Speech Function Following Deep Brain Stimulation of the Caudal Zona Incerta: Effects of Habitual and High-Amplitude Stimulation

Purpose: Deep brain stimulation (DBS) is often successful in alleviating motor symptoms of essential tremor (ET); however, DBS may also induce adverse speech effects. The caudal zona incerta (cZi) is a promising DBS target for tremor, but less is known about the consequences of cZi DBS for speech. This preliminary study examined how habitual cZi DBS and cZi stimulation at high amplitudes may affect speech function in persons with ET. Method: Fourteen participants with ET were evaluated: off stimulation, on habitual cZi DBS, and with unilateral cZi stimulation at increasing stimulation amplitudes. At each stimulation condition, the participants read three 16-word sentences. Two speech-language pathologists made audio-perceptual consensus ratings of overall speech function, articulation, and voice using a visual sort and rate method. Rated functions when off stimulation, on habitual cZi DBS, and at maximal-amplitude stimulation were compared using Friedman nonparametric tests. For participants with bilateral habitual DBS (n = 5), the effects of bilateral and unilateral stimulation were described in qualitative terms. Results: Habitual cZi DBS had no significant group-level effect on any of the investigated speech parameters. Maximal-amplitude stimulation had a small but significant negative effect on articulation. Participants with reduced articulatory precision (n = 9) had more medially placed electrodes than the nonaffected group (n = 5). Bilateral and unilateral left stimulation had comparable effects on speech. Conclusions: Findings from this preliminary study of cZi DBS indicate that speech is generally not affected by stimulation at habitual levels. High-amplitude cZi stimulation may, however, induce adverse effects, particularly on articulation. Instances of decreased articulatory function were associated with stimulation of more medial electrode contacts, which could suggest cerebello-rubrospinal involvement.