Experimental: Autonomic nervous system (ANS) regulation therapy

HRV biofeedback is a non-invasive intervention that focuses on increasing heart rate oscillations through real-time feedback and slow-paced breathing training (Lehrer et al., 2020; Laborde et al., 2022; Pizzoli et al., 2021). Participants will practice 20 min a day (2 sessions of 10 min) for 1 month. One weekly session will be organized in the clinic under the guidance of the therapist (individual sessions), the other sessions will be organized at home, and tracked by a chest strap heart rate monitor (Polar H10) and the Elite HRV app (elitehrv.com). First, the resonance frequency (RF) (i.e. respiration rate with the highest HRV) will be personalized for each participant. Biofeedback slow-paced breathing exercises in the app will then be customized based on this RF (40% inhale, 60% exhale: e.g. 6 bpm: 4s inhale, 6s exhale). Participants will be asked to breathe in through the nose and breathe out through pursed lips, following the breath pacer with visual feedback.

Active_comparator: conventional voice therapy (CVT)

The CVT will be based on Meerschman et al. (2019). This therapeutic program has been proven effective in voice therapy and is the standard clinical care for FD patients. The program is a combination of education, vocal hygiene, posture, local relaxation, costo-abdominal breathing, resonant voice, voice placing, forward focus, voice onset, semi-occluded vocal tract exercises and laryngeal manipulation. Identical to the ANS regulation therapy, participants will practice 20 min a day for 1 month. One weekly session will be organized in the clinic under the guidance of the therapist (individual sessions), the other sessions will be organized at home and tracked by the RedCap app.

Active_comparator: ANS regulation therapy + CVT

The third group will receive a combination of both therapies. The same frequency and duration of practice (20 min a day for 1 month) will apply. They will also receive one weekly session in the clinic under the guidance of the therapist (individual sessions) and the other sessions will be completed at home, tracked by the Elite HRV app and the RedCap app.