Spinal Muscular Atrophy (SMA) Testing

Spinal muscular atrophy (SMA) is classified into five types (0 to IV) based on the age symptoms begin and the highest level of movement a person achieves. SMA type 0 is the most severe form, presenting at birth with extreme weakness, difficulty breathing, and reduced movement in the womb; without treatment, babies rarely survive beyond six months. NCBI SMA type I (also called Werdnig-Hoffmann disease) is the most common form, appearing before six months of age — affected babies may briefly gain some head control but quickly lose it and, with supportive care alone, never learn to sit independently. NCBI SMA type II (intermediate SMA) develops between 6 and 18 months; children can learn to sit on their own but are never able to walk unaided, and typically survive into adolescence or longer. PubMed Central

SMA type III (Kugelberg-Welander disease) is a milder form where symptoms appear after 18 months of age; affected individuals can stand and walk without support, ScienceDirect though some may lose the ability to walk later in life. SMA type IV is the mildest form, with symptoms beginning in adulthood; individuals generally maintain the ability to walk and have a normal life expectancy. PubMed Central It’s important to note that the severity of SMA is closely linked to the number of copies of the SMN2 gene a person carries — more copies typically lead to a milder form of the condition. PubMed Central Thanks to newer treatments such as nusinersen, risdiplam, and gene therapy, the outlook for people with all types of SMA is improving significantly.

SMA is caused by variants in the SMN1 gene, which provides instructions for making a protein needed to keep motor neurones healthy. Without enough of this protein, motor neurones in the spinal cord deteriorate, leading to muscle weakness. The condition follows an autosomal recessive inheritance pattern, meaning a child must inherit two faulty copies of the gene to be affected.

If both parents are carriers of the faulty SMN1 gene, there is a 25% chance with each pregnancy that their child will have SMA. There is also a 50% chance the child will be a carrier like the parents, and a 25% chance they will not inherit the faulty gene at all. Carrier testing can help identify your risk before starting a family.

Symptoms of SMA vary depending on the type but generally include progressive muscle weakness, difficulty with movement, and problems with breathing and swallowing. In severe cases, babies may have difficulty holding their head up, rolling over, or reaching developmental milestones. Muscle weakness typically affects the legs more than the arms and worsens over time.

In milder forms, symptoms may not appear until later in childhood or adulthood. Individuals may experience difficulty walking, climbing stairs, or getting up from a seated position. Fatigue and muscle tremors are also common. Early diagnosis allows for prompt treatment, which can slow progression and improve quality of life.

SMA is diagnosed through genetic testing, usually a blood test that looks for mutations or deletions in the SMN1 gene. If a baby shows signs of muscle weakness or developmental delay, a healthcare professional may recommend testing to confirm the diagnosis. 

Carrier testing is available for adults who want to know if they carry the SMA gene before or during pregnancy. If both partners are found to be carriers, prenatal diagnostic testing such as CVS or amniocentesis can determine whether the baby is affected. Early diagnosis is crucial as treatments are most effective when started before symptoms appear.

Yes, there are now several treatments available for SMA that can slow or even halt the progression of the disease. Gene therapy, such as Zolgensma, works by replacing the faulty SMN1 gene with a functional copy. Other treatments, including Spinraza and Evrysdi, help increase the production of the SMN protein that motor neurones need to survive.

These treatments are most effective when given early, ideally before symptoms develop. This is why newborn screening and carrier testing are so important. While there is currently no cure for SMA, ongoing research continues to improve treatment options and outcomes for those living with the condition.

SMA carrier testing is a simple blood test that determines whether you carry a faulty copy of the SMN1 gene. Carriers do not have SMA themselves and typically have no symptoms, but they can pass the gene on to their children. If both you and your partner are carriers, there is a 1 in 4 chance your baby could be affected.

Carrier testing is for anyone planning a pregnancy, particularly if there is a family history of SMA or other genetic conditions. At London Pregnancy Clinic, we offer both targeted SMA carrier testing and expanded carrier screening that covers hundreds of genetic conditions in a single test.

Ideally, SMA carrier testing should be done before pregnancy so you have time to understand your options and make informed decisions. However, testing can also be carried out during pregnancy if you have not been screened beforehand. If both partners are found to be carriers, further diagnostic testing can be arranged to determine whether the baby is affected.

You should also consider testing if there is a family history of SMA or if you or your partner belong to a population group with higher carrier rates. Genetic counselling can help you understand your personal risk and guide you through the testing process. Early knowledge empowers you to plan for the future with confidence.