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Congenital anomaly of an ionic channel resulting in a multisystemic disease with anomalies of skeletal, smooth, and cardiac muscles. May cause mental deficiency and loss of hair. The more obvious features are muscle rigidity and lack of muscle relaxation after contraction. Onset 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: 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: 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.

Prevalence is 1:8000-10,000 general population. An estimated 98% of people with myotonic dystrophy have type I; the others have the milder form type II.

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 CTG repeating remains complex, and many unclarified issues remain. Normal CTG repeat is between 5 and 35. The effects of an expanded CTG repeat may be via abnormal RNA transcript processing. The phenomenon of anticipation is typical for myotonic dystrophy: cataracts at first generation, classic form at second generation, and neonatal form at third generation. Myotonic dystrophy is an ion channel disease: Cl- conductance is reduced and there are anomalies in the control of the refractory period of Na+ channels. This results in larger Na+ currents and altered muscle excitability.

Diagnosis of myotonic dystrophy is suspected in individuals with muscle weakness and atrophy, especially of the distal leg, hand, neck, and face; myotonia (sustained muscle contraction), which often manifests as the inability to quickly release a hand grip and which can be demonstrated by tapping with a reflex hammer on, for example, the thenar muscle group; and cataracts, which requires slitlamp examination for early detection. Myotonic dystrophy is suspected in neonates with some combination of hypotonia, facial muscle weakness, generalized weakness, clubfoot, and ...

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