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.

Neurophysiological assessment of the injured spinal cord: an intraoperative approach.

STUDY DESIGN: Prospective, observational study. OBJECTIVES: To assess the spinal cord function intraoperatively in subjects during spine stabilization for spinal cord trauma, by recording muscular (m-MEPs) and epidural motor evoked potentials (e-MEPs, D wave) along with cortical and epidural somatosensory evoked potentials (e-SEPs) and predicting the outcome of spinal cord injury (SCI). SETTING: Regional Trauma Center, Torino, Italy. METHODS: Fifty-five patients were intraoperatively studied during posterior spine stabilization surgery for traumatic SCI. In all, 21 of these had complete SCI, 14 an incomplete SCI-6 of them with central cord syndrome and 1 with central cord plus Brown Sequard syndrome-and 20 patients were neurologically uncompromised. RESULTS: The neurophysiologic profile of the complete SCI was the absence of both m-MEPs and e-MEPs caudally to the lesion site, associated with a lack of cortical and e-SEPs cranially to the lesion site. None of these patients recovered motor function in the follow-up. A clearly detectable caudal D wave was associated with motor recovery even in deeply paraparetic patients. In one neurologically incomplete patient a reversible deterioration of m-MEPs and e-MEPs was observed during the compression-distraction manoeuvre. CONCLUSION: Intraoperative neurophysiological evaluation of SCI patients can provide information about spinal cord function that is not retrievable by other clinical means and can correctly predict neurological outcome. Intraoperative testing during early stabilization of the spine of deeply paraparetic SCI patients provides additional information about their neurological profile.

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.

Transcranial and direct cortical stimulation for motor evoked potential monitoring in intracerebral aneurysm surgery.

STUDY AIM: To analyse the parallel use of transcranial electrical stimulation (TES) and direct cortical stimulation (DCS) for eliciting muscle motor evoked potentials (MMEPs) in intracranial aneurysm surgery; to correlate permanent or transient TES- and/or DCS-MMEP changes with surgical maneuvers and clinical motor outcome. PATIENTS AND METHODS: TES and DCS were intraoperatively performed in 108 patients (51.5+/-14.7 years); MMEPs were obtained in muscles belonging to the vascular territory of interest. Monopolar, anodal stimulation was achieved with a train of five stimuli consisting of an individual pulse width of 0.5ms, an interstimulus interval of 4ms, a train repetition rate of 0.5-2Hz, and maximum stimulation intensities up to 200mA (TES) versus 25mA (DCS). RESULTS: In 95/108 (88%) patients, no changes in MMEPs occurred and none of these patients suffered a permanent severe motor deficit. In 14/108 (12%) patients, we observed nine (64%) temporary changes, four (29%) permanent deteriorations and one (7%) permanent MMEP loss. Out of 14 MMEP changes, nine (64%) occurred with TES, compared to 13 (93%) with DCS (Fishers'p=0.165). Parallel changes in TES- and DCS-MMEPs occurred in 8/14 patients (57%), in which case a permanent loss was always followed by a permanent severe motor deficit. Sixty-seven percent of all permanent changes occurred with DCS-MMEPs, compared to 33% with TES-MMEPs (p=0.567, NS). DISCUSSION AND CONCLUSIONS: In aneurysm surgery, provided that close-to-motor-threshold stimulation and the most focal stimulating electrode montage are used, TES- and DCS-MMEPs do not differ in their capacity to detect an impending lesion of the motor cortex or its efferent pathways. TES stimulation can cause significant muscular contraction during surgery, potentially disrupting the operating surgeon. DCS maintains the singular advantage of stimulating a very focal and superficial motor cortex stimulation that does not result in patient movement.

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.

The importance of brainstem mapping in brainstem surgical anatomy before the fourth ventricle and implication for intraoperative neurophysiological mapping.

Brain stem mapping (BSM) is an intraoperative neurophysiological procedure to localize cranial motor nuclei on the floor of the fourth ventricle. BSM enables neurosurgeon to understand functional anatomy on the distorted floor of the fourth ventricle, thus, it is emerging as an indispensable tool for challenging brain stem surgery. The authors described the detail of BSM with the special emphasis on its clinical application for the brain stem lesion. Surgical implications based on the result of brains stem mapping would be also informative before planning a brain stem surgery through the floor of fourth ventricle. Despite the recent advancement of MRI to depict the lesion in the brain stem, BSM remains as the only way to provide surgical anatomy in the operative field. BSM could guide a neurosurgeon to the inside of brain stem while preventing direct damage to the cranial motor nuclei on the floor of the fourth ventricle. It is expected that understanding its advantage and limitations would help neurosurgeon to perform safer surgery to the brain stem lesion.

Neuroprotective role of neurophysiological monitoring during endovascular procedures in the spinal cord.

The endovascular treatment of spinal vascular malformations places the spinal cord at risk for ischemia. When these procedures are performed using general anesthesia, the neurophysiological monitoring methods currently available provide the only means by which to assess the functional integrity of sensory and motor pathways. Neurophysiological monitoring allows a warning for the neuroradiologist of impending irreversible neurological damage so that action may be taken for the prompt restoration of adequate spinal cord perfusion. Muscle motor evoked potentials (mMEPs) better reflect spinal cord perfusion in the anterior spinal artery territory than do somatosensory evoked potentials (SEPs), although their use during spinal endovascular procedures remains anecdotal in the literature. In the study reported here we assessed: (1) the feasibility of intraoperative neurophysiological monitoring, (2) the role of provocative tests with Amytal and Xylocaine, and (3) the specific but complementary role played by SEPs and mMEPs, during endovascular embolization of spinal vascular malformations and tumors. The results suggest that: (1) neurophysiological monitoring is feasible during most endovascular procedures in the spine and spinal cord under general anesthesia, (2) provocative tests enhance the safety of the procedure, (3) mMEPs are more feasible than SEPs and more sensitive than SEPs to provocative tests. We strongly suggest the use of multimodal neurophysiological monitoring and provocative tests during the endovascular treatment of spinal and spinal cord vascular lesions.

The role of intraoperative neurophysiology in the protection or documentation of surgically induced injury to the spinal cord.

Playing both neuroprotective and educational roles, intraoperative neurophysiology has become an intrinsic part of modern neurosurgery. In this article, we present evidence substantiating the neuroprotective role of intraoperative neurophysiology, specifically its capacity to help prevent injury to the corticospinal tracts and the dorsal columns during spinal cord injury.

Neurophysiological mechanisms underlying motor evoked potentials in anesthetized humans. Part 1. Recovery time of corticospinal tract direct waves elicited by pairs of transcranial electrical stimuli.

Direct (D) corticospinal tract discharges were recorded epidurally in patients at anesthetic depths suppressing indirect (I) activity and were elicited by two equal transcranial electrical stimuli. The recovery of amplitude of the second D wave (D2) was a function of the interstimulus interval (ISI) and the stimulus duration. For example, with a 100 micros pulse, there was no response at an ISI of 1.1 ms, but partial recovery occurred with a 500 micros pulse. This indicates a relative refractory component at this ISI. Both D2 amplitude and conduction time recovered completely using a 4 ms ISI, with evidence of increased amplitude and reduced conduction time (supernormality) at longer ISIs. These findings are relevant in explaining high frequency D and I discharges and facilitation of motor responses by two transcranial magnetic pulses. Furthermore, these data help to understand why an ISI of 4 ms would be optimal in eliciting limb muscle responses when a short train of transcranial stimuli elicits only D waves in anesthetized patients (Deletis et al., Clin Neurophysiol 112 (2001) 445).

Neurophysiological mechanisms underlying motor evoked potentials in anesthetized humans. Part 2. Relationship between epidurally and muscle recorded MEPs in man.

OBJECTIVE AND METHODS: Direct (D) and transynaptic, (i.e. indirect) (I) corticospinal tract (CT) discharges were simultaneously recorded epidurally with muscle motor evoked potentials (MEPs) in patients under different levels of anesthesia. The effects of the one, two or more equal electrical stimuli, applied transcranially or directly to the motor cortex, were studied at different interstimulus intervals (ISIs) to determine the optimal conditions for eliciting I and MEP responses. RESULTS AND CONCLUSION: At anesthetic levels permiting large D and I responses to single stimuli, optimal D and I wave facilitation and MEPs occurred with two stimuli at ISIs greater than 4 ms (e.g. at 5.9 and 8 ms). When single electrical stimuli elicit only a D response, optimal MEP responses are determined by the number of stimuli and the recovery of CT fibers excitability (e.g. at an ISI of 4 ms).

Interventional neurophysiological mapping during spinal cord procedures.

As part of interventional neurophysiology's growing armamentarium, we present two intraoperative mapping techniques that can be used to guide the neurosurgeon during spinal cord procedures. They are: (1) Dorsal Column Mapping (DCM) and (2) Mapping of the Corticospinal Tract (CT) within the spinal cord. These two techniques are currently in different stages of development and clinical trials.

Intramedullar stimulation of the facial and hypoglossal nerves: estimation of the stimulated site.

AIM: To determine the stimulation site of both facial and hypoglossal nerves after transcranial magnetic stimulation. METHODS: After surgical exposure of the brainstem in 22 patients with intrinsic pontine (n=9) or medullary (n=13) tumors, the facial colliculus and the hypoglossal triangle were electrically stimulated. The EMG responses were recorded with flexible wire electrodes from the orbicularis oculi/orbicularis oris muscles, and genioglossal muscles. Patients had no preoperative deficit of the nerves. RESULTS: The EMG mean latencies of the unaffected facial nerve were 5.2+/-0.6 ms for the orbicularis oculi, and 5.2+/-0.5 ms for the orbicularis oris muscle. After the stimulation of 18 possibly affected facial nerves, the EMG mean latencies were 5.3+/-0.3 ms for the orbicularis oculi (p=0.539, unpaired Student's t-test), and 5.4+/-0.2 ms for the orbicularis oris (p=0.122). The EMG mean latency of the unaffected hypoglossal nerve was 4.1+/-0.6 ms for the genioglossal muscle. After the stimulation of 26 possibly affected hypoglossal nerves, the EMG mean latency for the genioglossal muscle was 5.3+/-0.3 ms. There was a significant difference (p<0.001) in latency for genioglossal EMG responses between the patients with pontine and those with medullary tumors. CONCLUSION: Shorter EMG mean latencies of unaffected facial nerves obtained after direct stimulation of the facial colliculi confirm that magnetic stimulation is most likely to occur closer to the nerve's exit from the brainstem than to its entrance into the internal auditory meatus. The hypoglossal nerve seems to have the site of excitation at the axon hillock of the hypoglossal motor neurons.

Prognostic value of motor evoked potentials elicited by multipulse magnetic stimulation in a surgically induced transitory lesion of the supplementary motor area: a case report.

Surgery involving the supplementary motor area (SMA) places the patient at risk of transient motor deficit. To predict outcome in patients with early postoperative hypokinesis would be relevant to both the patient and the surgical team. A 15 year old girl with a large left thalamic tumour removed through a left transcallosal approach is described. Despite intraoperatively preserved muscle motor evoked potentials (mMEPs) from all limbs, elicited by multipulse electrical stimulation, she awoke with a right hemiplegia and mutism. On the first postoperative day, neurophysiological evaluation using a multipulse magnetic stimulation technique, with a train of four magnetic stimuli, confirmed the presence of mMEPs from the hemiplegic right limbs. Slight spontaneous motor activity of the right limbs and initial speech were seen later on the same day with dramatic improvement over subsequent days. It is concluded that multiple rather than single magnetic stimulation techniques may be needed to elicit mMEPs for an early postoperative differential diagnosis of SMA damage versus injury to the primary motor cortex or the corticospinal tract.