Nerve conduction measurement based on lumbosacral evoked potentials

Lumbosacral evoked potentials (EP) following stimulation of the posterior tibial nerve at the ankle were recorded with surface electrodes at 5 cm intervals from the lower lumbar to the lower thoracic levels and the site of maximum amplitude was determined by monopolar and bipolar recordings in 27 normal subjects, 37 epileptics receiving anticonvulsant therapy and 22 patients with subacute myelo-optico-neuropathy (SMON). The amplitude of EP in monopolar recording with a reference on the contralateral knee was usually maximum 10 cm above the Jacoby line, which nearly corresponded to the 1st lumbar or 12th thoracic vertebral level. The entire nerve conduction velocity (ENCV) was calculated by dividing the distance from the ankle to the level of maximum amplitude by the peak latency of the N21 or P17. The latency of N21 at the level of maximum evoked potential was used for determining ENCV. The mean ENCV calculated using the peak latency N21 was 49.8 (1.97) m/sec in monopolar recording and 51.2 (2.48) m/sec in bipolar recording, and these values were considered to correspond the sensory conduction along the entire posterior tibial nerve. In epileptic patients mainly treated with anticonvulsant drugs such as phenytoin and phenobarbital, the ENCV was reduced in those treated for over 13 years and in those whose illness was poorly controlled, or large doses were necessary for seizure control. In SMON patients, a significant decrease in ENCV was found in the group under 60 years old in comparison with the normal control group.(ABSTRACT TRUNCATED AT 250 WORDS)