Spinal muscular atrophy

If a couple has already had a child with SMA, while it is more likely that another affected sibling would have the same type, it is not definite.^[1] There are rare families reported in which marked differences in severity have been present in affected siblings with the same SMN2 copy number.^[2][3]

The genetics of SMA is complex, and distinction between subtypes of childhood-onset, autosomal recessive SMA is based on the age of onset, achieved milestones, and lifespan.^[2] While genetic testing of a fetus at risk can confirm a diagnosis of SMA, it cannot distinguish between the 3 types of childhood onset SMA (types I, II and III).^[1] Because of this, if a fetus is at risk or is diagnosed with SMA, the potential severity and prognosis cannot be predicted.^[4] For example, if 3 copies of the SMN2 gene are detected in a fetus, type I, II or III can result. Similarly, the presence of 2 copies is typical for type I, but could also result in type II. On the other hand, having a single copy of SMN2 is rare but is highly predictive of severe type I with a very poor prognosis.^[4]

Couples who have had a child with SMA are strongly encouraged to meet with a genetic counselor or other genetics professional. A genetics professional can evaluate personal genetic circumstances, assess recurrence risks, and address questions and concerns. He/she can also discuss testing and management options in current or future pregnancies. For example, there are often options for couples who want to avoid having another affected child, or who want to find out early in pregnancy whether a fetus is affected.

You can visit SMA Support UK's website for information about the genetics of SMA types I, II, and III, and information about future options in pregnancy.

To find a medical professional who specializes in genetics, you can ask your doctor for a referral or you can search for one yourself. Online directories are provided by the American College of Medical Genetics and the National Society of Genetic Counselors. If you need additional help, contact a GARD Information Specialist. You can also learn more about genetic consultations from Genetics Home Reference.