gms | German Medical Science

Objective: Thalamic deep brain stimulation (DBS) is an established therapy for patients with essential tremor (ET). Average improvement of tremor amounts to 60-80%, but outcomes are often variable. A highly relevant side-effect in thalamic neuromodulation is the evolution of gait disturbances which deteriorate daily activities. So far, the exact proportion of this delayed DBS failure has been unknown, and no study has analysed risk factors.

Methods: We collected ET subjects from 2 German DBS centers with stable and effective bilateral thalamic stimulation. Based on video analysis of every subject, the Scale for the Assessment and Rating of Ataxia (SARA) item 1-3 and the Tremor-Rating-Scale (TRS) were performed. Subjects were stratified in two groups for the presence of gait changes in long-term follow-up (SARA1-3 ≥4). For the correlation between the presence of gait disturbances and the anatomical location of stimulation, we simulated the volume of tissue activated (VTA) in subjects’ related MRI space based on their stimulation parameters. After transformation to common (MNI-) space only voxels overlapped by ≥6 VTAs were visualized to define a volume, where stimulation may provoke gait problems.

Results: 76 ET subjects were collected: 31 subjects (13 males) in ETgaitdisturbances, 45 subjects (27 males) in ETcontrol. A subset of 8 subjects obtain external support. Subjects with gait disturbances (2,6 years after chronic DBS) were older at the disease onset and at the surgery and had more gait difficulties preOP (all p<0.001). The amount of tremor control smaller (24.1(±42.3)% vs. 60.9(±20.1)% (p<0.001)). For individuals with gait changes, the spot most commonly stimulated was located more posterior and inferior compared to non-effected subjects (ETgaitdisturbances: lateral=11.90, posterior=3.42, inferior=3.4 vs ETcontrol: l=11.62, p=2.60, i=2.89 [based on AC-PC in mm]).

Conclusion: Changes in gait pattern were detected in 31 subjects (40,8%) of the cohort. Those changes do not necessarily affect patient’ daily routine, but 10,5% of patients show a relevant gait ataxia and obtain support for walking. Potential risk factors are gait difficulties before the surgery, a higher age at disease onset or at surgery combined with anatomical suboptimal placed stimulation therapy. Further investigations are needed to define strategies to prevent and overcome this adverse effect in thalamic DBS.