Many studies of deep brain stimulation (DBS) of the subthalamic nucleus (STN), globus pallidus internus (GPi), and ventral intermediate (Vim) nucleus of the thalamus have reported dysarthria and dysphagia as side effects (88-90). Several studies examined specific aspects of voice, speech, swallowing and related orofacial, and respira-tory-laryngeal functions associated with DBS treatment of PD. Santens et al. (91) found that left-brain stimulation had a profound negative effect on prosody, articulation, and intelligibility not seen with right-brain stimulation. With bilateral stimulation, no differences in speech characteristics were observed on- and off-stimulation. Wang et al. (92) also studied the effects of unilateral STN DBS on respiratory/phonatory subsystems of speech production in PD. Speech recordings were made in the medication-off state at baseline and three months post-DBS with stimulation-on and -off, in six right-handed patients. Three patients who received left-brain STN DBS showed a significant decline in vocal intensity and vowel duration compared with baseline, which the authors attributed to microlesions of the dominant hemisphere for speech.
Some studies indicate improvement in voice and speech functions with DBS. Gentil et al. (93) assessed the effects of bilateral STN DBS on hypokinetic dysarthria using force measurements of the articulatory organs and acoustic analysis in 16 PD
patients. They noted that STN DBS-reduced reaction and movement time of the articulatory organs, increased maximal strength and precision of these organs, and improved respiratory and phonatory functions. Gentil et al. (94) also compared the effects of bilateral STN DBS versus Vim DBS on oral control in 14 individuals with PD. They used force transducers to sample ramp-and-hold force contractions generated by the upper lip, lower lip, and tongue at 1- and 2-N target force levels, as well as maximal force. With STN stimulation, dynamic and static control of the artic-ulatory organs improved greatly, whereas with Vim stimulation it worsened. In another study of 26 individuals with PD treated with bilateral STN DBS, Gentil et al. (95), using acoustic analysis of voice, found that stimulation resulted in longer duration of sustained vowels, shorter duration of sentences, words, and pauses, increased variability in voice F0 in sentences, and increased stability of voice F0 during sustained vowels. There was no difference in vocal intensity between the on- and off-stimulation conditions. Pinto et al. (96) assessed the impact of bilateral STN DBS on forces and control of the upper lip, lower lip, and tongue in 26 dysarthric individuals with PD before and after DBS surgery. They reported that with stimulation, there was an improvement in the maximal voluntary force, reaction time, movement time, precision of the peak force, and the hold phase during an articulatory force task. They also reported that these beneficial effects of DBS on articulatory forces persisted up to five years.
Dromey et al. (97) studied the effects of bilateral STN DBS on acoustic measures of voice in seven individuals with PD. Acoustic recordings of voice were made before surgery in the medication-off and medication-on conditions and after surgery with and without stimulation in the medication-on and -off conditions. Six months after surgery, there were significant though small increases in vocSPL and F0 variability when on-medication with DBS. Rousseaux et al. (98) studied the effects of bilateral STN DBS on speech parameters and intelligibility in seven dysarthric PD patients. Speech was evaluated before and three months after surgery with stimulation-off and -on and with and without a suprathreshold levodopa dose. Modest beneficial effects were reported on several motor speech parameters, especially lip movements. Modulation of voice pitch and loudness improved mildly. Articulation was not affected and speech intelligibility was slightly reduced in the on-stimulation condition, especially when patients received levodopa. Marked negative effects on intelligibility were observed in two patients, due to increased facial and trunk dyskinesia.
In sum, DBS can result in a moderate benefit on the speech motor system during nonspeech tasks and minimal therapeutic or adverse effects on voice and speech functions. Although the follow-up studies suggest deterioration in speech following DBS, it is not clear to what extent this deterioration is due to the DBS surgery, to voltage spread from the stimulating electrodes (99), and to the natural progression of PD.
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