An SMA diagnosis must be confirmed through genetic testing.
SMA is usually diagnosed in one of three ways:
- Through genetic testing, after an infant or child shows symptoms of SMA
- Through a positive newborn screening result
- Through prenatal testing
Early Symptoms of SMA
A doctor may suspect SMA when a child is noticeably weak or has a delay in meeting developmental milestones. This may include delays in holding their head up, rolling over, sitting independently, standing, or walking.
SMA is not the only condition that can cause weakness or a delay in meeting milestones, so further examination and testing is needed to make an SMA diagnosis.
If a doctor suspects SMA, they may:
- Order genetic testing through a blood sample, or
- Refer the child to a neurologist who will also perform an examination, and then order genetic testing (again through a blood sample) to confirm the diagnosis.
A simple blood draw test can identify an estimated 95% of all SMA cases. The other 5% are caused by a rare mutation and must be identified through further testing.
When a baby is born, a small blood sample is taken. This sample is then screened for a number of genetic conditons. In 2018, some states began screening infants for SMA. If a newborn screening result is positive for SMA, follow-up testing is required to confirm the diagnosis.
Newborn screening allows an infant to begin treatment before symptoms appear, when research suggests it may be most effective. For more information on what to do about a positive SMA screen, visit our Support & Care section.
Prenatal testing is used to determine if a fetus has inherited a genetic disorder. After consultation with their doctor, some families may choose this type of testing if their child is known to be at risk for SMA. One of two different tests may be used:
- In amniocentesis, the most common form of prenatal testing, a very fine needle is inserted into the woman’s abdomen, and amniotic fluid is extracted. This fluid contains fetal DNA that can be tested for SMA. Amniocentesis can be performed after the 14th week of pregnancy, and is associated with a risk of miscarriage that may be as high as 1 in 200.
- Chorionic Villus Sampling can often be performed as early as the 10th week of pregnancy. Chorionic villi are small, fingerlike structures that form the placenta. Chorionic villi contain fetal DNA that can be extracted and tested for SMA. CVS is associated with a risk of miscarriage that may be as high as 1 in 100.