Spinal muscular atrophy (SMA) is caused by problems with nerve cells in the spine that control muscles. Tests for muscle health and nerve function are often abnormal in people with SMA. However, these tests are now rarely used to diagnose SMA. This is because genetic testing is widely available and more reliable.
These tests are now usually used when genetic testing rules out the major types of SMA. People who show symptoms of SMA but normal genetic testing results may have rare types of SMA or other neuromuscular conditions. These tests may be helpful to diagnose these cases.
When are tests used to diagnose SMA?
A genetic test is the only way to diagnose SMA with certainty. Genetic testing for SMN1 is reliable and readily available. Other tests are now rarely used to diagnose major types of SMA. As soon as doctors suspect SMA based on physical symptoms, they will order a genetic test.1
However, other tests may be helpful to diagnose milder types of SMA. When muscle weakness and other symptoms are mild, it may be difficult for doctors to pinpoint SMA. They may use tests for muscle health and nerve function to distinguish SMA from other neuromuscular conditions.2
Experts also recommend using these diagnostic tests if genetic tests do not reveal alterations in the SMN1 gene. People who show symptoms of SMA but have normal SMN1 genes may have other types of SMA or other neuromuscular conditions. Diagnostic tests can help in these cases.2
What are some tests used to diagnose SMA?
Electromyography
Electromyography (EMG) is a test that analyzes the health of muscles and motor neurons. These are nerve cells that send signals and cause muscles to contract. An EMG can look at these signals to determine if there are problems with nerves, muscles, or the communication between them.3
EMG is most often used to diagnose diseases that cause muscle weakness. Symptoms that may lead doctors to perform EMG include:3
- Numbness
- Low muscle tone
- Twitching
- Cramps
- Tingling
EMG was often used to diagnose SMA before genetic testing was possible. This is because EMG shows signs of motor neuron loss in people with SMA. EMG findings can also indicate the severity of SMA.4,5
EMG is a low-risk procedure, and complications are rare. The needle used in EMG may cause some discomfort or pain.3
Muscle biopsy
A muscle biopsy is a test used to see if muscles are healthy. Doctors will remove a small piece of muscle using a biopsy needle. They will view the muscle under a microscope. The muscle selected for biopsy depends on the location of weakness. The most common muscles used are from the upper arm, shoulder, or thigh.6
In SMA, a muscle biopsy will show unique signs of a loss of nerve supply to the muscle (denervation). This includes wasting away (atrophy) of certain types of muscle fibers. Muscle biopsies can also indicate the severity of SMA.4,7
Muscle biopsies are low-risk procedures. The most common side effects of the procedure are bruising and discomfort at the biopsy site.6
Creatine kinase levels
Creatine kinase (CK) is an enzyme released into the blood when there is muscle damage. Conditions that cause muscle damage may increase CK levels in the blood.8
CK levels are usually normal in SMA. However, doctors may use this test because people with similar conditions, like Duchenne muscular dystrophy, show high CK levels. If genetic tests rule out SMA, doctors may use a blood sample to test CK levels to diagnose other neuromuscular conditions.2,4