Spinal Muscular Atrophy Carrier Testing

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Spinal Muscular Atrophy Carrier Testing

What Is Spinal Muscular Atrophy?

Spinal muscular atrophy (SMA) is a nerve disease that causes severe muscle weakness that gets worse over time. It affects about 1 in 6,000 to 1 in 10,000 people, and is the second-most common fatal autosomal recessive disorder. It is caused by the loss of nerve cells that connect from the spinal cord to control muscle movement in the arms, legs, and other parts of the body. These nerve cells are called lower motor neurons. Treatments are available to help manage certain aspects of the disease, and may reduce or relieve some of the complications of the disease.

SMA can be divided into types based on how severe the disease is. The most severe type is diagnosed in babies who have low muscle tone (feel "floppy") and have trouble breathing and eating. Babies with severe SMA usually die before they turn two years old, often from breathing problems. Children with less severe SMA can learn to sit. People with the least severe type of SMA can walk. They have health problems but do live into teen years or young adulthood. SMA doesn't cause changes in intelligence.

Types of Spinal Muscular Atrophy (SMA)

Type of SMA
Age of Onset
Lifespan Signs and Symptoms
SMA 0 (proposed)Prenatal2 to 6 months after birth
  • Less fetal movement during pregnancy
  • Abnormal positioning of the knees and elbows found on ultrasound (joint contractures)
  • No motor development in infancy, such as holding the head up or rolling over
  • Severe weakness at birth


(Werdnig-Hoffman disease)

Before 6 months<2 years
  • Severe lack of muscle tone ("floppy baby")
  • Not able to sit without support
  • Paralysis that affects both sides of the body
  • Mild joint contractures
  • Difficulties with sucking and swallowing


(Dubowitz disease)

6 to 18 months~70% survive to 25 yrs
  •  Able to sit independently
  • Some are able to stand with assistance but none can walk
  • Curvature of the spine (scoliosis)
  • Trembling (shaking) of the fingers
  • Difficulty swallowing and gaining weight


(Kugelberg-Welander disease)

Over 12 months
  • Can walk independently, but may lose this ability with age
  • Curvature of the spine (scoliosis)


(20s or 30s)

  • Muscle weakness
  • Able to walk
  • No breathing or swallowing problems

(Table adapted from GeneReviews with information added from additional sources.)

Next: Genetics of Spinal Muscular Atrophy

Would you change your family planning or have prenatal testing if you knew that there was a higher chance for you to have a child with SMA?




SMA carrier screening makes the most sense for people who want the information to make different plans for having a family or who would consider prenatal diagnosis for SMA. Since you said that you might change your family planning, SMA carrier screening may be a good option for you.

There are more choices if carrier testing is done before pregnancy. If you are already pregnant, then your decision focuses on whether you would have prenatal testing to find out whether the baby has SMA. Some parents want that information to prepare for the birth of a baby with special needs. Other parents may choose not to continue the pregnancy if SMA is diagnosed. Genetic counseling is available for parents trying to understand a diagnosis and make such decisions.

If you are sure that you would not change your family plans if you were at-risk to have a child with SMA, you should consider whether or not carrier testing for SMA makes sense for you. Finding out that you and your partner are carriers means that you have a 25% chance to have a child with SMA. There are family planning options that can help at-risk couples lower their chances to have a baby with SMA, but there are no treatments for SMA during pregnancy or after the baby is born.

If you and your partner are carriers, there is a 25% chance with each pregnancy to have a child with SMA. There is no way to be sure whether that child would have more or less severe health problems. In making your choice about carrier screening, consider:

  • If you aren't pregnant now, would you ever consider egg or sperm donation or preimplantation genetic diagnosis to avoid a pregnancy with SMA? Do you consider adoption an option?
  • If you are pregnant now, how do you feel about prenatal diagnosis? CVS and amniocentesis can test for SMA during pregnancy, but both procedures have a risk for pregnancy loss. How concerned are you about those risks?
  • If you had a prenatal diagnosis of SMA, what would you do with that information? Would you want the information to prepare for the birth of a child with special needs? Would you consider ending a pregnancy if you knew the child would have SMA?