Our study shows that the total survival 10 years after VIM-DBS implantation in ET patients (Figure 1) is above 80%. Louis et al.  have indicated that the mortality in ET patients not treated with DBS is increased relative to a control group without ET. We found no significant increase in mortality in VIM-DBS treated ET patients compared to the general Norwegian population (calculated SMR 1.26).
There are several limitations to this comparison of mortality in ET patients. The total number of patients included in this study is relatively small. Because of this, a possible modest increase in mortality would not give a significant increase in SMR and could therefore escape recognition. However, the group of patients included is relatively large compared to most other studies of ET patients. Another factor that might affect our results could be that ET patients selected for surgery have less comorbidity than those excluded.
One of our VIM-DBS patients committed suicide within seven months after surgery. Increased suicide rate after DBS has been indicated by several authors, but we have found only one previous description of a suicide after VIM-DBS . Burkhard et al.  described a 4.3% suicide rate after DBS, and found no relationship to the underlying condition or DBS target. Most reports have discussed suicide risk after subthalamic nucleus DBS in PD patients , but a recent randomized, controlled multicentre study found no direct association between DBS surgery and an increased risk for suicide ideation and behaviors in PD patients . Therefore, it is still unclear whether, or to which extent, there is an increased suicide risk after DBS, and whether the risk varies between different DBS targets.
The results from the patient questionnaire are hampered by its retrospective design, with both recall- and selection bias. Another problem is the wide range of follow-up times due to the low number of patients operated each year. Patient satisfaction is difficult to assess, and there are no validated methods to measure this in our patient group. We used VAS-scores, since these have shown to have good validity and reliability for patient satisfaction after other interventions . Seventy percent of the eligible participants responded to the questionnaire. The median VAS-score for the postoperative effect of VIM-DBS on tremor reported was high (Table 2B). At follow-up, a modest, but significant effect reduction was reported compared to the postoperative score. In spite of this, a median VAS-score of 7.4 for the effect of VIM-DBS on tremor after median 6.0 years follow-up, indicate that VIM-DBS has a good long-term effect on ET in most patients (Table 2A). Median VAS-score of overall patient satisfaction with VIM-DBS treatment in the questionnaire responders was high.
VAS-scale was also used by Zhang et al.  in their long term study. At 56.9 months follow-up they found a 1.43 (±2.62) mean score for the effect of VIM-DBS on tremor in ET patients. Some other studies have also reported a decrease in activities of daily living (ADL) and an increase in tremor at long-term follow-up, indicating a loss of benefit of VIM-DBS over time or disease progression [10, 11]. Compared to these reports, our results seem to indicate a better long-term effect of DBS on tremor, similar to what was described in a few other long-term studies of ET [6, 7, 9].
Stimulation parameters were increased both for voltage and frequency from the first postoperative year until follow-up. These findings are similar to previous reports on ET [6, 7, 9]. Whether this increase in stimulation parameters and the loss of effect of VIM-DBS on tremor in ET patients is related to disease progression or tolerance to the VIM-DBS treatment, is still unclear.
Dysarthria was the most frequent reported adverse effect. Similar findings have been reported from other DBS studies, especially after bilateral stimulation [9, 19]. One of two responding patients with unilateral VIM-DBS also reported dysarthria in the follow up questionnaire. Disequilibrium and balance difficulty together with reported falls have also been used as a caution against bilateral stimulation . However, coordination problems and dizziness were not reported as severe problems in our study (Table 2C).
As in other studies of patients with ET [9, 10] there is a larger proportion of men compared to women treated with DBS (2/3), despite that the prevalence of ET seems to be equal in men and women . Whether this has a cultural or a gender dependent explanation is difficult to tell, but similar is observed also in PD patients treated with DBS [21–23].