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 carriers.
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. The determination of the type of SMA is based upon the physical milestones achieved. It is important to note that the course of the disease may be different for each child.
Thank you Goodens for this definition.
Danika
