What Is a Spinal Muscle Atrophy (SMA) Test and Who Needs One?

Spinal muscular atrophy (SMA) is a group of inherited diseases that affect the nerves and muscles. Tests to diagnose SMA may be done before birth, after birth, or in adulthood. SMA is progressive and has no cure.

Spinal muscular atrophy (SMA) affects nerve cells in the brain and spinal cord called motor neurons.

Over time, it causes muscle weakness in the arms and legs as well as in the face, chest, and throat. Symptoms may range from mild to severe. Some people may only have minor issues, while others may have difficulty walking, breathing, and swallowing.

The Centers for Disease Control and Prevention (CDC) report that around 1 in every 10,000 people may develop SMA. Here’s who should consider testing and which tests are available for diagnosis.

The genes that cause SMA are passed on during conception. Consider SMA testing if you have a family history of the disorder, as you may have the disease or be a carrier for the gene mutation.

Your doctor might also consider testing if you have a pattern of weakness and:

• weakness in the arms and legs
• muscle tremors
• scoliosis
• swallowing difficulties
• breathing problems

If you’re pregnant and have a family history of SMA, your doctor may suggest prenatal testing.

• Chorionic villus sampling (CVS):CVS involves taking a sample of cells from the placenta and may be performed anywhere between weeks 11 and 14 of pregnancy.
• Amniocentesis:Amniocentesis involves taking a sample of amniotic fluid and may be performed between weeks 15 and 20.

Both of these tests do carry some risks, like increasing the risk of miscarriage.

Carrier screening is an option before you become pregnant. Usually, one partner will have the screen done first. If they are a carrier for the gene, it’s recommended that the other parent be screened as well.

Many states screen for SMA after birth. A blood sample is collected from a baby’s heel within the first day or two of life. This sample helps detect a number of diseases, including SMA.

Otherwise, SMA may be diagnosed in infants and children through:

• a physical examination
• a detailed health history (to determine whether there’s a family history of SMA or similar conditions)
• genetic blood testing to look for changes in the SMN1 gene

Your child’s doctor may also order additional tests:

• Electromyography (EMG): In this test, your doctor will insert thin needles into the muscle to see how it’s working.
• Muscle biopsy: During a muscle biopsy, your doctor will take a small sample of muscle tissue for analysis.
• Nerve conduction velocity (NCV) study: In this test, your doctor will stimulate a nerve and record how it responds.

Diagnosis for adults is done in a similar way. Your doctor will ask what symptoms you are experiencing, give you a physical exam, and ask about your family health history.

A genetic blood test can help to confirm changes in the SMN1 gene.

Again, EMG, NCV, and muscle biopsy may be necessary for additional confirmation.

SMA is inherited in something called an autosomal recessive pattern. This means that for a person to develop SMA, he or she must inherit two affected genes — one from each parent.

A person who inherits just one affected gene is considered a carrier and will not develop symptoms of SMA. However, if you’re a carrier and your partner is also a carrier, you may pass on SMA to your children.

Think you may be a carrier? Your doctor may refer you to a genetic counselor so you can discuss your risks.

There’s no cure for SMA. Treatment is individual and directed at addressing the symptoms a person is experiencing and preventing complications.

• nusinersen (Spinraza)
• onasemnogene abeparovec-xioi (Zolgensma)
• risdiplam (Evrysdi)

• physical therapy
• occupational therapy
• speech therapy
• assistive devices (braces, wheelchairs, orthotics, etc.)

The outlook for someone with SMA depends on the type they have and the severity of their symptoms.

Some types may shorten a person’s life expectancy. Type 1, for example, is the most severe, with a life expectancy of around 2 years. People with type 2, on the other hand, may live into adulthood (to their 30s or 40s).

Types 3 and 4 do not typically shorten life expectancy.

At what age do SMA symptoms start?

The onset of SMA depends on what type a person has. Some types may begin at birth or in early childhood. Other types don’t start until a person is an adult.

Will all people with SMA eventually need wheelchair assistance?

SMA is progressive. Whether a person will have trouble walking depends on the age of onset and the severity of their symptoms. Not all people will need wheelchair assistance.

What is the percent risk of passing SMA to my child?

If both biological parents are carriers of SMA, their risk (for each pregnancy) is as follows:

• 50% chance that a child will be a carrier
• 25% chance that a child will have SMA
• 25% chance that a child will be unaffected

SMA is a lifelong condition that runs in families. If you or your partner have a family history of SMA, speak with your doctor about being screened before pregnancy or having your child tested after birth.

Likewise, if you are an adult and experiencing symptoms, like progressive muscle weakness, speak with your doctor about testing and treatment options.

Last medically reviewed on April 6, 2023

How we reviewed this article:

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• Overview: Spinal muscular atrophy. (2020).
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• Spinal muscular atrophy. (n.d.).
  dph.illinois.gov/content/dam/soi/en/web/idph/files/publications/sma-physician-fact-sheet.pdf
• Spinal muscular atrophy (SMA). (2023).
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• Spinal muscular atrophy (SMA). (2023).
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• Spinal muscular atrophy. (2023).
  ninds.nih.gov/health-information/disorders/spinal-muscular-atrophy
• What is newborn screening? (2023).
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