Share this on:
Spinal Muscular Atrophy (SMA) Month

.What is spinal muscular atrophy (SMA)?

Spinal muscular atrophy (SMA) is a genetic (inherited) neuromuscular disease that causes muscles to become weak and waste away. People with SMA lose a specific type of nerve cell in the spinal cord (called motor neurons) that control muscle movement. Without these motor neurons, muscles don’t receive nerve signals that make muscles move. The word atrophy is a medical term that means smaller. With SMA, certain muscles become smaller and weaker due to lack of use.

How common is spinal muscular atrophy?

Approximately 10,000 to 25,000 children and adults are living with SMA in the United States. It’s a rare disease that affects one out of 6,000 to 10,000 children.

Who might get spinal muscular atrophy?

A person with SMA inherits two copies of a missing or faulty (mutated) survival motor neuron 1 (SMN1) gene. One faulty gene comes from the mother and the other comes from the father. An adult can have a single copy of the defective gene that causes SMA and not know it.

About six million Americans (1 in 50) carry the mutated SMN1 gene. These carriers have one healthy SMN1 gene and one missing or defective SMN1 gene. Carriers don’t develop SMA. There's a 1 in 4 chance that two carriers will have a child with SMA.What are the types of spinal muscular atrophy?

There are four primary types of SMA:

  • Type 1 (severe): About 60% of people with SMA have type 1 , also called Werdnig-Hoffman disease. Symptoms appear at birth or within an infant’s first six months of life. Infants with type 1 SMA have difficulty swallowing and sucking. They don’t meet typical milestones like holding up their heads or sitting. As muscles continue to weaken, children become more prone to respiratory infections and collapsed lungs (pneumothorax). Most children with type 1 SMA die before their second birthday.
  • Type 2 (intermediate): Symptoms of type 2 SMA (also called Dubowitz disease) appear when a child is between six months and 18 months old. This type tends to affect the lower limbs. Children with type 2 SMA may be able to sit up but can’t walk. Most children with type 2 SMA live into adulthood.
  • Type 3 (mild): Symptoms of type 3 SMA (also called Kugelbert-Welander or juvenile-onset SMA) appear after a child’s first 18 months of life. Some people with type 3 don’t have signs of disease until early adulthood. Type 3 symptoms include mild muscle weakness, difficulty walking and frequent respiratory infections. Over time, symptoms can affect the ability to walk or stand. Type 3 SMA doesn’t significantly shorten life expectancy.
  • Type 4 (adult): The rare adult form of SMA doesn’t typically appear until the mid-30s. Muscle weakness symptoms progress slowly, so most people with type 4 remain mobile and live full lives.What causes spinal muscular atrophy?

    People with SMA are either missing part of the SMN1 gene or have a changed (mutated) gene. A healthy SMN1 gene produces SMN protein. Motor neurons need this protein to survive and function properly.

    People with SMA don’t make enough SMN protein, and so the motor neurons shrink and die. As a result, the brain can’t control voluntary movements, especially motion in the head, neck, arms and legs.

    People also have SMN2 genes that produce a small amount of SMN protein. A person may have up to eight copies of an SMN2 gene. Having multiple copies of the SMN2 gene typically leads to less severe SMA symptoms because the extra genes make up for the missing SMN1 protein. Rarely, non-SMN gene mutations (non-chromosome 5) cause SMA.How is spinal muscular atrophy managed or treated?

    There isn’t a cure for SMA. Treatments depend upon the type of SMA and symptoms. Many people with SMA benefit from physical and occupational therapy and assistive devices, such as orthopaedic braces, crutches, walkers and wheelchairs.

    These treatments may also help:

    • Disease-modifying therapy: These drugs stimulate production of SMN protein. Nusinersen (Spinraza®) is for children ages 2 to 12. Your provider injects the drug into the space around the spinal canal. A different medication, risdaplam (Evrysdi®), helps adults and children older than two months. People take risdaplam daily by mouth (orally).
    • Gene replacement therapy: Children younger than two may benefit from a one-time intravenous (IV) infusion of a drug called onasemnogene abeparvovec-xioi (Zolgensma®). This therapy replaces a missing or faulty SMN1 gene with a functioning gene.