Deep brain stimulation of the ventral intermediate nucleus (VIM) is performed to suppress medically-resistant essential tremor. However, a stimulation induced speech disorder, a so-called dysarthria, is a common side effect, limiting the extent to which tremor can be suppressed. We investigated the effect of VIM stimulation on speech production in patients with essential tremor. Speech analysis focused on acoustic and articulatory speech data (electromagnetic articulography).

Our findings reveal that stimulation-induced dysarthria is more pronounced under bilateral compared to unilateral VIM stimulation. Dysarthric features are a reduced glottal control and imprecise oral articulation as well as timing deficits of slowed-down speech movements. Furthermore, the syllable coordination is deteriorated under stimulation, leading to timing deficits in syllables with higher complexity.

We attempted to explore stimulation-induced dysarthria during monopolar review in patients who had received thalamic deep brain stimulation for the treatment of essential tremor. As part of the examination we conducted a speech assessment that included the reading of a standardized text. Intelligibility of the patients was evaluated by blinded, naive listeners based on the readings.

Discriminative fiber analysis for stimulation-induced worsening of intelligibility using linear mixed effect models was conducted. Fibers determined as discriminative for stimulation-induced worsening of intelligibility were mainly connected to the ipsilateral precentral gyrus as well as to both cerebellar hemispheres and the ipsilateral brain stem. The analysis also revealed, that fibers for tremor control ran in close proximity, but more anterolateral to the fibers associated with stimulation-induced worsening of speech that we identified. The overlap of the respective clinical stimulation setting's volumes of tissue activated with these fibers explained 62.4 % (p < 0.001) of the variance of stimulation-induced change in intelligibility in a leave-one-out analysis.

In summary, the project could demonstrate, that both, connectivity to the motor cortex and the cerebellum are involved in generating motor speech disturbance in patients with essential tremor and deep brain stimulation.

The aim of this project was to evaluate the effect of deep brain stimulation of the posterior subthalamic area (PSA) and of the ventral intermediate nucleus of the thalamus (VIM) on speech in patients with essential tremor. Patients were analyzed before the implantation of a deep brain stimulation and three months after implantation of stimulation electrodes implanted in the VIM and PSA. The stimulation was randomly activated in one of the two targets and later in the other. Speech assessments included a reding text and fast syllable repetitions. Results indicate, that stimulation of PSA and VIM lead to slower articulation rates and imprecise articulation. Both, PSA and VIM stimulation affect speech in a comparable way.