Modulation of Brainstem-Cortical Pathways by Injection Laryngoplasty in Unilateral Vocal Fold Paralysis: A Longitudinal Diffusion MRI Case Study.

Objective: This study investigates the potential role of injection laryngoplasty with hyaluronic acid in supporting spontaneous reinnervation and the recovery of vocal fold mobility, following unilateral vocal fold paralysis (UVFP). Building on previous findings suggesting functional activation of the brainstem during a sustained vowel task before vocal fold mobility recovery (Dedry, Dricot, Van Parys, Boucquey, Delinte, van Lith-Bijl, Szmalec Maryn & Desuter, 2022), this study aims to explore the neural plasticity in response to this peripheral intervention. Specifically, we hypothesize that this intervention may not only activate brainstem regions of interest (ROIs) but also induce changes in the motor and sensory pathways associated with phonation.

Methods: A longitudinal single-case study was conducted on a 54-year-old woman (P3) with right-sided UVFP. P3 received a hyaluronic acid injection laryngoplasty into the paralyzed vocal fold, along with a sham behavioral voice therapy. Assessment sessions, including MRI scans (structural, functional, and diffusion), were conducted at four time points to examine potential neural changes. Diffusion MRI and tractography were used to assess microstructural changes in brainstem-to-cortex motor and sensory pathways. The same assessment protocol was followed by a matched healthy control participant for comparison. Diffusion data were preprocessed and modeled using the DIAMOND model, focusing on the corticobulbar and medial lemniscal tracts.

Results: A marked increase in weighted fractional anisotropy (wFA) was observed in left and right anterior motor tracts of interest (TOIs) between the first and second sessions, shortly after the hyaluronic acid injection. These values remained stable thereafter. In contrast, the posterior sensory tracts showed minimal change over time, with wFA values remaining lower than those of the control participant.

Conclusion: Although limited to a single case, the bilateral improvements observed in motor tract integrity, reflected by increased FA shortly after the hyaluronic acid injection, suggest a possible neuroplastic response, particularly within the corticobulbar projections. These findings indicate that early injection laryngoplasty may promote adaptive structural changes in motor pathways involved in phonation, potentially supporting reinnervation following UVFP.