What is Spinal Muscular Atrophy?

Spinal muscular atrophy (SMA) is a genetic disease that affects the nervous system and, mostly, the muscles it controls. It weakens muscles and can lead to problems breathing, eating, and walking. SMA can strike at any age, from infants through adults. More than 25,000 Americans have the disease.

SMA is the No. 1 genetic cause for death in infants. But how severe the disease is varies widely. Many of those with SMA will lead long, productive lives with a normal life expectancy.

What Causes It?

SMA happens because of low levels of a protein known as SMN. It feeds special cells that transmit information to other cells or muscles (called neurons) that help your muscles work the way they should.

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The SMN shortage is a result of problems with a couple of different genes, called the SMN1 or SMN2 genes.

Spinal muscular atrophy is an inherited disease; both parents have to have the faulty genes to pass the disease onto their child. If both parents are carriers of the faulty gene, their child has a 1 in 4 chance of having SMA.


Your doctor may call for a blood test to look for the faulty genes.

They also might order:

  • Saliva tests
  • Muscle biopsies (where a small portion of a muscle is removed and looked at under a microscope)
  • An electromyogram (a test that measures electrical activity in a muscle or a group of muscles)
Types of SMA

SMA is broken down into categories based on the symptoms you have and the age you are when they start.

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Generally speaking, the more SMN protein that is produced and the later in life that the symptoms begin, the milder the disease.

Type 0: Very little SMN is made in a child with this type. A mother may notice a slowdown in her child's movement even before birth. She may give birth earlier, too. A child with type 0 might have heart defects and breathing problems. This is the rarest, most severe form of the disease, and babies with it rarely live past 3 months of age.

Type 1: This type usually presents itself at birth, or shortly after. Babies with it have problems breathing, swallowing, and sucking. They also won't be able to sit up without help, and they'll have trouble lifting their head.

Type 2: This typically happens when kids are about 7 to 18 months old. They'll be able to sit up. But because muscle weakness is especially apparent in the legs, they will need help to walk. Scoliosis (a curved spine) and breathing issues are also possible. Type 2 is less severe than types 0 and 1.

Type 3: This hits older kids and teens who have learned to walk but can’t later. They may also be affected by respiratory weakness and scoliosis.

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Type 4: Older teens or adults get type 4. They can walk, but they might have general muscle weakness, trembling or twitching, minor difficulty breathing, or a walking disability.

SMA can't be prevented, and so far, there is no cure. It affects about one in every 11,000 births. But you can take a blood test to see if you're a carrier, which will help you find out your chances of having a child with SMA.

There are FDA-approved treatments for SMA that can ease symptoms and slow the progression of the disease. Talk with your doctor about which treatments might be best for you.

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