Diagnosing SMA

SMA is diagnosed primarily through a blood test, which looks for the presence or absence of the SMN1 gene, in conjunction with a suggestive history and physical examination.

Normally, individuals have two genes called Survival Motor Neuron 1 and 2. In approximately 95% of patients with SMA there is an absence of the SMN gene sequence, which is present in normal individuals. Sometimes the SMN1 gene is not missing, but mutated. The numbers of copies of SMN2, a near identical backup copy of the SMN1 gene, is related to the severity of the disease, but does not reliably predict a specific SMA type in a given individual. SMA type is generally determined from the clinical examination evaluating the child’s degree of weakness and ability to achieve major motor milestones such as sitting independently or walking.

Occasionally, doctors may request muscle biopsy or EMG (electromyography) testing (Miller had both of these. He had the muscle biopsy first but you don’t get results from that for 2-4 weeks, so the next day we did the EMG. That’s what initially gave us the dreaded diagnosis. After we had been home for a couple of weeks we got the results from the genetic blood test AND the muscle biopsy and both of those were also conclusive for SMA). Since the genetic blood test became available, a muscle biopsy is almost never indicated and is valuable mainly in cases where the blood DNA test is negative.