Originally appeared: November 2020 Newsletter
A pediatrician or pediatric neurologist will order molecular genetic testing to confirm the diagnosis of SMA or rule out the disorder. Genetic testing for the SMN1 and SMN2 genes is highly reliable and is the first test to be carried out when SMA is suspected.
If your child does not have both full SMN1 copies, this confirms the diagnosis of SMA. If only one full copy is present and the clinical findings suggest SMA, the remaining SMN1 gene is investigated to look for other mutations. If both full SMN1 copies are present, a diagnosis of SMA is highly unlikely. If testing shows an intact SMN1 gene along with clinical findings that might suggest SMA, it is likely that another motor neuron disease is the cause.
Although the number of SMN2 copies is not essential to diagnose SMA, it is usually assessed as it is an important factor influencing how severe the SMA is. Most infants and children with non-sitter (Type 1) SMA carry two SMN2 copies. Children with sitter (Type 2) and walker (Type 3) SMA which began before the age of three have three SMN2 copies. Those with walker (Type 3) SMA which began after age three have four SMN2 copies, and those with adult (Type 4) SMA have four to six copies.
REFERENCES:
Mercuri E, Finkel R, Muntoni F et al. Diagnosis and management of spinal muscular atrophy: Part 1: Recommendations for diagnosis, rehabilitation, orthopedic and nutritional care. Neuromuscular Disorders. 2018;28(2):103-115.