Spinal muscular atrophy type 3 (SMA3), also known as juvenile SMA, is one type of a group of conditions known as spinal muscular atrophies (SMAs), which are inherited disorders that cause a child 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 child loses control of muscle movement, has muscle weakness, and loses muscle tissue (atrophy). SMA3 is considered to be a more mild form of SMA and symptoms usually present after the age of 12 months. The muscle weakness in SMA3 affects both arms and legs, but legs are typically more severely affected. The disease can sometimes cause a curvature of the spine (scoliosis). Most children with SMA3 can walk without help, but they may fall often. People with SMA3 may need to use a wheelchair later in life.
SMA3 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. SMA3 is inherited in an autosomal recessive way, which means a child must have a mutation in both copies of their SMN1 gene to have the condition.
SMA3 is usually considered in a child who has significant muscle weakness that is noticed around 12 months of age. Genetic testing of the SMN1 gene is used to confirm the diagnosis. There is no cure for SMA3. Treatment options, such as braces to help support the back or legs, are available to help with complications of SMA3. If your child has been diagnosed with SMA3, talk to their doctor about all treatment options. Support groups are available for additional information and to connect with other families affected by SMA3.