The subthalamic nucleus, hemiballismus and Parkinson's disease: reappraisal of a neurosurgical dogma

The subthalamic nucleus (STN) currently is considered to play a key role in the pathophysiological origin of the parkinsonian state and is therefore t he main target for surgical treatment of Parkinson's disease. The authors r eview the incidence of hemichorea/ballism (HCB) as a complication of thalam otomy, pallidotomy or campotomy procedures before the introduction of levod opa therapy, including the few reported cases accompanied by a neuropatholo gical study. The literature shows that only a small number of parkinsonian patients with HCB had a lesion of the STN, Preliminary data in Parkinson's disease patients submitted to a subthalamotomy with current functional ster eotaxy also indicate that HCB is a very rare complication, To explain this observation, we suggest that the parkinsonian state is characterized by an increased threshold for the induction of dyskinesia following STN lesioning , This arises as a consequence of reduced activity in the 'direct' GABA pro jection to the globus pallidus medialis (GPm) which accompanies dopamine de pletion. Lesioning of the STN reduces excitation of the GPm, and theoretica lly this should induce dyskinesias. However, STN lesion also, simultaneousl y, further reduces the hypoactivity in the globus pallidus lateralis (GPI) that is feature of Parkinson's disease, and hence may compensate for GPm hy poactivity, thus self-stabilizing basal ganglia output activity and reducin g the risk of HCB, We conclude that lesioning of the STN in Parkinson's dis ease is a feasible approach in some circumstances.