What is SMA?

Spinal Muscular Atrophy (SMA) is a disease that causes motor neurons, which control voluntary movements such as walking, talking, and swallowing, to function abnormally. SMA leads to progressive muscle weakness and atrophy, especially in the muscles of the torso, upper legs, and upper arms. Symptoms can begin prior to six months of age, in childhood and, more rarely, in adulthood. In the most common form of the disease, lifespan is often less than two years of age.

How is SMA Inherited?

SMA is inherited in an autosomal recessive manner. In most cases, both parents must be carriers of the condition in order to have a child with SMA. Carriers are not themselves affected by the disease, but have a chance of having a child with the disease. If one parent is found to be a carrier, screening is recommended for their partner. If the partner screens negative, then the couple’s risk to have an affected child is reduced (but not eliminated). If both parents are carriers of SMA, there is a 25% chance in each pregnancy of having a child with SMA, a 50% chance of having a child who is an unaffected carrier of the condition, and a 25% chance of having a child who is not a carrier and is unaffected.

Who Should be Offered SMA Testing?

Since SMA is a common and severe genetic condition, the American College of Medical Genetics (ACMG) and the American College of Obstetrics and Gynecology (ACOG) recommend that SMA carrier testing be offered to all couples prior to conception or early in pregnancy. If you have no family history of SMA, your risk to be a carrier is based on your ethnicity. SMA occurs in all ethnic backgrounds. If you have a family member with SMA, your risk to be a carrier is increased.

Caucasian 1/35
Ashkenazi Jewish 1/41
Asian 1/53
African-American 1/66
Hispanic 1/117

Is There Prenatal Diagnosis for Spinal Muscular Atrophy?

Yes. Prenatal testing by chorionic villi sampling or amniocentesis can be performed to determine whether or not a fetus is affected with SMA.