Influence of hypokinetic dysarthria severity level on the long-term outcome of repetitive transcranial magnetic stimulation therapy

Abstract

Introduction: Hypokinetic dysarthria (HD) is a common and disabling symptom of Parkinson's disease (PD) for which established pharmacological and surgical treatments bring only limited, often short-term improvement in speech. The aim of this study was to investigate whether the long-term acoustic effects of repetitive transcranial magnetic stimulation (rTMS) over the right superior temporal gyrus (STG) depend on the baseline severity of HD. Methods: 26 PD patients were randomized to active STG-targeted rTMS (n = 15) or sham stimulation (n =11) and followed across five recording sessions over 14 weeks, alongside one session of matched healthy controls. HD severity at baseline was quantified using 3F test subscores and used to stratify patients into milder (0) and more severe (1) subgroups. The STG 1 subgroup combined greater motor and speech impairment with shorter disease duration. Multiple acoustic features from sustained phonation and free monologue were extracted, normalized to controls, adjusted for sex, and analyzed using non-parametric statistics and descriptive visualisations. Results: Patients with more severe HD receiving active stimulation (STG 1) showed consistent, long-lasting improvements in phonation-related parameters, whereas milder HD (STG 0) and sham groups (SHAM 0, SHAM 1) exhibited only limited or transient changes. Conclusion: Given this observation, STG-targeted rTMS appears particularly beneficial for PD patients with more advanced HD, supporting auditory-motor network modulation as a therapeutic strategy and motivating larger, multi-centre trials to confirm these stratified effects and their clinical impact.