Voice modulation, self-perception and motor branch of the superior laryngeal nerve

Vocal morbidity resulting from damage to the motor branch of the superior laryngeal nerve (SLN) after endocrine surgery is well known, but diagnosis is often delayed. The present study aimed to quantify these vocal changes acoustically (main objective), and correlate this with the vocal complaints of patients with suspected SLN motor impairment (secondary objective).

Thirty females patients with suspected injury of the SLN cricothyroid branch (CT-) were compared to 30 patients without postoperative vocal impairment (CT+) and to 30 control subjects. Mean, minimal and maximal fundamental frequencies (F0mean, F0min and F0max) and vocal range were measured on /e/ at high frequency, sirens (glissandi), a reading text, and minimal intonation pairs. Subjective vocal impairment was evaluated on the Voice Handicap Index (VHI).

A lowering of F0mean associated with vocal range reduction by one fifth (in the reading text) seemed to be specific to CT- patients. Production of questions was affected, with differences in melodic curve and attack. Thyroidectomy within 2 months in itself (without suspected SLN cricothyroid branch injury) also affected these parameters, but to a lesser degree. CT- patients reported greater voice impairment than CT+ patients or controls (P=0.0004).

Alterations in speech intonation, quantified on minimal pair test, and self-assessed vocal handicap (VHI) are tools that can easily be used in daily practice to screen for SLN motor branch lesion.