Pallidal stimulation for dystonia

The net output of the basal ganglia is tightly regulated by the activity and balance of driving and inhibitory circuitry. In pathologic states, disrupted activity in the main outflow nucleus, the globus pallidus interna (GPi), is relayed to the motor areas of the thalamus and brainstem. The behavior of these targets receiving this disrupted outflow is consequently also disrupted, which in turn produces the profound disturbances in motor function that are characteristic of parkinsonian states and certain forms of dystonia. Therapeutic efforts are directed at reversing or canceling the pathologic basal ganglia output. When drugs are ineffective or have shortcomings, surgical approaches can be considered. It is interesting and paradoxical that elimination of this abnormal activity with destruction of the motor GPi is usually well tolerated and produces little in the way of overt motor deficit. Indeed having no motor pallidum appears to be preferable to having a pallidum generating and transmitting pathologic inputs to down-stream targets. This observation brings into question the mysterious role of the GPi in normal motor function. Nevertheless, bilateral pallidal lesions can be associated with significant adverse effects including speech difficulties and cognitive disturbances. It is for this reason that neurosurgeons have sought to develop surgical procedures that offer the efficacy of selective pallidal lesions but have a better index of safety. With the introduction of DBS to treat first chronic pain and then PD, it became logical to apply DBS to treat dystonia. There is now increasing experience in the use of DBS to treat various forms of dystonia. The initial results suggest that certain primary dystonias can show a strong improvement with GPi DBS.