Not paralysis, but dystonia causes stridor in multiple system atrophy.

Electromyography (EMG) was performed in 10 patients with multiple system atrophy, laryngeal or pharyngeal symptoms, or both. In patients with stridor, EMG during quiet breathing revealed persistent tonic activity in both abductor and adductor vocal cord muscles. In patients with dysphagia, the cricopharyngeal muscle showed persistent EMG activity throughout all phases of swallowing. Botulinum toxin injection into the adductor muscle determined subjective improvement and reduced tonic EMG activity. Therefore, the cause of stridor in multiple system atrophy is dystonia of the vocal cords.