What is spinal muscular atrophy?

Spinal muscular atrophy (SMA) refers to a group of inherited neuromuscular diseases that affect the nerve cells (motor neurons) and the control of voluntary muscles.

SMA, the leading genetic cause of death in infants and toddlers, causes lower motor neurons in the base of the brain and the spinal cord to disintegrate, preventing them from delivering the necessary signals for normal muscle function.

Involuntary muscles, such as those that control bladder and bowel function, are not affected in SMA. Hearing and vision are not affected, and intelligence is normal or above average.

Types of SMA

The three major childhood-onset forms of SMA are now usually called Type 1, Type 2, and Type 3. All three types are also known as autosomal recessive SMA, meaning both parents must pass on the defective gene in order for their children to inherit the disease.

All forms of SMA affect the skeletal muscles of the trunk and limbs. In general, those muscles closer to the center of the body are more affected than those farther away.

SMA Type 1, the most severe form, mostly affects the neurons controlling the mouth and throat muscles and therefore involves more problems with chewing and swallowing. Respiratory muscles are involved to varying degrees in all forms of the disease. In SMA Type 1, the onset of the disease is noted within the first six months of the child’s life. Children with SMA Type 1 are unable to sit without support, and death usually occurs before age two.

SMA Type 2 is an intermediate form of the disease. Onset is between seven and eighteen months. Children with SMA Type 2 usually learn to sit without support, but they don’t learn to stand or walk without aid. The child’s survival depends in large part on the degree of respiratory and swallowing difficulties.

SMA Type 3 is a milder form of this condition. Onset occurs after the age of eighteen months and most often between the ages of five and fifteen. Weakness of the muscles of chewing and swallowing is rare, and respiratory effects are generally not as severe as in the first two forms. These children may live into adulthood. Respiratory complications, if they occur, pose the most serious threat to life.

Treatment

At present, there is no known treatment that will stop or reverse SMA. Physical therapy and orthopedic devices can help preserve walking function. Braces or surgery may also help to counteract scoliosis, or curvature of the spine.

Researchers around the world have collaborated to find the causes of SMA, which in most cases result from a deficiency of a protein called SMN (survival of motor neuron). This deficiency occurs when a mutation is present in both copies of the SMN1 gene – one on each chromosome 5.

Scientists hope to characterize the genes, study gene function and disease course, and find ways to prevent, treat, and, ultimately, cure these diseases.

Resources

If you are looking for more information on spinal muscular atrophy or have a specific question, our information specialists are available business weekdays, Monday through Friday, toll-free at 800-539-7309 from 9am to 5pm ET.

Additionally, the Reeve Foundation maintains a SMA fact sheet with additional resources from trusted Reeve Foundation sources. Check out our repository of fact sheets on hundreds of topics ranging from state resources to secondary complications of paralysis.

We encourage you to reach out to spinal muscular atrophy support groups and organizations, including:

  • Spinal Muscular Atrophy Foundation hopes to accelerate the development of a treatment or cure for SMA. Toll-free 1-877-FUND-SMA.
  • Families of Spinal Muscular Atrophy (FSMA) raises funds to promote research into the causes and treatment of the spinal muscular atrophies; supports families affected by SMA. Toll-free 1-800-886-1762.
  • Muscular Dystrophy Association (MDA) provides services and supports research for a group of hereditary muscle-destroying disorders, including spinal muscular atrophies. Toll-free 1-800-572-1717; search under "Diseases."

This project was supported, in part by grant number 90PR3002, from the U.S. Administration for Community Living, Department of Health and Human Services, Washington, D.C. 20201. Grantees undertaking projects under government sponsorship are encouraged to express freely their findings and conclusions. Points of view or opinions do not, therefore, necessarily represent official Administration for Community Living policy.