Kugelberg Welander Spinal Muscular Atrophy Workup
- Author: Joyce L Oleszek, MD; Chief Editor: Stephen Kishner, MD, MHA more...
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- Molecular genetic testing[13, 14] - Routine diagnostic testing for spinal muscular atrophy (SMA) involves targeted mutation analysis to detect deletion of exons 7 and 8 of SMN1. Approximately 95-98% of individuals with a clinical diagnosis of SMA lack exon 7 in both copies of SMN1 (ie, they are homozygous for the deletion). Approximately 2-5% of individuals with a clinical diagnosis of SMA are compound heterozygotes for deletion of SMN1 exon 7 and an intragenic mutation of SMN1. Routine genetic testing detects only patients with the homozygous deletion. However, additional testing is available that includes SMN1 sequence analysis for the detection of point mutations in the SMN1 gene. Genetic testing has also been developed that uses the quantitative polymerase chain reaction to determine the SMN2 gene copy number; however, such testing is not yet widely available. The SMN2 gene copy number is variable, rangingfrom0-5.
- Other testing - Serum creatine kinase levels may be elevated, but usually not to the extent that they are elevated in persons with muscular dystrophy. Serum aldolase levels also are commonly elevated in persons with types III and IV SMA.
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- Ultrasonographic imaging of the muscles has been used to assess for neurogenic atrophy in spinal muscular atrophy (SMA), but it is fairly nonspecific. Ultrasonography has lost favor as a diagnostic tool for SMA. Neuroimaging of patients with SMA reveals no brain abnormalities.
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- Muscle biopsy reveals evidence of neurogenic atrophy and chronic reinnervation in spinal muscular atrophy (SMA). Skeletal muscle changes include atrophy with a combination of narrow fibers and large, hypertrophic fibers. These fibers are separated by abundant fat and fibrous tissues. Increase in the sarcolemmal nuclei with preservation of striations is observed. The phrenic (C3-C5) and sacral (S2-S4) sphincter motor neurons are spared. Typical findings consistent with neurogenic atrophy also are seen on biopsy and are discussed above in the Pathophysiology section.
- Electromyography (EMG) and nerve conduction studies (NCS) can be very useful for the physician in the diagnosis of SMA. Diffuse abnormalities on EMG are seen in the extremities and bulbar musculature. The findings are consistent with axonal degeneration. Fibrillation potentials, positive sharp waves, and complex, repetitive discharges are common. Large motor unit potentials are typical, but small amplitudes also have been seen. Upon recruitment, polyphasic motor unit potentials, decreased recruitment, fast firing, and synchronization of motor units are seen. A marked increase in jitter on electromyograms often is seen and helps to differentiate SMA types III and IV from ALS. Motor nerve conduction velocities are normal or slightly decreased. Motor unit action potentials (MUAPs) progressively decrease in amplitude. Sensory nerve action potentials (SNAPs) are normal.
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