The purpose of this study is to examine effects (good and bad) of gamma knife radiosurgery for essential tremor. The gamma knife places a small lesion in the brain to suppress tremors.

This research is being done because although multiple studies evaluating gamma knife for essential tremor show good results, few studies evaluating patients prospectively have been done. Prior studies have looked at patient outcomes following treatment - also known as retrospective studies. In this study, data collection will initiate before treatment, to obtain consistent baseline evaluations from all study participants undergoing the gamma knife treatment and at specific intervals following treatment.

Inclusion Criteria:

• Men and women age 18 years or older
• Diagnosis of essential tremor as confirmed from clinical history and examination by a neurologist or neurosurgeon
• Tremor following prior first line therapy with either propranolol or primidone
• Must be able to undergo MRI of brain and CT of head for treatment planning
• Postural or intention tremor severity score of greater than or equal to 2 in one hand/arm as measured by the CRST rating scale

Exclusion Criteria:

• Prior ipsilateral stereotactic radiosurgical ablation of the thalamus, ipsilateral deep brain stimulation, radiofrequency ablation of a ipsilateral thalamic target, or ultrasound ablation of an ipsilateral thalamic target
• Prior whole brain radiation therapy
• Pregnancy