Akira Ofuji

The effect of current direction induced by transcranial magnetic stimulation on the corticospinal excitability in human brain

Evoked spinal cord potentials (ESCPs) from the cervical epidural space and motor evoked potentials (MEPs) from the hand muscles were recorded simultaneously in 6 subjects following transcranial magnetic stimulation in two different coil orientations on motor cortex. The onset latency of the MEPs was approximately 1 ms shorter when the induced current flowed in a latero-medial direction (L-M stimulation) on the motor cortex as compared to a postero-anterior direction (P-A stimulation). Hence, L-M stimulation elicited an earlier component of the ESCPs than that induced by P-A stimulation. During general anesthesia with Sevoflurane, only the first component of the ESCPs could be elicited routinely following L-M stimulation. In contrast, all components of the ESCPs were dramatically attenuated following P-A stimulation. Moreover, first component latency of the ESCPs induced by L-M stimulation was almost the same as that induced by transcranial anodal electrical stimulation. These results suggest that if the induced current following transcranial magnetic stimulation flows in a latero-medial direction on motor cortex, it preferentially stimulates the corticospinal tract non-synaptically (producing a D-wave). However, if the induced current flows in a postero-anterior direction, it preferentially stimulates the corticospinal tract trans-synaptically (producing I-waves). Therefore, the direction of magnetically induced current is crucial in determining corticospinal excitability in the human brain.

Cutaneous silent period in syringomyelia

Painful cutaneous nerve stimulation can suppress measured. F waves over 100 mV in peak-to-peak amplitude were considered significant. electromyographic activity in voluntarily contracting muscle.3–5,10,11 This phenomenon is designated the cuMotor evoked potentials (MEPs) following transcranial magnetic stimulation were recorded from the taneous silent period (CSP). In the present study we investigated a clinical application of CSP in syringoAPB during 30% of maximal contraction. The center of a round coil 14 cm in diameter (Magstim, Whitmyelia associated with Chiari I malformation, where the loss of pain sensation is the most common early land, UK, Model 200) was held over the vertex. Induced current was applied to target motor cortex in neurologic sign.7,8 a posteroanterior direction. The stimulus intensity was set at 30% above the threshold of the MEPs. MATERIALS AND METHODS Somatosensory evoked potentials (SSEPs) followFive male patients (mean age 18 years) with cervical ing median nerve stimulation at the wrist were resyringomyelia were studied. Dysesthesias and recorded. Spinal responses were recorded with active duced pain sensation were the only symptoms in all electrodes placed over the spinous process of the patients. Fifteen healthy subjects (12 men, 3 women; sixth cervical vertebra (Cv6) and reference electrodes mean age, 32 years) were studied as controls. The placed above the thyroid cartilage.9 Peripheral nerve experimental procedure was explained and informed potentials from Erb’s point were recorded with the consent was obtained. same reference. Cortical responses were recorded Several CSPs following index finger stimulation from the parietal scalp with both ears connected as were recorded from the abductor pollicis brevis a reference (A1 1 A2). (APB) using self-adhesive electrodes over the belly All potentials except for SSEPs were recorded and tendon. Electrical stimuli were applied randomly with a bandpass filter between 20 Hz and 5 kHz. from ring electrodes using 0.5 ms square pulses at 10 SSEPs were filtered between 20 Hz and 3 kHz. The times the sensory threshold. The degree of isometric parameters of all the responses were expressed as contraction was maintained at about 30% of maximean 6 SD. mum using an audiovisual biofeedback system. From sagittal and axial T1and T2-weighted magCompound muscle action potentials (CMAPs) netic resonance imaging (MRI), the location and and F waves following supramaximal stimulation to laterality of the syrinxes was confirmed. the median nerve at the wrist were recorded. Onset latency and baseline-to-negative peak amplitude of RESULTS CMAPs were measured. The persistence and shortest latency for F waves among 20 responses also were Controls. The onset and duration of CSPs were 74.7 6 8.1 ms and 44.9 6 10.6 ms, respectively. CMAPs latency and amplitude were 3.5 6 0.3 ms and 7.2 6 2.8 mV, respectively. F-wave latency was 26.8 6 2.2 ms with 40.3 6 10.5% persistence. MEPs *Correspondence to: Dr. K. Kaneko latency and amplitude (peak-to-peak) were 19.5 6 CCC 0148-639X/97/070884-03  1997 John Wiley & Sons, Inc. 1.5 ms and 4.0 6 2.5 mV, respectively. For SSEPs,

New method to measure central motor conduction time using transcranial magnetic stimulation and T-response

Measuring central motor conduction time (CMCT) is one of the useful methods to detect an impaired level of the spinal segment in cervical myelopathy patients. We modified a new technique to calculate the CMCT using tendon reflex latency (T-response) and investigated its accuracy. Motor-evoked potentials (MEPs) following transcranial stimulation were recorded in 19 patients with cervical myelopathy caused by a single level of spinal cord compression. CMCT was measured by subtracting the peripheral conduction time, which was calculated by using the T-response for the biceps brachii muscle (Biceps), the compound muscle action potentials (CMAPs) and the F-wave of the abductor digiti minimi muscle (ADM). In the control subjects, the mean value of CMCT of the Biceps and ADM was 3.8 and 7.0 ms, respectively. The accuracy of the determination of the CMCT for Biceps using T-response was investigated beforehand in the unilateral brachial plexus palsy patients and thoracic spinal cord myelopathy patients. The calculated CMCT (3.88+/-0.65 ms) for Biceps was close to the N2 latency (4.06+/-0.3 ms) of the evoked spinal cord potentials which were recorded from the epidural space on the C3-4 vertebral level following transcranial magnetic stimulation. The CMCT of both the Biceps and ADM was delayed in all cases of C1-2 cord compression. In patients with cord compression on the C3-4 level, two of four patients showed CMCT prolongation in Biceps. The prolongation of CMCT was observed only in ADM in patients with C4-5 or C5-6 cord compression. Measurement of the CMCT using T-responses was useful in proximal limb muscles. Comparison of the CMCT in Biceps and ADM could allow us to better detect the functional level diagnosis for compressive cervical myelopathy.

Effect of coil position and stimulus intensity in transcranial magnetic stimulation on human brain

Evoked spinal cord potentials (ESCPs) from the cervical and high thoracic epidural space following transcranial magnetic stimulation were recorded from eight subjects in awake and anesthetized condition. Motor evoked potentials (MEPs) from the right abductor digiti minimi (ADM) and rectus femoris (RF) muscles were simultaneously recorded during voluntary contraction. The stimulus intensity was at 30% above the MEPs threshold of the ADM when the coil center was fixed on 10-20 international Cz position. In awake condition, multiple ESCP components (greater than 3) were recorded from the cervical epidural space but no or minimal components were recorded from the upper thoracic epidural space. When the coil was moved anteriorly so that the posterior edge of the coil was positioned on Cz, the amplitude of the first ESCP component was significantly increased (P < 0.02) and shortened (not significant) at cervical levels. In addition, several ESCP components were more evident at high thoracic levels. Although the amplitude of the ADM was not enhanced, that of the RF was enhanced. During general anesthesia with volatile anesthetics (sevoflurane), only the first component of the ESCPs (D-wave) was elicited. Its amplitude was enhanced (P < 0.02) when the coil edge was fixed on Cz, similar to results in awake condition. This enhancement of the first ESCP component was accompanied by enhancement of those recorded from the high thoracic epidural space. However the amplitude of D-wave was the same in the two different coil positions when the stimulus intensity was set a 100% of the output. These results suggest that at low stimulus intensity, positioning the coil edge on Cz is optimal in inducing D-wave effectively but at high stimulus intensity, D-wave generation can be achieved in either if the two different coil position.

Differential recording of upper and lower cervical N13 responses and their contribution to scalp recorded responses in median nerve somatosensory evoked potentials

To distinguish the different origins of cervical N13 potentials in median nerve somatosensory evoked potentials (SSEPs), cervical N13 potentials were recorded by two different montages. The abnormal patterns of the SSEPs were compared to the abnormal evoked spinal cord responses (ESCPs) recorded from posterior epidural space in 13 patients with various cervical lesions. SSEPs from the posterior cervical surface were recorded from the mid-cervical level with anterior neck reference (Cv5-AN) and from the upper cervical level with inion reference (Cv2-IN). Scalp responses were recorded from the parietal region contralateral to the stimulating side with non-cephalic reference (shoulder contralateral to stimulating side). ESCPs were recorded from the posterior epidural space using catheter electrodes or needle electrodes inserted into the ligamentum flavum. Lower cervical N13 (LC-N13) recorded from the Cv5-AN montage showed similar latency to upper cervical N13 (UC-N13) recorded from the Cv2-IN montage. The latency of the early part of the P13-P14 complex in the scalp montage was similar to that of the UC-N13 and the negative peak latency of the ESCPs recorded at the C2-3 level. Attenuation of the LC-N13 and relatively preserved UC-N13 and P13-P14 were characteristic in patients with cervical syringomyelia and compression cervical myelopathy at the mid-cervical levels. Attenuation of the UC-N13 with normal LC-N13 was characteristic in patients with cervical spondylotic myelopathy who showed conduction blockade of the ESCPs at the C3-4 level. In a patient with schwannoma at the C1-2 level, conduction blockade of the ESCPs was observed at the C1-2 level. P13 was normal but P14 was prolonged. UC-N13 and P13 latencies were similar to the negative peak latency of the ESCPs at the C2-3 level. We demonstrated that two different cervical N13 potentials can be recorded by two different montages and they represent different behavior in various spinal cord lesions. In addition, at least the early part of the P13-P14 complex originates in the upper cervical region. To distinguish two different cervical N13, it is useful to detect not only the cervical pathology but also the symptomatic cervical cord compression level in patients with cervical myelopathy.

Spatial distribution of corticospinal potentials following transcranial electric and magnetic stimulation in human spinal cord

To investigate the spatial distribution of the human corticospinal tract in the spinal cord, evoked spinal cord potentials (ESCPs) following transcranial electrical and magnetic stimulation were recorded simultaneously from both the anterior and posterior epidural space in five anesthetized patients. One ESCP component following transcranial electrical stimulation (D-wave) and at least two ESCP components (initially D-wave and later I-wave) following transcranial magnetic stimulation were recorded in all subjects. The negative peak latency of all the potentials recorded from the posterior epidural space was the same as that recorded anteriorly. The amplitude ratio of the ESCP following electrical stimulation (posterior/anterior) was 1.10+/-0.12, while that of ESCPs following magnetic stimulation was 1.08+/-0.12 (N1) and 1.15+/-0.16 (N2). These results suggest that lateral corticospinal tract descending dorsolateral fasciculus in the spinal cord is main corticospinal pathway and spatial distribution of D and I-waves are similar in the human cervical cord.