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Myotonic Dystrophy Type 1, DM1 - DMPK

Clinical Features

Myotonic dystrophy is the most common muscular dystrophy in adulthood and is characterized by distal muscular weakness, particularly in the legs, and atrophy of the facial muscles (facies myotonica), and of the pharyngeal and neck muscles. Delayed muscular relaxation following contraction (myotonic reaction) is an early sign. Myocardial involvement manifests as arrhythmias or rarely as cardiomyopathy. Additional signs are early cataract and frontal balding, particularly in men, as well as endocrine changes. The congenital form is characterized by severe muscular hypotonia ("floppy infant") and by a severe disease course with a variable degree of psychomotor and mental retardation.

Genetic Information

Myotonic dystrophy is an autosomal dominant condition. 90% of the cases are caused by an expansion of a CTG trinucleotide repeat in the DM kinase gene (chromosome 19). Healthy individuals carry up to 30 CTG repeats on both alleles. Expansions of more than 50 repeats lead to a mild, protracted disease (e.g. only early cataracts). The higher the number of repeats the earlier is disease onset and the more severe are the symptoms. However, an individual exact prognosis concerning disease onset and course is not possible. Congenital manifestation is observed in individuals carrying more than 800 repeats. Repeat number increases from one generation to the next (anticipation), especially by inheritance through the maternal germline. Thus, there may be an earlier onset and more severe symptoms in off-spring. Congenital manifestation only occurs via inheritance through the mother who is usually already symptomatic.

Prevalence

Approx. 1 : 8 000

Diagnostic