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 conditions. 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, finger-like 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.
Carriers of Spinal Muscular Atrophy
Most people have two functioning copies of the SMN1 gene. People with one faulty copy and one functioning copy are called “carriers.”
Carriers generally do not show signs and symptoms of SMA, but could be at risk to have a child affected with the condition.
Approximately 1 in 50 people is a genetic carrier for SMA. Most carriers have no idea they are carriers until they have a child born with SMA.
How is SMA Inherited?
SMA is an autosomal recessive genetic condition. This means that a child must inherit two non-working copies of the SMN1 gene, typically one from each parent, in order to have SMA.
When two parents are carriers, there is:
- A 25% chance that their child will be unaffected
- A 50% chance that their child will be a carrier
- A 25% chance that their child will have SMA
If only one parent is a carrier, the child is usually not at risk for SMA, though they do have a 50% risk of being a carrier. However, in very rare cases, spontaneous genetic changes in the SMN1 gene can occur during egg or sperm production. In this situation, only one parent will be a carrier. In addition, a small percentage of carriers have genetic changes that cannot be identified through current testing technology. In this case, it will appear as though the disease has been caused by a single carrier.
A DNA test is the only way to know if a person is a carrier. The DNA test is a simple procedure, based on a blood test. In the general population, this test can detect about 95% of carriers. However, in African-American populations, detection is closer to 70%. This is because a difficult to detect mutation is seen more frequently in African-American populations than in other races.
The American College of Obstetricians and Gynecologists recommends that all women who are thinking about becoming pregnant or who are already pregnant be offered carrier screening for SMA and other genetic conditions. In addition, individuals with a family history of SMA are encouraged to have carrier screening. Deciding whether or not to undergo genetic testing is highly personal, and we strongly recommend discussing it with a physician or genetic counselor. Carrier screening via saliva testing is also available as an alternative to a blood test.
For couples who are carriers, reproductive decisions can be sensitive. A number of options are available, such as no testing, prenatal testing, adoption, and pre-implantation genetic diagnosis (PGD). PGD screens embryos for genetic disorders and selects the unaffected embryos for implantation.
We believe that your family has the right to choose whatever option is best for you.
We help families understand their options and provide resources to support their decision-making process. We do not advocate any specific course of action, nor do we pressure families to choose one way or the other.
We encourage each family to discuss their situation with a an expert physician and a genetic counselor and only then take a final call.