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
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 ...