Diagnosing spinal muscular atrophy 

Tests related to spinal muscular atrophy (SMA) can be carried out before, during and after pregnancy.

Tests before pregnancy

If one partner in a couple has a family history of SMA or is known to carry the faulty gene, the other partner can have a blood test to check if they also carry the faulty gene.

If both partners carry the faulty gene, every pregnancy carries a one in four chance that the child will have SMA.

Couples at risk of having a child with SMA should be offered information and advice, known as genetic counselling, to help them understand their options.

Read more about genetic testing and counselling.

Pre-implantation genetic diagnosis

Pre-implantation genetic diagnosis (PGD) may be an option for couples who are both carriers of SMA.

PGD involves using in-vitro fertilisation (IVF), where eggs are removed from a woman's ovaries before being fertilised with sperm in a laboratory. After a few days, the resulting embryos can be tested for SMA and a maximum of two unaffected embryos are transferred into the uterus.

Funding for PGD is decided on an individual basis. For example, a couple may not be considered for PGD on the NHS if they already have unaffected children or if the chances of success are thought to be low. In these cases a couple can choose to fund PGD privately, although it's likely to cost between £6,000 and £10,000.

Tests during pregnancy

Tests can also be carried out during pregnancy to check if an unborn child has SMA. These are offered to people who already have a child with SMA as, in the majority of cases, there's a one in four chance with every pregnancy that the child will also have the condition.

There are two different tests that can be used at different points in a pregnancy, including:

  • chorionic villus sampling (CVS) – where a sample of cells from the placenta are removed and tested, usually between weeks 11 and 14 of pregnancy
  • amniocentesis – where a sample of amniotic fluid is removed and tested, usually during weeks 15 to 20 of pregnancy

Both these tests can slightly increase the chance of having a miscarriage.

If tests show a child is likely to have SMA, a couple can decide to either continue the pregnancy or have an abortion. Support will be offered to couples facing this decision.

Tests after birth

If SMA isn't diagnosed before birth and a child or adult has typical symptoms of SMA, genetic testing can be used to confirm the condition. This involves analysing a blood sample for the faulty gene that causes SMA.

The doctor will also ask about the family's medical history to see if anyone has had a condition affecting the nerves and muscles (neuromuscular conditions).

A physical examination will also be carried out to look for signs of SMA, such as:

  • muscle weakness and wastage
  • reduced or absent tendon reflexes
  • twitching of individual muscle fibres

In some cases, other tests may be used to confirm the diagnosis. For example:

  • electromyography – where a thin needle is inserted into a muscle to detect its electrical currents at rest and during activity
  • muscle biopsy – where a small sample of muscle tissue is taken for analysis

Page last reviewed: 09/03/2015

Next review due: 01/04/2017