Myotonic dystrophy (DM) is a genetic, multisystem disorder causing progressive muscle weakness and delayed muscle relaxation, with two main types: DM1 and DM2.
Overview
Myotonic dystrophy is a form of muscular dystrophy that affects skeletal muscles and multiple other organs, including the heart, eyes, endocrine system, and central nervous system australiannmdregistry.org.au+1. The hallmark symptom is myotonia, where muscles contract normally but have difficulty relaxing. DM is inherited in an autosomal dominant pattern, meaning each child of an affected individual has a 50% chance of inheriting the condition Wikipedia.
Types
- Type 1 (DM1 or Steinert disease): Caused by a mutation in the DMPK gene on chromosome 19, involving an expanded CTG triplet repeat australiannmdregistry.org.au+1. DM1 can present in four forms: congenital, childhood-onset, classic adult-onset, and mild. Symptoms include muscle wasting, weakness in the face, neck, and limbs, myotonia, cataracts, heart conduction defects, diabetes, and sometimes intellectual disability australiannmdregistry.org.au+2. Severity often correlates with the number of triplet repeats.
- Type 2 (DM2 or proximal myotonic myopathy): Caused by a mutation in the CNBP (ZNF9) gene on chromosome 3, involving a different triplet repeat expansion australiannmdregistry.org.au+1. DM2 generally has a milder course, with muscle pain (myalgia), weakness, and risk of cardiomyopathy and diabetes, but usually without cataracts or swallowing difficulties australiannmdregistry.org.au+1.
Symptoms
Symptoms vary widely and may appear at any age, from birth to adulthood mda.org.au. Common features include:
- Progressive muscle weakness and atrophy, especially in distal muscles for DM1 and proximal muscles for DM2
- Myotonia (delayed muscle relaxation)
- Cardiac issues, such as arrhythmias or conduction defects
- Cataracts
- Endocrine disturbances, including diabetes
- Respiratory difficulties, particularly in congenital DM1
- Cognitive or developmental delays in congenital or childhood-onset DM1 mda.org.au+1
Diagnosis
Diagnosis is confirmed through genetic testing, identifying the specific triplet repeat expansions in DMPK (DM1) or CNBP (DM2), Wikipedia. Clinical evaluation includes muscle strength testing, electromyography (EMG), and assessment of systemic involvement.
Management
There is no cure for myotonic dystrophy, but treatments focus on symptom management and improving quality of life clevelandclinic.org:
- Muscle relaxation: Medications like mexiletine or carbamazepine can reduce myotonia
- Pain management: NSAIDs or tricyclic antidepressants
- Supportive care: Physical therapy, braces, wheelchairs, and respiratory support
- Cardiac monitoring: Pacemakers or defibrillators for conduction defects
- Regular screening: For cataracts, diabetes, and other systemic complications
Prognosis
The disease is progressive, with severity and onset varying by type and genetic mutation. Many individuals with DM1 or DM2 live a normal lifespan, especially in milder cases, while congenital DM1 can be life-threatening in infancy due to respiratory complications mda.org.au.
Key Points
Genetic counseling is recommended for affected families due to the autosomal dominant inheritance pattern
DM1 is generally more severe than DM2 and can present at birth or in adulthood
Both types are caused by unstable triplet repeat expansions in specific genes
Management is multidisciplinary, addressing muscle, cardiac, respiratory, and endocrine complications