Prognostic value of globus pallidus internus volume in primary dystonia treated by deep brain stimulation.

OBJECT: Given that improvement is variable from one patient to another, the authors analyzed the impact of globus pallidus internus (GPi) volume on the result of deep brain stimulation (DBS) by comparing highly and less improved patients with primary dystonodyskinetic syndromes.

METHODS: A stereotactic model was developed to visualize and quantify the relationship between the isofield lines generated by the DBS lead and GPi target. The model was used in 30 right-handed selected patients with primary dystonodyskinetic syndromes who had been treated using bilateral stimulation of the sensorimotor GPi. Ten healthy control individuals were also included in the study. First, the authors compared the GPi volumes between patients and healthy controls. Second, the stimulated GPi volumes, that is, the intersection between the volume of each isofield value and the GPi volumes, were compared between less improved and highly improved patients.

RESULTS: Improvement in the Burke-Fahn-Marsden Dystonia Rating Scale's motor score was rated > 90% in 20 patients (97 +/- 4.6%) and < 60% in 10 patients (56.9 +/- 6%). The mean volume of the right (461.8 +/- 81.8 mm(3)) and left (406.6 +/- 113.2 mm(3)) GPi in patients showing less response to DBS was significantly smaller than the GPi volume of patients who responded well (right 539.9 +/- 86.6 mm(3), left 510.6 +/- 88.7 mm(3)) and healthy controls (right 557.8 +/- 109.1 mm(3), left 525.1 +/- 40.8 mm(3)).

CONCLUSIONS: On the left side, the mean stimulated volumes (isofield line range 0.2-1 V/mm) were significantly larger in highly improved than in less improved patients. In this model, the threshold for functional effect was calculated at 0.2 V/mm.