Duchenne and Becker muscular dystrophy: SAQ

This boy has the classic presentation of Duchenne muscular dystrophy, with the delayed walking, the toe-walking, the calf pseudohypertrophy, the Gowers sign, the reduced reflexes, and a creatine kinase of 28,000 units per litre that is off the scale. The mildly raised transaminases are muscle-derived, not liver-derived, because the bilirubin and the synthetic function are normal. The creatine kinase above 10,000 points to a dystrophic process, and the first step after it is the DMD genetic test. [3]

Question 1 (10 marks)

Outline the diagnostic confirmation and the immediate management plan for this boy, justifying each step with the relevant evidence. [3]

A full-mark answer addresses the genetic confirmation, the glucocorticoid decision, the systems surveillance, and the family counselling, each with the correct number or dose. [4]

Genetic confirmation (3 marks). The creatine kinase of 28,000 units per litre confirms a dystrophic process, and the diagnosis is now settled by genetic testing rather than a muscle biopsy. The first-line test is multiplex ligation-dependent probe amplification, which detects the exon deletions and duplications that account for the majority of cases, and if it is normal, whole-gene sequencing finds the point and splice mutations. The reading-frame hypothesis of Koenig predicts that an out-of-frame deletion produces Duchenne, while an in-frame deletion produces Becker, and the mutation type also decides the eligibility for the precision therapies. [2][3]

The glucocorticoid decision (3 marks). The boy is four years old and showing the motor plateau, the point at which the care considerations say to start the glucocorticoid, typically between four and seven years. The FOR-DMD trial established prednisolone 0.75 mg per kg per day and deflazacort 0.9 mg per kg per day as equivalent and superior to placebo, with deflazacort producing less weight gain, so either is started now and continued through the loss of ambulation. For this 17 kg boy, prednisolone is about 13 mg per day. The bone protection with calcium and vitamin D begins at the same time, and the family is taught the sick-day rule for stress-dose hydrocortisone. [7][4]

Systems surveillance and counselling (4 marks). A baseline electrocardiogram and echocardiogram are done at diagnosis and repeated at least annually, and an angiotensin-converting-enzyme inhibitor is started by the age of ten even before symptoms, because the cardiomyopathy is progressive and silent in the young boy. Spirometry begins from age five or six and the forced vital capacity is tracked. The family is counselled that the disease is X-linked recessive, that about two thirds are inherited and one third new, and that the mother, the sisters, and the maternal aunts are offered genetic testing and cardiac surveillance. The multidisciplinary neuromuscular centre coordinates the neurology, the cardiology, the respiratory, the rehabilitation, the endocrine, and the psychosocial care. [5]

Question 2 (10 marks)

Explain the reading-frame hypothesis and critically appraise the role of glucocorticoids and gene therapy using the trial evidence. [2]

A full-mark answer reproduces the molecular logic and appraises the two trials by their conclusion and what they changed. [2]

The reading-frame hypothesis (3 marks). The DMD gene is read in groups of three bases, and a deletion that removes a whole number of triplets keeps the reading frame intact, so translation continues past the gap and produces a shorter but folded protein, the in-frame lesion of Becker. A deletion that throws the reading frame out of step introduces a premature stop codon, the ribosome falls off, and almost no protein is made, the out-of-frame lesion of Duchenne. This explains the severity gradient and is the basis of exon-skipping therapy, which skips a neighbouring exon to restore the reading frame and produce a shorter functional dystrophin. [2]

Glucocorticoid appraisal (4 marks). The FOR-DMD trial of Griggs and colleagues in 2016 randomised 196 ambulatory boys aged four to seven years to deflazacort 0.9 mg per kg per day, prednisone 0.75 mg per kg per day, or placebo over 52 weeks, and it found that both steroids improved the muscle strength score over placebo with a comparable gain in motor function, while deflazacort produced significantly less weight gain than prednisone. The implication is that prednisolone 0.75 mg per kg per day and deflazacort 0.9 mg per kg per day are the equivalent, evidence-based glucocorticoid doses, started at the motor plateau and continued through the loss of ambulation, with deflazacort preferred where weight gain is the dominant concern. The harms, the weight, the growth retardation, the behavioural change, the osteoporosis, and the adrenal suppression, are anticipated and managed. [7]

Gene therapy appraisal (3 marks). The adeno-associated-virus gene therapy of delandistrogene moxeparvovec delivers a micro-dystrophin to the muscle, and the EMBARK phase 3 trial of Mendell and colleagues in 2025 tested its effect on the motor function in boys with a confirmed DMD mutation. The therapy is chosen by the mutation and the age, and it is offered alongside the glucocorticoid backbone rather than instead of it, because the micro-dystrophin is shorter than the native protein and the long-term durability is still being defined. The exon-skipping antisense oligonucleotides, eteplirsen for exon 51 and golodirsen and viltolarsen for exon 53, restore a small amount of dystrophin by the reading-frame logic, and ataluren promotes read-through of the nonsense mutation. [4]