Bilateral subthalamic stimulation effects on oral force control in Parkinson's disease

Dysarthria in Parkinson's disease (PD) consists of articulatory, phonatory and respiratory impairment. Bilateral subthalamic nucleus (STN) stimulation greatly improves motor disability, but its long-term effect on speech within a large group of patients has not been precisely evaluated. The aim of this study was to determine the effect of bilateral STN stimulation on oral force control in PD. We measured forces of the upper lip, lower lip and tongue in twenty-six PD patients treated with bilateral STN stimulation. Measurements of the articulatory organ force, as well as a motor evaluation using the Unified Parkinson's Disease Rating Scale (UPDRS), were made with and without STN stimulation. Maximal voluntary force (MVF), reaction time (RT), movement time (MT), imprecision of the peak force (PF) and the hold phase (HP) were all improved with STN stimulation during the articulatory force task, as well as the motor examination scores of the UPDRS. It seems that the beneficial STN stimulation-induced effect on articulatory forces persisted whatever the duration of post-surgical follow-up. However, dysarthria evaluated by the UPDRS was worse in two subgroups of patients with a one to two year and three to five year post-surgical follow-up, in comparison with a subgroup of patients with a three month follow-up. STN stimulation has a beneficial long-term effect on the articulatory organs involved in speech production, and this indicates that parkinsonian dysarthria is associated, at least in part, with an alteration in STN neuronal activity. Nevertheless, to confirm the persistence of the beneficial effect of STN stimulation on parkinsonian dysarthria, a longitudinal evaluation is still needed.