Median and tibial somatosensory evoked potentials. Changes in short- and long-latency components in patients with lesions of the thalamus and thalamo-cortical radiations.

Alterations in short- and long-latency components of median and tibial somatosensory evoked potentials (SEPs) were studied in patients with lesions in the thalamus and thalamo-cortical radiations. When the lesions were located primarily in the ventro-posterior thalamus, the SEP changes consisted of the following combination: absence of response; decrease in response amplitude; delay in peak latency; and attenuation of median N20-P25 and tibial P40. The laterally situated ventro-posterior lesions tended to preferentially affect tibial SEPs whereas the medially situated lesions tended to preferentially affect median SEPs. The lateral thalamic lesions affected primarily the long-latency SEP components, whereas the medial thalamic lesions affected primarily the mid-latency or the mid- and long-latency SEP components. Corona radiata infarcts produced SEP changes similar to those with the ventro-posterior thalamic lesions except that absence of evoked responses was not observed. Subcortical infarcts tended to affect the mid- and long-latency SEP components with relative preservation of the short-latency components. The present data indicate that only the lesions involving the primary thalamic relay area affected all SEP components, particularly the short-latency components, and that the lesions in other thalamic areas can also influence the SEPs, particularly the mid- and long-latency components. The present study further demonstrates that a combined use of median and tibial SEPs is useful in delineating the topographic organization of the somatosensory system in the thalamus.