Long-term Thalamic Deep Brain Stimulation for Essential Tremor: Clinical Outcome and Stimulation Parameters.

BACKGROUND: The reasons underlying the loss of efficacy of deep brain stimulation (DBS) of the thalamic nucleus ventralis intermedius (VIM-DBS) over time in patients with essential tremor are not well understood. METHODS: Long-term clinical outcome and stimulation parameters were evaluated in 14 patients with essential tremor who underwent VIM-DBS. The mean ± standard deviation postoperative follow-up was 7.7 ± 3.8 years. At each visit (every 3-6 months), tremor was assessed using the Fahn-Tolosa-Marin tremor rating scale (FTM-TRS) and stimulation parameters were recorded (contacts, voltage, frequency, pulse width, and total electrical energy delivered by the internal generator [TEED 1sec]). RESULTS: The mean reduction in FTM-TRS score was 73.4% at 6 months after VIM-DBS surgery (P < 0.001) and 50.1% at the last visit (P < 0.001). The gradual worsening of FTM-TRS scores over time fit a linear regression model (coefficient of determination [R2] = 0.887; P < 0.001). Stimulation adjustments to optimize tremor control required a statistically significant increase in voltage (P = 0.01), pulse width (P = 0.01), frequency (P = 0.02), and TEED 1sec (P = 0.008). TEED 1sec fit a third-order polynomial curve model throughout the follow-up period (R2 = 0.966; P < 0.001). The initial exponential increase (first 4 years of VIM-DBS) was followed by a plateau and a further increase from the seventh year onward. CONCLUSIONS: The current findings suggest that the waning effect of VIM-DBS over time in patients with essential tremor may be the consequence of a combination of factors. Superimposed on the progression of the disease, tolerance can occur during the early years of stimulation.