This is a pilot study to evaluate the safety and efficacy of unilateral gamma knife subthalamotomy for Parkinson's disease in patients deemed poor candidates for deep brain stimulation.

The goal of the study is to enroll 20 subjects who could benefit from unilateral subthalamotomy due to the presence of unilateral rigidity, tremor, bradykinesia and/or dyskinesias (as opposed to axial symptoms such as walking) but who are suboptimal candidates for deep brain stimulation (DBS) because of advanced age (>74), medical comorbidities precluding surgery, or patient aversion to DBS, and who show more than 30% improvement in Unified Parkinson's Disease Rating Scale (UPDRS) score in the ON versus OFF medication state.

Secondary objectives will focus on the efficacy of Gamma Knife (GK) subthalamotomy for Parkinson's disease (PD), as determined by changes in the patient's clinical status and any improvement following the treatment. A full neurologic exam as well as a screen for depression will be administered at every postoperative visit. The patient's neurologic function will be assessed with a full neurological examination and objectively quantified using the UPDRS focusing on the contralateral side, as well as overall UPDRS.

The clinician will complete a Clinical Global Impression (CGI) and Mini-Mental State Examination (MMSE) at each visit to record overall impression of the patient's disease progression and severity, and to evaluate cognitive function. Pre- and post-treatment assessment of the Levodopa Equivalent Daily Dose (LEDD) (quantification of medication regimen in levodopa equivalents) will be obtained and recorded to evaluated the efficacy of gamma knife subthalamotomy (GKS) as a treatment for Parkinson's disease (PD). A Parkinson's Disease Questionnaire-39 (PDQ-39) to determine the patient's independence and quality of life will be filled out at each visit. Finally, the patient will complete a Beck Depression Inventory at each clinic visit to monitor for development or progression of depressive symptoms.

Inclusion Criteria:

1. Otherwise appropriate DBS candidates but deemed suboptimal for DBS on account of one or more of the following:

   • advanced age greater than 74 years of age

   • medical comorbidities

     • need for continual anticoagulation
     • poor candidates for general anesthesia based on cardiac, hepatic ,renal or pulmonary status
     • high infection risk if internal DBS device hardware is placed (ie. Poor diabetes mellitus control)

   • patient aversion to DBS

2. Patients with asymmetric disease symptoms including:

   • rigidity
   • tremor
   • bradykinesia
   • dyskinesia

3. More than 30% improvement in UPDRS score in ON versus OFF medication state, except:

   • in the presence of disabling contralateral dyskinesias/dystonia limiting dose tolerance
   • tremors that are medically refractory

4. Patients who are on a stable medication regimen and demonstrating stable disease symptoms (absence of significant improvement or deterioration) for at least 3 months at the time of GKS, as determined by review of medical record

5. MMSE score greater than or equal to 26

Exclusion Criteria:

• Patients with prominent axial symptoms including postural instability and gait disturbances, defined as a score of 3 or greater on Questions 28, 29 & 30 on the UPDRS
• Patients whose symptoms do not improve by more than 30% in UPDRS score with medication, unless patient experiences dose-limiting side effects
• The presence of dementia or inability to provide informed consent for participation in the study, as determined by a mini-mental status examination score below 25
• Patients who have had prior brain radiation
• Patients with improvement and satisfactory symptom management will not be offered this procedure. We will only include patients without satisfactory symptom management.

Primary care clinic