Myotonia refers to delayed relaxation after muscle contraction, which can cause muscle stiffness. Muscular dystrophies are inherited, progressive muscle disorders resulting from defects in one or more genes needed for normal muscle structure and function; dystrophic changes (eg, muscle fiber necrosis and regeneration) are seen on biopsy specimens.
Myotonic dystrophy affects about 1/8000 in the general population. Inheritance is autosomal dominant with variable penetrance. Two types are recognized, with different genetic loci. DM 1 involves a mutation of the DMPK gene located on chromosome 19. DM 2 is a milder form that involves a mutation of the ZNF9 gene located on chromosome 3. Both types involve abnormal gene expansion.
Both types affect voluntary muscles and DM 1 also affects involuntary muscles (eg, of the gastrointestinal tract, uterus).
Occasionally, myotonic dystrophy is present at birth in children of mothers and, rarely, fathers with DM 1 mutations. Offspring may have a severe form of myotonia referred to as congenital myotonic dystrophy. This form is characterized by severe hypotonia (floppy infant), feeding and respiratory difficulties, skeletal deformities, facial weakness, and delayed psychomotor development. Up to 40% of infants do not survive, usually because of respiratory failure and perhaps cardiomyopathy. Up to 60% of survivors have intellectual disability. Congenital myotonic dystrophy should not be confused with myotonia congenita, a separate disorder.
Symptoms and signs of myotonic dystrophy begin during adolescence or young adulthood and include myotonia (delayed relaxation after muscle contraction, which may be asymptomatic or described as muscle stiffness), weakness and wasting of distal limb muscles (especially in the hand) and facial muscles (ptosis is especially common), and cardiomyopathy. Intellectual disability, cataracts, and endocrine disorders can also occur.
Because of involuntary muscle involvement, patients with DM 1 may also have dysphagia and constipation; uterine muscle abnormalities may cause problems for women during labor and delivery.
Death is most commonly due to respiratory and cardiac disease, and patients who develop cardiac arrhythmias and severe muscle weakness at a younger age are at increased risk of premature death. Mean age at death is 54 years.
Myotonia may respond to membrane-stabilizing drugs (eg, mexiletine, procainamide, quinidine, phenytoin, carbamazepine). Of these, mexiletine has been shown to significantly reduce myotonia in nondystrophic myotonia and is thus the first-line drug for myotonic dystrophy patients who have functionally limiting myotonia. Because mexiletine can rarely precipitate arrhythmias in patients with underlying ventricular arrhythmias, the drug is contraindicated in patients with 2nd- or 3rd-degree atrioventricular block; consultation with a cardiologist is recommended before initiating mexiletine therapy, particularly in those with an abnormal ECG.
However, it is weakness, for which no treatment is available, and not myotonia that usually disables the patient; braces for footdrop are usually required as the disease progresses.
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