Myotonic Muscular Dystrophy
Myotonic muscular dystrophy (DM, or dystrophia myotonica) is the most common adult-onset muscular dystrophy, having a frequency of one per twenty thousand persons in the general population. Myotonia, the delayed relaxation of a voluntary muscle after it is contracted, and muscle weakness are the hallmarks of the disorder. For example, a person with DM using a hammer will not immediately be able to release his grip on the handle when finished. It is an autosomal dominant disorder, but there is great variability in the disorder's severity and in the number of manifestations it leads to.
A unique feature of this dystrophy is a genetic phenomenon called pleiotropy, or multisystem involvement, despite the single genetic defect. The potential involvement includes multiple organs and organ systems other than skeletal muscle, including the cardiac, respiratory, gastrointestinal, central nervous, endocrine, and dermatologic systems, as well as bone or eyes. A congenital variety occurs in which infants are born floppy, often require respiratory assistance, have extremity deformities, and are both physically and mentally retarded.
Patients often initially complain of a loss of hand strength (they have difficulty twisting off caps from bottles, for example) or of tripping while walking or climbing stairs, due to the weakness of muscles that extend the feet and toes. Weakness may progress to involve the shoulder and hip girdles and, in some cases, is severe enough to necessitate the use of a wheel-chair. Droopy eyelids, wasting of facial and neck muscles, and frontal balding frequently occur, producing atypical facial appearance.
The gene for DM is a protein kinase gene (known as DMPK) and is located on the long arm of chromosome 19. The disorder arises from a repeated sequence of three nucleotides—cytosine (C), thymine (T), and gua-nine (G)—in the gene. Individuals without DM have C-T-G repeats that contain between 5 and 37 iterations of the triplets. By contrast, repeats that are between 40 and 170 iterations long are found in the mild phenotype, repeats between 100 and 1,000 iterations are found in the "classic pheno-type," and repeats of between 500 and 3,000 are found in the congenital phenotype. A number of laboratories in the United States perform this triplet repeat assay for diagnostic, prenatal, and presymptomatic testing.
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