Spinal Muscular Atrophy (SMA) is a rare inherited neuromuscular condition that causes progressive muscle weakness and loss of movement. Although it was once considered one of the most common genetic causes of infant death, remarkable advances in genetic medicine have transformed the outlook for many people living with SMA. Early diagnosis and timely treatment can make a significant difference, highlighting the important role of genetic testing and genetic counselling.
What is Spinal Muscular Atrophy?
SMA affects specialised nerve cells called motor neurons, which are located in the spinal cord. These neurons send signals from the brain to the muscles, allowing us to move, breathe, swallow, and maintain posture. In people with SMA, motor neurons gradually degenerate and die, leading to muscle weakness and wasting.
The condition primarily affects muscles closest to the centre of the body, including those of the shoulders, hips, legs, and trunk. In more severe forms, the muscles involved in breathing and swallowing may also be affected.
What Causes SMA?
SMA is caused by pathogenic variants in the SMN1 (Survival Motor Neuron 1) gene. This gene produces the SMN protein, which is essential for the survival and function of motor neurons. When both copies of the SMN1 gene are not functioning correctly, insufficient SMN protein is produced, resulting in the progressive loss of motor neurons.
Humans also have a related gene called SMN2, often referred to as the “backup” gene. While SMN2 produces only small amounts of functional SMN protein, the number of SMN2 copies a person has can influence disease severity. Individuals with more SMN2 copies often have milder forms of SMA, although this relationship is not absolute.
SMA follows an autosomal recessive inheritance pattern. This means a child must inherit one non-working copy of the SMN1 gene from each parent to develop the condition. Parents who carry a single altered copy are usually healthy and unaware they are carriers.
What Are the Different Types?
SMA is classified into several types based on the age symptoms begin and the highest motor milestone achieved.
- Type 1 is the most severe and typically presents in infancy with poor muscle tone, feeding difficulties, and breathing problems.
- Type 2 usually begins between 6 and 18 months of age. Children can often sit independently but generally do not walk without assistance.
- Type 3 develops later in childhood or adolescence. Many individuals can walk independently but may gradually lose mobility over time.
- Type 4 is a rare adult-onset form with relatively mild symptoms and slower progression.
The severity varies considerably between individuals, particularly since the introduction of modern treatments.
How Is SMA Diagnosed?
Diagnosis usually begins with a clinical assessment, followed by genetic testing to confirm pathogenic variants in the SMN1 gene. Genetic testing is highly accurate and can usually establish the diagnosis without the need for more invasive investigations.
Australia has newborn screening for SMA, allowing affected infants to be identified before symptoms develop. This enables treatment to begin as early as possible, which has been shown to significantly improve outcomes. In recent years, Australia has also introduced Medicare – funded carrier screening for SMA.
Treatment Has Changed the Outlook
Over the past decade, several disease-modifying therapies have revolutionised SMA care. These treatments aim to increase the amount of functional SMN protein produced or replace the missing gene function.
While these therapies are not a cure, many children treated before symptoms develop achieve motor milestones that would previously have been considered unlikely. Early diagnosis remains one of the most important factors influencing treatment success.
Can SMA Be Prevented?
Carrier screening allows prospective parents to determine whether they carry a pathogenic variant in the SMN1 gene before or during pregnancy. In Australia, carrier screening for SMA is available alongside testing for cystic fibrosis and fragile X syndrome through Medicare-funded reproductive carrier screening for eligible individuals.
If both partners are carriers, they can discuss reproductive options with a genetic counsellor, including prenatal testing or IVF with preimplantation genetic testing.
How Rosalind Genetics Can Help
At Rosalind Genetics, we provide expert genetic counselling for individuals and families affected by SMA or those considering reproductive carrier screening. We can help by ordering the genetic test, explain genetic test results, assess family risks, discuss inheritance, and guide you through your testing and reproductive options.
Whether you have a family history of SMA, are planning a pregnancy, or have received a new diagnosis, our experienced team is here to provide accurate information, compassionate support, and personalised care every step of the way.
