In SMA types 1 through 4, symptoms vary on a continuum from severe to mild, based on how much SMN protein there is in the nerve cells called motor neurons. (SMN stands for survival of motor neuron.) The more SMN protein there is, the later in life symptoms begin and the milder the course of the disease is likely to be.
Children who have noticeable SMA symptoms at or shortly after birth usually are very weak, have difficulty breathing, sucking and swallowing, and never reach the developmental milestone of being able to sit on their own (type 1 SMA or Werdnig-Hoffmann disease). In the past, children with type 1 SMA usually didn't survive more than two years, but today this is not always the case. With technology such as mechanical ventilation and feeding tubes to assist with breathing and nutrition, children with type 1 SMA can survive for a number of years.
When SMA symptoms begin in babies at approximately 7 to 18 months of age, who learn to sit unassisted but not to stand or walk independently, the disease usually is called type 2 SMA, or intermediate SMA. Although respiratory complications are a constant threat, children with type 2 SMA usually live to young adulthood and many live longer.
When muscle weakness begins in older children and teens, who learn to stand and walk but lose the ability later in life, the disease may be labeled type 3 SMA (also known as mild SMA and Kugelberg-Welander disease). Although some with type 3 stop walking in adolescence, others walk well into their adult years. SMA that comes on in late teens or adulthood is called type 4, or adult-onset SMA. Life span is generally normal in these two types.
In SMA types 1 through 4, the muscles closer to the center of the body (proximal muscles) usually are more affected, or at least affected much sooner, than the muscles farther away from the center. For example, the muscles of the thighs are weaker than the muscles of the lower legs and feet.
|Scoliosis (spinal curvature) is a common problem in SMA and should be corrected.|
Legs tend to weaken before arms. Hands may weaken eventually, but they usually stay strongest the longest, and, even if they do weaken, they usually remain strong enough for typing on a computer keyboard and other basic functions of modern life.
The most serious danger in SMA comes from the weakness of muscles necessary for breathing. Careful attention to respiratory function is needed throughout life, with prompt attention to infections.
Your doctor can help you with details of maintaining respiratory health, including clearance of secretions and perhaps assisted ventilation  (not necessarily around the clock).
Another medical complication in SMA is spinal curvature, usually a side-to-side type of curvature called scoliosis. Scoliosis occurs because of weakness of the muscles that normally support the spine, which is a flexible column.
Scoliosis can be very uncomfortable, interfere with position and mobility and damage a child’s (or adult’s) body image. Some studies have shown that spinal curvatures, if they’re severe, can interfere with breathing.
Many children with SMA start to show a scoliotic curve early in life, which is often treated with a brace until the right time for surgery is reached.
Surgeons generally like to wait until growth is complete or nearly so before surgically straightening and fusing the spine . They also take into account the child’s pulmonary function and how fast the curve is likely to progress.
Some forms of SMA are not linked to chromosome 5 or SMN deficiency. These forms vary greatly in severity and in the muscles most affected. While most forms, like the chromosome 5-related form, affect mostly the proximal muscles, other forms exist that affect mostly the distal muscles, those farther away from the body’s center, at least at the beginning.