Speech and Deep Brain Stimulation in Parkinson's Disease, Essential Tremor, and Dystonia: A Systematic Review and...

Deep brain stimulation (DBS) effectively treats motor symptoms in movement disorders but often compromises speech through incompletely defined mechanisms. We conducted a PROSPERO-registered systematic review and meta-analysis of publications through August 2024 (CRD42024527738). Among 2726 screened records, we included 184 studies: 131 in Parkinson's disease (PD), 32 in essential tremor (ET), and 21 in dystonia, assessing perceptual, acoustic, and patient-reported speech outcomes. Meta-analyses showed that subthalamic nucleus DBS in PD resulted in poorer speech intelligibility compared with best medical treatment (effect size -0.24; 95% confidence interval: -0.46 to -0.03; P = 0.027), with state-dependent decline under active stimulation on longitudinal analysis (Unified Parkinson's Disease Rating Scale Part III item 18 monthly change: medication on +0.016, P < 0.001; medication off +0.002, P = 0.50). In ET, DBS consistently suppressed vocal tremor but increased the risk of dysarthria, particularly with bilateral stimulation. Dystonia outcomes showed greater heterogeneity. Across disorders, sustained phonation measures improved, whereas connected speech performance worsened, indicating selective vulnerability of complex motor tasks. Neuroanatomical mapping identified two nonexclusive mechanisms: current spread to corticobulbar fibers producing spastic speech features and to the cerebellothalamocortical pathway producing ataxic features. Hypokinetic and stuttering-like phenotypes also occurred but likely reflect network-level interactions rather than tract-specific spread. These tract-mediated and network-level alterations appear to interact with hemispheric lateralization, medication state, and longer-term plasticity to produce complex clinical phenotypes. We outline a clinical framework integrating systematic screening, phenotype identification, and targeted programming adjustments. Enhanced speech assessment, precise field mapping, and adaptive DBS paradigms may promote individualized care that optimizes speech and motor function in precision DBS therapy. © 2026 The Author(s). Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.