Spinal muscular atrophy diagnosis

Editors-in-Chief: C. Michael Gibson, M.S., M.D.; Priyamvada Singh, MBBS

Diagnostics methods
The diagnosis of SMA is done by a combination of different methods. The commonly used methods are – Genetic testing, blood tests (Enzyme levels), Nerve conduction study, Needle EMG, muscle biopsy. In order to be diagnosed with Spinal muscular atrophy, symptoms need to be present. In most cases a diagnosis can be made by the SMN gene test, which determines whether there is at least one copy of the SMN1 gene by looking for its unique sequences (that distinguish it from the almost identical SMN2) in exons 7 and 8. In some cases, when the SMN gene test is not possible or does not show any abnormality, other tests such as an electromyography (EMG) or muscle biopsy may be indicated.

Genetic testing- It is currently performed with PCR-based targeted mutation analysis using a restriction enzyme that digests exon 7 (the specific area of chromosome 5 that is affected in the disease). This test can diagnose SMA patients who are homozygous deleted (both allele deleted), compound heterozygotes (deletion in one SMN1 allele and an intragenic point mutation in the other).

Creatine Kinase (CK) – SMA patients may have 2-4 fold serum CK elevation.

Nerve conduction studies - May show decreased compound motor action potential (CMAP) amplitudes.

Needle EMG- In needle EMG a thin needle electrode is inserted into the muscle. The electrode on the needle picks up the electrical activity of the muscles. This electrical activity is displayed on a monitor called an oscilloscope or heard through a speaker. The presence, size, and shape of the action potential provides information about muscle's ability to respond when the nerves are stimulated. It Shows denervation in SMA I patients without evidence of reinnervation changes.

Muscle biopsy- shows atrophy of muscle groups. .