Effects of expiratory muscle strength training on speech in patients with Parkinson's disease

Abstract

Parkinson's disease (PD) is a progressive neurodegenerative disease due to the loss of dopamine-synthesising cells in the basal ganglia. Without sufficient supply of dopamine, a neurotransmitter for motor control circuits, coordination of movements of body parts will be disrupted. Tremor at rest, rigidity in muscles, bradykinesia and postural instability are the major motor symptoms associated with PD which affect patients' limb movement, respiration, swallowing and speech. In addition to the motor symptoms, speech disturbance is commonly seen that over 80% of PD patients have complaints of speech problems. Hypokinetic dysarthria, characterized by reduced vocal loudness, breathy and hoarse voice, monotonicity, mono-loudness and imprecise consonants, is the signature motor-speech disorder of PD primarily due to the inadequate respiratory support and movement initiation difficulty. Pharmaceutical and surgical approaches are regarded as the effective managements for PD targeting limb movement, balancing and postural stability. However, for functions which are not purely dopaminergic and involve multiple systems, such as speech production, medications and surgeries were not yielding consistent outcomes as compared with those seen in limb movements. Behavioural treatments, instead, can help patients cope with functional disabilities by training of muscle strength and/or motor-skills. Expiratory muscle strength training (EMST) is a recently developed instrumental treatment principled from exercise science which aims to improve one's respiratory and related functions (including speech and swallowing). Through intensive strength training using a pressure-threshold device, neural and muscle adaptations would take place in expiratory muscles and as a result muscle coordination is improved and power output of expiration is increased. Studies on healthy individuals and patient populations proved increased expiratory pressure, improved vocal loudness and reduced swallowing risks after receiving treatment of EMST. The current study examined if EMST can potentially be an effective treatment for PD speech. Three pioneering experiments were carried out: 1) to test the treatment efficacy in multi-modalities, 2) to confirm the working mechanism in the view of aerodynamic performance and, 3) to refine the treatment paradigm by incorporating strength training of EMST with skill training of voice. The overall results confirmed the positive effects of EMST on PD speech especially in amplifying speech loudness and improving perception of speech and outlined the working mechanism of EMST as it compensated the impaired pressure transducing process from lung pressure to speech pressure with increased expiratory muscle strength. In addition, it supported the notion that strength training and skill training are supplementary of each other as EMST and voice therapy can yield a better treatment outcome in terms of speech loudness of different tasks. In conclusion, the current study conducted a comprehensive investigation of EMST on PD speech. The positive and consistent treatment outcomes supported that EMST should be included in a treatment paradigm for PD speech. In practice, a combined treatment of EMST and skill training of speech will be more beneficial to patients which facilitate a better carryover of the enhanced strength. Further investigations including detraining studies and the possibility of group therapy are encouraged to contribute a better design of treatment.