- Clinical science
Myotonic syndromes are a heterogenous group of inherited disorders with similar pathological mechanisms. Myotonic dystrophies, the most prevalent myotonic syndromes, are one of the most common forms of adult-onset muscular dystrophy. Both types, myotonic dystrophy type I (Curschmann-Steinert disease) and myotonic dystrophy type II (proximal myotonic myopathy), are autosomal dominant conditions with CTG trinucleotide repeat and CCTG tetranucleotide repeat expansions respectively. Type I is a severe (often life-threatening) form of disease, while type II is usually mild. Both present with skeletal muscle weakness and myotonia, muscle pain, heart conduction defects, cataracts, testicular atrophy, and frontal balding. Electromyography may confirm myotonia that is not identified during clinical examination; however, genetic tests usually confirm the diagnosis. As no curative therapy exists, treatment is symptomatic. Except for DM1, patients with myotonic syndromes have a normal lifespan.
(DM1, Curschmann-Steinert disease)
(DM2, proximal myotonic myopathy)
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- Electromyography: (EMG): identifies myotonia
- Genetic diagnostics (confirmatory test): detection of trinucleotide repeat expansion mutation in leukocytes
- Biopsy: to distinguish between an inflammatory and a metabolic cause of the myopathy
- Other supportive tests
|Myotonic dystrophies||Non-dystrophic myotonic syndromes|
|Type 1 (DM1; Curschmann-Steinert disease)||Type 2 (DM2; Proximal Myotonic Myopathy, PROMM)|| |
|Onset|| || || || || || || |
|Defect|| || || |
|Clinical features|| |
The differential diagnoses listed here are not exhaustive.
- The course of DM1 is chronic progressive. Cardiac complications reduce life expectancy.
- DM2 has a milder disease course.