[Cervical magnetic motor-evoked potential--the site of the stimulation]

Magnetic stimulation over the cervical spine using the 8-shaped coil was studied to determine the optimal site and intensity of stimulation. Twenty-one healthy volunteers and 9 patients with acute inflammatory demyelinating polyradiculoneuritis (AIDP) or chronic inflammatory demyelinating polyradiculoneuritis (CIDP) were examined. Each wing of the 8-shaped coil had a 5 cm outer diameter and the field (maximum 2.2 tesla) was produced with MAGSTIM200 (MAGSTIM company). The coil was placed along the line from the seventh cervical prominence to the acromion on the recorded side (C7 standard line) to give orthodromic stimulation to the motor nerve. The coil position was marked at the contact point of each wing. Motor evoked potential (MEP) was recorded from abductor digitorum minimi muscle by using surface electrodes. In normals, the latency of MEP did not change while the coil was moved vertically from 2 cm rostral to 2 cm caudal of C7 standard line and horizontally from the midline to 6 cm lateral. Increasing the stimulus intensity, MEP latency never got shorter when the intensity was over 180% of motor threshold. On the other hand, the MEP amplitude was highest when the stimulation was given at 4 cm lateral from the midline on the C7 standard line, but our equipment were unable to give the supramaximal stimulation. Our conclusion was therefore that the optimal stimulus point was 4 cm lateral from the midline on the C7 standard line, and the minimal latency of MEP was obtained by the stimulation with the intensity beyond 180% of motor threshold.(ABSTRACT TRUNCATED AT 250 WORDS)