Muscle biopsy, radiography, and EMG can each provide valuable information when diagnosing SMA:
Muscle biopsy: Patients with SMA typically have uniformly small muscle fibers, although patients with type III (usually diagnosed between age 2 and 15 years) may have predominantly small fibers with some larger fibers present.
Radiography: In patients with SMA type II (and many with type III), radiographs reveal a C-shaped scoliotic curve to the spine. Many patients with SMA type II also have dislocated hips.
EMG: In most cases, sensory nerve conduction is normal. In paraspinal and limb muscles, fibrillation potentials may be present. Compound motor action potentials amplitude may be reduced owing to muscle wasting, and polyphasic motor unit potentials are typically prolonged.
Expanded genetic testing, such as whole-exome sequencing, may also identify mutations in other genes that are associated with the SMA phenotype.
CK levels are typically within normal limits in patients with SMA type I, but may be slightly elevated in other SMA types—especially in late-onset disease. Serum aldolase levels are commonly elevated in patients with SMA types III and IV. Regardless of type, aldolase levels may become mildly elevated in advanced cases of SMA or older age at disease onset. It is important to note that elevations of CK and aldolase in SMA are typically mild—in contrast to Duchenne muscular dystrophy or Becker muscular dystrophy, in which significant elevations in CK and aldolase are expected.
Aside from providing normal results to rule out other conditions, the other tests are unnecessary to diagnose SMA.
For more on additional testing for SMA, read here.
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Cite this: Stephen L. Nelson. Fast Five Quiz: Can You Diagnose the Different Types of Spinal Muscular Atrophy? - Medscape - Dec 06, 2018.