Somatosensory evoked potentials: variability analysis in unilateral hemispheric disease.

A new method to study SEP using signal-to-noise estimate was applied to 35 patients (age 3 days-81 years, mean 12.1 years) with unilateral supratentorial cerebral lesions. The patients were grouped according to severity of deficits: minimal, mild, moderate and severe. Based on the observation that the SEPs were much more variable over the diseased hemisphere, we proposed the following hypothesis: normal neurons when stimulated generate a series of wave forms which are stable and reproducible. These are seen as stable and 'clean' scalp EPs. Diseased neurons on the other hand are malfunctioning and may tend to depolarize erratically. This can be seen as variable, 'noisy' and poorly reproducible EPs. To test this hypothesis, we measured the degree of variability of the SEPs on both the normal and affected sides, using the technique of Wong and Bickford (1980). Overall, 30 of the 36 SEPs (83%) had a greater variability on the diseased side. The prediction accuracy was greatest with the severely affected cases and least in the minimally affected cases. There are minor suggestions that the variability parameter may be useful for long-term follow-up. Clearly, if used with care, the amount of 'noise' in an EP can be a useful parameter.