Effects of subthalamic nucleus stimulation on parkinsonian dysarthria and speech intelligibility.

Subthalamic stimulation is known to improve tremor, akinesia and rigidity in Parkinson's disease. However, other signs such as hypophonia and swallowing disorders can be relatively resistant to this technique. The effect on dysarthria remains unclear. The aim of this study was to investigate the effects of implantation of electrode and stimulation of the subthalamic nucleus (STN) on parkinsonian dysarthria. Seven patients were prospectively included. Electrodes (Medtronic) were implanted in both STN. The electrode contacts and stimulation parameters were adjusted to provide best relief of symptoms with fewest side effects. Assessment used global scales (Unified Parkinson Disease Rating Scale, UPDRS II and III), dyskinesia scale, exhaustive dysarthria assessment (bucco-facial movements, voice, articulation, intelligibility) and the 'dysarthria' item from the UPDRS III. Evaluations were performed in six conditions: before and three months after surgery (pre-op, post-op) stimulation turned off or on (off-stim, onstim), and without or with a suprathreshold levodopa dose (offdrug, on-drug). Performance level on the UPDRS III significantly improved following electrode implantation and stimulation. For dysarthria, modest beneficial effects were observed on several motor parameters, especially lip movements. Voice mildly improved, especially for the modulation in loudness and pitch. Articulation was not affected. Furthermore, intelligibility was slightly reduced in the on-stimulation condition, especially when patients received levodopa. At an individual level, negative effects on intelligibility were observed in two patients, and this was associated with a discrete increase in facial and trunk dyskinesias, but not with the electrode position or stimulation parameters. In conclusion, surgery had weak effects on dysarthria. Intelligibility can be worsened, especially in the on-drug condition. Thus, adaptation of the stimulation parameters can be difficult.