• Type I, or Werdnig-Hoffman Disease, is the most severe form of SMA. Children with Type I tend to be weak and lack motor development, rendering movement difficult. Children afflicted with Type I cannot sit unaided and have trouble breathing, sucking and swallowing. Type I SMA strikes infants between birth and six months.

• Type II is slightly less severe. Type II patients may be able to sit unaided or even stand with support and usually do not suffer from feeding and swallowing difficulties. However, they are at increased risk for complications from respiratory infections. Type II SMA affects infants between seven and 18 months old.

• Type III, also known as Kugelberg-Welander disease, is the least deadly form of childhood-onset SMA. Type III patients are able to stand, but weakness is prevalent and tends to eventually sentence its victims to a wheelchair. Type III SMA strikes children after the age of 18 months, but can surface even in adulthood.

• Type IV is the adult form of the disease in which symptoms tend to begin after age 35. Symptoms usually begin in the hands, feet and tongue, and spread to other areas of the body.

• Adult Onset X-Linked SMA, also known as Kennedy's Syndrome or Bulbo-Spinal Muscular Atrophy, occurs only in men. Facial and tongue muscles are noticeably affected. In addition, these men also often have breast enlargement known as gynecomastia. Like all forms of SMA, the course of the disease is variable, but in general tends to progress slowly.

SMA does not affect sensation and intellectual activity in patients. It commonly is observed that patients with SMA are unusually bright and sociable.