[Correlation between the analgesic effect by thalamic relay nucleus stimulation and somatosensory evoked potentials...

Electric stimulation of the thalamic sensory relay nucleus (Vc) has an analgesic effect on deafferentation pain, however, the analgesic effect differs from patient to patient. Electrode position and state of the substrate stimulated are considered important factors influencing the analgesic effect. In order to determine the best position for the stimulating electrodes, we recorded somatosensory evoked potentials (SEPs) from stimulating electrodes implanted in the Vc and compared thalamic SEPs with the analgesic effect of Vc stimulation. The subjects were thirteen patients with deafferentation pain, four patients with thalamic lesions, seven patients with suprathalamic lesions and two patients with infrathalamic lesions. We inserted the electrode array into the Vc stereotactically, and fixed it so that stimulation-induced paresthesia would cover the painful frea. The electrode array consisted of the four contact points of four electrodes spaced at 2 mm intervals within 10 mm from the tip. Using bipolar combinations of the four electrodes (twelve combinations in all), we stimulated the Vc for about half an hour with each combination. We then rated the degree (%) of analgesia as 100% when pain disappeared and 0% when there was no change. Thalamic SEPs elicited after stimulation of the contralateral median nerve were recorded from all four contact points simultaneously. The latencies, amplitudes and recorded positions of large early positive components (P1) followed by large negative components (N1) with latencies between 10 and 20 msec were then analyzed and compared with the best electrode combination for optimal pain relief and with the degree of analgesia.(ABSTRACT TRUNCATED AT 250 WORDS)