Spinal muscular atrophy is a genetic disorder of the motor neurons that cau
ses profound hypotonia, severe weakness, and often fatal restrictive lung d
isease. Patients with spinal muscular atrophy present a spectrum of disease
from the most severe infantile-onset type, called Werdnig-Hoffmann disease
(type 1), associated with a mortality rate of up to 90%, to a late-onset m
ild form (type 3), wherein patients remain independently ambulatory through
out adult life. Although many clinicians agree that patients with spinal mu
scular atrophy lose motor abilities with age, it is unknown whether progres
sive weakness occurs in all patients with spinal muscular atrophy. we prese
nt here results of the first prospective study of muscle strength in patien
ts with spinal muscular atrophy. There was no loss in muscle strength as de
termined by a quantitative muscle test during the observation period. Howev
er, motor function diminished dramatically in some patients with spinal mus
cular atrophy. Explanations for this loss of function could not be determin
ed from our data. Decrease in motor function could be caused by factors oth
er than loss of strength. Therefore, it is not clear from our results wheth
er spinal muscular atrophy is a neurodegenerative disease. We conclude that
treatment trials in spinal muscular atrophy should be designed with consid
eration of the natural history of strength and motor function in this disor
der.