If you have questions or are worried about anything, call the Neurogenetics Unit at 617-726-5732. You can also reach the genetic counselor at 617-726-4878.

What is spinal muscular atrophy (SMA)?

Spinal muscular atrophy (SMA) is a rare genetic disorder in which the nerves in the spinal cord slowly become weaker or stop working. This can affect your child’s ability to crawl, walk, eat or breathe.

What causes SMA?

SMA is caused by a mutation (change) in the SMN1 gene (piece of genetic material). The change causes lower levels of survival motor neuron protein (a protein that helps certain nerves and muscles work properly). Without this protein, nerves in the spine do not work as they should.

A child can inherit SMA in an autosomal recessive manner. This means that he receives 2 copies (one from each parents) of the non-working SMN1 gene to have SMA.

What are the different types of SMA?

There are four different types of SMA:

  • SMA type I is the most common and most severe type. Babies with SMA type 1 usually have low muscle tone, trouble breathing, or delayed milestones. Symptoms usually appear by your baby’s first 6 months of life.
  • SMA type II. Muscle weakness is the most common symptom of SMA type II. Symptoms appear between 6 months and 2 years of age.
  • SMA type III. Some children show symptoms of SMA type III between 18 months and 3 years of age, but some symptoms might not show up until the teen years.
  • SMA type VI usually affects adults age 30 and older.

Rev. 3/2018. Reviewed by the MGfC Family Advisory Council. This webpage is intended to provide health information so that you can be better informed. It is not a substitute for medical advice and should not be used to treatment of any medical conditions.