Spinal muscular atrophy type 4 (SMA4) is one type of a group of conditions known as spinal muscular atrophies (SMAs), which are inherited disorders that cause a person to lose a specific type of cell in their spinal cord (motor neurons) and a part of their brain that is connected to the spinal cord (brain stem). Without these important parts of the brain and spinal cord, an affected person loses control of muscle movement, has muscle weakness, and loses muscle tissue (atrophy). SMA4 is one of the milder forms of SMA. Symptoms typically do not occur until after age 30 and include weakness in the muscles that are closest to the center of the body (proximal muscle weakness), which include the upper arms and legs. Other symptoms include tremors, twitching, and mild breathing issues.
SMA4 is caused by changes (mutations) in the SMN1 gene. This gene provides the instructions to make a protein that helps motor neurons work. Motor neurons are cells that send signals from the brain to the muscles. When there are mutations in the SMN1 gene, the motor neurons die off, leading to the symptoms of this condition. SMA4 is inherited in an autosomal recessive way, which means a person must have a mutation in both copies of their SMN1 gene to have the condition.
SMA4 is usually considered in an adult who begins to have proximal muscle weakness. Genetic testing of the SMN1 gene is used to confirm the diagnosis. Muscle weakness seen in SMA4 is similar to the muscle weakness seen in other genetic conditions. A doctor may collect and examine a small muscle sample (biopsy) to look for clues for the diagnosis. There is no cure for SMA4. Treatment options focus on addressing the muscle weakness and can include physical therapy. If you or your child has been diagnosed with SMA4, talk with a doctor about all treatment options. Support groups are available for additional information and to connect with others affected by this condition.