The onset is usually in childhood, and the course
tends to be irregularly progressive. The presenting symptom is usually a
perceptive hearing loss caused by bilateral sensorineural deafness of slow or rapid
onset. This is followed by signs of lower cranial nerve motor palsies.
Intercurrent illness or physiologic stress, including surgery or
pregnancy, may precipitate or exacerbate the condition. The severity of the
condition varies from mild to severe, resulting in death. Neurologic
involvement includes bulbar palsy, loss of cranial and spinal motor nerve
function, wasting and fasciculation of the tongue, widespread muscle wasting
with weakness and hypotonia most obviously in neck and lower limb girdle.
Coordination and sensation remain intact. Respiratory complications include
nocturnal hypoventilation because of marked diaphragmatic weakness (forced
vital capacity [FVC] is severely reduced) and obstructive sleep apnea. Bulbar
nerve involvement causes laryngeal and pharyngeal dyscoordination, nasal
speech, and dysphagia with failure to protect the laryngeal inlet resulting
in aspiration episodes. Scoliosis and cervical subluxation have been