Background: Up to 70% of the patients with Parkinson's Disease (PD) experience speech problems, which cause a diminished intelligibility. A reduced intelligibility has a profound negative impact on social interaction and quality of life. Since pharmacological treatment only has limited effects on speech, non-pharmacological treatment, like speech therapy, is particularly relevant. Cochrane reviews (Herd et al., 2012a; Herd et al., 2012b) showed that evidence for speech therapy in PD is increasing, but is still inconclusive. Moreover, only very intensive standardized treatment programs have been studied, which are only feasible for people with mild to moderate PD, but too intensive for people with advanced PD. Here, the investigators will perform the first large-scale study to demonstrate the efficacy of speech therapy in PD patients in all disease stages on quality of life and speech quality.

Objective: The aim is to demonstrate the effectiveness of personalized and home-based speech therapy on quality of life, intelligibility and social participation for people with Parkinson's disease who have a reduced intelligibility of speech.

Methods: The investigators will perform a single blind, randomized and controlled trial. A total of 215 patients (18 years and older) with PD in all disease stages who have difficulty with intelligibility affecting daily communication will participate in this study. The patients will be randomly allocated to either speech therapy or a waiting list control group (1:1 ratio). Speech therapy using telerehabilitation will be provided for 8 weeks which consists of 12-16 sessions. The control group will receive deferred treatment after 8 weeks. The measurements will take place before the randomization (To), after 8 weeks (T1), and for the experimental group also after 24 weeks (T2). The primary outcome measure is quality of life, as measured using the total score on the PDQ-39. Secondary outcome measures include speech and voice, speech intelligibility, non-motor symptoms and caregiver burden.

Hypothesis: The investigators hypothesize that patients in all disease stages can improve their speech intelligibility by using the explicit feedback from external cues provided by instructed caregivers plus a dedicated smartphone/ tablet app (the Voice Trainer app).

Inclusion Criteria:

• A diagnosis of idiopathic PD;
• Problems in intelligibility affecting daily communication (as indicated by the patient and/or the caregiver);
• A desire for improvement;
• willing and able to receive online treatment

Exclusion Criteria:

• Recent (<1 year) speech therapy;
• Voice or speech problems due to other causes;
• Communication difficulties based on language problems without predominantly reduced intelligibility;
• inability to receive online treatment

We will make relevant and anonymised data available in a validated database (DANS Easy).

After publication of the main results of our trial.

Access to the data is restricted, meaning that researchers who are interested in re-use of the data are asked to contact the central contact person for permission. Approval is given after a signed agreement.

The assessors will be blinded for treatment allocation.