Duchenne muscular dystrophy

Duchenne muscular dystrophy is a common and possibly the best-known muscular dystrophy. It follows an aggressive and progressive course.

The disease is inherited in an X-linked manner although up to 30% of cases represent new mutations. The gene for Duchenne muscular dystrophy is the dystrophin locus at Xp21.

• occurs in approximately 1 in 3500 male births, and in about 1 in 50,000,000 female births
• females with a mutant X chromosome will not have DMD, but they will be carriers of the disorder
  • those very rare females who have DMD are believed to either carry mutant genes on both X chromosomes or have an inactivated healthy X chromosome (1)

Cardiac and respiratory complications are seen, especially with disease progression, and if untreated DMD typically leads to death in the late teens (3).

Multidisciplinary care, particularly the initiation of corticosteroids and assisted ventilation supported by regular cardiac and respiratory monitoring, has led to increasing numbers of patients living into adulthood in recent years - however, death often occurs in the mid-20s (1,3,5).

Study evidence has shown that (8):

• among patients with Duchenne muscular dystrophy, treatment with daily prednisone or daily deflazacort, compared with intermittent prednisone alternating 10 days on and 10 days off, resulted in significant improvement over 3 years in a composite outcome comprising measures of motor function, pulmonary function, and satisfaction with treatment

Key points (1,2,3,4):

• DMD should be considered in boys with features such as: such as:
  • delayed motor milestones such as:
    • poor head control
    • not walking independently by 18 months
    • not running by age 3
  • frequent trips or falls
  • positive Gowers' sign (unable to get up from the floor without using support from arms to push up)
  • abnormal gait
    • waddling gait
    • tiptoe gait
  • muscle pains
  • calf hypertrophy
  • unexplained elevated liver enzymes
  • learning difficulties
  • behavioural problems, or speech and language delay
• creatine kinase (CK) should be requested for any patient you suspect of having a neuromuscular condition (4)
  • if CK is markedly raised, this requires an urgent referral to a neuromuscular specialist
  • a mildly raised creatine kinase level of 1-2 times the normal range should be confirmed through a repeated test and followed up
    • expert advice should be sought if any concern or doubt
• early diagnosis is important for effective management of boys with DMD (and support for families)
• if DMD then refer for prompt specialist review if
  • breathlessness, palpitations and arrhythmias, morning headaches (secondary to nocturnal hypoventilation), repeated chest infections or weight loss

Life expectancy for patients with DMD has improved during the last decades, however, it is still poor, with a 2020 systematic review and meta-analysis of studies in Organisation for Economic Co-operation and Development (OECD) countries finding a median life expectancy between 21.0 and 39.6 years, provided patients received ventilatory support (6)

• similarly, a 2016 chart review of all deaths in the DMD population in North East England found that the mean age of death caused by underlying cardiac or respiratory failure was 23.9 years (7)

Females with DMD

• in general, carrier females show no evidence of muscular weakness (9)
  • approximately 2.5% to 20% of female carriers may be affected
  • can be explained by the Lyon hypothesis in which the normal X chromosome becomes inactivated, and the X chromosome with the mutation is expressed

Notes:

• the prevalence of Duchenne's muscular dystrophy varies with respect to different populations. However, in a study of neuromuscular disease in Northern Ireland, Duchenne muscular dystrophy was found to have the second highest prevalence, after myotonic dystrophy (2).

Reference:

1. Webb CL. Parents' perspectives on coping with Duchenne muscular dystrophy. Child Care Health Dev. 2005 Jul;31(4):385-96.
2. Hicks MI et al. The prevalence of inherited neuromuscular disease in Northern Ireland.Neuromuscul Disord. 1996 Jan;6(1):69-73.
3. Rodger S et al. Adult care for Duchenne muscular dystrophy in the UK. J Neurol. 2015; 262(3): 629–6
4. Fox H et al. Duchenne muscular dystrophy. BMJ 2020;368:l7012
5. Lisak RP, Truong DD, Carroll W, Bhidayasiri R (2011). International Neurology. Wiley. p. 222. ISBN 9781444317015.
6. Landfeldt E, Thompson R, Sejersen T, McMillan HJ, Kirschner J, Lochmuller H. Life expectancy at birth in Duchenne muscular dystrophy: a systematic review and meta-analysis. European Journal of Epidemiology. 2020;35:643-653.
7. Van Ruiten HJA, Marini Bettolo C, Cheetham T, et al. Why are some patients with Duchenne muscular dystrophy dying young: An analysis of causes of death in North East England. European Journal of Paediatric Neurology. 2016;20(6):904-909
8. Guglieri M, Bushby K, McDermott MP, et al. Effect of Different Corticosteroid Dosing Regimens on Clinical Outcomes in Boys With Duchenne Muscular Dystrophy: A Randomized Clinical Trial. JAMA. Published online April 05, 2022. doi:10.1001/jama.2022.4315
9. Venugopal V, Pavlakis S. Duchenne Muscular Dystrophy. [Updated 2023 Jul 10]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK482346/