Recording Of Motor Evoked Potentials Research Articles

Real-Time Evaluation of Anterior Choroidal Artery Patency During Aneurysm Clipping.

Inadvertent occlusion of the anterior choroidal artery during aneurysm clipping can cause a disabling stroke in minutes. We evaluate the clinical utility of direct cortical motor evoked potential (MEP) monitoring during aneurysm clipping, as a real-time assessment of arterial patency, prior to performing indocyanine green videoangiography. Direct cortical MEPs were recorded in seven patients undergoing surgery for aneurysms that involved or abutted the anterior choroidal artery. The aneurysms clipped in those seven patients included four anterior choroidal artery aneurysms and six posterior communicating artery aneurysms. Serial MEP recordings were performed during the intradural dissection, aneurysm exposure, and clip placement. A significant change in MEPs after clip placement would prompt immediate inspection and removal or repositioning of the clip. If the clip placement appeared satisfactory and MEP recordings were stable, then an intraoperative indocyanine green videoangiogram was performed to confirm obliteration of the aneurysm and patency of the arteries. Seven patients underwent successful clipping of anterior choroidal artery aneurysms and posterior communicating artery aneurysms using direct cortical MEP monitoring, with good clinical and radiographic outcomes. In six patients, no changes in MEP amplitudes were observed following permanent clip placement. In one patient, a profound decrease in MEP amplitude occurred 220 seconds after placement of a permanent clip on a large posterior communicating aneurysm. An inspection revealed that the anterior choroidal artery was kinked. The clip was immediately removed, and the MEP signals returned to baseline shortly thereafter. A clip was then optimally placed, and the patient awoke without neurologic deficit. Direct cortical MEPs are a useful adjunct to standard electrophysiologic monitoring in aneurysm surgery, particularly when the anterior choroidal artery or lenticulostriate arteries are at risk. When these arteries are occluded, infarction may occur before the occlusion is detected by indocyanine green videoangiography or intraoperative angiography. The use of MEPs allows real-time detection of ischemia to subcortical motor pathways.

Motor Evoked Potentials for Femoral Nerve Protection in Transpsoas Lateral Access Surgery of the Spine

ABSTRACTDetecting potential intraoperative injuries to the femoral nerve should be the main goal of neuromonitoring of lateral lumber interbody fusion (LLIF) procedures. We propose a theory and technique to utilize motor evoked potentials (MEPs) to protect the femoral nerve (a peripheral nerve), which is at risk in LLIF procedures. MEPs have been advocated and widely used for monitoring spinal cord function during surgical correction of spinal deformity and surgery of the cervical and thoracic spine, but have had limited acceptance for use in lumbar procedures. This is due to the theoretical possibility that MEP recordings may not be sensitive in detecting an injury to a single nerve root considering there is overlapping muscle innervation of adjacent root levels. However, in LLIF procedures, the surgeon is more likely to encounter lumbar plexus elements than nerve roots. Within the substance of the psoas muscle, the L2, L3, and L4 nerve roots combine in the lumbar plexus to form the trunk of the femoral nerve. At the point where the nerve roots become the trunk of the femoral nerve, there is no longer any alternative overlapping innervation to the quadriceps muscles. Insult to the fully formed femoral nerve, which completely blocks conduction in motor axons, should theoretically abolish all MEP responses to the quadriceps muscles. On multiple occasions over the past year, our neuro-monitoring groups have observed significantly degraded amplitudes of the femoral motor and/or sensory evoked potentials limited to only the surgical side. Most of these degraded response amplitudes rapidly returned to baseline values with a surgical intervention (i.e., prompt removal of surgical retraction).

TMS-evoked N100 responses as a prognostic factor in acute stroke.

Rehabilitation programs, to be efficiently tailored, need clear prognostic markers. In acute stroke, neurophysiological measures, such as motor evoked potentials (MEPs), have been proposed, although with discordant results. The aim of this study was to identify a reliable neurophysiological measure of recovery in acute post-stroke individuals by combining MEPs and the N100 component of transcranial magnetic stimulation-evoked potentials (TEPs). Nine acute post-stroke subjects were included. Clinical evaluation performed in the first week after the event included administration of the European Stroke Scale and Barthel Index and recording of MEPs and TEPs; administration of the clinical scales was repeated after one and three months. The presence/absence of MEPs and TEPs showed correlations with motor outcome. Individuals with a poorer outcome showed absence of both MEPs and TEPs; absence of MEPs alone was related to a partial recovery. Given the results of this exploratory study, further investigation is needed to define the accuracy of combined use of MEPs and TEPs as an approach for predicting motor recovery after acute stroke.

Direct Comparison of the Effect of Desflurane and Sevoflurane on Intraoperative Motor-evoked Potentials Monitoring

During spinal surgery, intraoperative monitoring of motor-evoked potentials (MEPs) is a useful means of assessing the intraoperative integrity of corticospinal pathways. However, MEPs are known to be particularly sensitive to the suppressive effects of inhalational halogenated anesthetic agents. To investigate the effects of increasing end-tidal concentrations of desflurane and sevoflurane anesthesia in a background of propofol and remifentanil with multipulse cortical stimulation on intraoperative monitoring of MEPs. In this randomized crossover trial, 14 consecutive patients (7 in each arm) undergoing major spine surgery, under a background anesthetic of propofol (75 to 125 mcg/kg/min) and remifentanil (0.1 to 0.2 mcg/kg/min), were randomly assigned to receive the sequence of inhalational agents studied: either DES-SEVO (desflurane followed by sevoflurane); or SEVO-DES (sevoflurane followed by desflurane). Multiples (0.3, 0.5, and 0.7) of minimum alveolar concentration (MAC) of desflurane and sevoflurane were administered. After a washout period of 15 minutes using high fresh oxygen/air flows, each of the patients then received the other gas as the second agent. Cortical stimulation was achieved with a train of 5 equivalent square pulses, each 0.05 ms in duration, delivered at 2 ms intervals. MEP recordings were made in the upper limb (UL) from first dorsal interosseus and lower limb (LL) from tibialis anterior with subdermal needle electrodes. At 0.3 MAC desflurane, there was no statistical significant difference in transcranial-evoked MEP amplitudes from the baseline in both UL and LL stimulation. However, this was not the case for sevoflurane for which even a low concentration at 0.3 MAC significantly depressed MEP amplitudes of LL (but not UL) from baseline value. Desflurane at 0.5 and 0.7 MAC depresses LL MEP to 58.4% and 59.9% of baseline, respectively (P<0.05), whereas sevoflurane at 0.3, 0.5, and 0.7 MAC depresses LL MEP to 66.2%, 41.3%, and 25.3% of baseline, respectively (P<0.05). There was no difference in latency of the responses at any MAC. Inhalational anesthetic agents (sevoflurane >desflurane) suppress MEP amplitudes in a dose-dependent manner. The use of 0.3 MAC of desflurane (but not sevoflurane) provided good MEP recordings acceptable for clinical interpretation for both upper and LLs. The LL appears to be more sensitive to anesthetic-induced depression compared with the UL. All patients studied had normal neurological examination hence, these results may not be applicable to those with preexisting deficits.

P39. Fast versus slow charge cortical stimulation for motor evoked potential

Fast charge (FC) and slow charge (SC) stimulation protocols are both available on modern neuromonitoring machines in order to induce motor evoked potentials (MEP). In order to define the best strategy for MEP recording, we have compared the efficiency and the feasibility of both stimulation protocols for the induction of motor evoked potential in neurosurgical patients that present without myelopathy. Fifteen consecutive adult patients (13 arthrodesis and 2 kyphoplasties) without clinical motor deficit undergoing spine surgery under neuromonitoring were included in this study. Fast (50 μs) and slow (500 ms) charge biphasic cortical stimulation was applied between C3 and C4 electrodes in trains of 5 at 500 Hz. Muscular response was quantified as the area under the curve between the MEP start and end time. We were able to deliver FC and SC in all patients. SC induced always-measurable MEP response up to 140 V. On the contrary, in some patients FC induced large recording artefacts that precluded measurement. Current delivery saturated between 100 (129 ± 32 mA) and 140 (117 ± 62 mA) V for SC while this was not the case for FC up to 450 V (660 ± 123 mA). In the same way MEP area under the curve value reached a plateau between 120 and 140 V for SC (96% of maximal response) while FC induced MEP area under the curve varied linearly with stimulation current (R2 = 0.998). When both stimulations were directly compared, FC was able to induce a maximal response that was approximately 1.3 times larger than SC for the same muscle in the same patient. In our experience, FC cortical stimulation is more efficient and more potent than SC stimulation. The only drawback of FC compared to SC is the large stimulation artefact when the ground electrode is placed on the scalp. The value of displacing the ground electrode to the sternum or the coccyx to minimize stimulation artefacts needs to be studied in the future. The results of FC stimulation also needs to be verified in myelopathic patients.

Central motor conduction studies in patients with spinal cord disorders: a review.

Topical review of the literature. The evaluation of patients with myelopathies requires radiological investigations; however, for the correct interpretation of the neuroimaging findings, the functional assessment of corticospinal conduction is helpful or even mandatory in many conditions. The objective of this review article was to assess the utility of the motor evoked potentials (MEPs) in diagnosis and management of the most frequent spinal cord disorders. Salzburg (Austria) and Merano (Italy). A MEDLINE search was performed using following terms: 'motor evoked potentials', 'transcranial magnetic stimulation', 'central motor conduction', 'compressive myelopathy', 'spinal cord infarction', 'spinal cord injury', 'syringomyelia', 'myelitis', 'hereditary spastic paraparesis', 'subacute combined degeneration' and 'hepatic myelopathy'. Central motor conduction abnormalities can be detected also in the absence of neuroradiological abnormalities-for example, in patients with subacute combined degeneration or hepatic myelopathy. In the most frequent patients with compressive myelopathies, MEPs were found to be very helpful in determining the functional significance of neuroimaging findings. MEP recording can supplement clinical examination and neuroimaging findings also in the assessment of the spinal cord injury level. In patients with spinal cord infarction, the MEP study can demonstrate spinal involvement even when radiological evidence for spinal cord damage is absent or equivocal, thus allowing an important early diagnosis. MEPs represent a highly sensitive and accurate diagnostic tool in many different spinal cord disorders. MEPs can also be useful in follow-up evaluation of motor function during treatment and rehabilitation.

P 193. Excitability modulation of the motor system induced by transcranial direct current stimulation

Question Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique that modulates cortical excitability and activity in a polarity-dependent way. In the human motor system, such cortical modulations are inferred through changes in the amplitude of motor evoked potentials (MEPs). To directly evaluate tDCS-induced changes at the cortical level, we investigated polarity-dependent tDCS-induced effects on the motor system, evaluating changes in MEPs, TMS-evoked potentials (TEPs) and in the EEG oscillatory activity. Methods Sixteen young healthy right-handed subjects participated in this study. Two experimental sessions were performed for each subject in randomized order: anodal and cathodal tDCS (a- and c-tDCS). The EEG activity was recorded from 10 scalp electrodes while EMG activity was recorded from the right FDI. Corticospinal excitability and cortical reactivity were investigated through the recording of MEPs and TEPs, whereas the cortical state was evaluated through the acquisition of the EEG activity. All the measures were collected before tDCS, immediately and 30 min after tDCS, to evaluate short and long-lasting tDCS effects. The tDCS was applied for 13 min (1 mA) over the left primary motor cortex. The TEP-MEP block consisted of 100 TMS pulses (intensity of 110% of the RMT), delivered with a random inter stimulus interval of 2–4 s. The EEG block consisted of 3 min of recording during a resting state. To determine the tDCS induced changes in the cortical evoked potentials, a local mean field power analysis was computed. To characterize tDCS-induced changes in cortical oscillatory activity, the EEG power density was estimated by means of the Fast Fourier transform. Results The application of a- and c-tDCS over M1 induced respectively a short-term increase and decrease of MEPs amplitude. The MEPs changes persisted 30 min after a-tDCS but not after c-tDCS. The TEPs analysis highlighted short term changes induced by tDCS polarities. Particularly, we found a significant pattern of topographically specific and current-dependent changes. a- and c-tDCS induced consistent differences in cortical reactivity only on the stimulated area. The long lasting changes partially overlapped those observed in the short-term analyses. Finally, the EEG frequency analysis revealed a significant main effect only in the theta and alpha bands, suggesting a general increase in their power density after tDCS. These changes were reduced 30 min after stimulation. Conclusions a-tDCS over primary motor cortex induced an enhancement of corticospinal excitability, whereas c-tDCS produced an excitability reduction. More interestingly, the cortical reactivity resulted increased after anodal stimulation whereas cathodal stimulation produced a decrease over the stimulated area. These cortical reactivity changes lasted for at least30 min. Moreover, a general increase in the power density of theta and alpha frequencies was also present over all scalp sites for both the stimulation polarities. These results shows direct evidence that tDCS induces polarity-dependent changes on brain activity at cortical level.

P 200. Coupling of the sensorimotor cortical activation explored by blood-oxygenation-level-dependent signal navigated and traditional magnetic evoked potential recordings

Introduction The blood-oxygenation-level-dependent (BOLD) signal in functional magnetic resonance imaging (fMRI) reflects associated hemodynamics for a specific motor task. Motor evoked potentials (MEPs) represent the excitability of the motor cortex. Recently the MEP recording can be visually guided by the BOLD signal in fMRI. However, the two methods may represent diverse physiological mechanisms ( Krivanekova et al., 2012 , Antal et al., 2011 ). Objectives To study the coupling of the sensorimotor cortical activation observed in fMRI BOLD signals and measured by traditional MEP recordings. Materials and methods Ten healthy, right-handed subjects (7 men, mean age: 34.6 ± 7.6 years) participated in this study. Functional imaging was acquired by using a 3.0T GE MR scanner with a single-shot EPI sequence. The subjects were requested to perform simple, repetitive abduction movement at right index finger while they saw a go-signal appeared on the projection screen and to take a rest during no-go signal periods. Transcranial magnetic stimulation (TMS) was delivered through a focal figure-of-eight stimulating coil. The coil was held tangential to the scalp with the handle pointing backwards and ∼45° away from the midline. Twenty single TMS trials were measured with a randomly varying intertrial interval ranging from 7.5 to 12.5 s at four coil positions. The electrophysiological optimal (EO) position was determined as the site where TMS at a slightly suprathreshold intensity produced consistently the largest MEPs in right first dorsal interosseus (FDI). The site was marked on the scalp to assure a constant placement of the coil throughout this session. The other three positions were determined by a frameless MR-navigation system (Visor™). The maximal BOLD signals at left primary motor cortex (BM1), left primary sensory cortex (BS1) and the lateral margin of the BOLD activity (BL) were targeted by the guidance of the navigation system. Two TMS intensities which were adjusted to produce MEPs of about 0.5 mV (low intensity) and 1 mV (high intensity) in peak-to-peak amplitude in the right FDI muscle were tested. In total 8 recording sessions including 4 positions and 2 intensities were completed by a randomized order for each subject. The mean MEP amplitude of each session was analyzed by a two-way repeated measures analysis of variance (rmANOVA) with the within-subject factors of POSITION and INTENSITY. Results All subjects demonstrated a clear BOLD signal in the left sensorimotor cortex with response to the right finger movement. RmANOVA of the MEP amplitudes revealed significant main effects of POSITION and INTENSITY, and a significant interaction between POSITION and INTENSITY. The post hoc analysis revealed a significant MEP difference between the four positions except EO-BM1 and BS1-BL at high intensity stimulation (all p Conclusion Findings suggest that the coupling of the BOLD signals and the MEPs can be TMS-intensity dependent. A distinct localization of the BOLD signal in M1 or S1 by the TMS-navigation system may significantly vary the MEP findings under a high TMS-intensity condition.

Effect of neck flexion on somatosensory and motor evoked potentials in Hirayama disease

Hirayama disease (HD) is a rare motor disorder mainly affecting young men, characterized by atrophy and weakness of forearm and hand muscles corresponding to a C7–T1 myotome distribution. The weakness is usually unilateral or asymmetric and progression usually stops within several years. The etiology of HD is not well understood. One hypothesis, mainly based on MRI findings, is that the weakness is a consequence of cervical flexion myelopathy. The aim of this study was to explore the function of corticospinal and ascending somatosensory pathways during neck flexion using evoked responses. Materials and methods15 men with HD and 7 age-matched control male subjects underwent somatosensory evoked potentials (SSEP) and motor evoked potentials (MEP) studies with the neck in neutral position and fully flexed. SSEP studies included electrical stimulation of median and ulnar nerves at the wrist, and tibial nerve at the ankle with recording over the ipsilateral Erb's point, cervical spine, and contralateral sensory cortex. MEP recordings were obtained by magnetic stimulation of the motor cortex and the cervical lower spinal roots; the evoked responses were recorded from the contralateral thenar and abductor hallucis muscles. ResultsMEP recordings demonstrated significant lower amplitudes, and slightly prolonged latencies in HD patients on cervical stimulation, compared to control subjects. During neck flexion, MEP studies also demonstrated a statistically significant drop in mean upper limb amplitude on cervical stimulation in HD patients, as well as in control subjects, although to a lesser degree. In contrast, no significant differences were found in SSEP studies in HD patients compared to control subjects, or between neutral and flexed position in these groups. ConclusionThe study shows a negative effect of cervical flexion on MEP amplitudes in HD patients as well as in control subjects, requiring more studies to investigate its significance. Neck flexion did not have an influence on any SSEP parameters in patients or controls.

Transcranial direct current stimulation treatment protocols: should stimulus intensity be constant or incremental over multiple sessions?

Interest in transcranial direct current stimulation (tDCS) as a new tool in neuropsychiatry has led to the need to establish optimal treatment protocols. In an intra-individual randomized cross-over design, 11 healthy volunteers received five tDCS sessions to the left primary motor cortex on consecutive weekdays at a constant or gradually increasing current intensity, in two separate weeks of testing. Cortical excitability was assessed before and after tDCS at each session through peripheral electromyographic recordings of motor-evoked potentials. Both conditions led to significant cumulative increases in cortical excitability across the week but there were no significant differences between the two groups. Motor thresholds decreased significantly from Monday to Friday in both conditions. This study demonstrated that, in the motor cortex, administration of tDCS five times per week whether at a constant intensity or at a gradually increasing intensity was equally effective in increasing cortical excitability.

Transcranial motor evoked potential monitoring outcome in the high-risk brain and spine surgeries: Correlation of clinical and neurophysiological data - An Indian perspective

Objective:The objective of this study is to assess the safety, feasibility and clinical value of transcranial motor evoked potential (MEP) monitoring by electrical stimulation.Setting:Clinical neurophysiology department of tertiary reach hospital.Materials and Methods:MEP monitoring was attempted in 44 “high risk” patients. Intraoperative surgical, anesthesia and neurophysiological findings were documented prospectively. MEP monitoring results were correlated with motor outcome.Results:The success for reliable MEP recording from the lower limbs was 75%. Incidence of new permanent post-operative motor deficit was zero. Nearly, 76.5% of the cases (13 out of 17 cases) who showed unobtainable and unstable MEP outcome had lesion location in the spine as compared with 23.5% (4 out of 17 cases) that had lesion location in the brain. Chi-square test demonstrated a statistically significant difference between these two groups (P = 0.0020). Out of these 13 spine surgery cases, 8 (62%) were operated for deformity. Seven out of 12 (60%) patients less than 12 years of age had a poor MEP monitoring outcome suggesting that extremes of age and presence of a spine deformity may be associated with a lesser incidence of successful MEP monitoring. No complications related to the repetitive transcranial electrical stimulation for eliciting MEP were observed.Conclusion:MEP monitoring is safe. The protocol used in this study is simple, feasible for use and has a fairly high success rate form the lower limbs. Pediatric age group and spine lesions, particularly deformities have an adverse effect on stable MEP recording.

Ischemia changes and tolerance ratio of evoked potential monitoring in intracranial aneurysm surgery

ObjectiveWe assessed the relationship between cerebral ischemia-induced changes in evoked potentials and the degree of ischemia tolerance. Methods47 patients underwent somatosensory evoked potential (SEP) and motor evoked potential (MEP) monitoring in intracranial aneurysm surgery. Three duration parameters (time) were recorded: Time 1, from the starting of temporary occlusion unavoidable in aneurysm surgery to the time the evoked potentials decrease from basal level to reaching the warning criterion; Time 2, from evoked potentials reaching the warning criterion to the time the blood flow was resumed; Time 3, after resuming the blood flow, the time it took the evoked potentials to recover to baseline. All three times can be reliably calculated in the SEP recording, but not in the MEP recording which consisted of either unchanged amplitudes or abruptly changing amplitudes, making it impossible to obtain Time 1. The ischemic tolerance ratio (ITR) was calculated as ITR=time 2/time 1×100%. New decreasing myodynamia and fresh infarction after the surgery were employed for evaluating neurological deficits postoperatively, and their correlations with the ischemia-induced changes of evoked potentials recorded during the surgery were analyzed. ResultsWe found a change in SEPs in 12 patients whose cerebral ischemia was induced by temporary occlusion of the aneurysm's parent artery. We also found the development of postoperative neurological deficits in 4 patients whose ischemic tolerance ratio (ITR) reached over 80%, while no deficits were found in the other 8 patients whose ITR was less than 50%. MEP changes were seen in 4 patients whose cerebral ischemia was caused by accidentally clamping the perforating branches, causing the development of postoperative neurological deficits but not necessarily leading to significant SEP changes. ConclusionThe Ischemia tolerance ratio (ITR) in SEP recordings is valuable to predicting postoperative neurological deficits caused by temporary occlusion of aneurysm's parent artery. Maintaining the ITR under 50% during operation can effectively avoid postoperative neurological deficits, while an ITR above 80% reliably forecasts postoperative neurological deficits. Complementary to SEPs, MEP recordings are particularly valuable in monitoring ischemic effects caused by accidentally clamping perforating branches. Taken together, this system of monitoring makes it possible to promptly adjust surgery procedures and minimize postoperative neurological deficits.

Representation of cricothyroid muscles at the primary motor cortex (M1) in healthy subjects, mapped by navigated transcranial magnetic stimulation (nTMS)

ObjectivesTo establish a methodology for mapping of primary motor cortex (M1) for cricothyroid (CTHY) muscles in a group of healthy subjects using three-dimensional (3D) magnetic resonance imaging (MRI) navigated transcranial magnetic stimulation (nTMS). MethodsTwo independent measurements were performed. Twelve right-handed healthy subjects were included in the study. In the first measurement, mapping of the abductor pollicis brevis (APB) muscle was followed by mapping of the M1 for CTHY. This was performed in 11 subjects. Second, to avoid bias concerning using a hand knob as a landmark, mapping of M1 for CTHY muscle was followed by mapping of M1 for APB. This was performed in three healthy subjects.The nTMS was used, with selective recordings of motor evoked potentials (MEPs) from APB muscle and corticobulbar motor evoked potentials (CoMEPs) from the CTHY muscle. For recording the responses from the CTHY muscle two hook wire electrodes (the size of 76 μm of diametre passing through 27 gauge needle) were inserted in the muscle. For the recording of MEPs from APB muscle, surface electrodes were used. ResultsFirst measurement: Stimulation over the left M1 for APB muscles elicits MEPs in the contralateral APB muscle with a mean latency of 22.8±1.69ms. Stimulation over the left M1 for the CTHY muscle elicits CoMEPs in the contralateral CTHY muscle with a mean latency of 11.89±1.26ms. The distance between the cortical representation for APB and CTHY was 25.19±6.52mm, with CTHY muscle representation lateral to the APB muscle.Second measurement: The results of second measurement of the distance between M1 for CTHY and M1 for APB and their cortical localisation were comparable to the results of the first measurement. ConclusionThis is the first study with the aim to determine the exact cortical localisation of CTHY muscle with nTMS. Mapping of M1 for CTHY and APB muscles by nTMS was successfully performed in all healthy subjects. The exact location of the stimulating points over M1 muscles eliciting responses in CTHY and APB muscles was determined and superimposed over 3D MRI images. The data show that M1 for CTHY muscle is about 25mm more lateral with regard to M1 for the APB muscle. SignificanceMapping of M1 for CTHY muscle might represent an important neurophysiologic marker for facilitating preoperative mapping of motor speech-related cortical areas due to the proximity of motor cortical representation for laryngeal muscles and opercular part of the Broca area.

Effect of Neck Flexion on Somatosensory and Motor Evoked Potentials in Hirayama Disease (P03.169)

Objective: Determining the effect of neck flexion on measured SEP and MEP parameters, regarding the hypothesis that Hirayama disease (HD) is a consequence of cervical flexion myelopathy. Background Hirayama disease (HD) develops mainly in young men, characterized by atrophy and weakness of the forearm and hand muscles, in a C7-T1 myotome distribution, usually unilaterally or asymmetrically, followed by spontaneous arrest within several years. The etiology of HD is not well understood. One hypothesis, mainly based on MRI findings, is that HD is a consequence of cervical flexion myelopathy. Concerning this hypothesis, previous neurophysiological and pathological studies produced conflicting findings. Design/Methods: 15 Israeli male patients, mean age 23 ± 6.5 years (range 17-31), with definite HD underwent somatosensory evoked potentials (SEP) and motor evoked potentials (MEP) studies in neutral position and with the neck fully flexed. SEP studies included stimulation of median and ulnar nerves at the wrist, and tibial nerve at the ankle with recording over ipsilateral erb9s point and cervical spine, and contralateral sensory cortex. MEP recordings were obtained by magnetic stimulation of the motor cortex and the cervical spinal cord; the evoked responses were recorded from the contralateral thenar and abductor hallucis muscles. Results: No significant differences were found in SEP studies for the mean values of median and ulnar N9, N13, N20, and tibial P38 latencies and amplitudes in HD patients between neutral and flexed position. MEP studies detected a statistically significant drop in mean upper limb cervical amplitude, and prolongation in lower limb latency during neck flexion compared to the neutral position. Conclusions: In this large cohort of patients neck flexion had no effect on measured SEP parameters, but influenced slightly part of the MEP variables, supporting the view of a negative effect of neck positioning in HD. Disclosure: Dr. Abraham has nothing to disclose. Dr. Gotkine has nothing to disclose. Dr. Drory has nothing to disclose. Dr. Blumen has nothing to disclose.

Improvement of motor evoked potentials monitoring is required during thoracic or thoracoabdominal aortic aneurysm surgery under hypothermic cardiopulmonary bypass

Keywords Motor evoked potential Thoracoabdominalaortic aneurysm HypothermiaMonitoring of motor evoked potentials (MEPs) can provideinformation about the functional integrity of a motorpathway and, therefore, has been widely used during sur-gery in which a motor pathway is at risk of injury,including craniotomy, spine/spinal surgery, and thoracic orthoracoabdominal aortic aneurysm (TAA or TAAA) sur-gery [1, 2]. Because myogenic MEPs are very sensitive tosuppression by anesthetic agents and neuromuscularblockade, careful attention should be paid to anestheticmanagement during monitoring of MEPs. In general, totalintravenous anesthesia with propofol and opioids is used,although ketamine-based anesthesia may be applied incases in which recording of myogenic MEPs is not reliablebecause of suppression by anesthetic agents or influence bypreoperative motor dysfunction.Of the cases in which MEPs monitoring is required,TAA/TAAA surgery is the most challenging for reliableMEP monitoring. In addition to the effects of anesthetics,a variety of factors can affect the recording of MEPs,including hypothermia, aortic clamp, and induction ofcardiopulmonary bypass (CPB). Hypothermia may havedirect effects on the MEP itself and also indirect effects onMEPs by changing the pharmacokinetics and pharmaco-dynamics of anesthetic agents, modified by the effects ofaortic clamp and the induction of CPB. Leslie et al. [3]demonstrated that a temperature reduction of 3 C increasedblood propofol concentration by 30 C during a constantrate infusion. Kakinohana et al. [4] reported that plasmapropofol concentration increased and bispectral indexvalues decreased rapidly after aortic cross-clamping inthoracic aortic aneurysm repair surgery during propofolanesthesia. Yoshitani et al. [5] demonstrated that, in patientsreceiving propofol infusion at a rate of 5 or 6 mg/kg/h,plasma propofol concentration increased and burst sup-pression rate on electroencephalogram increased after theinduction of normothermic CPB, compared with thosevalues before CPB. These findings suggest that interpre-tation of MEP changes could be difficult because anestheticlevel can easily fluctuate during TAAA surgery withhypothermic CPB.Data regarding the effects of hypothermia on myogenicMEPs have been limited to animal experiments. Meylaertset al. [6] investigated the effects of regional spinal cordhypothermia on myogenic MEPs induced by transcranialtrain-pulse stimulation in pigs anesthetized with ketamine,sufentanil, clonidine, and nitrous oxide. They demonstratedthat progressive cooling resulted in an increase of MEPs at28 –30 C and was followed by a progressive decrease inMEP amplitude. MEP amplitude decreased below 25% atapproximately 14 C. Sakamoto et al. [7] also investigatedthe effects of systemic hypothermia on myogenic MEPsinduced by a single pulse and a train of pulses under

Neurophysiological Changes in Deformity Correction of Adolescent Idiopathic Scoliosis With Intraoperative Skull-Femoral Traction

Retrospective review of 36 consecutive patients undergoing coronal plane deformity correction with intraoperative skull-femoral traction between 2005 and 2008 with motor evoked potential (MEP)/somatosensory evoked potential monitoring. To determine the prevalence and significance of neurophysiological changes with intraoperative skull-femoral traction in adolescent idiopathic scoliosis. Intraoperative skeletal traction can be associated with spinal cord stretching and ischemia with resultant electrophysiological changes. The prevalence and risks of such changes and their clinical significance is unknown. Thirty-seven procedures involving 36 patients (27 females and 9 males) with a mean age of 14.8 (12-18) years were divided into two groups on the basis of the presence (group 1, n = 18 procedures) or absence (group 2, n = 19) of significant MEP changes with surgery. They were compared with patients undergoing correction without traction (group 3). Significant differences among the groups were observed in mean preoperative Cobb angle (86° vs. 70° vs. 59°), mean intraoperative posttraction Cobb angle (50.0° vs. 34.6°), traction index (0.41 vs. 0.50), flexibility index (0.14 vs. 0.27 vs. 0.25), and presence of primary lumbar curves (0% vs. 32% vs. 14%). Initial onset of MEP amplitude loss (group 1) occurred at a mean of 94 (1-257) minutes from the onset of surgery, was bilateral in 13 procedures, and improved at a mean of 5.5 (1-29) minutes after decreasing or removing the traction. At closure, complete bilateral recovery to baseline was observed in 10 procedures, recovery to >50% baseline in five, and recovery to <50% baseline in three procedures. There were no neurologic deficits in this series. Intraoperative traction is associated with frequent changes in MEP monitoring. The thoracic location of the major curve, increasing Cobb angle, and rigidity of major curve are significant risk factors for changes in MEP with traction. The presence of any MEP recordings irrespective of its amplitude at closure was associated with normal neurological function. Somatosensory evoked potential monitoring did not correlate with the traction induced MEP amplitude changes.

Evaluation of Combined Use of Transcranial and Direct Cortical Motor Evoked Potential Monitoring During Unruptured Aneurysm Surgery

The feasibility and reliability of combined use of transcranial and direct cortical motor evoked potential (MEP) monitoring during unruptured aneurysm surgery were evaluated. Forty-eight patients with unruptured cerebral aneurysms underwent craniotomy and neck clipping accompanied by muscle MEP monitoring. MEPs were elicited successfully by transcranial electrical stimulation in all patients. Direct cortical stimulation elicited MEPs in 44 patients. Reduction in MEP amplitude to less than 50% of baseline was considered significant. No postoperative motor paresis occurred in 39 patients in whom transcranial and direct MEPs remained unchanged. Four patients in whom direct MEPs could not be recorded had no intraoperative abnormality in transcranial MEPs and no postoperative motor dysfunction. Four of the other 5 patients manifested significant transient direct MEP changes without transcranial MEP changes. The transient MEP changes were observed in 3 patients during temporary clipping of the parent artery and in one patient with inadequate clipping of an middle cerebral artery aneurysm, and were considered due to insufficiency of blood flow. Decrease or disappearance of direct MEP waves recovered immediately after re-application of the clip and release of the temporary clip. Direct MEP waves disappeared and did not recover until the end of microsurgical procedures in one patient, although transcranial MEP amplitude remained at less than 50% of baseline. She developed hemiparesis postoperatively, which recovered within 6 hours. The duration of temporary occlusion in patients with direct MEP changes was significantly longer than that in patients without (p < 0.05). Direct MEP was sensitive in detecting ischemic stress to descending motor pathways during aneurysm surgery. Transcranial MEPs could be elicited in patients in whom direct MEPs could not be obtained, and during periods such as craniotomy or after dural closure, in which direct MEPs could not be recorded. These findings suggest that combined transcranial and direct cortical MEP recording may improve the feasibility and reliability of MEP monitoring during unruptured aneurysm surgery.

Case Scenario: Neurologic Complication after Continuous Interscalene Block

A 33-YR-OLD woman, American Society of Anesthesiologists risk classification I, was referred for surgical treatment of left shoulder instability. Her medical history was unremarkable, but she had undergone two previous shoulder surgeries: anterior stabilization of left shoulder 3 yr ago followed by a posterior Bankart stabilization 2 yr later because of recurrent luxation and pain. At examination, she still complained about pain in her left shoulder, and movement was severely limited. A left Bankart operation and capsulotomy were scheduled. A preoperative interscalene catheter was placed according to the modified lateral approach. The interscalene brachial plexus was identified using a nerve stimulator (Stimuplex® HNS II, B. Braun Melsungen AG, Melsungen, Germany) connected to the proximal end of the metal inner part of a short beveled needle. A contraction of the deltoid muscle was observed with a current output of 0.35 mA, with an impulse duration of 0.1 ms. The insertion of the perineural catheter was performed using the cannula-over-needle technique with a plastic cannula (Polymedic®, Polyplex N-50 T, 20 gauge external diameter, te me na, Bondy, France). The catheter was introduced distally between the anterior and middle scalene muscle up to 3 cm. A bolus of 30 ml ropivacaine (0.5%) was administered, and the procedure was uneventful. No pain, dysesthesias, or paresthesias occurred at any time. Fifteen minutes later, a complete sensorimotor block of the left arm was observed. Then, general anesthesia with targetcontrolled infusion (TCI Diprifusor, SIMS Graseby Ltd., Watford, Herts, United Kingdom) of propofol and remifentanil was administered. The patient was positioned in the beach chair for surgery. The procedure lasted 3 h 30 min and was described by the surgeon as difficult. Postoperative analgesia was performed with patient-controlled interscalene analgesia using ropivacaine (0.2%) and was started 4 h after the end of the surgery. The pump settings were basal rate of 5 ml/h with supplementary boli of 4 ml for every 20 min. The patient was pain free, and the patient-controlled interscalene analgesia was stopped after 54 h. Even after 36 h of termination of patient-controlled interscalene analgesia, no motor or sensory recovery was observed. An ultrasonography of the brachial plexus was performed to exclude a hematoma and revealed no irregular finding. A complete neurologic examination including neurophysiologic recordings on the following day (72 h after surgery) demonstrated a motor and sensory incomplete lesion of nerve fibers originating from the upper brachial plexus (involving the following muscles: deltoid M0/5, biceps M0/5, and triceps M3/5). Motor nerve conduction studies (figs. 1 and 2) and somatosensory-evoked potentials (SSEPs) of the median and ulnar nerves that originate from the lower brachial plexus revealed normal recordings. Electromyography at the initial examination did not show signs of denervation (fibrillation or positive waves) but resulted in complete paralysis (no retrievable voluntary electromyography activity). The later finding at this early stage after injury can be in line with a neurapraxia (conduction failure) or axonotmesis (structural neural damage). Eventually, 1 month later, at the first follow-up, electromyographies showed signs of denervation most severe within the deltoid and biceps muscles, disclosing objective signs of neural damage (fig. 3) congruent to the clinical condition in which no motor improvement was observed. Six months later, although no clinical improvement occurred, a surgical exploration was performed that revealed no pathologic findings. At this time, a perioperative stimulation of the trunks of the brachial plexus elicited reliable responses for the three trunks. In repeated electromyographic recordings during a period of 2 yr, increasing signs of motor recovery could be observed, although the patient still demonstrated severe proximal muscle weakness that could not be * Professor and Chief-of-Staff, † Consultant, Department of Anesthesiology, ‡ Professor and Chairman, Spinal Cord Injury Center, University of Zurich, Balgrist University Hospital, Zurich, Switzerland.

Efferent and afferent evoked potentials in patients with adrenomyeloneuropathy

Objective This paper investigates efferent and afferent conductions of the central nervous system by various evoked potentials in patients with adrenomyeloneuropathy (AMN). Patients and methods Ten pure AMN patients without cerebral involvement were studied. Motor evoked potentials (MEPs), somatosensory evoked potentials (SEPs), auditory brainstem response (ABR), and pattern reversal full-field visual evoked potentials (VEPs) were recorded. For MEP recording, single-pulse or double-pulse magnetic brainstem stimulation (BST) was also performed. Results Abnormal MEP was observed in all ten patients, abnormal SEP in all ten, abnormal ABR in nine, and abnormal VEP in only one. Brainstem latency was measured in three of the seven patients with central motor conduction time (CMCT) prolongation. The cortical–brainstem conduction time was severely prolonged along the normal or mildly delayed brainstem–cervical conduction time in those three patients. Conclusions The pattern of normal VEP and abnormal MEP, SEP, ABR is a clinically useful electrophysiological feature for the diagnosis. BST techniques are helpful to detect, functionally, intracranial corticospinal tract involvement, probably demyelination, in pure AMN patients.

Significance of intraoperative motor function monitoring using transcranial electrical motor evoked potentials (MEP) in patients with spinal and cranial lesions near the motor pathways

Intraoperative motor evoked potential (MEP) monitoring in patients with spinal and cranial lesions is thought to be a valuable tool for prevention of postoperative motor deficits. Aim of this study was to investigate its diagnostic value in a spinal and a cranial patient group. Ninety-six patients, 31 with spinal and 65 with intracranial lesions, were studied. Transcranial stimulation was performed with a high-frequency electrical train stimulation using two subdermal needle electrodes. MEPs were recorded from the pathology-related muscles. Decreasing amplitudes of 50% or more, increasing stimulus intensities of 20% or more or increased latencies were taken as warning criteria. MEP recording was possible in 90% of the spinal and 98% of the cranial group. With two further exclusions, 28 patients of the spinal and 62 of the cranial group were analyzed. We saw a temporary maximum amplitude reduction of 50% or more and an increase in stimulation intensity of 20% or more in 8 spinal and 29 cranial patients. Five of the spinal and nine of the cranial patients deteriorated in motor function postoperatively. One patient with normal MEP monitoring showed a temporary motor weakness postoperatively. Latencies were normal in all patients. Given both warning criteria, intraoperative MEP changes had a sensitivity of 83%/ 100% and a specificity of 86%/ 62% (spinal/ cranial group). The positive predictive value of MEP changes for postoperative motor function deterioration was 63%/ 31%, and the negative predictive value was 95%/ 100%. Transcranial electrical monitoring of MEP is a practicable and safe method. However, there are many events, which can cause amplitude changes of MEP independent from surgical manipulations. Although sensitivity is high for both groups, this results in a moderate specificity for the cranial group and a low positive predictive value for both groups.