Senichi Ishii

Warning thresholds on the basis of origin of amplitude changes in transcranial electrical motor-evoked potential monitoring for cervical compression myelopathy

Study Design. A retrospective analysis of prospectively collected data from consecutive patients undergoing transcranial electrical motor-evoked potential (TCE-MEP: compound muscle action potentials) monitoring during cervical spine surgery. Objective. To divide the warning threshold of TCE-MEP amplitude changes on the basis of origin into the spinal tract and spinal segments and decide warning thresholds for each. Summary of Background Data. The parameter commonly used for the warning threshold in TCE-MEP monitoring is wave amplitude, but amplitude changes have not been examined by anatomical origin. Methods. Intraoperative TCE-MEP amplitude changes were reviewed for 357 patients with cervical myelopathy. Most of the patients were monitored by transcranial electrical stimulated spinal-evoked potential combined with TCE-MEP. The warning threshold of TCE-MEP was taken as waveform disappearance. For each patient, amplitude changes were separated, according to origin, into the spinal tract and spinal segments and compared with clinical outcome. Results. Assessable TCE-MEP waves were obtained in 350 cases. Disappearance of TCE-MEP waves, which were innervated by the spinal levels exposed to the surgical invasion, was seen in 11 cases. Disappearance of TCE-MEPs, which were innervated by the spinal levels inferior to them, was seen in 43 cases. There was no postoperative motor deficit in those cases. However, such deficits caused by spinal segment injury were seen in 2 cases, which showed that intraoperative amplitude decreased to 4.5% and 27%. Conclusion. If we had established the warning threshold as 30% of the control amplitude, we would likely have prevented both cases of postoperative motor deficits, but 106 (30.3%) cases would have become positive cases. If we had established the warning threshold separately as wave disappearance for the spinal tract and 30% of the control amplitude for the spinal segments, sensitivity and specificity would have been 100% and 83.7%, respectively. Dividing the warning threshold on the basis of origin of amplitude changes could reduce false-positive cases and prevent intraoperative injuries.

Functional Imaging of Spinal Cord Electrical Activity From Its Evoked Magnetic Field

This paper investigates dynamic source imaging of the spinal cord electrophysiological activity from its evoked magnetic field by applying the spatial filter version of standardized low-resolution brain electromagnetic tomography (sLORETA). Our computer simulation shows that the sLORETA-based spatial filter can reconstruct the four current sources typically associated with the elicitation of the spinal cord evoked magnetic field (SCEF). The results from animal experiments show that significant changes in the latency and intensity of the reconstructed volume current arise near the location of the artificial incomplete conduction block. The results from the human SCEF show that the SCEF source imaging can visualize the dynamics of the volume currents and other nerve electrical activity propagating along the human spinal cord. These experimental results demonstrate the potential of SCEF source imaging as a future clinical tool for diagnosing cervical spinal cord disorders.

Efficacy of biphasic transcranial electric stimulation in intraoperative motor evoked potential monitoring for cervical compression myelopathy.

Study Design. Retrospective analysis of prospectively collected data from consecutive patients undergoing 2 methods of transcranial electrical motor evoked potential (TCE-MEP) monitoring during cervical spine surgery. Objective. To investigate the efficacy of biphasic transcranial electric stimulation, the deviation rate, amplitude of TCE-MEPs, complications, and sensitivity and specificity of TCE-MEP monitoring were compared between the biphasic and conventional monophasic stimulation methods. Summary of Background Data. With biphasic stimulation, unlike monophasic stimulation, measurement time can be reduced considerably because a single stimulation elicits bilateral responses almost simultaneously. However, no study has yet reported a detailed comparison of the 2 methods. Methods. Examination 1: Amplitude and derivation rate of TCE-MEPs was compared for monophasic and biphasic stimulation in the same 31 patients with cervical compression myelopathy. Examination 2: Sensitivity, specificity, and complications of TCE-MEP monitoring were compared in 200 patients with cervical compression myelopathy who received monophasic or biphasic stimulation (100 patients each) during intraoperative monitoring. Results. Examination 1: Derivation rates of biphasic stimulation in the deltoid, biceps brachii, abductor digiti minimi, and flexor hallucis brevis muscles were the same or higher than for monophasic stimulation. TCE-MEP amplitudes elicited by biphasic stimulation compared with monophasic stimulation were significantly larger in the biceps (paired t, P < 0.0001), but similar in the other 3 muscles. Examination 2: In the biphasic and monophasic stimulation groups, warnings were issued to surgeons in 10 and 11 cases, for a sensitivity of 100% for both groups and specificity of 97.8% and 96.7%, respectively. No complications related to stimulation were observed in any of the 200 patients. Conclusion. Biphasic stimulation had similar or higher derivation rates and equivalent sensitivity and specificity than monophasic stimulation. No complications were observed for either stimulation method. Biphasic stimulation is an effective TCE-MEP monitoring method for cervical spine surgery that may also reduce measurement time. Level of Evidence: 4

Conductive neuromagnetic fields in the lumbar spinal canal.

OBJECTIVE To measure neuromagnetic evoked fields in the lumbar spinal canal. METHODS Using a newly developed superconducting quantum interference device (SQUID) fluxmeter, neuromagnetic fields of 5 healthy male volunteers were measured at the surface of the lower back after stimulation of the tibial nerves at the ankles. For validation, we inserted a catheter-type electrode percutaneously in the lumbar epidural space in 2 of the subjects and measured cauda equina action potentials after tibial nerve stimulation. RESULTS Neuromagnetic fields propagating from the intervertebral foramina into the spinal canal were measured, and the latencies of the magnetic fields corresponded largely with those of the cauda equina action potentials. CONCLUSIONS We successfully measured ascending neuromagnetic fields originating at the nerve root and the cauda equina with high spatial resolution. Future studies will determine whether neuromagnetic field measurement of the lumbar spine can be a useful diagnostic method for the identification of the disordered site in spinal nerves. SIGNIFICANCE We successfully measured neuromagnetic fields in the lumbar spinal canal, which have previously been difficult to verify. Future studies will determine whether neuromagnetic field measurement of the lumbar spine can be a useful diagnostic method for identifying disorders of spinal nerves.

Diagnosis of incomplete conduction block of spinal cord from skin surface using spinal cord evoked magnetic fields

BackgroundWe previously reported the usefulness of neuromagnetic recordings for the diagnosis of disorders in peripheral nerves or the spinal cord. However, there have been no reports on incomplete conduction block of the spinal cord, which is clinically common in conditions such as cervical myelopathy. Here, we estimated the usefulness of measuring spinal cord evoked magnetic fields for evaluating incomplete conduction block.MethodsIncomplete conduction block models of the spinal cord of the rabbit were established using a Fogarty balloon catheter that was inserted into the epidural space of the cervical spine. Electrical stimuli were applied to the lower thoracic spinal cord with an epidural catheter electrode. Spinal cord evoked potentials were recorded using epidural electrodes. Spinal cord evoked magnetic fields were recorded over the skin surface of the neck using a biomagnetometer.ResultsThe decrease in the conduction velocity and amplitude at the compression site could be detected by spinal cord evoked potentials from the epidural space, confirming the spinal cord lesion. The waveforms of the magnetic fields showed a biphasic configuration. The distribution of magnetic fields showed a characteristic quadrupolar pattern propagating from caudal to cranial. After compression, the amplitude and the conduction velocity of the magnetic fields decreased, and the distribution of magnetic fields were attenuated and decelerated near the compression site especially in the trailing magnetic fields. Diagnosis of the incomplete conduction block was thus possible.ConclusionsWe report the first measurement of the spinal cord evoked magnetic field in the intact spinal cord from the skin surface and that it can be applied to incomplete conduction block of the injured spinal cord. The use of a biomagnetometer is promising as a less-invasive method for clinically evaluating spinal cord function.

Magnetospinography visualizes electrophysiological activity in the cervical spinal cord

Diagnosis of nervous system disease is greatly aided by functional assessments and imaging techniques that localize neural activity abnormalities. Electrophysiological methods are helpful but often insufficient to locate neural lesions precisely. One proposed noninvasive alternative is magnetoneurography (MNG); we have developed MNG of the spinal cord (magnetospinography, MSG). Using a 120-channel superconducting quantum interference device biomagnetometer system in a magnetically shielded room, cervical spinal cord evoked magnetic fields (SCEFs) were recorded after stimulation of the lower thoracic cord in healthy subjects and a patient with cervical spondylotic myelopathy and after median nerve stimulation in healthy subjects. Electrophysiological activities in the spinal cord were reconstructed from SCEFs and visualized by a spatial filter, a recursive null-steering beamformer. Here, we show for the first time that MSG with high spatial and temporal resolution can be used to map electrophysiological activities in the cervical spinal cord and spinal nerve.

17. Spatial filter imaging of nerve electrical activities in the lumbar spine

The aim of this studywas to develop amethod for functional imaging of human nerve electrical activities in the lumbar spine from evoked neuromagnetic measurements. The source reconstruction of nerve electrical activities was carried out over a curved surface that contained the nerve fiber in the lumbar spine. This curved surface was determined by using subject’s lateral X-ray image. We proposed to apply the Unit-Gain Constraint MinimumNorm filter with a Recursively Update Gram matrix spatial filter to reconstruct source activities on this surface. Reconstructed source intensity reflected cauda equine travel. This linewas determined by using reconstructed source imaging of the front direction. Reconstructed sources propagated from caudal to rostral along the lumbar spinal canal. During the time courses at five selected voxels along the nerve travel, each time course showed a clear peak and into latency indicated the instants when nerve activity arrived at five voxel locations. Our proposed imaging method can clearly visualize the dynamics and the temporal intensity changes of nerve electrical activities in the lumbar spine.

S28-5 Multimodal intraoperative spinal cord monitoring during cervical spine surgery

through: (1) Somatosensory evoked potential phase reversal technique; (2) DCS with a short train of 5 7 monopolar stimuli (0.5 ms duration, ISI 4.1, intensity up to 20 mA) at 1 2 Hz. To monitor motor pathways during tumor removal, MEPs are recorded from controlateral limb muscles after either DCS and/or transcranial electrical stimulation. When approaching tumor borders, MEP monitoring is combined with periodical direct subcortical stimulation (DSS) to localize the corticospinal tract (CT) and guide resection.A more than 75% drop in transcranial MEP amplitude at the end of surgery, and a DSS thresholds lower than 3 4 mA are associated to post-operative neurological worsening. However these two neurophysiological warning signs are not significantly related and do not necessarily identify the same patients, suggesting that continuous MEP monitoring and periodical subcortical mapping should be combined to minimize risks. Recently, thanks to the introduction of diffusion tensor imaging and neuronavigation, there has been an increasing interest for the correlation of fiber tractography with subcortical stimulation. This work is providing some degree of reliability in the assessment of subcortical current spreading. Preliminary data suggest that 1 mA = 1 mm may be an acceptable practical parameter to judge on the distance from the CT, when using subcortical mapping.

P33-26 Neuromagnetic field measurement in the lumbar spine

cue. The timing of the cue was used as the trigger and 50 trials were performed. The data were analyzed by the equivalent current dipole (ECD) procedure and b band event-related desynchronization (ERD). The estimated location of the ECD, peak of the distribution of b-band ERD sources, and hand motor cortex in MRI were compared in a threedimensional coordinate system. To compare the motor imaginary and motor execution, the hand clenching task was assigned consecutively in the same subjects and analyzed by the same methods. Results: In motor imaginary, event-related activations were recognized in the contralateral frontal cortex approximately 50 250 ms after the cue in ECD and 0 600 ms after the cue in b band ERD. These activated areas were well-matched with the motor cortex in MRI, and considered to show activation in the primary motor area. There were no differences in the coordinate axis in comparing with the motor execution task. However, both of the analyses had low sensitivities for detection of the primary motor area. The low sensitivity in ECD was thought to be because the timing of the cue and the execution of motor imaginary could not be matched precisely. In ERD, this was because there were some cases in which significant ERD did not emerge, and widespread ERD in the occipital area by the visual stimulus masked the ERD around the motor cortex. Conclusion: We detected the activated area by two different methods of analysis in magnetoencephalography, i.e., ECD and ERD. The location of the area matched that of the primary motor cortex. This corresponded well with the results of previous functional MRI studies.

33. Diagnosis of incomplete conduction block in compressive spinal cord using spinal cord evoked magnetic fields from peripheral nerve stimulation

and nerve conduction velocities in 73 consecutive patients with ALS, 12 with spinal muscular atrophy (SMA), 12 with spinal-bulbar muscular atrophy (SBMA), and 36 with axonal neuropathy due to vasculitis (VN). Multiple axonal excitability measurements were performed for the median nerve in the ALS patients. The DML was prominently prolonged (=5.4 ms: over 125% of the upper limit of normal) in 8 ALS patients (11%), 1 SMA/SBMA patient (4.2%) and no VN patients (0%). In threshold electrotonus, the results of the ALS patients with markedly prolonged DML showed a tendency that the axonal resting membrane potential shifts in the depolarizing direction. Our findings suggest that, in ALS, there is a subgroup with prolonged DML unexplained by merely loss of the fastest axons, which is rare in other motor neuron diseases and neuropathies. The robust resting membrane depolarization in motor axon may result in inactivation of sodium channels, and thereby distal nerve conduction slowing in the terminal stage of ALS.