Spinal muscular atrophy (SMA) is an inherited neurodegenerative disorder characterized by progressive muscle weakening and wasting due to the gradual loss of motor neurons, or nerve cells, that control muscles. Depending on the type of SMA, the age of onset and severity of the disorder can vary.

Several therapies exist that may help to manage the disease or its symptoms. An early and accurate diagnosis is important to patients accessing the best treatment possible as soon as possible, which can improve therapy effectiveness.

Genetic testing is one of the most accurate ways of diagnosing SMA. It involves taking a blood sample from the patient and checking for known mutations associated with this disorder.

Genetic tests for SMA

When SMA is suspected, an individual may be referred to undergo a genetic test.

Around 95 percent of SMA cases are caused by a mutation to both copies of the survival motor neuron 1 (SMN1) gene. This is commonly a deletion or conversion mutation on exon 7 in that gene, and results in little or no SMN protein being produced. Deletion or conversion mutations in SMN1 are first checked for when SMA is suspected.

The test is generally non-invasive, as it only involves taking a small sample of blood from the patient to be sent to a laboratory for testing. It can detect 95 percent of all SMA cases, and results are usually available in about a month.

However, rare types of SMA can be also caused by point mutations, which affect only a few nucleotides of the SMN1 gene. These mutations are not part of standard SMA genetic tests, but may be requested if a diagnosis is strongly suspected and initial tests are negative.

Genetic testing may include checking how many copies a person has of the related gene, SMN2. This gene produces a very low amount of a functional SMN protein, and the more copies an individual has the less severe disease symptoms may be.

Carrier testing

SMA is inherited in an autosomal recessive manner. This means that a child must inherit two copies of the mutated gene, one from the mother and one from the father, to develop the disease. Generally, individuals with only one mutated copy will not have disease symptoms, but have a 50 percent risk of passing the mutated gene to their children. These people are said to be SMA carriers.

If both parents are carriers, their child has a 25 percent risk of being born with SMA. People who suspect they may be a carrier — perhaps because they have a family member with SMA —  may choose to get tested for a mutation in their SMN1 gene.

Prenatal screening

Prenatal screening for SMA is not standard, although this is changing as disease-modifying treatments are coming into practice.

If a couple know that they are carriers, they may request prenatal screening to learn whether their unborn child has inherited the disease. It is recommended that people considering prenatal screening first speak with a genetic counselor to ensure they fully understand the potential results and their ramifications. One of two procedures may be used to obtain a sample for testing: chorionic villus sampling (CVS) or amniocentesis.

CVS may be performed as early as the 10th week of pregnancy, but is normally carried out between weeks 11 and 14. It involves removing cells from inside the placenta that contain the fetus’ DNA. This is done by either inserting a needle through the abdomen or by inserting a catheter through the cervix.

Amniocentesis cannot be performed until later in the pregnancy, after week 15. A needle is inserted through the abdomen to take a small sample of amniotic fluid, the liquid that fills the placenta and surrounds the fetus.

Both procedures may be uncomfortable but are generally low risk. In less than 1 percent of cases, they may result in a miscarriage.

Newborn screening

Newborn screening can help identify the existence of disease before symptoms begin, enabling early treatment. This is thought to help improve a patient’s outcomes.

SMA is now part of the Recommended Uniform Screening Panel (RUSP) for newborns in the United States, and screening can be carried out as alongside standard tests for other conditions. But each state decides if SMA will be included in its screening panel, alongside standard tests for other conditions. As of September 2018, six states test now test for SMA are Indiana, Illinois, Minnesota, Missouri, New York, and Utah; four others — North Carolina, Georgia, Massachusetts, and Wisconsin — do screenings under a pilot program that may be adopted.

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SMA News Today is strictly a news and information website about the disease. It does not provide medical advice, diagnosis, or treatment. This content is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read on this website.