What is the role of lab testing in the workup of spinal muscle atrophy (SMA)?

Updated: Aug 11, 2020
  • Author: Ashish S Ranade, MBBS, MS, MRCS; Chief Editor: Jeffrey A Goldstein, MD  more...
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Answer

A simple blood test can confirm whether the child has a mutation that causes spinal muscle atrophy (SMA; also known as spinal muscular atrophy). The SMN1 deletion test is recommended as the first diagnostic step for a patient suspected of having SMA. The deletion status can be tested by using polymerase chain reaction (PCR) to determine if both copies of SMN1 exon 7 are absent, a finding that is noted in 95% of affected individuals. PCR can reliably and accurately measure SMN1 and SMN2 copy numbers over a wide range (ie, 0-8 copies).

If the survival motor neuron (SMN) gene test is positive, the diagnosis is confirmed. However, 5% of children with the symptoms of SMA can have a negative SMN gene test and may require additional diagnostic testing. These tests can include electromyography (EMG), a nerve conduction study (NCS), or muscle biopsy and additional blood tests to help rule out other forms of muscle disease. A congenital hypotonia panel may be ordered to test for SMA. Clinical laboratories may offer panels that include tests for disorders such as SMA, myotonic dystrophy (type 1), Prader-Willi syndrome, Angelman syndrome, and maternal uniparental disomy. [25]

In contrast to findings in patients with Duchenne muscular dystrophy and Becker muscular dystrophy, aldolase and serum creatine kinase (CK) findings are within reference ranges in patients with SMA. In later-onset SMA, these muscle enzymes may be slightly elevated.

Diagnostic delays are common in SMA. A systematic review of the literature conducted to diagnose diagnostic delay reported both age of onset and age at confirmed diagnosis; the delay to diagnosis ranged from months to years. [28] Earlier identification of newborns with SMA will also allow infants to begin treatment even before showing symptoms, when research in human and mouse models suggests it may be most effective.


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