Electrical stimulation of the posterior limb of the internal capsule was performed in seven patients with central pain due to vascular lesions of the thalamus or due to multiple sclerosis. Three patients suffered from thalamic hemorrhage and three others from thalamic infarction, and one from multiple sclerosis. These patients developed dysesthesia, hyperpathia and spontaneous burning pain several months after the onset of the disorders. The severe pain was not relieved by any anticonvulsant, psychopharmacological drug or analgesic drug, nor even by the intrathecal infusion of morphine.
Stimulating electrodes were implanted stereotaxically in the posterior limb of the internal capsule 25 mm lateral to the posterior commissure for one-week stimulation trials. The patients reported pain relief and sensations such as warmth or comfortableness in the appropriate portion of the body during the electrical stimulation with a square pulse of 2-3 v, 0.2 msec, 50 Hz. In the patients whose pain was relieved successfully, a receiving unit manufactured by Medtronic Co. was internalized for transduced transcutaneous stimulation. The patient with multiple sclerosis felt no pain relief during stimulation of either the posterior limb of the internal capsule or the posterior ventral nucleus of the thalamus, so the electrode was removed. Satisfactory relief of pain was obtained by 30 minute stimulation twice or three times a day for 10 to 14 days after surgery. Long-term observations from three months to two years showed that three patients had good results (incomplete pain relief with the stimulator with no medication required), two had fair results (incomplete pain relief with the stimulator with small doses of medication) and another had poor results in which the pain recurred within one month. No remarkable side effects were seen. These results indicate that internal capsule stimulation is more effective and lasts longer than the various ablative methods to relieve central pain. The mechanism of pain relief by internal capsule stimulation was discussed briefly.