Myotonic dystrophy testing

Last updated: Wednesday, 11, March, 2009

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Item Process
Specimen10-20 mL blood in EDTA tube.

PCR screen followed by Southern blot analysis to detect an expanded triplet repeat mutation in the myotonic dystrophy protein kinase gene (DMPK).


Used to detect the myotonic dystrophy type 1 (DM1) mutation in symptomatic or asymptomatic people.

Prenatal diagnosis for myotonic dystrophy is available if the mother has been shown to have an abnormal gene.


Myotonic dystrophy type 1 is caused by a CTG expansion located in the 3' untranslated region of DMPK. In the normal population, the triplet repeat region is less than 30 triplets (90 nucleotides) in length.

More than 99% of patients with typical myotonic dystrophy have more than 50 triplets in this region.

The identification of the characteristic expansion of the triplet repeat is diagnostic for the presence of the myotonic dystrophy mutation.

The degree of expansion of the triplet region is correlated with the age of onset and severity of the disorder, but considerable variation exists.

Molecular genetic studies may be indicated in family members.


Ranum LPW & Day JW. Am J Hum Genet 2004; 74:793-804