A new method to determine pudendal nerve motor latency and central motor conduction time to the external anal sphincter.

By placing the earth electrode between the site of the stimulus and the site of derivation, peripheral motor latency to the external anal sphincter can be determined and thus central motor conduction time (CMCT) can be calculated. In 18 volunteers we found a total motor conduction time of 19.4 msec (S.D. 1.71) after stimulation of the motor cortex and recording above the external anal sphincter. Latency was 5.6 msec (S.D. 0.66) when stimulated above L1 and pudendal latency (MEPuL) after stimulation above S3 was 2.5 msec (S.D. 0.32). CMCT to L1 (TMCT minus MCT to L1) was 13.8 msec (S.D. 1.13) and to S3 (TMCT minus MEPuL) it was 16.9 msec (S.D. 1.67). This method allows us to locate spinal dysfunction more precisely and also improves diagnosis of a possible neuropathy of the pudendal nerve.