[Clinical significance of calculating the coefficients of Lyapunov in the objective assessment of dysphonia].

Measurement of the vibration instability by means of jitter in cases of dysphonia fails to take into account certain features of the vocal signal, such as modulation or bitonatily. These abnormalities appear in jitter measurements as instability, without allowing any differentiation from random instability (to give 'true' instability). The authors suggest the use of the coefficient of Lyapunov, a measurement which is used for nonlinear dynamics. The vocal product of 179 normal and dysphonic subjects has been analysed. The coefficient of Lyapunov and the jitter were measured during the stable period of a sustained 'a' vowel. For the coefficient of Lyapunov, the authors used the algorythm which has been published previously [Giovanni et al, J. Voice 1999;13:341-354]. The measurements were compared with the perceptive analysis of the reading voice by a panel, and divided into four grade (G0 to G3). The coefficient of Lyapunov appeared to be more relevant than the jitter in distinguishing the various perceived grades. Precise recognition of the grade was obtained in 46% using the coefficient of Lyapunov as against only 36% using jitter. The addition of this new index to the existing puriparametric methods of voice analysis seems to be promising.