Introduction

Duchenne muscular dystrophy is an x-linked disorder associated with progressive proximal muscle weakness in children.

Epidemiology

  • Incidence: 0.15 cases per 100,000 person-years
  • Peak incidence: 1-5 years
<1 1-5 6+ 16+ 30+ 40+ 50+ 60+ 70+ 80+

Pathophysiology

Overview
  • X-linked recessive disorder that affects around 1 in 5,000 male births
  • Due to mutation in the gene encoding dystrophin, dystrophin gene on Xp21
  • Dystrophin is part of a large membrane associated protein in muscle which connects the muscle membrane to actin, part of the muscle cytoskeleton
  • In Duchenne muscular dystrophy there is a frameshift mutation resulting in one or both of the binding sites are lost leading to a severe form
  • In Becker muscular dystrophy there is a non-frameshift insertion in the dystrophin gene resulting in both binding sites being preserved leading to a milder form

Clinical features

The average age of diagnosis is 4.3 years. Parents are typically concerned for around 18 months before an official diagnosis is made.

Clinical features
  • Proximal muscle weakness, of a proximal to distal pattern, typically develops from 2-3 years of age
  • Motor milestones may be delayed
  • Gait abnormalities (e.g. waddling gait) and frequent falls are common
  • Calf pseudohypertrophy
  • Gower's sign: child uses arms to stand up from a squatted position
  • 30% of patients have intellectual impairment

Investigations

Creatinine kinase (CK)
  • Is nearly always raised and serves as a good screening test prior to more specialist assessment
  • It should be remembered that other neuromuscular conditions may have a normal CK if there are ongoing concerns assessment by a specialist may still be warranted

Other enzymes such as LDH, ALT and AST may also be raised.

Genetic testing is used to confirm the diagnosis

Management

Overview
  • multidisciplinary care anticipating and responding to complications
  • corticosteroids
    • oral prednisolone is used to improve muscle function

Complications

Cardiac
  • dilated cardiomyopathy
  • annual echocardiograms should be undertaken in all patients

Respiratory
  • respiratory failure secondary to muscular weakness

Complications can be seen secondary to longterm corticosteroid use:
  • osteoporosis
  • impaired glucose tolerance
  • obesity
  • Addisonian crisis triggered by suddenly stopping steroids or intercurrent illness

Prognosis

Many patients now live into their 30s.