Myotonic Dystrophy

Myotonic dystrophy is the most common heritable neuromuscular disorder, inherited in an autosomal dominant manner. It is also known as Dystrophica Myotonica, DM1 or Steinert disease.

Epidemiology

• Incidence: 1/8000
• Male = Female

Clinical presentation

• Clinical presentation of myotonic dystrophy is extremely variable, even within families.
• Varies from severe respiratory insufficiency in infancy to cataracts alone in adulthood.
• Most common age of symptom onset is 20-30 years.

Physical Signs

• Frontal balding
• Myotonia: elicited by inability to rapidly relax clenched fist or by tapping thenar eminence
• Facial muscles: ptosis, inability to frown, clench teeth, smile.
• Peripheral muscles: reduced power in sternocleidomastoids, distal arm and leg, reflexes may still be present if only mild disease presentation
• Bradycardia/heart block
• Lens opacities

Complications

• Myopathy: poor mobility, difficulty swallowing
• Lens opacities
• Heart conduction defects
• Gastrointestinal dysfunction
• Obstructive sleep apnoea and daytime hypersomnolence
• Adverse response to general anaesthetics: (prolonged recovery, aspiration, dysrythmias)
• In pregnancy: higher miscarriage rate, polyhydramnios, prema-turity, failure to progress in labour, post-partum haemorrhage

Life expectancy:

• Mean age of death 60 years
• Mortality most commonly due to pneumonia and cardiac dys-rythmias

Congenital myotonic dystrophy:

There is a risk of congenital myotonic dystrophy in babies born to affected mothers.  This is due to gross triplet repeat expansion.  Congenital myotonic dystrophy presents with hypotonia, respiratory and feeding difficulties and has a 20% mortality.  Surviving children have learning difficulties.