a genetic disease in which loss of nerve cells in the spinal cord called motor neurons affects the part of the nervous system that controls voluntary muscle movement.
Weakness is most severe in muscles closest to the center of the body, such as those of the shoulders, hips, thighs and upper back. Respiratory muscles can also be involved. Spinal curvature often develops.
A deficiency of a motor neuron protein called SMN, for survival of motor neurons, resulting from a faulty gene on chromosome 5.
Usually progresses slowly, and survival into adulthood is common if respiratory status is closely monitored. Most doctors now consider SMN-related SMA to be a continuum of severity and prefer not to make rigid predictions about life expectancy or weakness.