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 Autosomal Dominant Genetic disorders determined by a single gene (Mendelian disorders) are easiest to analyze and the most well understood. If expression of a trait requires only one copy of a gene (one allele)... read more with variable penetrance. Two types are recognized, with different genetic loci. Type 1 DM (DM1) involves expansion of a CTG trinucleotide repeat of the DMPK gene located on chromosome 19. Type 2 DM (DM2) is milder and involves a CCTG repeat expansion mutation of the cellular nucleic acid binding protein gene CNBP (previously known as ZFN9) on chromosome 3q21.3.
Both types affect voluntary muscles, and DM1 also affects involuntary muscles (eg, of the gastrointestinal tract, uterus).
Congenital myotonic dystrophy
Occasionally, myotonic dystrophy is present at birth in children of mothers and, rarely, fathers with DM1 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 Overview of Cardiomyopathies A cardiomyopathy is a primary disorder of the heart muscle. It is distinct from structural cardiac disorders such as coronary artery disease, valvular disorders, and congenital heart disorders... read more . Up to 60% of survivors have intellectual disability Intellectual Disability Intellectual disability is characterized by significantly subaverage intellectual functioning (often expressed as an intelligence quotient 70 to 75) combined with limitations of adaptive functioning... read more . Congenital myotonic dystrophy should not be confused with myotonia congenita Myotonia Congenita Myotonia congenita is an inherited disorder causing muscle stiffness and hypertrophy beginning during childhood. There are 2 main types with different modes of inheritance and manifestations... read more , a separate disorder.
Symptoms and Signs of Myotonic Dystrophy
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 DM1 may also have dysphagia and constipation; uterine muscle abnormalities may cause problems for women during labor and delivery.
Diagnosis of Myotonic Dystrophy
DNA mutation analysis
Diagnosis of myotonic dystrophy is indicated by characteristic clinical findings, age at onset, and family history and is confirmed by DNA testing.
Prognosis for Myotonic Dystrophy
Death is most commonly due to respiratory and cardiac disease, and patients who develop cardiac arrhythmias Overview of Arrhythmias The normal heart beats in a regular, coordinated way because electrical impulses generated and spread by myocytes with unique electrical properties trigger a sequence of organized myocardial... read more and severe muscle weakness at a younger age are at increased risk of premature death. Mean age at death is 54 years.
Treatment of Myotonic Dystrophy
Myotonia may respond to membrane-stabilizing drugs (eg, mexiletine, lamotrigine, carbamazepine, phenytoin). 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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