Spinal Muscular Atrophy

A new guideline was published in March by the American College of Obstetricians and Gynecologists stating that individuals planning to start a family or pregnant should be offered carrier screening for Spinal Muscular Atrophy (SMA).^1,2

Some questions that may come to mind are:

What is SMA?
What does it mean to be a carrier?
How do I decide if I should get screening?

What is Spinal Muscular Atrophy?

Spinal Muscular Atrophy (SMA) is a genetic condition that affects the nerves located within the spine. These nerves are responsible for signaling muscles to move, whether it’s to nod your head, walk, or breathe.

As this condition progresses, the nerves weaken and may die, leading to muscle damage.

Although there are different types of SMA, people with the most common form usually pass away from breathing complications before age four.

*The muscles closer to the center of the body are usually more affected in spinal muscular atrophy than are the muscles farther from the center. [source]*

What does it mean to be a carrier of Spinal Muscular Atrophy?

SMA is caused when a gene, in this case SMN1, does not work properly. Genes tell our body how to perform a task. SMN1 plays a role in helping nerves signal to muscles.

Individuals normally have two copies of SMN1, one from each parent. Carriers usually have only one copy of SMN1, but carriers of SMA do not have symptoms and do not have SMA. Both copies of SMN1 do not work in individuals with SMA.

Gene function for carriers is like a toaster with two slots for bread. If one slot does not work, bread can still be toasted, but if both slots do not work, no toast can be made.

How does carrier screening work?

Carrier screening uses a blood sample to see if you are a carrier of one or two SMN1 genes. Carrier screening is designed to identify most people that carry SMA, but does not find all people who carry SMA.

Below is a table indicating the chance of being a carrier based on your ethnicity and the chance of being a carrier following a negative carrier screening result.

Ethnicity	Carrier Risk	Risk After Negative Carrier Result
Caucasian	1 in 35	1 in 632
Ashkenazi Jewish	1 in 41	1 in 350
Asian	1 in 53	1 in 628
Black	1 in 66	1 in 121
Middle Eastern	1 in 50	1 in 560
Hispanic	1 in 117	1 in 1,061

What if I am a carrier of SMA?

If you are a carrier, then genetic counseling and carrier screening can be offered to your partner. If both you and your partner are carriers, then there is a 25% chance your child will have SMA, a 50% chance your child will be a carrier of SMA, and a 25% chance that your child will not have or carry SMA.

How do I decide if I should get carrier screening?

Having carrier screening is a personal choice that is often influenced by beliefs, family dynamics, and personal preference. Some people opt for screening because they want to be prepared in case their future child has SMA or want reassurance that their child will not likely have SMA. Others do not want screening because the information will not help them prepare and may cause anxiety.

Sometimes if both parents are carriers, couples may choose different reproductive options. There is no universally right decision, just the decision that is right for you and your family.

Carrier Screening in the age of genomic medicine. Committee Opinion No. 690. American College of Obstetricians and Gynecologists. Obstet Gynecol 2017; 129:e35-40.
Carrier Screening for genetic Committee Opinion No. 691. American College of Obstetricians and Gynecologists. Obstet Gynecol 2017; 129:e41-55.