Voice improvement in unilateral laryngeal paralysis during loud voicing: theoretical impact

Voice of patient with unilateral laryngeal paralysis (ULP) shows a nonlinear behaviour with sudden octave jumps, bifurcations and chaos. Such a behaviour may be due to an increased number of freedom degrees in the glottal system. We hypothesized that voice intensity (with increasing sub glottal pressure) could improve vocal signal stability with less freedom degrees in vibrating system, and then a decrease of nonlinearities. A prospective study of 32 consecutive voices of patients with ULP and severe dysphonia was conducted. Jitter and Lyapunov exponent from vocal signals were compared at comfortable and loud voicing with Wilcoxon’s test. In 23 out of 32 patients, jitter significantly decreased from 5 (median) in normal voice to 1.2 in loud voice (P < 10⁻³), Lyapunov exponent decreased from 1,495 bit/s (median) to 708 bit/s (P < 10⁻⁴). Two patients had paradoxical results regarding jitter (higher in loud voice) and 2 regarding Lyapunov exponent. From the 23 cases of voice improvement, 15 cases showing a marked improvement of the acoustic analysis supported our hypothesis (65%). Nonlinear phenomena detected in vocal signals of ULP with severe dysphonia may be reduced in loud voice.