Dualistic effect of pallidal deep brain stimulation on motor speech disorders in dystonia

Although pallidal deep brain stimulation (GPi-DBS) is an effective treatment for dystonia, it may cause important stimulation-induced side-effects such as hypokinetic dysarthria or stuttering. However, the reasons behind the occurrence of these side-effects remain unknown.

To objectively investigate the impact of GPi-DBS on patients with dystonia on speech fluency, intelligibility, and key aspects of hyperkinetic and hypokinetic dysarthria.

Speech was systematically evaluated in 19 dystonic patients with GPi-DBS. Each patient was tested twice within one day in both the GPi-DBS ON and GPi-DBS OFF stimulation conditions. A control sample of 19 matched healthy speakers underwent the same speech assessment.

We observed an improvement of hyperkinetic dysarthria symptoms in 47% and an aggravation of hypokinetic dysarthria symptoms in 26% of patients with the GPi-DBS switched ON. A higher stimulus intensity was found in a group of patients in whom the hypokinetic dysarthria worsened with the GPi-DBS ON when compared to other dystonic patients (p = 0.02). Furthermore, we revealed a significant increase of dysfluent words in the GPi-DBS ON when compared to OFF condition (p = 0.001) associated with the shorter distance of the active contact localization along the medio-lateral direction (r = -0.70, p = 0.005).

This study provides evidence of dualistic effects of GPi-DBS on speech in dystonia manifested as an improvement of hyperkinetic or a deterioration of hypokinetic dysarthria. Our findings suggest that lower stimulation parameters and placement of active contacts more laterally in the internal globus pallidus should be preferred to avoid the possible side effects of hypokinetic dysarthria and dysfluency.