On the complexity matching and multiscale nonlinear perspective of voice restoration via fat injection laryngoplasty in unilateral vocal fold paralysis

Voice disorders represent a common medical condition affecting up to 16.9% of the general population, with unilateral vocal fold paralysis (UVFP) being particularly severe. This condition causes breathy dysphonia, maladaptive articulatory behaviours, and cortical alterations in sensory processing. Although injection laryngoplasty (IL) is considered an effective minimally invasive treatment, traditional acoustic analysis often does not provide robust metrics to evaluate its effectiveness. Here, a multiscale nonlinear approach, including the concept of complexity matching computed through the correlation of scaling factors obtained from Multi-Fractal Detrended Fluctuation Analysis, is introduced to better detect improvements in vocal fold vibratory patterns and vocal tract resonance in 69 UVFP patients, treated by autologous fat IL. This method revealed that mildly recovered (MR) patients show stronger similarity of physiological complex characteristics between pre- and post-operative conditions than fully recovered ones. This outcome shows agreement between perceptual and objective evaluations, indicating that the chaotic properties of voice production are still preserved in the MR group. This finding could assist clinicians in recognising patients requiring further treatment, fostering a patient-centred care approach. Moreover, sample entropy (SE) emerged as the most reliable parameter in this study as it could consistently identify vocal recovery at both phonatory and articulatory levels. This result suggests that using multiscale SE could be a unique metric to support and simplify traditional acoustic analysis procedures.