title: Factors involved in long-term efficacy of deep brain stimulation of the thalamus for essential tremor.
abstract: OBJECTIVE
  Although nucleus ventralis intermedius stimulation has been shown to be safe and efficacious in the treatment of essential tremor, there is a subset of patients who eventually lose benefit from their stimulation. Proposed causes for this phenomenon include tolerance, disease progression, and suboptimal location. The goal of this study was to assess the factors that may lead to both stimulation failure, defined as loss of meaningful tremor relief, and less satisfactory outcomes, defined as leads requiring voltages>3.6 V for effective tremor control.
  
  
  METHODS
  The authors present their clinical outcomes from 31 leads in 27 patients who had effective tremor control for >1 year following nucleus ventralis intermedius stimulation. All patients postoperatively had a mean decrease in both the writing and drawing subscales of the Fahn-Tolosa-Marin Tremor Rating Scale (p<0.001).
  
  
  RESULTS
  After a mean follow-up of 40 months, 22 patients continued to have tremor control with stimulation. Four patients eventually lost efficacy of their stimulation at a mean of 39 months. There was no difference in age, duration of disease, or disease severity between the groups. Examination of perioperative factors revealed that suboptimal anteroposterior positioning as evidenced on intraoperative fluoroscopy occurred significantly more frequently in patients with stimulation failure (p=0.018). In patients with less satisfactory outcomes, no difference was seen between group demographics. Fluoroscopy again revealed suboptimal positioning more frequently in these patients (p=0.005).
  
  
  CONCLUSIONS
  This study provides further evidence that suboptimal lead position in combination with disease progression or tolerance may result in less satisfactory long-term outcomes.
authors.affiliation: Department of Neurosurgery, Rush University Medical Center, Chicago, Illinois 60612, USA.
authors.forename: Julie G
authors.initials: JG
authors.lastname: Pilitsis
authors.forename: Leo Verhagen
authors.initials: LV
authors.lastname: Metman
authors.forename: John R
authors.initials: JR
authors.lastname: Toleikis
authors.forename: Lindsay E
authors.initials: LE
authors.lastname: Hughes
authors.forename: Sepehr B
authors.initials: SB
authors.lastname: Sani
authors.forename: Roy A E
authors.initials: RA
authors.lastname: Bakay
mesh_terms: D000368:Aged; D046690:Deep Brain Stimulation; D020329:Essential Tremor / Q000601:surgery* / Q000628:therapy*; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D008297:Male; D008499:Medical Records; D008875:Middle Aged; D011183:Postoperative Complications; D016896:Treatment Outcome; D020651:Ventral Thalamic Nuclei / Q000502:physiology*