Essential tremor (ET) is the most common type of movement disorder, affecting approximately four out of 1000 people, and is significantly more common, though less recognized, than Parkinson’s disease. ET affects men and women equally and is inherited as an autosomal-dominant condition in about 60 percent of cases.
Although often referred to as benign essential tremor, it is hardly benign in patients who may not be able to write legibly, hold a glass of water or use a knife and fork. ET is primarily an action tremor of the upper extremities but may involve resting tremor of the head and neck and/or lower jaw, and also tremor of the voice. The latter may be so severe that speech becomes unintelligible.
Medication and surgical treatment options
Primidone and beta blockers are useful in reducing tremor in the early stages of ET, but as the tremor progresses, medical management often becomes less effective or side effects can prevent the use of adequate doses of medication. ET patients then are candidates for surgical or radiosurgical treatment.
The mainstay of the surgical treatment of ET is deep brain stimulation (DBS), in which an electrode is implanted in the ventral intermediate nucleus (VIM) of the thalamus. Neurosurgeons Peter Nora, M.D., and Ryder Gwinn, M.D., have been implanting DBS electrodes at Swedish Medical Center for several years. The treatment is effective, but it requires implantation of permanent hardware (wires and batteries) into the brain and chest wall. Patients who take anticoagulants or have severe cardiovascular disease are not suitable candidates for DBS. These patients, however, may be candidates for radiosurgical treatment.
A new option for difficult-to-treat patients
A Gamma Knife® device is used to focus high-energy gamma rays precisely on the VIM in a single outpatient treatment. This results in the death of the brain cells that generate ET. We recently published our experience in the radiosurgical treatment of more than 200 patients with ET. More than 80 percent of the treated patients showed significant reduction in tremor, including nearly 50 percent who were entirely free of tremor. Patients could write legibly, drink without spilling and, in general, perform activities of daily living with minimal or no disability (see figure). These results are comparable to the data published for the use of DBS in the treatment of ET. Follow-up MRI scans in these patients demonstrated the development of lesions of about 6 mm in diameter precisely located in the VIM thalamus. Eight percent of patients developed lesions which were larger than expected. These large lesions occasionally resulted in sensory loss, hemiparesis or speech difficulty. In about half of these patients, the neurological symptoms recovered over time, so the permanent rate of complications was about 4 percent. At present we have not been able to identify what factors may predict the development of an excessively large lesion.
Gamma Knife radiosurgery offers a means for the treatment of ET that is safe and effective without the need for implanted hardware. The rate of tremor relief and the complications compare favorably with DBS. Radiosurgery can be offered to all patients who are candidates for surgical treatment of ET, but it is particularly useful for patients who are not candidates for DBS.