Overview
Duchenne muscular dystrophy (DMD) is the most common and rapidly-progressing muscular dystrophy. Muscular dystrophies (or dystrophinopathies) are genetic neuromuscular diseases characterised by the breakdown of skeletal muscle over time.
DMD is an X-linked recessive mutation in the gene for the dystrophin protein, which is important in maintaining muscle structure and integrity. In DMD, no dystrophin is produced (unlike Becker’s muscular dystrophy, where there is abnormal dystrophin function). This leads to muscle fibre death and replacement with fat and connective tissue.
DMD can also arise as a result of a new mutation.
Epidemiology
- DMD affects around 1 in 3500 newborn boys
Presentation
- Progressive proximal muscle weakness – presents at around 3 years old
- Delayed walking and toe-walking – due to Achilles tendon shortening as it compensates for weakness in knee extension
- Not walking by 18 months is a red flag and may suggest DMD
- Gower’s sign – the child walks their hands up their legs to stand upright from sitting
- Pseudohypertrophy of the calves, tongue, deltoid, and quadriceps – muscle tissue is replaced with fat and connective tissue
- Global developmental delay
Investigations
Overview
- Serum creatine kinase (CK):
- Elevated – due to muscle breakdown
- Genetic testing:
- Has now replaced muscle biopsy and identifies Xp21 mutations which correlate to the dystrophin gene
Management
Overview
There are unfortunately no effective treatments for DMD, and its diagnosis can be devastating. Genetic counselling, information and support, and multidisciplinary management are the mainstay of care.
Complications and Prognosis
- Affected patients are unable to walk by 12 years old and die from respiratory or cardiac complications in their 20s-30s
- Respiratory failure is the most common cause of death:
- Due to progressive weakness of the respiratory muscles
- Dilated cardiomyopathy is present in all patients with DMD at differing degrees:
- This is because cardiac muscle is also affected by DMD
- Becker’s muscular dystrophy tends to progress more slowly than DMD and develops after the age of 10 years. Dilated cardiomyopathy is the more common cause of death in Becker’s muscular dystrophy.
