Integrative action of axonal membrane explored by trains of subthreshold stimuli applied to the peripheral nerve.

OBJECTIVE: The objective of this study was to provide evidence on the integrative action of axonal membrane in humans and its ability to integrate multipulse subthreshold stimuli and generate action potential. METHODS: The median nerve was stimulated at the wrist in six healthy subjects and 17 patients who underwent low spine surgery by means of percutaneous electrodes, with trains of one to nine near-threshold constant-current stimuli of 500-µs duration. The interstimulus interval between stimuli was 2 or 4 ms. The compound muscle action potential (CMAP) was recorded from the abductor pollicis brevis muscle using subcutaneous needle electrodes in patients and surface electrodes in healthy subjects. Total intravenous anesthesia (TIVA) without a muscle relaxant was used in all patients, and measurements were performed at the end of surgery. RESULT: A single near-threshold stimulus did not generate CMAP either in the healthy subjects or in the patients. However, when the number of near-threshold stimuli was increased to two to nine stimuli, and packed into a short train with interstimulus intervals of 2 or 4 ms, a CMAP of varying amplitude from 100 to 200 µV was successfully elicited. CONCLUSION: We concluded that the described phenomenon might be explained by the integrative action of the axonal membrane, which is able to summate the trains of subthreshold stimuli, increasing the resting potential to the firing level, and consequently generating CMAP. This is because the subthreshold stimuli make the axonal membrane hyperexcitable. SIGNIFICANCE: This phenomenon is not very well explored in clinical neurophysiology, and it needs to be studied further. This can explain some neurophysiologic phenomena during intraoperative monitoring.

Intraoperative monitoring of carotid endarterectomy by transcranial motor evoked potential: a multicenter study of 600 patients.

OBJECTIVE: This study was to investigate the utility of motor evoked potential monitoring elicited by transcranial electrical stimulation (tcMEP) during CEA in addition to the established median nerve somatosensory evoked potentials (mSSEPs). METHODS: We retrospectively reviewed data from 600 patients undergoing CEA under general anesthesia with monitoring of mSSEPs and tcMEPs in a multicenter study. MSSEP and tcMEP parameters were recorded during internal carotid artery (ICA) cross clamping and compared with the postoperative motor outcome, demographic and patient history data. RESULTS: The intraoperative monitoring of tcMEPs was successful in 594 of the patients (99%) and selective shunt was performed in 29 of them (4.83%). Nine of the patients showed a transient contralateral loss of tcMEPs, without changes in mSSEPs and required intervention (1.5% "false-negative"). Three of them showed postoperative motor deficits. The time period from tcMEP loss to intervention was significantly longer (p = 0.01) in this group compared to the patients without postoperative motor deficit. CONCLUSION: TcMEPs during CEA may be an adjunct to mSSEP monitoring to avoid "false-negative" mSSEP results, as mSSEPs seem to lack specificity for detecting isolated ischemia of corticospinal pathway. SIGNIFICANCE: TcMEPs seem to improve postoperative outcome, especially in case of a timely correction of cerebral ischemia.

Intra-operative neurophysiology during microvascular decompression for hemifacial spasm.

There is evidence that primary hemifacial spasm (HFS) in the majority of patients is related to a vascular compression of the facial nerve at its root exit zone (REZ). As a consequence, the hyperexcitability of facial nerve generates spasms of the facial muscles. Microvascular decompression (MVD) of the facial nerve near its REZ has been established as an effective treatment of HFS. Intra-operative disappearance of abnormal muscle responses (lateral spread) elicited by stimulating one of the facial nerve branches has been used as a method to predict MVD effectiveness. Other neurophysiologic techniques, such as facial F-wave, blink reflex and facial corticobulbar motor evoked potentials (FCoMEP), are feasible to intra-operatively study changes in excitability of the facial nerve and its nucleus during MVDs. Intra-operative neuromonitoring with the mentioned techniques allows a better understanding of HFS pathophysiology and helps to optimise the MVD.

Monitoring of scoliosis surgery with epidurally recorded motor evoked potentials (D wave) revealed false results.

OBJECTIVE: To elucidate the mechanism behind D wave amplitude changes after surgical correction of scoliosis. METHODS: We collected D wave and muscle MEP data from 93 patients (78 female, 15 male, age range 4-19 years, mean age 15.9 years), who underwent surgical correction of scoliosis. D waves were recorded via a catheter electrode inserted epidurally through the flavectomy. Muscle MEPs from lower limb muscles were also recorded. Muscle MEPs/D wave were elicited by short trains/single transcranial electrical stimuli. SEPs were elicited through bilateral percutaneous stimulation of the tibial nerves at the ankle and an averaged response from 100 to 200 single sweeps were recorded over the scalp at Cz'/Fz. In addition, we analyzed intraoperatively obtained X-ray images of the spine in 9 patients and preoperative spinal MRI in two of those nine. RESULTS: After surgical correction of scoliosis in 25 of 93 (27%) patients, the D wave amplitude changed by more than 20% of its baseline value. A decremental change occurred in 21 (84%) and an incremental change in 4 (16%) patients. D wave decrements of more than 50% were observed in 5 patients without significant SEP changes in any of these cases. In 9 patients, intraoperatively obtained X-rays of the spine (before and after correction of spine curvature) showed no catheter displacement. Muscle MEPs did not change and postoperative sensory-motor status was normal. In 2 patients, preoperative MRI revealed displacement of the spinal cord towards the concave side of the scoliotic curvature. CONCLUSIONS: During scoliosis surgery, D wave amplitude changes should be interpreted cautiously until the definitive cause(s) of these changes are found. One possible mechanism to explain D wave changes during scoliosis correction could involve rotation of the spinal cord within the spinal canal, and the relative position of the epidural recording catheter (ERC). Rotation of the spinal cord after correction of scoliosis could introduce a new relationship between the ERC and the corticospinal tracts (CTs). Due to high incidence of false D wave amplitude changes we suggest that this methodology should not be used to assess the functional integrity of the CTs during scoliosis surgery. SIGNIFICANCE: This study provides new insight into the methodology of D wave monitoring as well as strong evidence of a high incidence of false positive results using D wave monitoring during surgical correction of scoliosis.

Difference of disability between electrophysiologic subgroups of essential tremor.

OBJECTIVE: The aim of the study was to test the validity of the controversial subdivision of essential tremor (ET) patients into electrophysiological subgroups. METHODS: We evaluated a hundred patients with ET using surface electromyographic (EMG) recordings of antagonists forearm muscles and distinguished three groups: the first group showed synchronous activity of antagonistic muscles, the second showed alternating activity of antagonist muscles; and the third group consisted of patients whose EMG recordings were not compatible with the other two groups. We compared patients with synchronous and alternating activity in terms of sex, age at onset, duration of illness, family history of tremor, symmetry and frequency of tremor, and the scores of a disability scale. RESULTS: The only significant difference between the patients with synchronous and alternating activity was that the patients with synchronous activity were more disabled. CONCLUSION: This result adds to the evidence for distinct electrophysiological subgroups of ET with distinct clinical properties.

Effect of thigh flexion on somatosensory evoked potentials in meralgia paresthetica.

Standing with the thigh extended or lying still provokes and stepping or sitting relieves the symptoms in some patients with meralgia paresthetica. We performed this study to confirm this clinical feature with electrophysiological measures. Twenty-one symptomatic and 17 asymptomatic legs of 19 patients were evaluated by somatosensory evoked potential studies in both extended and flexed thigh positions. In the symptomatic group, thigh flexion significantly reduced the cortical latency. This finding is parallel with the relief of the symptoms.

Cavernous angioma presenting with hemidystonia.

We present a 34-year-old woman with right hemidystonia. She had a cavernous angioma (CA) of left thalamo-mesencephalic junction, revealed by magnetic resonance imaging (MRI). Her symptoms were moderately alleviated by biperiden 20 mg daily. We identified 11 patients reported in the literature to have movement disorders due to a cavernous angioma. None of them presented with hemidystonia. Some clinical properties of patients with CA associated with a movement disorder were evaluated.