SMA Testing And Diagnosis

Spinal muscular atrophy (SMA) is an inherited disorder that can be diagnosed using a genetic test. Genetic tests use a blood sample to identify alterations in a certain gene. Our genes make up our DNA. More than 95 percent of people with SMA can be diagnosed by a deleted portion of the SMN1 gene.

Genetic testing for SMA is usually done after symptoms appear. SMA is a rare disease. Early symptoms are similar to other neuromuscular disorders. Many families experience a delay in diagnosis as doctors rule out other conditions.

An early diagnosis is important because newly approved treatments are more effective when started earlier. To address this, the United States recommends that every newborn should be tested for SMA.

When is genetic testing done to diagnose SMA?

SMA is diagnosed through genetic testing in 1 of 3 ways:

  • After showing symptoms
  • Through newborn screening
  • Through prenatal testing

The test uses a blood sample to check if the SMN1 gene is present.1-3

Testing after symptoms start

Unless there are previous cases of SMA in the family, diagnosis usually begins after symptoms appear. Parents often first notice early symptoms of SMA in their baby. The first symptoms may be weakness or a delay in meeting motor milestones, such as holding their head up.4,5

Early symptoms of SMA are similar to other neuromuscular disorders. Families may go through many referrals and tests before SMA is suspected. Once your doctor suspects SMA, they will order genetic testing through a blood sample. At the same time, children will be referred to a specialist to begin physical therapy and treatments.5

Newborn screening

Newborn screening identifies conditions at birth where early diagnosis improve long-term health. This allows babies to begin treatment before symptoms appear. In the United States, babies are screened for many conditions using blood from a heel prick.1

Newly approved disease-modifying treatments for SMA are more effective when started earlier. Screening newborns for SMA helps them start these treatments as early as possible. In 2018, SMA was added to the federal Recommended Uniform Screening Panel (RUSP) for newborn screening.6,7

As of early 2021, 33 U.S. states screen for SMA, which is about 68 percent of American newborns. The CureSMA foundation website offers more information about newborn screening. The foundation also provides more details about SMA in each state.8

Prenatal testing

Prenatal testing can determine if an unborn baby has inherited SMA. This test is done if the child is at risk for SMA. For example, if both parents are carriers of SMA, the child has a 25 percent chance of having SMA.1

The most common procedure for prenatal testing is amniocentesis. This can be done after the 14th week of pregnancy to test the unborn baby’s DNA for SMA. The decision to undergo prenatal testing is a personal one. Talk to your doctor or a genetic counselor about the risks and benefits.1

Carrier screening

People who have 1 normal copy of the SMN1 gene and 1 abnormal copy of the SMN1 gene are “carriers.” They show no symptoms but can pass on the abnormal copy to their children. Most people do not know they are carriers until they have a child with SMA.9,10

If you have a family member who has or carries SMA, talk to a genetic counselor. They can discuss your chances of being a carrier and having a child with SMA. They can perform genetic testing to determine if you are a carrier.9,10

What other tests are done to diagnose SMA?

After diagnosis, your doctor will refer you to a specialist. This is usually a pediatric neurologist. This is a doctor who specializes in childhood nerve diseases. They may order more tests to confirm the diagnosis.1

They may perform an electrical study called an electromyograph (EMG) or a muscle biopsy. These tests are now rarely used because genetic testing is more available.2,4

Doctors may perform genetic testing for the number of copies of the SMN2 gene. This is the back-up gene that people with SMA rely on for motor neurons to function. The number of SMN2 copies helps predict disease severity. It is also often used as criteria for clinical trial enrollment. Having more copies of the SMN2 gene usually means milder SMA. For example, people with:2,5

  • Type 0 usually have 1 copy
  • Type 1 usually have 2 copies
  • Type 2 usually have 3 copies
  • Type 3 usually have 3 to 4 copies
  • Type 4 usually have 4 to 5 copies

How are physical exams used for diagnosis?

Physical exams help doctors distinguish SMA from other neuromuscular disorders. Many neuromuscular conditions show low muscle mass and weakness. Some physical signs that help diagnose SMA include:5

  • Abnormal breathing and heart rate
  • Tongue twitches
  • Absent reflexes
  • Weak cry and cough
  • Bell-shaped chest
  • Slow feeding and swallowing problems

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Written by: Matt Zajac | Last reviewed: August 2021