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Spinal Muscular Atrophy (SMA)
Case Study A
A 3-year-old boy was able to stand and cruise at age 13-months, but lost this ability at age 18-months. There was a strong suspicion of mental retardation due to delayed speech and social development. Exam showed diffuse muscle weakness and atrophy, diminished reflexes, and absence of spasticity. EMG and muscle biopsy were typical for Spinal Muscular Atrophy (SMA) type II. A SMA DNA test demonstrated absent PCR reaction product for exon 8, while that for exon 7 was present. Nerve conduction studies demonstrated normal sensory nerve action potential amplitudes and normal motor nerve conduction velocities with a diminished compound motor action potential amplitude. EMG demonstrated delayed recruitment and fast firing of high amplitude, as well as long duration motor unit potentials. Muscle biopsy demonstrated severe atrophy of both type I and type II muscle fibers, with a single fascicle with enlarged fibers having type I histochemistry. While the presence of other neurological problems had precluded diagnosis by formal criteria, the DNA test resulted in a diagnosis of SMA type II/III, Idiopathic Mental Retardation. Mental retardation, cerebral palsy, seizures, or other relatively common disorders of the CNS may co-exist with SMA, or may be a sign of more global neurological disease.
Using the findings as described above, one could have determined that SMA type II was a strong diagnostic possibility, despite the mental retardation, and that an SMA gene test was useful to order. This is illustrated using the SimulConsult Neurological Syndromes decision support program. Access to the software is free with registration, which is not needed if you work at a hospital with institutional access. Videos describing how to use the software are online. Then: - Click here to launch the software with the findings from this case already entered, illustrating that SMA II is the leading diagnosis (clicking the "Patient's findings" tab shows the pertinent positives and pertinent negatives used).
- Click the "Add findings" tab to see a list of useful clinical findings, and uncheck the "Clinical > labs" checkbox to see a list of useful lab tests, in which the SMA gene test (SMN1) test is high in the list.
- Click the SMN1 button to see the link in the yellow box at the bottom of the screen leading to details of how to order the test.
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