Spinal muscular atrophy (SMA) is a genetic disease that causes weakness and wasting of the voluntary muscles. It most often develops during infancy and childhood, although rare adult onset SMA can have late onset of the genetic disease. Majority of cases are due to an abnormal survival motor neuron (SMN) protein on chromosome 5. SMN type 1 will either be missing completely or cause defects. For those with milder cases of SMA, such as SMN type 2 that will try to makeup for the missing or inadequate proteins. Yet, some rarer SMA instances are caused by other chromosomal mutations other than the SMN, and known as non-chromosome 5 SMA.

SMN proteins are an important part of keeping spinal cord motor neurons functioning. A lack of SMN protein causes the spinal cord neurons to shrink and eventually die, and the patient to eventually lose control of their voluntary muscles (i.e., legs, arms, neck, and head). Muscles will weaken and waste away causing movements like walking, crawling, sitting, head control, swallowing, and breathing difficult, or impossible. SMA also causes respiratory issues, such as frequent respiratory infections, breathing, swallowing and eating issues, in which eating and aspiration is an issue, as well as pneumonia from the aspiration of food or drink. The four main types of spinal muscle atrophy include the following:

1. SMA Type 1: Severe SMA
Also referred to as Werdnig-Hoffmann Disease, the onset of SMA type 1 is 0-6 months of age. This is the most severe form and children usually will not live past the age of 2. Children with this type typically cannot move, have a hard time feeding and swallowing, holding their head up and breathing. Children with Werdnig-Hoffmann Disease will never be able to sit up.

2. SMA Type 2: Intermediate SMA
The onset of SMA type 2 is 7-18 months of age. The progression rate and life expectancy will vary greatly between patients, however, patients will never be able to stand up on their own.

3. SMA Type 3: Mild SMA
Also known as Kugelberg-Welander or Juvenile Spinal Muscular Atrophy, the onset ranges from 18-months into early adulthood. Children with Juvenile Spinal Muscular Atrophy have a life expectancy that falls within the average range. The child or adult may be able to walk and stand, but will have difficulty going from sitting to other positions.

4. SMA Type 4: Adult SMA
SMA type 4 typically occurs between 20-30 years of age. This is an extremely rare form and patients will have a more typical life expectancy. Generally, those with this form of the disorder are able to walk, but will have progressive motor weakness as they age.