Abstract
Deep brain stimulation (DBS) is an effective treatment for reducing symptoms of tremor. A common and typically subjectively determined adverse effect of DBS is dysarthria. Current assessment protocols are driven by the qualitative judgments of treating clinicians and lack the sensitivity and objectivity required to optimize patient outcomes where multiple stimulation parameters are trialed. To examine the effect of DBS on speech in patients receiving stimulation to the posterior sub-thalamic area (PSA) via randomized manipulation of amplitude parameters. Six patients diagnosed with tremor receiving treatment via DBS of the PSA were assessed in a double-blinded, within-subjects experimental protocol. Amplitude (i.e., voltage or current) was randomly adjusted across 10 settings, while speech samples (e.g., sustained vowel, counting to 10) were recorded to identify the patient-specific settings required for optimal therapeutic benefit (reduced tremor) with minimal adverse effects (altered speech). Speech production between stimulation parameters was quantified using acoustic analysis. Speech changed as a response to DBS but those changes were not uniform across patients nor were they generally in line with changes in amplitude with the exception of reduced vocal control and increased mean silence length in two patients. Speech outcomes did not correlate with changes in tremor. Intra-individual changes in speech were detected as a response to modified amplitude; however, no clear pattern was observed across patients as a group. The use of objective acoustic measures allows for quantification of speech changes during DBS optimization protocols, even when those changes are subtle and potentially difficult to detect perceptually.
Highlights
Deep brain stimulation (DBS) is well established as a safe and effective treatment option for reducing tremor severity where drug therapy is ineffective (Plaha et al, 2004; Blomstedt et al, 2010)
The changes in speech observed from DBS amplitude variation are often subtle in isolation
The cumulative effect of speech timing, vocal control, and quality deficits can lead to reduced intelligibility and increased dysarthria severity
Summary
Deep brain stimulation (DBS) is well established as a safe and effective treatment option for reducing tremor severity where drug therapy is ineffective (Plaha et al, 2004; Blomstedt et al, 2010). Qualitative assessment protocols do not allow for precise comparison between the effects of stimulus parameter settings. They limit the capacity of clinicians to evaluate the comparative influence of tremor and speech effects when determining a clinical endpoint. The limitations of subjective evaluation of speech (i.e., bias and error) are well known (Kent, 1996), including limited capacity of listeners to determine the size of change from one production to the the influence of experience on the capacity of a listener to identify (and quantify) changes in speech, and the limited inter- and intra-rater reliability of auditory-perceptual judgment (Vogel and Maruff, 2014). Current assessment protocols are driven by the qualitative judgments of treating clinicians and lack the sensitivity and objectivity required to optimize patient outcomes where multiple stimulation parameters are trialed
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