Alterations of motor evoked potentials by thalamotomy.

To evaluate the effect of stereotactic thalamotomy on the function of the corticospinal tract, we studied motor evoked potentials (MEPs) recorded by surface electromyography (EMG) in the left extensor carpi radialis (ECR) and flexor carpi radialis (FCR) with magnetic stimulation of the contralateral motor cortex in a 43-year-old patient with a severe postural and resting tremor of the left hand. The patient was diagnosed eight years previously with left hemiparkinsonism. The tremor was unresponsive to various medications. After thalamotomy the tremor had disappeared, confirmed by EMG studies. MEP latencies at rest were normal and did not change after thalamotomy. Volitional contraction of either ECR or FCR shortened the latency of the corresponding MEP before and after thalamotomy. However, before thalamotomy responses at rest were less well synchronized and followed by EMG silence with subsequent long duration tonic after discharges. Furthermore, during voluntary contraction the responses only slightly enhanced. After surgery MEPs at rest in both muscles were more synchronized and after-discharges had disappeared. Moreover, with volitional contraction of either ECR of FCR, the MEPs enhanced more dramatically. The silent periods (SPs) following the MEP during sustained voluntary contraction were longer after thalamotomy. The consistent MEP latencies suggest that the conduction of the pyramidal tract is unaffected by thalamotomy. The better synchronized responses, the alleviation of after-discharges and the longer SPs in this patient with hemiparkinsonism following thalamotomy suggest an improved sensorimotor integration, which may be the result of a reduced thalamic input onto suprasegmental levels.