Health Library

What Is Spinal Muscular Atrophy (SMA)?

Spinal muscular atrophy (SMA) is a genetic condition that causes muscle weakness and atrophy (when muscles get smaller).

SMA can affect a child’s ability to crawl, walk, sit up, and control head movements. Severe SMA can damage the muscles used for breathing and swallowing.

There are four types of SMA. Some show up earlier and are more severe than others. All types of SMA need ongoing treatment by a medical care team. There’s no cure for SMA, but treatments can help children with SMA live a better life.

What Happens in SMA?

In SMA, the nerves that control muscle strength and movement break down. These nerves (called motor neurons) are in the spinal cord and lower part of the brain. They can’t send signals from the brain to the muscles to make them move. Because the muscles don’t move, they get smaller (or atrophy).

What Causes SMA?

Most kinds of SMA are caused by a problem with a gene called the SMN1 gene. The gene does not make enough of a protein needed for the motor neurons to work normally. The motor neurons break down and can’t send signals to the muscles.

A child with SMA gets one copy of the SMN1 gene from each parent. A child who gets the SMN1 gene from only one parent probably won’t show any signs of SMA, but could pass the gene to his or her kids.

Genetic testing of people with SMA and their parents can help determine how likely it is for someone to have a child with SMA.

What Are the Signs and Symptoms of SMA?

The signs of SMA can vary. Some babies with SMA are “floppy” and don’t learn to roll or sit at the expected age. An older child might fall more often than kids of the same age or have trouble lifting things.

Kids with SMA can develop scoliosis (a curved spine) if the back muscles are weak. If SMA is severe, a child might not be able to stand or walk and may need help to eat and breathe.

What Are the Types of SMA?

The four types of SMA are categorized by the disease’s severity and the age when symptoms begin:

1. Type I, sometimes called infantile onset SMA or Werdnig-Hoffmann disease. Type I begins to affect infants from birth up to 6 months of age, with most babies showing signs of the disease by 3 months. This is the most severe form of SMA.
2. Type II begins to affect children between 7 and 18 months old. Children can sit independently, but cannot walk. This form can be moderate to more severe.
3. Type III, also called Kugelberg-Welander syndrome or juvenile SMA, begins to affect kids as early as 18 months of age or as late as adolescence. Children can walk independently, but have weakness in their arms and legs and may fall often. This is the mildest form of SMA in children.
4. Type IV is the adult form of SMA. Symptoms usually begin after age 35 and slowly get worse over time. Because it develops slowly, many people with type IV SMA don’t know that they have it until years after symptoms begin.

How Is SMA Diagnosed?

When they think a child might have SMA, doctors may order:

• genetic testing: This is the most common way to test for SMA. Testing checks for a deletion or variation in the SMN1 gene.
• a muscle biopsy: This is when doctors take a small sample of muscle to check under a microscope.

How Is SMA Treated?

Although there’s no cure for SMA, these treatments can help kids who have it:

• Nusinersen (or Spinraza™), a new treatment for SMA that was approved in 2016. This medicine increases the amount of protein the body needs from the missing SMN1 gene. It works by making the “back-up” gene, the SMN2 gene, look more like the SMN1 gene and produce the needed protein. This is given through a spinal tap. Four doses are given over 2 months, and then every 4 months after that. Studies have shown significant improvement in breathing, motor function, and survival.
• gene therapy trials. These are ongoing and have shown promising results in improving overall function in SMA.
• breathing support through a mask/mouthpiece or a breathing machine. If a breathing machine is needed, a tube may be placed into the windpipe (called a tracheostomy).
• treatments to help kids cough and clear mucus, which can help prevent infections
• proper nutrition. Sometimes a feeding tube is place through the nose into the stomach (an NG tube) or directly into the stomach (called a gastrostomy tube, or G-tube). This way, feedings can go right into the stomach.
• medicine
• a splint, brace, or sometimes surgery for scoliosis
• physical therapy and occupational therapy
• counseling and support groups

Looking Ahead

Research shows that kids with SMA do best with a team approach to their care. Parents, doctors, nurses, therapists, counselors, and a dietitian are all important members of the team. This approach and recent medical advances have improved the outlook for kids with SMA.