N THE neurosurgical treatment of parkinsonian tremor, a major question to be ,answered is, where is the optimum site of target for the lesion? The present study is an attempt to evaluate the posterior ventrolateral area of the thalamus, internal capsule, medial region of the globus pallidus and posterior subthalamus. The comparative value of lesions in these structures was determined by (1) noting differences in reduction of tremor associated with insertion of electrodes, and (~) a comparison of the amount of radio-frequency electrical energy (expressed as Boyle-time factor) necessary to obtain over 75 per cent reduction in tremor. The primary objective of this presentation is to demonstrate that lesions in the posterior subthalamus are the most efficient for reducing parkinsonian tremor. Technique Patients. From a group of 150 parkinsonian patients with varying degrees of tremor and rigidity, only those patients having unilateral or bilateral tremor as the major finding were utilized for this study. Tremor was bilateral in the majority of the 58 patients although usually it was more pronounced on one side. The mean age of the patients was 58 and ranged from 41 to 80 years. All patients had been receiving a variety of medications before operation which did not satisfactorily control the tremor. Patients with bilateral tremor were replaced on medication postoperatively. Since these observations were made in patients distributed over the past 7 years, the traditionally desired 5-year detailed follow-up was not possible. The shortest period of follow-up was 4 months (3 patients). Operation. A total of 7~ operations were performed in the 58 patients. Bilateral operations were done in 6 patients and in 1 it was repeated. Repeated unilateral operations were performed in 6 and the rest of the patients had unilateral procedures. Operations were done with patients

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SUMMARY The effects of surgically placed thalamic lesions on cognitive skills, as measured by psychological tests, are dependent upon the nature of the skill and the site of the lesion. Lesions within the limits of the centrum medianum nucleus produce few deficits, whereas lesions outside its limits result in deficits for a greater number of skills. In general, the more complex skills revealed more impairment and the deficits were long lasting.The function of the thalamus in relation to cognition, as reflected by psychological tests, was evaluated in patients treated by thalamotomy for Parkinsonism, seizure, and pain disorders.The psychological tests are considered under four categories and correlated with lesions placed within and outside the limits of the centrum medianum nucleus. The age of patients with Parkinson's disease ranged from 37 to 72 years and those with seizure and pain disorders from 20 to 62 years. METHOD SURGICAL PROCEDURE Lytic lesions were made stereotaxically under local anaesthesia. A 20 gauge barrel electrode with a 3 to 4 mm non-insulated tip was inserted into the diencephalon through a frontal burr hole at an angle of approximately 40 to 45°. Electrical stimulation followed by an electrolytic lesion was performed as previously described (Andy, Jurko, and Sias, 1963;Andy, 1966 TEST RESULTS AND LESION SITE The data were analysed by plotting results derived from each of the tests in relation to the locus of the lesion. Scattergrams were used for plotting the lesions on horizontal planes through the thalamus at the level of the centrum medianum nucleus (+ 3-5 mm). This plane was chosen because the maximum size of the lesion was at that approximate location of the thalamus, and the greater variability in these patients was in relation to the mediolateral and frontoposterior positioning of the lesion. The statistics used, where applicable, were the 2 x 2 x2 test, and the two-tailed binomial expansion test. RESULTS

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