The motor neurons affect the voluntary muscles that are used for
activities such as crawling, walking, head and neck control, and
swallowing. It is a relatively common "rare disorder": approximately 1
in 6000 babies born are affected, and about 1 in 40 people are genetic
SMA affects muscles throughout the
body, although the
proximal muscles (those closest to the trunk of one’s body - i.e.
shoulders, hips, and back) are often most severely affected. Weakness
in the legs is generally greater than in the arms. Sometimes feeding
and swallowing can be affected. Involvement of respiratory muscles
(muscles involved in breathing and coughing) can lead to an increased
tendency for pneumonia and other lung problems. Sensation and the
ability to feel are not affected. Intellectual activity is normal and
it is often observed that patients with SMA are unusually bright and
sociable. Patients are generally grouped into one of four categories,
based on certain key motor function milestones.