How is spinal muscular atrophy (SMA) diagnosed?

Genetic testing is the most accurate way to confirm whether or not an individual has the SMN1 gene mutation that causes SMA. This test is also referred to as an SMN gene deletion test. The copy number of the SMN2 gene can also be determined. Approximately 95% to 98% of individuals with a clinical diagnosis of spinal muscular atrophy have a mutation in the SMN1 gene. The SMN gene deletion test is performed by several diagnostic laboratories. The results can be achieved in approximately 3 to 4 weeks, though this may vary by center.

Since hypotonia is a common finding in several genetic conditions, a doctor may order a congenital hypotonia panel. This will test for spinal muscular atrophy as well as myotonic dystrophy (type 1), Prader-Willi syndrome, Angelman syndrome, and maternal uniparental disomy 14. The turnaround time for a congenital hypotonia panel is approximately 3 weeks.

Because the signs and symptoms of SMA occur at different ages and with different severity, an initial diagnosis can be delayed. A recent survey of parents and doctors showed that the recognition of SMA symptoms and finding a diagnosis required numerous visits to their pediatrician and specialists. 

Healthcare providers may use other diagnostic tests, including additional genetic testing, electromyography, or a blood test to measure the level of creatine kinase—an enzyme that leaks out of deteriorating muscles. Unlike a genetic molecular blood test, these tests may not accurately confirm a spinal muscular atrophy diagnosis. However, they may be used to rule out other forms of muscle disease.