It is a congenital anomaly of an ionic channel resulting in a multisystemic disease with anomalies of skeletal, smooth, and cardiac muscles. It may cause mental deficiency and alopecia. Clinically, the most obvious features are muscle rigidity and lack of muscle relaxation after contraction. The onset of the disease occurs during early adulthood. However, it may occur at any age and is extremely variable in degree of severity. Progression of the disease is slow, sometimes evolving over 50 to 60 years. There appear to be at least two forms.
Myotonia Dystrophica; Dystrophia Myotonica; Steinert Disease; Curschmann-Steinert Disease; Curschmann-Batten-Steinert Disease.
Myotonic Dystrophy, Type I (Rossolimo-Curschmann-Batten-Steinert Syndrome): Characterized by a multisystem disorder that affects skeletal and smooth muscle, including the eyes, heart, endocrine system, and central nervous system. The clinical expression varies according to three phenotypes: mild, classic, and congenital. Mild myotonic dystrophy is characterized by cataract and mild sustained muscle contraction. The life span is normal. Classic myotonic dystrophy is characterized by muscle weakness and wasting, myotonia, cataract, and often by cardiac conduction abnormalities. Adults may become physically disabled and the life span can be significantly reduced. Congenital myotonic dystrophy is characterized by hypotonia and severe generalized weakness at birth, often with respiratory insufficiency and early death. Mental retardation is common.
Myotonic Dystrophy, Type II (Ricker Syndrome; PROMM Syndrome): Characterized by progressive muscle weakness, prolonged myotonia, cataracts, cardiac abnormalities, balding, and infertility. Increased sweating, particularly of the hands and trunk, is common in Type II myotonic dystrophy. It typically appears in adulthood. It tends to be milder than Type I.
The exact incidence remains unknown; however, it is estimated about 1:20,000. The prevalence of the disease, without consideration for the type, is estimated at 1:7,000 to 10,000 in the general population. The prevalence in the Province of Quebec, Canada, is estimated as high as 1 in 500, but it is suspected that there is a “founder effect” (is defined as the loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population). It is estimated that 98% of people with myotonic dystrophy are affected with Type I form. The others have the milder form of myotonic dystrophy Type II. However, there are indications that Type 2 may be as common as Type 1 in Germany and Finland. It affects males and females approximately equally. About 30,000 people in the United States are known to have the disease.
Myotonic dystrophy is inherited as an autosomal dominant manner; the responsible gene is located on chromosome 19q13.
DMPK (myotonin protein kinase) is the myotonic dystrophy gene. The effect of the “triplet” or “trinucleotide repeat” (CTG) ...