As of 2024, all 50 states in the U.S. screen newborns for spinal muscular atrophy (SMA) at birth. However, milder types of SMA may develop later in life and take longer to diagnose.

SMA is a neuromuscular disorder that causes worsening muscle weakness over time. Depending on the type, symptoms of SMA can include breathing problems, lack of motor function, inability to stand or walk, and difficulty chewing and swallowing. Here are some of the different tests you might expect along the way.

How Do Doctors Test for SMA?

Newborn screening for SMA is standard at every U.S. hospital. Newborn screening includes drawing a blood sample (often via a heel stick) that’s checked for any conditions listed on the state’s screening panel. This expanded newborn screening makes it possible to start treatment sooner, including gene replacement therapy in babies under age 2.

For those who develop SMA symptoms later in life, the process of diagnosing SMA depends on when the symptoms start. Diagnosing adult-onset SMA types like type 4 or other motor neuron disorders like Kennedy disease (also called X-linked spinal and bulbar muscular atrophy) may start after noticing mild symptoms like hand tremors or leg weakness.

Physical Exam

If potential SMA symptoms appear later in life, testing often starts with a physical exam to check for muscle weakness or missing reflexes. If a primary care doctor suspects SMA or another rare disease, they may refer you or your child to a specialist for more testing.

For those who develop SMA symptoms later in life, the process of diagnosing spinal muscular atrophy depends on when the symptoms start.

Family Medical History

A thorough family history helps determine if neuromuscular diseases run in the family. Based on the physical exam and family history, the next step may be genetic testing.

Genetic Testing for SMA

SMA is diagnosed through molecular genetic testing, which involves a blood test. In molecular genetic testing, a single gene is evaluated for mutations (or changes) associated with a genetic disease. After SMA bloodwork, the results may take a bit of time to come back. In some cases, you may receive them within five days.

You may meet with a genetic counselor as part of the genetic testing process. A genetic counselor is trained to help people understand their risk of inherited conditions, support them through testing, and explain what the results mean for them and their family.

If SMA runs in your family, you can get carrier testing to see if you carry the gene. Prenatal testing is also available.

When diagnosing SMA, molecular genetic testing focuses on the survival motor neuron 1 (SMN1) gene. Specifically, this gene is found on chromosome 5. Mutations in the SMN1 gene cause SMA types 0 through 4. These types account for nearly all cases of SMA.

The SMN1 gene produces survival motor neuron (SMN) proteins. These proteins are important for the motor neurons to work. Motor neurons are nerve cells. They transmit signals from the brain and spinal cord to the muscles. Without these proteins, the brain and the muscles can’t communicate. As a result, the muscles become weaker.

Other types of lower motor neuron disorders are caused by mutations in different genes. There’s specialized genetic testing for other types.

Additional Testing

Before genetic testing was available to diagnose SMA, doctors used to do a muscle biopsy. A biopsy is a procedure where a tiny piece of muscle tissue is removed and tested in a lab.

Another test, called electromyography, was also used. Electromyography measures the electrical activity of muscles. A combination of these two tests was previously used to diagnose SMA.

If the genetic test results are unclear, a doctor may order an electromyography or, rarely, a muscle biopsy to rule out other conditions.

Today, a genetic test is enough to diagnose most cases of SMA. If the genetic test results are unclear, a doctor may order an electromyography (EMG) or, rarely, a muscle biopsy to rule out other conditions.

Spinal Muscular Atrophy Diagnosis: Q&A

The steps to diagnosing SMA have evolved. With more newborn screening and better genetic tests available, it’s easier to get the right diagnosis now. New technology and more awareness of SMA should continue to help improve diagnostic testing.

Here are the answers to some common questions about diagnosing SMA.

What Is Carrier Screening for SMA?

Carrier testing is a type of genetic test. It shows if someone has a recessive genetic mutation, meaning they have genetic mutations for SMA, but they don’t have signs of the disease. SMA and other recessive genetic disorders happen when a child inherits a mutated copy of the disease-causing gene from each parent. There’s no way to know if you’re a carrier of the mutated gene without genetic testing.

Genetic screening for SMA is available before and during pregnancy. Screening is recommended for anyone with a family history of SMA.

Can SMA Be Diagnosed During Pregnancy?

Yes. Prenatal testing can show if a fetus has inherited the genes that cause SMA. Prenatal genetic testing can be done through amniocentesis (testing the amniotic fluid) or chorionic villus sampling (testing the placental tissue).

Testing comes with some risks and is usually only recommended if both partners carry the gene that causes SMA. A genetic counseling session can help you understand your options before and during pregnancy.

Talk With Others Who Understand

On mySMAteam, people talk about their experiences with spinal muscular atrophy, get advice, and find support from others who understand.

What was the path to diagnosis like for your family? How did genetic testing and counseling affect your life? Share your thoughts in the comments below.