Post-thyroidectomy superior laryngeal nerve injury
Voice dysfunction after thyroidectomy may be caused by damage to laryngeal nerves or lesions to strap muscles with laryngo-tracheal movement impairment. Injury to an external branch of the superior laryngeal nerve (EBSLN) is sometimes difficult to recognize clinically and its electromyographic incidence ranges from 0% to 58%. In this study we evaluated, 12–18 months postoperatively, 45 patients who had undergone thyroid surgery (6 total lobectomy, 5 subtotal thyroidectomy, and 34 total thyroidectomy), using a subjective interview, laryngeal videostroboscopy and spectrographic analysis with a multidimensional voice program. Vocal parameters included fundamental frequency, jitter, shimmer, noise-to-harmonic-ratio (NHR) and degree of sub-harmonics. Laryngeal electromyography (LEMG) of the cricothyroid (CT) muscles was performed in 21 subjects with voice problems (35 EBSLNs) using a modified method for the CT recording. In 3 patients of this group (14%) LEMG documented a unilateral EBSLN injury. Easy voice fatigue and decreased pitch range were the most common symptoms after surgery. Average values of vocal parameters pre- and post-operatively in patients without neural damage (n = 42) were: jitter 0.64% and 0.78%, shimmer 3.25% and 3.54%, and NHR 0.12% and 0.13%, respectively (P > 0.05). Acoustic analysis revealed altered patterns in some patients with no objective evidence of damage to EBSLNs, suggesting an extralaryngeal cause of vocal dysfunction, such as laryngo-tracheal fixation or lesions to strap muscles. We conclude that laryngeal videostroboscopy and spectrographic analysis are very useful to assess voice problems after thyroidectomy, including in patients without LEMG-proven neural lesions, in order to suggest early speech rehabilitation, especially in professional voice users.
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