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Spinal Muscular Atrophy (SMA)

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 five types of SMA. Some show up earlier and are more severe than others. All types need ongoing treatment by a medical care team. There's no cure for SMA, but treatments can help children who have it 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?

Motor neurons need a protein called SMN (survival motor neuron) to work. Most kinds of SMA are caused by a problem with a gene that makes this protein, called the SMN1 gene. When this gene mutates (changes in some way), it can’t make enough protein for the motor neurons to work properly. The motor neurons break down and can't send signals to the muscles.

Normally, a child gets one copy of the SMN1 gene from each parent. In a child with SMA, both genes will be mutated. If a child gets one healthy gene from a parent and one mutated gene from the other parent, they probably won't show any signs of SMA, but they could pass the mutated gene to their children. A person who has one healthy gene and one mutated gene is called a "carrier."

Genetic testing can help someone find out if they're a carrier and how likely it will be for them to have a child with SMA.

What Are the Signs & Symptoms of Spinal Muscular Atrophy?

The signs of SMA can vary. In general, the later the symptoms appear, the less severe they are.

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

  1. Type 0, also called prenatal SMA, affects babies before they are born. This is very rare, but also very severe. The baby is born extremely weak and has trouble breathing. They may not live longer than a few months.
  2. Type I, sometimes called infantile-onset SMA or Werdnig-Hoffmann disease, 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 also considered a severe form of SMA. These babies look “floppy” and don’t move a lot. They can have trouble eating and breathing, and don’t learn to roll or sit at the expected age. Without treatment, children with this type don’t usually live more than 2 years. With treatment, children with type I SMA are starting to live longer.
  3. Type II begins to affect children between 6–18 months old. Children can sit independently but cannot walk. This form can be moderate to more severe.
  4. 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.
  5. 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.

Problems that can happen because of SMA include:

  • scoliosis (a curved spine) if the back muscles are weak
  • aspiration (breathing in) of fluid or food into the airways
  • deformed or very stiff joints (contractures)
  • fragile bones leading to bone fractures

How Is SMA Diagnosed?

When they think a child might have spinal muscular atrophy, doctors will order genetic testing to look for mutations in the SMN1 gene.

Newborn babies are screened for many diseases, and in most U.S. states, SMA is one of them. This simple blood test, done shortly after birth, looks for different health conditions in newborns who have no symptoms and appear healthy. If the screening test shows that a baby is likely to have SMA, the doctor will order more tests to confirm the diagnosis. Newborn screening is important — the earlier spinal muscular atrophy is diagnosed, the better the chances for the child to get early treatment and possibly avoid serious problems.

How Is Spinal Muscular Atrophy Treated?

There's no cure for SMA, but a few treatment options are available. What doctors use depends on a child’s age and how severe the symptoms are.

Treatment options include:


  • Disease-modifying therapy: These medicines increase the amount of SMN protein in the body. One medicine, Nusinersen (or Spinraza™), is given through a spinal tap. Others can be given by IV infusion or by mouth.
  • Gene replacement therapy: This therapy involves replacing the mutated SMN1 gene with a normal SMN1 gene. It is given one time by IV infusion.

Supportive care:

  • 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.
  • a splint, brace, or sometimes surgery for scoliosis
  • physical therapy and occupational therapy
  • counseling and support groups

What Else Should I Know?

Some children with SMA can take part in clinical trials that are looking at new types of treatments. These are ongoing and have shown promising results in improving overall function in people who have the condition.

Research shows that kids with spinal muscular atrophy 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.

Reviewed by: Elana Pearl Ben-Joseph, MD
Date Reviewed: Sep 20, 2023

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