Medial thalamic permanent electrodes for pain control in man: an electrophysiological and clinical study.

In 7 patients with medial thalamic electrodes implanted for relief of chronic pain, clinical observations and electrophysiological recordings showed: (1) Clinically, paresthesia in the pain area and contralateral half of the body was reported as well as some unpleasant side effects. The EEG was not changed during medial thalamic stimulation. (2) Threshold for non-painful and painful perception of electrical stimuli were not significantly raised by contralateral medial thalamic stimulation. (3) After electrical median nerve stimulation, evoked thalamic potentials started after 17 msec. With ipsilateral stimulation the early components (up to 40 msec) showed longer latencies and the late components slightly shorter latencies as compared to the contralateral stimulation. (4) Median nerve evoked cortical SSEPs are not significantly changed either by a 500 msec lasting conditioning stimulation of the dorsal columns or by such stimulation of the medial thalamic structures. A facilitation at about 90 msec was found in the cortical SSEP after medial thalamic conditioning in only one of three patients tested. (5) Visual and auditory evoked potentials can be recorded in the medial thalamus as well.