Onset, natural history and outcome in idiopathic adult motor neuron disease.

Cases of adult-onset idiopathic motor neuron disease (MND) identified from January 1970 through December 1986 were studied in a defined area of California. The patients were followed prospectively throughout the illness in 99% of cases. Among 708 cases aged 25-74 years at onset, the most common type (86%) was typical, sporadic amyotrophic lateral sclerosis (SporALS). The risk of bulbar onset and shorter survival times increased with age in both men and women. About 4%, mainly younger men, experienced unusually long courses with milder paralysis, but could not be identified early in the illness. They probably represent one extreme of the ALS spectrum rather than a distinct subtype. Familial ALS (FamALS) was diagnosed in 7%. It developed earlier in life but ran a slightly longer course, which suggests a different disease process. Overall there was a statistically significant predominance of males, especially in 17 cases (2%) of progressive muscular atrophy (PMA). There were 26 cases (4%) classified as primary lateral sclerosis (PLS). Progressive bulbar palsy was not found; that diagnosis usually denotes merely the bulbar onset of ALS.