Objectives: To demonstrate the importance of detailed clinical analysis in the differential diagnosis of unilateral vocal fold paralysis, and to provide an update on current knowledge and treatment of myasthenia gravis.

Case report: A female patient presented with left unilateral vocal fold immobility. Diagnostic investigation revealed a 10 mm thyroid adenoma, but no other abnormality likely to cause unilateral vocal fold paralysis. Follow-up flexible endoscopy at three months showed laryngeal remobilisation with persistent left vocal fold bowing and vertical asymmetry of the vocal folds on phonation. Over the following months, voice quality varied between normal and breathy, with the breathy periods lasting from three days to one month. Laryngeal electromyography (EMG) showed a slight bilateral paradoxical activation of both posterior crico-arytenoid muscles on phonation. Magnetic resonance imaging of the brain and brainstem was normal. A diagnostic test for myasthenia gravis with intravenous edrophonium bromide (Tensilon®) lead to an immediate improvement in voice quality. The patient was subsequently treated with pyridostigmine bromide, with complete resolution of dysphonia.

Conclusions: Myasthenia gravis affecting the larynx may mimic unilateral vocal fold paresis or paralysis. A personal or family history of auto-immune disease, fluctuating symptoms, motor deficits in cranial nerve territories, and normal or subnormal laryngeal EMG results should lead the physician to reconsider a diagnosis of idiopathic unilateral vocal fold paralysis and to perform specific testing.