consistent with previous observations in anaesthetized humans and anaesthetized dogs and show that the decrease in latency of the SSEP associated with hypocapnia is not due to changes in the depth of anaesthesia. These effects of hypocapnia may contribute to small variations in the latency of the SSEP when monitoring is performed during surgery, but are unlikely to be large enough to be of clinical concern.     

对青少年特发性脊柱侧弯患者的体感诱发电位检查

本体感觉神经传导异常被认为与青少年特发性脊柱侧弯有关。用体感诱发电位检查在青少年特发性脊柱侧弯患者中是否合并存在本体感觉传导通道的功能异常。研究包括147例青少年特发性脊柱侧弯患者及31位同年龄分布正常对照。对每一位受试者检查胜后神经体感皮质诱发电位，电信号缺失、传导时间延长或双侧传导时间不对称为本体感觉传导通道结构性异常诊断指标。在脊柱侧弯患者中有7人电信号单或双侧缺失，其余140例脊柱侧弯患者中10例传导时间延长，其中4例双侧延长，6例单侧延长。结果证实，部分青少年特发性脊柱侧弯患者同时有本体感觉传导异常，提示青少年特发性脊柱侧弯可进一步分为有本体感觉传导异常及无异常两组。