Latency and amplitude abnormalities of the scalp far-field P14 to median nerve stimulation in multiple sclerosis. A SEP study of 122 patients recorded with a non-cephalic reference montage

The frequency and characteristics of P14 abnormalities were investigated in 122 patients with probable (68), or definite (54) multiple sclerosis by recording SEPs to median nerve stimulation with a non-cephalic reference montage. The most frequent SEP abnormality found in our series (62% of abnormal results) combined latency increase and amplitude reduction of P14. Interindividual variability, inherent in absolute amplitude measurements, was by-passed by calculating the ratio between the amplitudes of far-field P9 and P14 components, which proved to be normally distributed in controls. In spite of the strong association (P much less than 0.001) between the P9-P14 interpeak interval (IPL) and the P9/P14 amplitude ratio in MS patients, the latter parameter was found to be the only abnormality in 12 patients whose P9-P14 and P14-N20 IPLs were normal. Also IPLs were increased in 12 patients with normal P14 amplitudes. These results suggest that adding the P9/P14 amplitude criterion to standard IPL data might be useful to detect conduction troubles in MS patients.