Loss of large myelinated nerve fibres of the recurrent laryngeal nerve in patients with multiple system atrophy and vocal cord palsy.

OBJECTIVES: Vocal cord palsy seen in some patients with multiple system atrophy may result from neuronopathy of the recurrent laryngeal nerve. METHODS: Six controls and six patients with multiple system atrophy, four with and two without vocal cord palsy were studied. The number of myelinated nerve fibres were counted and fibre diameter histograms were established for the motor and sensory divisions of the laryngeal branch of the recurrent laryngeal nerve. RESULTS: Although both groups of patients with multiple system atrophy showed selective loss of the myelinated fibres in the motor branch, the change was greater in those with vocal cord palsy than in those without. The small myelinated nerve fibres (diameter < 7 microm) were decreased in number in both multiple system atrophy groups, whereas the large myelinated nerve fibres (diameter < 8 microm) were decreased only in those with vocal cord palsy, and preserved in those without the symptom. CONCLUSION: In multiple system atrophy, the small myelinated fibres innervating the vocal cord are affected first, without obvious clinical signs. The patient develops vocal cord palsy only after the loss of the large myelinated fibres, which mostly comprise the alpha motor axons that innervate the intrinsic laryngeal muscles.