Spinal muscular atrophy - Diagnosing spinal muscular atrophy

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

These can determine if a person has SMA, or if a couple is at risk of having a child with the condition.

Tests before pregnancy

If there are concerns your children may inherit spinal muscular atrophy (SMA), tests can be carried out to assess your risk of having a child with the condition.

This will usually involve having a blood test to check for the faulty gene that can cause SMA (see causes of spinal muscular atrophy for more information).

This can be particularly useful if a partner has a family history of SMA or if it's already known that one partner carries the gene, and the couple would like to know if the other partner is affected.

If the test shows you may be at risk of having a child with SMA, you should be offered information and advice known as genetic counselling.

Read more about genetic testing and counselling.

Pre-implantation genetic diagnosis

For couples at risk of having a child with SMA, pre-implantation genetic diagnosis (PGD) may be an option.

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.

However, funding for PGD is decided on an individual basis. For example, you may not be considered for PGD on the NHS if you already have unaffected children or if the chances of success are thought to be low. In these cases you can choose of fund PGD yourself, although it is 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 will develop SMA. These are often used for people who already have a child with SMA, as there is a chance any other children they have will also have the condition.

There are several different tests which 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 10 and 13 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 your chances of having a miscarriage.

If tests show your child is likely to have SMA, you can choose to either continue the pregnancy or have an abortion.

Tests after birth

If SMA is not diagnosed before birth and a child has typical symptoms of SMA, there are a number of tests that can check for the condition.

Most cases can be confirmed with genetic testing, which involves analysing a blood sample for the faulty gene that causes SMA.

If you or your child is referred for genetic testing, the doctor will also ask about your family's medical history to see if there are any conditions that affect the nerves and muscles (neuromuscular conditions) in your family.

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

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

If a diagnosis is not entirely certain, several other tests – such as an electromyography test or a muscle biopsy – may be carried out. These are described in more detail below.

Electromyography

During an electromyography test, a thin needle is inserted through your skin and into your muscle.

The needle is used to detect the electrical currents of the muscle, both at rest and during activity. This can help determine whether any muscle weakness is caused by a disease in the muscle itself or is the result of a lack of nerve supply. A lack of nerve supply indicates SMA.

Due to the availability of genetic tests, EMG is now very rarely carried out in typical cases of SMA.

Muscle biopsy

During a muscle biopsy, a small sample of muscle tissue is taken for analysis. The sample, which is usually taken from the thigh, is examined under a microscope. However, with the wide availability of genetic testing, muscle biopsies are rarely done nowadays to diagnose SMA.