Background. Although psychological factors have been implicated in patients with functional dysphonia (FD), conventional treatment typically targets the aberrant voice symptoms exclusively. Yet, symptomatic/conventional voice therapy (CVT) is not always successful, and in view of the significant adverse quality of life (QoL) impact combined with the substantial financial burden on the healthcare system and society, research is needed to elucidate the underlying pathogenesis and psychophysiology of FD and improve treatment outcomes. Given that (1) the Vagus nerve not only innervates the larynx, but helps to regulate the autonomic nervous system (ANS), (2) autonomic dysfunction is well recognized in the fields of psychology and psychiatry, but remains relatively understudied in the area of voice disorders, and, (3) many of the psychological symptoms/features commonly observed in patients with FD may reflect ANS dysregulation, we intend to investigate ANS dysfunction as a potential psychophysiological mechanism underlying FD; and, to evaluate the comparative effectiveness of a novel therapy that aims to improve ANS regulation in patients with FD.

Objectives. The first objective of this research project is to compare the occurrence and frequency of symptoms and/or disorders related to autonomic dysfunction in patients with FD with gender- and age-matched vocally healthy controls, using a case-control study. The second objective is to compare the effects of a novel therapy based on ANS regulation (i.e., ANS therapy: heart rate variability biofeedback), for FD versus CVT alone or in combination with ANS regulation therapy (i.e., ANS therapy + CVT), using a longitudinal randomized controlled trial (RCT).

Methods. Case-control study: Autonomic (dys)function of patients with FD will be compared with gender- and age-matched vocally healthy controls, using both physiological measures (e.g. heart rate variability, skin conductance level) and psychological patient-reported outcome measures (PROMs, e.g. Neuroception of Psychological Safety Scale, Depression Anxiety and Stress Scale). RCT: The FD group will be randomly assigned to the innovative ANS therapy group, the CVT group or the ANS therapy + CVT group. All patients will receive 1 month of treatment with 20min of daily practice. Both the autonomic assessment and the voice assessment will be performed pretherapy, immediately after therapy and at 3 months follow-up by assessors blinded to group allocation and study phase.

Expected results. Higher occurrence of symptoms and/or disorders related to autonomic dysfunction are expected in patients with FD compared with vocally healthy controls. Physiological outcomes: Lower heart rate variability, lower cardiac pre-ejection period, higher respiration rate and higher skin conductance level are hypothesized in patients with FD compared with vocally healthy controls. Psychological PROMs: Higher self-report of feelings/symptoms related to autonomic dysfunction (e.g. perceived stress, anxiety) is expected in patients with FD compared with vocally healthy controls. Autonomic function is hypothesized to improve more after the ANS therapy and the ANS therapy + CVT compared with the CVT only. Voice function is expected to improve more after the ANS therapy + CVT compared with the ANS therapy and the CVT alone.