Although there are different types of SMA according to the age of onset and severity, SMA affects most all physical movement including walking, sitting without assistance, swallowing, and breathing.
The disease is characterized by a progressive weakening and shrinking of the muscles. The muscles closest to the center of the body (shoulders, hips and back) are usually the first and most severely affected. In most cases, the disease is diagnosed before age 2.
Symptoms vary from mild to severe in the different SMA types (from 1 to 4), and they are based on how much SMN protein there is in the motor neurons.
The more SMN protein exists, the later the symptoms will begin and the milder the evolution of the disease will be3.
Most of the people with SMA will have to a lesser or greater extent:
• Poor muscle tone, that will lead to lack of motor development
• Breathing difficulties, that may lead to respiratory complications
• Scoliosis, a spinal curvature
Because SMA types 1, 2, 3, and 4 are inherited in an autosomal recessive pattern in families, a person with SMA inherits two mutated copies of the SMN1 gene from his or her parents. The parents of a person with SMA are carriers of one copy of the altered gene, but because they also carry a normal version of the gene they do not have signs or symptoms of SMA.
Finkel type SMA is an autosomal dominant pattern. This means that the person has one copy of the altered gene in each cell2.