Myotonic dystrophy
Myotonic dystrophy is an autosomal dominant trinucleotide repeat disorder causing myotonia, progressive distal muscle weakness, and multisystem involvement including cataracts, cardiac conduction disease, and endocrinopathy. Diagnosis is clinical, confirmed by genetic testing for the DMPK (type 1) or CNBP (type 2) repeat expansion; management is multidisciplinary, with mexiletine for myotonia and mandatory cardiac screening given the risk of sudden death.
- Topic reviewThe full article, organized the way the exam asks it.
- ABIM-format MCQsVignettes with an explanation for every wrong answer.
- Key factsThe numbers and thresholds worth memorizing, up front.
- AI tutorMedBrain answers the question you're stuck on.
What this article covers
- Overview
Autosomal dominant, multisystem trinucleotide repeat disorder: myotonic dystrophy causes myotonia (impaired muscle relaxation) together with progressive skeletal, cardiac, and smooth muscle weakness…
- Epidemiology
Typical DM1 onset is in the 20s to 30s, though the disease spans a spectrum from a mild late-onset form to a severe congenital form.
- Risk factors
Family history of myotonic dystrophy: autosomal dominant inheritance with a 50% transmission risk to offspring of an affected parent.
- Pathophysiology
Repeat expansion produces a toxic RNA gain-of-function effect: the expanded CTG repeat (DM1) or CCTG repeat (DM2) is transcribed into RNA that accumulates as nuclear foci rather than being properly translated.
- Clinical features
Myotonia is the defining sign: delayed muscle relaxation after voluntary contraction, classically demonstrated as grip myotonia (inability to rapidly release a handshake) and percussion myotonia…
- Differential diagnosis
Myotonia congenita (Thomsen or Becker type): pure myotonia from a chloride channel (CLCN1) mutation without the multisystem features (no cataracts, cardiac disease, or endocrinopathy) of myotonic dystrophy.
- Diagnosis
The diagnosis is clinical, based on myotonia with the characteristic distribution and systemic features (cataracts, frontal balding, cardiac conduction disease, endocrinopathy) plus a compatible…
- Management
There is no disease-modifying cure; management is multidisciplinary and centers on systemic screening plus symptomatic therapy.
- Complications
Cardiac conduction disease and arrhythmia, the leading cause of death: progressive atrioventricular block and bundle branch disease can culminate in sudden cardiac death, often without preceding symptoms.
- Prognosis
Classic DM1 shortens lifespan, with death often occurring in the fifth to sixth decade, predominantly from cardiac or respiratory failure.
- Prevention
No primary prevention exists for this inherited disorder; prevention is focused on genetic counseling and surveillance for complications.
Read the whole article
Subscribing to Internal Medicine opens every article, the full question bank and MedBrain, on the web, iPhone and Android.
Related articles
- Duchenne muscular dystrophy
Duchenne muscular dystrophy is an X-linked recessive dystrophinopathy from absent dystrophin, causing early-onset, fulminant proximal weakness with loss of independent ambulation by the early teens and progressive cardiomyopathy. Diagnosis is by genetic testing for a DMD gene deletion/duplication; management is glucocorticoid therapy plus multidisciplinary cardiac, respiratory, and orthopedic surveillance.
- Becker muscular dystrophy
Becker muscular dystrophy is a milder, later-onset X-linked dystrophinopathy causing progressive proximal weakness and dilated cardiomyopathy from partially functional dystrophin. Diagnosis rests on elevated creatine kinase and confirmatory dystrophin genetic testing, and management centers on cardiac surveillance with ACE-inhibitor and beta-blocker therapy, physical therapy, and genetic counseling rather than the glucocorticoid regimen used in Duchenne muscular dystrophy.
- Charcot-Marie-Tooth disease
The most common inherited peripheral neuropathy, comprising more than 70 genetic subtypes that cause slowly progressive distal weakness, sensory loss, areflexia, and foot deformity; diagnosis rests on nerve conduction studies and confirmatory genetic testing, and management is supportive since there is no disease-modifying drug therapy.