Diagnosing spinal muscular atrophy

SMA can be diagnosed during pregnancy, just after a child is born or later in life depending on a few key factors.

Spinal muscular atrophy (SMA) — a group of inherited diseases that damages nerve cells in the spinal cord — may be diagnosed in a number of ways. Your medical history, where you live and the age at which symptoms start can all influence how and when you receive an SMA diagnosis.

Prenatal testing

Spinal muscular atrophy is passed on to children by parents through abnormal genes. If you know your unborn child is at risk of SMA due to your family medical history, your doctor may suggest getting a sample of the child's DNA during pregnancy (prenatal genetic testing).

Most often, this is done by collecting a sample of the fluid that surrounds the baby (amniocentesis). Sometimes, it may involve collecting some of the structures (chorionic villi) that make up the sac that surrounds the baby.

Both types of samples can be collected through a thin, hollow needle inserted through your abdomen into your uterus. But sometimes chorionic villi samples are collected through a thin, hollow tube (catheter) inserted through your vagina into your uterus.

Amniocentesis is usually done between 15 and 20 weeks of pregnancy. Chorionic villi samples can be collected as early as 10 weeks.

After it's been collected, the DNA sample will be tested for an abnormal or missing survival motor neuron 1 (SMN1) gene, which plays a role in most spinal muscular atrophy.

Newborn screening

In the United States, all states require that newborns get a blood test to look for certain genetic and metabolic abnormalities. Some states include genetic testing for spinal muscular atrophy as part of their routine newborn screening. If you live in one of those states, your child may receive an SMA diagnosis as a result of such testing.

This allows your child to receive care and treatment for SMA right away — before symptoms appear — when it may be most helpful.

Testing after symptoms start

A child with spinal muscular atrophy may look normal at birth. But symptoms may appear as early as a few months after a child is born. Rarely, symptoms start in adulthood.

Children showing early symptoms may:

  • Seem weak and have a lack of muscle tone (floppiness)
  • Breathe rapidly, with more belly than chest movement
  • Not reach out or fully lift their arms from their shoulders
  • Not kick or stretch their legs from the hip and knee
  • Position their legs in a frog-like posture
  • Not lift their heads when lying on their bellies
  • Not move their heads from side to side when lying on their backs

Once a person has symptoms of SMA, a diagnosis likely involves:

  • A physical exam
  • Questions about family history
  • A genetic test (blood test) that looks for an abnormal SMN1 gene — present in 95% of people who have SMA

If there aren't any abnormalities of the SMN1 gene, more tests may be needed to rule out other conditions. These could include:

  • Creatine kinase test. This blood test measures the level of an enzyme that leaks out of damaged muscles. Creatine kinase levels are usually normal or mildly elevated if the cause of symptoms is SMA, but can be very high if the cause is another condition with similar symptoms.
  • Electromyography (EMG). This test uses short needles inserted directly into a muscle to record the electrical activity in the muscle.
  • Nerve conduction study. This test uses electrode stickers applied to the skin (surface electrodes) to measure the speed and strength of signals traveling along nerves.
  • Muscle biopsy. Your doctor may take a small sample of muscle tissue, usually from the thigh, to examine under a microscope.
Sept. 23, 2020