Patient Guide to Spinal Muscular Atrophy

Spinal Muscular Atrophy: Testing and Diagnosis

Getting an early spinal muscular atrophy diagnosis can mean more effective treatment. Here’s how SMA is diagnosed and how you can get a diagnosis for your baby.

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There may be nothing more important than getting a diagnosis when your baby starts to show the scary signs of spinal muscular atrophy (SMA). Early diagnosis of SMA may allow for early treatment before symptoms start or worsen. 

If a baby shows poor muscle tone (weakness or floppiness), SMA testing should be done right away to either diagnose SMA or rule it out, according to Claudia A. Chiriboga, MD, MPH, Professor of Neurology and Pediatrics at Columbia University Medical Center in New York, NY.

Spinal muscular atrophy diagnosisEarlier is better when it comes to SMA diagnosis.

“Time is of the essence in SMA,” Dr. Chiriboga says. “The earlier we diagnose and treat SMA, the better the response.”

How Do I Know I Need a SMA Diagnosis?

Remember, SMA is a genetic disease. If you and your partner know you carry the gene, you’ll want to get your baby tested right away, even if there are no symptoms.

SMA symptoms can appear as early as in the first six months and as late as 18 months. Look out for SMA symptoms in your child, which can include:

  • Muscle weakness and decreased muscle tone
  • Trouble breathing
  • Trouble eating or swallowing
  • Delayed motor skills
  • Scoliosis

If you notice any of these symptoms—especially if they appear together—you’ll want to get your baby tested. An SMA diagnosis is usually made in one of 3 ways:

  1. Genetic testing in a baby or person who shows symptoms
  2. Newborn screening
  3. Prenatal screening

Genetic Testing

Genetic testing looks for mutations in the SMN1 gene and is the best way to diagnose SMA, with an accurately of 95%. SMA disease is caused by inheriting two defective (missing or mutated) copies of the SMN1 gene—one from each parent.

Spinal muscular atrophy diagnosis genetic testingFind out if you and your partner have faulty SMN1 and SMN2 genes with a genetic test when you're planning your family.

If the standard genetic test appears normal but your child has symptoms suggestive of SMA, genetic sequencing should be used to look for more subtle mutations (changes) on the SMN1 gene. Genetic testing also counts the number of SMN2 gene copies, which helps determine the type of SMA that’s present.

“In general, the less of these extra SMN2 copies you have, the earlier the symptoms start and the more severe the disease,” Dr. Chiriboga says. For example, babies with SMN type 1 generally have two SMN2 copies, while people with SMN type 4 have up to six copies.

Newborn Screening

Spinal muscular atrophy newborn screening is increasingly being used across the U.S. More states are adding SMA to their newborn screening panel to allow for early identification and treatment of this disease.

The test is performed shortly after a baby is born using a drop of blood collected from the baby’s heel (known as a heel prick test). If the screening is positive, your doctor will order more testing to confirm the diagnosis. This screening tests for all SMA types, even those that occur later in life.

Spinal muscular atrophy diagnosis heel prickA heel prick test can detect a number of diseases in newborns, including SMA.

Prenatal Screening

Spinal muscular atrophy screening can be performed in babies before birth (prenatal) using amniocentesis, in which a small amount of fluid surrounding the baby is removed, or by chorionic villus sampling, in which a small piece of the placenta (the organ that grows in the uterus to feed the baby) is removed and examined. Recently, noninvasive prenatal testing that uses the mother’s blood during pregnancy has shown promise in predicting SMA risk. 

SMA Carrier Screening

The American College of Obstetrics and Gynecology now recommends preconception carrier screening to women who are thinking about becoming pregnant or who are already pregnant.

If a woman is a carrier for SMA (meaning that she has 1 defective copy of the SMN1 gene), her partner should be screened as well. If both parents are carriers, the likelihood of having a baby with SMA is 25%.

You can use this information to guide family planning decisions. If two carriers become pregnant, they may choose prenatal screening to see if the baby has SMA.

Other Tests

If the results of SMA genetic testing are negative or unclear, other diagnostic tests may be used to look for other neuromuscular disorders, including electromyography (which measures electrical activity in muscles) and nerve conduction studies (which measure the how well nerves send electrical signals).

Muscle biopsy also may be used to confirm an SMA diagnosis or help rule out other neuromuscular disorders. Additionally, high levels of an enzyme called creatine kinase in your baby’s blood suggest that muscles have been damaged. This test is not specific for SMA as elevated creatine kinase levels can occur in other neuromuscular diseases, but it may be a starting point for your baby’s medical team.

Where Should I Go for an SMA Diagnosis?

If your baby is showing symptoms of SMA disease, you’ll want a referral to a neurologist or neuromuscular specialist right away.

“Ideally, a baby should be referred to a neuromuscular center that has experience with SMA and can diagnose the baby very quickly and get treatment started immediately,” Dr. Chiriboga says.

In other settings, delays often result from the use of broad genetic panel tests, which take longer to process, after which a referral is made for treatment. 

Delays in SMA Diagnosis

“The striking thing about all SMA types is how much of a delay there is in diagnosis,” according to Dr.  Chiriboga. The later the disease starts in life, the later it is diagnosed, she says. 

Typically, if a baby starts showing symptoms at two months of age, an accurate diagnosis isn’t made until about six months of age, according to Dr. Chiriboga. For people with SMA type 3, almost three years may pass between the start of symptoms and diagnosis, because milder symptoms are harder to recognize as SMA, she says. 

Why is treatment so delayed? Often, pediatricians and family practice clinicians may not consider SMA right away when a baby, child, or young adult shows muscle weakness and decreased movement.

Parents may first be referred to an orthopedist—a physician who treats bone, joint, and muscle problems—or the child may be tested for other more common conditions first, Dr. Chiriboga says. In fact, many families report going through many visits with pediatricians and specialists before getting a SMA diagnosis. 

All of these delays allow the SMA symptoms to worsen, Dr. Chiriboga notes. “By the time families get referred to us, the symptoms have progressed and are clearly a neuromuscular problem,” she says.

“We’ve been trying to dispel the antiquated belief that babies may outgrow low muscle tone,” Dr. Chiriboga says. “If doctors have any doubt of what is going on with a baby of low tone, they should order a genetic test for SMA.”


Updated on: 10/19/20
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