[A clinico-physiological study on the generators of short latency somatosensory evoked potential]

The purpose of this paper is to locate the generators of each wave component in the short latency somatosensory evoked potential (SSEP), by means of cumulative analysis of the SSEP obtained from various localized lesions in the upper cervical cord through brain stem and cerebral subcortical structures. Since there are considerable inconsistency of naming each component in the literature, SSEP to median nerve stimulation of 10 normal subjects were examined by two different recordings, i.e. recording from an electrode at the parietal scalp with a reference electrode on Erb's point (Par.-Erb), and the other at the frontal scalp with a reference electrode on Cv 7 (Fro.-Cv 7), and the SSEP was carefully studied. In normal subjects, the SSEP by Par.-Erb lead yielded 5 negative components (N 7, N 11, N 16, N 18 & N 26) and 4 positive components (P 9, P 13, P 22 & P 42), while by Fro.-Cv 7 5 negative components (N 7, N 10, N 12, N 16 & N 28) and 5 positive components (P 9, P 11, P 13, P 20 & P 44). Thirty three patients were subjected to analyse the influence of localized lesions to each component of the SSEP and the recording was evaluated in regard to (a) identification of each component, (b) latency of each component and inter-peak latency difference exceeding 2 SD, and (c) over 50% asymmetry and laterality of the amplitude. Cervical spondylotic myelopathy, high cervical cord tumor, tonsillar herniation, pontine infarct and hemorrhage, circumscribed thalamic lesion, and vascular lesion of centrum semiovale were carefully examined with CT scan and the findings were compared with neurological findings periodically. SSEP was taken repeatedly, especially before and after operative intervention, and alteration of the component was referred to the clinical progress of the lesion. In conclusion, results obtained from our present observation indicated that P 9 was the extramedullary projection, P 11 was intramedullary origin of the lower cervical cord, P 13 was medulla oblongata origin and P 13-N 16 was projection from medulla oblongata to thalamus. N 16-N 18 and N 26 were considered projection from thalamus to hand area of the parietal lobe with some association area and N 28 had the generator widely based on frontal projection system. These findings appeared to be quite useful for topographic diagnosis and functional evaluation of the lesions in central nervous system.