An approach to intraoperative neurophysiologic monitoring of thoracoabdominal aneurysm surgery.

Thoracoabdominal aneurysm surgery carries an approximate 10% risk of intraoperative paraplegia. Abrupt cord ischemia and the confounding effects of systemic alterations and limb or cerebral ischemia challenges neurophysiologic spinal cord monitoring. This investigation sought a rapid differential monitoring approach to predict or help prevent paraplegia. Thirty-one patients were monitored with motor evoked potentials (MEPs) and median and tibial somatosensory evoked potentials (SSEPs). MEPs involved single-pulse transcranial electrical stimulation with D wave recording (n = 16), arm and leg muscle MEPs following multiple-pulse transcranial electrical stimulation (n = 12), or both (n = 3). D wave recordings required averaging, invasive epidural electrode insertion, and produced both false positives and false negatives. Muscle MEPs were instantaneous and reliably sensitive and specific for cord ischemia. Cortical and peripheral nerve SSEPs provided rapid detection of systemic alterations and cerebral or limb ischemia. Cord and subcortical SSEPs required excessive averaging time. In conclusion, bilateral arm and leg muscle MEPs with median and tibial peripheral nerve and cortical SSEPs provide sufficiently rapid detection and differentiation of cord ischemia from confounding factors. There were two predicted intraoperative spinal cord infarctions (6.5%) and nine circumstantial examples of possible contributions to deficit prevention.     

The role of magnetic stimulation in the diagnosis of multiple sclerosis.

Magnetic stimulation was used to measure motor conduction time (MCT) between head and neck, and head and lumbar region, as well as amplitude of the motor evoked potential (MEP) in normal subjects and patients with multiple sclerosis (MS). Patients with definite MS had significantly longer MCTs and smaller amplitude MEPs than normal subjects when recording from arm and leg muscles. In a comparison with visual evoked potential (VEP) recordings, head to neck MCTs were abnormal less often than VEPs, and VEPs detected more silent lesions. Recording from leg as well as arm muscles significantly increased the yield of abnormal MCT measurements. The detection of silent lesions in the patients with definite MS was improved, but there was no improvement in the non-definite cases. Amplitude measurements provided very little extra diagnostic information over MCT measurements alone and did not improve the detection of silent lesions. Interside MCT differences yielded extra abnormalities when recording from the arms but not the legs. Interside MCT abnormalities increased the detection of silent lesions in both the definite and non-definite categories. It was concluded that the majority of useful diagnostic information in patients with MS should be obtainable from bilateral MCT (head to neck) measurements, together with estimation of interside MCT differences. However, VEP recording is a better diagnostic test for MS than MEP recording as more silent lesions are detected. This may be because MCT abnormalities tend to reflect the degree of pyramidal disability.     

SEPs and CNS magnetic stimulation in syringomyelia.

Somatosensory evoked potentials (SEPs) and motor evoked potentials (MEPs) to transcranial and spinal stimulation from upper and lower limb muscles were elicited in 13 patients with syringomyelia. Seven had an associated Chiari type I anomaly. Diagnosis was confirmed by MRI. In 5 cases, SEPs and MEPs were performed before and after surgical treatment. Prolonged central motor conduction times or absent motor responses in upper or lower limbs were found in most patients. The greatest number of abnormalities was disclosed by measurement of CMCT followed by SEPs after tibial nerve stimulation. Two of 5 cases undergoing surgery improved clinically and showed reduction in CMCT after surgical treatment. Our study shows that MEPs were useful in the evaluation of neurophysiological status in syringomyelia patients, helping to estimate anterolateral spinal cord function.     

Neurophysiological evaluation of the central nervous impulse propagation in patients with sensorimotor disturbances

Motor evoked potentials (MEPs) to transcranial stimulation (TCS) and somatosensory evoked potentials to median nerve stimulation (MN-SEPs) were examined in 74 patients affected by multiple sclerosis (MS = 49 cases), amyotrophic lateral sclerosis (ALS = 9 cases), cervical cord lesions (7 cases), Parkinson's disease (PD = 5 cases), Huntington's chorea (HC = 2 cases), Wilson's disease (WD = 1 case), subacute combined degeneration (SCD = 1 case). MN-SEPs were altered in 38% of arms in MS with a higher incidence in clinically affected than in clinically 'silent' arms (= 77.8% vs. 27.5%). MEP alterations were found in 54% of examined arms, mostly because of a prolongation of the motor CCT. This index was invariably altered in the affected arms, whilst it was involved in 40% of the 'silent' ones. Twelve out of 18 arms displayed abnormal MEPs in ALS. These were mainly due to an absent response, even if moderate motor CCT prolongation and 'giant' MEPs were also encountered. MN-SEPs were altered in 3/18 arms. By recording MEPs from proximal and distal upper limb muscles, cues on the level of abnormal propagation were obtained in patients suffering from 'focal' lesions of the spinal cord. Combining SEP records enhanced the diagnostic yield in this field. Both MEPs and SEPs were normal in patients with PD and HC, whilst abnormally prolonged CCTs were found in the case with WD. MEP and SEP recording revealed central propagation abnormalities coupled to a severe clinical picture of the peripheral nerve involvement (as in the case of SCD).     

Direct cortical stimulation but not transcranial electrical stimulation motor evoked potentials detect brain ischemia during brain tumor resection

Motor evoked potentials (MEPs) elicited by both direct cortical stimulation (DCS) and transcranial electrical stimulation are used during brain tumor resection. Parallel use of direct cortical stimulation motor evoked potentials (DCS-MEPs) and transcranial electrical stimulation motor evoked potentials (TCeMEPs) has been practiced during brain tumor resection. We report that DCS-MEPs elicited by direct subdural grid stimulation, but not TCeMEPs, detected brain ischemia during brain tumor resection. Following resection of a brainstem high-grade glioma in a 21-year-old, the threshold of cortical motor-evoked-potentials (cMEPs) increased from 13 mA to 20 mA while amplitudes decreased. No changes were noted in transcranial motor evoked potentials (TCMEPs), somatosensory evoked potentials (SSEPs), auditory evoked potentials (AEPs), anesthetics, or hemodynamic parameters. Our case showed the loss of cMEPs and SSEPs, but not TCeMEPs. Permanent loss of DCS-MEPs and SSEPs was correlated with permanent left hemiplegia in our patient even when appropriate action was taken. Parallel use of DCS- and TCeMEPs with SSEPs improves sensitivity of intraoperative detection of motor impairment. DCS may be superior to TCeMEPs during brain tumor resection.     

Motor evoked potentials (MEPs) in lacunar syndromes.

Motor evoked potentials (MEPs) evoked in the biceps, thenar and tibialis anterior muscles by electrical stimulation of the scalp and of the spinal regions were recorded in 32 patients with focal deficits due to minor cerebral ischemia of the lacunar type and in a control group. Somatosensory evoked potentials (SEPs) to median nerve stimulation were also recorded. The central motor conduction times (CMCTs) and the threshold intensities for eliciting MEPs in the relaxed muscles were significantly increased on the affected side. Central motor conduction, for at least one muscle, was altered in 18 patients. MEP abnormalities were related to pyramidal signs (though they could be observed also in a patient without any motor impairment) and occurred independently of a specific clinical picture or a radiologically confirmed lacunar lesion. SEPs were less frequently altered than MEPs.     

Neurophysiological study in Pelizaeus-Merzbacher disease

The neurophysiological characteristics of Pelizaeus-Marzbacher disease (PMD) were studied in four Japanese patients aged between 5 and 13 years. Pendular spontaneous nystagmus was always recorded with a frequency between 2.5 and 4 Hz, and abnormal saccades with an almost twofold prolongation in onset time and 50% decrease in velocity were noted. Brainstem auditory evoked potentials consistently demonstrated severely altered waves II to V, following a normal wave I, despite normal hearing acuity. Somatosensory evoked potentials (SEPs) were always absent between brainstem components and early cortical responses. Late cortical components of SEPs and visual evoked potentials with significantly prolonged latencies were recorded in the three younger cases having normal sensory and visual acuity (N35 of SEP, 73.1 ± 2.1 ms; N75 of VEP, 129.0 ± 12.7 ms; mean ± S.D.), while these peaks were absent in the oldest case having the most severe handicap. In motor evoked potentials (MEPs), R1 of blink reflex with significantly prolonged latency (14.9 ± 1.48 ms) was always obtained, and no subsequent R2 was elicited. Magnetic transcortical stimulation elicited no MEPs of the thenar even in the facilitating condition on voluntary contraction despite mild weakness of the thenar, while normal MEPs were always elicited on cervical stimulation. These electrophysiological findings were consistent with extensive conduction slowing involving the brainstem to the cerebrum, which seemed to be accompanied by conduction block in motor systems rather than sensory systems. Although each of the results was not specific, in combination they suggested the characteristics of diffuse brain dysmyelination in PMD.     

Evaluation of afferent pain pathways in adrenomyeloneuropathic patients

Objective

Patients with adrenomyeloneuropathy may have dysfunctions of visual, auditory, motor and somatosensory pathways. We thought on examining the nociceptive pathways by means of laser evoked potentials (LEPs), to obtain additional information on the pathophysiology of this condition.

脊髓型颈椎病病人经颅磁电刺激运动诱发电位的对比研究

目的探讨磁电刺激运动诱发电位（ＭＥＰ）在脊髓型颈椎病（ＣＳＭ）的应用价值，并对其临床相关性进行分析。方法采用经颅磁、电刺激对３０例脊髓型颈椎病病人以及年龄性别等相配匹的３０名健康成人分别于外展小指肌、肱二头肌及下肢展短肌表面进行ＭＥＰ的检测。结果全部病人的ＭＥＰ都出现异常，表现为潜伏期、中枢传导时间（ＣＭＣＴ）延长，时限增宽，波辐降低或不能引出。磁刺激ＭＥＰ的ＣＭＣＴ和皮层刺激潜伏期与脊髓型颈椎病临床日本整形外科协会（ＪＯＡ）评分间有密切相关性，能较好地反映ＣＳＭ病人的病情。结论ＭＥＰ在检测ＣＳＭ病人运动功能方面具有定量评价作用。与电刺激相比，磁刺激ＭＥＰ能更好地反映ＣＳＭ病人的病情。     

Value of somatosensory and motor evoked potentials in predicting arm recovery after a stroke

OBJECTIVES: Prediction of motor recovery in the arm in patients with stroke is generally based on clinical examination. However, neurophysiological measures may also have a predictive value. The aims of this study were to assess the role of somatosensory (SSEPs) and motor (MEPs) evoked potentials in the prediction of arm motor recovery and to determine whether these measures added further predictive information to that gained from clinical examination. METHODS: Sixty four patients who had had a stroke and presented with obvious motor deficit of the arm were examined in terms of three clinical variables (motor performance, muscle tone, and overall disability) and for SSEPs and MEPs. Clinical and neurophysiological examinations were done at entry to the study (2 to 5 weeks poststroke), and at about 2 months after stroke. Further clinical follow up was conducted at 6 and 12 months after stroke. RESULTS: Neurophysiological measures made in the acute phase were of little use alone in predicting motor recovery of the arm at 2, 6, and 12 months after stroke. At 2 months, the absence of SSEPs and MEPs indicated a very poor outcome. Conversely, if the responses were preserved, a great variation in motor outcome was found. Multiple regression analysis showed that the addition of SSEPs and MEPs to the clinical examination increased the possibility of predicting arm recovery in the long term. In the acute phase, the combination of the motor score and SSEPs were best able to predict outcome. The long term outcome based on variables taken at 2 months, was best predicted through incorporating the three clinical measures and MEPs. CONCLUSIONS: Neurophysiological measures alone are of limited value in predicting long term outcome. However, predictive accuracy is substantially improved through the combined use of both of these measures and clinical variables.     

Neurophysiological assessment of the motor and sensory spinal pathways in chronic spinal cord injury

Introduction of transcranial magnetic stimulation (TMS) has provided means to study non-invasively corticospinal functions in humans. The purpose of the present study was to obtain an objective evaluation of spinal cord functions in spinal cord injury (SCI) subjects using TMS, multichannel surface EMG and somatosensory-evoked potentials (SSEP). Multichannel surface EMG recording was performed during reinforcement manoeuvres and during vibratory tonic reflex. Twenty-five post-traumatic clinically incomplete (ambulatory, AMB, and non-ambulatory, nAMB) SCI subjects were studied and compared to a control group of seven subjects. After preliminary analysis of neurophysiological studies they were divided into four groups according to presence or absence of motor-evoked potentials (MEP) in response to TMS in muscles below the level of the lesion and according to their ability to ambulate. TMS was delivered at vertex at 100% intensity and recorded from the large muscles of the upper and lower limbs. Surface EMG was recorded during reinforcement manoeuvres (RM) in the leg muscles and EMG activity was scored. SSEP were recorded at T12, L2, L4 and SI spinous processes and at Cz' on the scalp following tibial nerve stimulation at popliteal fossa. The prevalence of EMG responses during RM was higher in group with present MEPs (AMB/MEP+ and nAMB/MEP+) than in the group without MEPs. The group with present MEPs also showed better preserved functions of the ascending tracts compared to subjects without MEPs. Groups with present MEPs had 5/10 normal, 2/10 abnormal and 3/10 absent cortical SSEPs, whereas groups without MEPs showed 1/11 normal, 4/11 abnormal and 6/11 absent cortical SSEPs. Sustained function of ascending tracts was also positively correlated with preserved ability to ambulate. It was concluded that TMS in combination with multichannel surface EMG monitoring and sensory evoked potentials may prove feasible in assessing the functional capacity of the spinal cord after spinal cord lesion.     

Utility of somatosensory evoked potentials in chronic acquired demyelinating neuropathy

Chronic acquired demyelinating polyneuropathy (CADP) is a heterogeneous syndrome that may be classified into a number of subtypes. Somatosensory evoked potentials (SSEPs) assess proximal segments of sensory nerves, inadequately assessed by routine nerve conduction studies (NCSs). The aim of the present study was to determine the utility of SSEPs in diagnosing and classifying different CADP subtypes. Forty-seven patients with CADP were studied and classified in five groups based on conventional NCSs and SSEPs. Some patients in Group 1 were initially misdiagnosed as having either motor neuron disease or multifocal motor neuropathy due to normal sensory NCSs, but they exhibited abnormal tibial and median nerve SSEPs, as evidenced by marked prolongation or absence of peripheral potentials (N9-median nerve, and N20-tibial nerve). These were reclassified as having chronic inflammatory demyelinating neuropathy (CIDP). In CIDP patients (Group 2), SSEPs were abnormal, thereby confirming the presence of demyelination in the proximal peripheral nerves. Patients with distal acquired demyelinating neuropathy (DADS) (Group 3), as defined by conventional NCS, exhibited abnormal SSEPs when anti-MAG antibodies were present. Anti-MAG-negative DADS patients (Group 3) had normal SSEPs. In the pure sensory ataxic group (Group 4), SSEP studies disclosed poorly formed and delayed cortical potentials with absent lumbar (N20) potentials, thereby suggesting the presence of proximal demyelination. SSEPs were normal in the pure motor CIDP and multifocal motor neuropathy patients (MMN) (Group 5), thereby differentiating asymmetric forms of CIDP from MMN. These findings suggest that SSEPs may be an important complementary investigation to conventional NCSs in the diagnosis of CADP.     

Electrophysiological studies in cerebrotendinous xanthomatosis.

Seven patients with cerebrotendinous xanthomatosis (CTX) were studied by electrophysiological techniques. The percentages of abnormalities detected in nerve conduction studies and electroencephalograms were 28.6% (two patients) and 100%, respectively. All patients showed prolonged central conduction times in short latency somatosensory evoked potentials (SSEPs) by tibial nerve stimulation but normal SSEPs by median nerve stimulation. Brain stem auditory evoked potentials and visual evoked potentials were abnormal in three (42.9%) and four patients (57.1%), respectively. These electrophysiological parameters were correlated with the ratio of serum cholestanol to cholesterol concentration. The results of SSEPs suggest that the polyneuropathy in CTX is caused by distal axonopathy affecting longer axons before shorter axons (central-peripheral distal axonopathy).     

Transcranial magnetic stimulation in patients with cervical spondylotic myelopathy: Clinical and radiological correlations

Motor evoked potentials (MEPs) were studied in 28 patients with cervical spondylotic myelopathy. MEPs after cortical stimulation were abnormal in 27 patients, the responses in the leg muscles being affected the most often. Clinically asymptomatic motor lesions were detected in 7 patients (25%). The central motor conduction time (CMCT) for the abductor digiti minimi muscles correlated significantly with the clinical disability, whereas the radiological findings did not correlate with the clinical and neurophysiological parameters. In 9 patients MEPs were also recorded in the biceps muscles. The 7 patients with an abnormal CMCT for the biceps muscles had the most severe stenosis at the C-4–C-5 level or higher. The 2 patients with normal MEPs of the biceps muscles both had a stenosis at the C-5–C-6 level. The results of this study suggest that MEPs are useful for detecting spinal cord dysfunction and for localizing the level of the lesion. Some recommendations regarding the possible use of MEPs in the clinical evaluation of patients with cervical spondylotic myelopathy are given. © 1994 John Wiley & Sons, Inc.     

Dissociation of somatosensory and motor evoked potentials in non-comatose patients after head injury.

OBJECTIVES: This study was performed to evaluate the clinical value of combined use of somatosensory evoked potentials (SEPs) and motor evoked potentials (MEPs) in patients with different brain lesions after head trauma. METHODS: A total of 64 patients with minor and moderate head injury were investigated by means of SEPs recorded over the parietal and frontal areas and MEPs following single-pulse transcranial magnetic stimulation (sTMS) and slow-rate repetitive transcranial magnetic stimulation (rTMS). RESULTS: In almost 50% of the patients, a dissociated impairment of somatosensory and motor evoked potentials was found. This dissociation was related to different distribution of SEP and MEP abnormalities in head injury subgroups. The higher threshold to sTMS and increased variability of the MEP amplitude during slow-rate rTMS were the most prominent features in patients with focal brain contusions, suggesting impairment of the cortical excitability. SEP abnormalities, as well as central conduction impairments, were more noticeable in patients with diffuse brain injury. CONCLUSIONS: A combined analysis of SEPs and MEPs may improve the assessment of cortical dysfunctions and central conduction abnormalities in non-comatose patients with head injury. A slow-rate rTMS may be considered as a complementary technique to the evaluation of the threshold in assessment of the excitability of the motor cortex in minor and moderate head injury.     

Electrophysiological abnormalities in cases of dystrophia myotonica.

EMG, nerve conductions, repetitive nerve stimulation and SSEPs were studied in 14 patients with dystrophia myotonica. Fibrillation potentials were seen in two of these patients. Motor nerve conduction velocities of the medial popliteal nerve were decreased in 4 of the 13 patients studied. Three of these patients also showed decrease in the antidromic sensory conduction velocities. Motor conduction velocities of the lateral popliteal nerve were similarly reduced in four patients. Taking both the nerves into consideration, 6 of the 14 patients showed a decrease in the motor nerve conduction velocities of either, or both, of these nerves. Repetitive nerve stimulation showed abnormal responses in 13 of these 14 cases. SSEPs from stimulation of the lateral popliteal nerve showed increased latencies in three of the ten cases studied. Sensory conduction velocities as measured from SSEPs were decreased in three of the four cases. SSEPs for the medial popliteal nerve showed similar values. The significance of these abnormalities has been discussed.     

Ellen R. Grass Lecture: Motor evoked potential monitoring

Because somatosensory evoked potentials (SSEPs) recorded from the brain are carried solely within the dorsal columns of the spinal cord, SSEP monitoring may fail to detect damage to the spinal cord motor pathways, and techniques for directly monitoring the motor pathways have been developed. Transcranial magnetic brain stimulation is useful for extraoperative evaluation of the motor system, but anesthetic effects on cortical synaptic activity limit its usefulness for intraoperative monitoring. Another proposed monitoring technique, rostral spinal cord stimulation with recording from peripheral nerves, predominantly reflects retrograde conduction within the dorsal columns; like SSEPs, this technique may fail to detect motor pathway damage. The true motor evoked potential (MEP) monitoring technique preferred by most institutions involves transcranial electrical brain stimulation. This stimulates the corticospinal tract axons directly, producing a D-wave in them. There may be additional corticospinal tract volleys, reflecting indirect activation of the pyramidal cells via synapses from cortical interneurons; these I-waves are largely suppressed by anesthesia. Temporal summation at the alpha motor neuron synapse is necessary to elicit myogenic MEPs (M-waves) under anesthesia; this is accomplished with brief trains of stimuli that produce multiple pyramidal tract volleys. High stimulus intensities are required to stimulate the brain through an intact skull; with proper precautions, this is acceptably safe. Myogenic MEPs are quite sensitive to anesthesia and show considerable run-to-run variability. Therefore, criteria for evaluating MEPs differ from those for SSEPs, and the anesthetic regimen and degree of neuromuscular blockade must be carefully controlled. SSEPs should be monitored concurrently with MEPs, because the dorsal columns may be affected without compromise of the motor tracts, and also because this provides redundancy, with two independent assessments of spinal cord function.     

Evoked potentials in HIV infection]

The study of the literature data on the multimodal evoked potentials in HIV infected patients shows many abnormalities as well in asymptomatic subjects without AIDS as in AIDS subjects with or without neurological signs. Visual evoked potentials (VEPs) reveal prolonged P100 wave latency in 22% of HIV asymptomatic subjects and in 26% of HIV symptomatic subjects; brainstem auditory evoked potentials (BAEPs) reveal an increase of the interpeak latency I-V in 16% of asymptomatic subjects and in 32% of symptomatic subjects; somatosensory evoked potentials (SEPs) by median nerve stimulation reveal prolonged central conduction time in 6% of asymptomatic subjects and in 11% of symptomatic subjects; somatosensory evoked potentials (SEPs) by tibial nerve stimulation reveal prolonged central conduction time in 4% of asymptomatic subjects and in 45% of symptomatic subjects; motor evoked potentials (MEPs) by magnetic stimulation reveal prolonged central motor conduction time in 46% of asymptomatic subjects.     

Neurophysiologic intraoperative monitoring in children with Down syndrome

Objective

Neurophysiological monitoring during complex spine procedures may reduce risk of injury by providing feedback to the operating surgeon. This tool is a well-established and important surgical adjunct in adults, but clinical data in children are not well described. Moreover, to the best of our knowledge, neurophysiologic intraoperative monitoring data have not been reported in children with neurodevelopmental disorders, such as Down syndrome, who commonly present with craniocervical instability requiring internal fixation. The purpose of this study is to determine the reliability and safety of neurophysiologic intraoperative monitoring in a group of children with Down syndrome undergoing neurosurgical spine procedures.

Methods

A total of six consecutive spinal procedures in six children with Down syndrome (three boys and three girls; mean age 10 years, range 4–16 years) were analyzed between January 1, 2008 and June 31, 2011. Somatosensory evoked potentials were stimulated at the ulnar nerve and tibial nerve for upper and lower extremities, respectively, and recorded at Erb’s point and the scalp. Motor evoked potentials were elicited by transcranial electrical stimulation and recorded at the extensor carpi ulnaris muscle and tibialis anterior muscle for upper and lower extremities, respectively. A standardized anesthesia protocol for monitoring consisted of a titrated propofol drip combined with bolus dosing of fentanyl or sufentanil.

Results

Somatosensory and motor evoked potentials were documented at the beginning and end of the procedure in all six patients. Changes during the surgery were recorded. Five patients maintained somatosensory potentials throughout surgery. One patient demonstrated a >10 % increase in latency or >50 % decrease in amplitude suggesting spinal cord dysfunction. A mean baseline stimulation threshold for motor evoked potentials of 485?+?85 V (range 387–600 V) was used. Four patients maintained motor evoked potentials throughout surgery. One patient had loss of left lower somatosensory evoked potentials (SSEPs) and motor evoked potentials (MEPs) after rod placement; upon removal of the rod, SSEPs returned but not MEPs. Another patient did not have consistent MEPs on one side and had absent MEPs on the contralateral side throughout the case. Loss of MEPs in these two patients did not correlate with postoperative neurological status. There were no complications directly related to neurophysiologic intraoperative monitoring technique.

Conclusions

Neurophysiologic intraoperative monitoring during neurosurgical procedures in children with Down syndrome may be reliably and safely implemented. Changes in neurophysiologic parameters during surgery must be carefully interpreted, and discussed with the neurosurgeon, neurophysiologist, and neuroanesthesiologist, and may not correlate with postoperative clinical changes. These changes may be related to abnormal physiology rather than an insult at the time of surgery. Nonetheless, the authors advocate routine neurophysiologic intraoperative monitoring in this special group of children undergoing neurosurgical spine procedures.     

20 and 40 somatosensory evoked potentials: Thalamic lesions and subcortical origin

Three patients with well-defined clinical and radiographic lesions have been studied with somatosensory evoked potentials (SSEPs). The data indicate that the primary scalp potentials from stimulation of both the arm and leg (N20 and P40) can be lost with thalamic lesions and would be consistent with these potentials being generated in either the thalamus or thalamocortical radiations.