Central motor conduction times using transcranial stimulation and F wave latencies

The purpose of this study was to determine central motor conduction times between the motor cortex and the C8 spinal cord level (MC/C8), the motor cortex and S1 cord level (MC/S1), and C8 and S1 (C8/S1). We stimulated 29 normal subjects with a transcranial high voltage (300-500 V), short duration spike waveform (time constant 50 mu sec) and recorded over the hypothenar or calf muscles. F wave and M response latencies were used to estimate peripheral conduction time. The mean MC/C8 conduction time was 4.4 +/- 0.6 msec. Out of 26 subjects tested, 14 had discernable responses in the calf. The mean MC/S1 conduction time was 13.1 +/- 2.5 msec, and the mean estimated C8/S1 conduction time was 8.5 +/- 2.3 msec. This technique accurately measures conduction time from the motor cortex to the cervical anterior horn cells but is less reliable for monitoring conduction to the sacral cord. These values will facilitate future studies in patients with suspected lesions of the descending motor pathways.