Background: Speech disorders are among the most common adverse effects after subthalamic nucleus deep brain stimulation (STN-DBS) in Parkinson's disease (PD) patients. However, longitudinal speech changes after STN-DBS are not fully understood. Objective: We performed a two-year prospective study on PD patients who underwent STN-DBS and analyzed changes in speech function to clarify factors predicting for speech deterioration. Methods: Twenty-five PD patients were assessed before and up to two years after STN implantation. Speech function was evaluated in the on-stimulation condition and 30 min after stimulation cessation using auditory-perceptual assessment. Patients who experienced overall worsening in speech intelligibility or naturalness ≥1 point during follow-up were classified into a deteriorated group (n = 16), with the remaining subjects being classified into a stable group (n = 9). Cognitive and motor functions were also assessed. Results: The stable group had significantly better values of low volume, monoloudness, and asthenic voice subscores of the auditory-perceptual assessment in the on-stimulation condition compared with the off-stimulation condition. Imprecise consonants, excess loudness variation, and strained voice subscores were improved via cessation of stimulation in both groups. Before surgery, the deteriorated group had significantly lower scores in the Stroop Color-Word Test and Digit Span compared to the stable group. Conclusions: During follow-up, some subscores showed significant worsening in the on-stimulation condition in both groups. However, beneficial effects of STN-DBS on speech appeared to counterbalance negative effects of STN-DBS on speech function only in the stable group. Worse cognitive function may be a potential predictor for speech deterioration after STN-DBS in PD patients.
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