Which genetic test is required to establish the diagnosis in this 59-year-old with distal muscle weakness, mild myotonia, and persistently elevated CPK values (200-300 U/L)?
Answer Choices:
A. PCR fragment analysis of CNBP
B. Muscular dystrophy gene panel
C. PCR fragment analysis of DPMK
D. Myositis specific antibody panel
Answer:
C. PCR fragment analysis of DPMK
Reference(s) / Additional Reading:
The images demonstrate ring fibers with fluorescence with congo red staining under polarized light. Distal muscle weakness, mild myotonia, ring fibers, and mild CK elevation are consistent with myotonic dystrophy, DM1. DM1 is caused by repeat expansion of the DPMK gene. Muscular dystrophy gene panel testing generally does not include testing for myotonic dystrophies and this testing has to be ordered separately due to clinical or pathologic suspicion. CNBP is mutated in DM2 which generally presents with proximal muscle weakness. No inflammation is seen in the specimen to suggest a myositis.
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