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Deep brain stimulation (DBS) can relieve dyskinesias effectively and safely. This modality is applied most commonly in the treatment of dyskinesias associated with levodopa therapy for Parkinson disease. The subthalamic nucleus (STN) and globus pallidus internus (GPi) are the most common surgical targets. Deep brain stimulation of the GP has a direct antidyskinetic effect, whereas relief of dyskinesias by DBS of the STN depends on postoperative reduction of dopaminergic medications. Outcomes are similar for DBS in these two sites despite the different mechanisms by which the stimulation relieves dyskinesias. Deep brain stimulation of the STN has become the surgical treatment of choice in many movement disorders programs but this modality has not been compared with DBS of the GPi in randomized controlled trials, and the superiority of one site over the other remains unproven. In the absence of data demonstrating superiority, selection of the stimulation target should be individualized to meet the needs of each patient. Selection of the target should be based on the patient's most disabling symptoms, response to medications (including side effects), and the goals of therapy, with consideration given to the different antidyskinetic effects of DBS of the STN and GPi.
The introduction of levodopa in the 1960s provided dramatic relief of symptoms for many patients with PD and had a profound impact on surgical therapies for this disease. Parkinson disease became viewed as a medical rather than a surgical disorder. Within a decade, however, the limitations of levodopa became apparent. It did not prevent disease progression and was frequently associated with complications and side effects, including progressive shortening of the duration of action with predictable loss of drug effect throughout the day ("wearing off"); unpredictable, abrupt loss of drug effect throughout the day ("on-off" fluctuation); and involuntary movements (dyskinesias and dystonias). These complications are observed in as many as 50% of individuals who have been treated with levodopa for more than 5 years and in as many as 70% of people who have been treated with levodopa for 10 years or more. For many patients these complications may be as disabling as the symptoms of the underlying disease and may compromise the utility of levodopa in the management of PD.
Dyskinesias can be functionally and cosmetically disabling. Reduction of dyskinesias improves motor function and the performance of activities of daily living, and it can have a significant beneficial effect on the self-esteem of individuals who are affected by these involuntary movements. Dyskinesias associated with levodopa therapy for PD can be alleviated by adjustment of the medication dose. In some cases this requires reduction of dopaminergic medications at the expense of loss of control of cardinal PD symptoms. Individuals whose PD is managed in this fashion are faced with a difficult choice between accepting more severe dyskinesias with better control of PD symptoms or less severe dyskinesias with worse control of PD symptoms. Surgical treatment can be very helpful for these individuals.
Reduction of dyskinesias by surgery can be accomplished in either of two ways. Surgical intervention (for example, DBS of the STN or subthalamotomy) can be aimed directly at reducing PD symptoms, with the hope that medications can then be reduced to provide relief from levodopa-related side effects including dyskinesias. Alternatively, surgical intervention (for example, pallidotomy, DBS of the GPi, thalamotomy, and thalamic DBS) can be aimed at treating the dyskinesias directly. In contemporary practice, DBS has become the preferred surgical treatment for movement disorders. Deep brain stimulation of the STN and the GPi has become the preferred surgical treatment for dyskinesias associated with PD. In this review these two variations of DBS treatment will be discussed in detail.
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