Case 1: Back pain in a boy with Duchenne muscular dystrophy

A 15-year-old boy, diagnosed with Duchenne muscular dystrophy (DMD) at four years of age, presented with a three-day history of progressive neck and upper back pain without preceding trauma. The patient had been on corticosteroid therapy for 10 years; his current regimen was deflazacort 0.6 mg/kg per day taken orally. He had been wheelchair-bound since 12 years of age. He had severe osteoporosis, previous T9–10 and L1–5 vertebral compression fractures and respiratory insufficiency, with a forced vital capacity of 50% of predicted. The patient was followed by a paediatric cardiologist and previous electrocardiograms (ECGs) showed sinus tachycardia and left ventricular hypertrophy, but normal heart size and function on serial echocardiograms. On this presentation, he was diagnosed with a new T1 compression fracture and discharged home on oral morphine. Twelve hours later, he returned due to worsening back pain, and associated nausea and vomiting. At the emergency department, his heart rate was 102 beats/min, blood pressure was 142/105 mmHg, respiratory rate was 16 breaths/min, oxygen saturation was 99% on room air and temperature was 36.8°C. The patient received morphine for his pain and was admitted. During the admission, the patient remained hemodynamically stable. Further investigations during his admission revealed the cause of his symptoms. During the admission, a chest x-ray was performed, which showed low lung volumes, normal cardiac contours, osteopenia and multiple mild-to-moderate thoracic compression fractures. An ECG showed Q waves in leads I and aVL, and ST wave elevation in leads I, II and V2–V6. Creatine kinase MB was 153.4 μg/L (normal 0 μg/L to 7.9 μg/L) and troponin I was 46 μg/L (normal <0.06 μg/L). The diagnosis of acute myocardial injury was made, with evidence of acute lateral infarct, and inferior and anterior subendocardial injury. An echocardiogram showed moderate hypokinesis of the anterior-apical left ventricle, normal ejection fraction and a minor pericardial effusion. Additional blood work revealed a mildly elevated white blood cell count of 14.8×109/L with predominant neutrophilia, normal hemoglobin level, platelet count, serum electrolyte, calcium, magnesium, phosphate, urea and creatinine levels, and liver function tests. Serum lipid profile was normal except for slightly low high-density lipoprotein (1.18 mmol/L; normal 1.4 mmol/L to 5 mmol/L). A prothrombotic work-up, including erythrocyte sedimentation rate, lupus anticoagulant, protein C and protein S, antinuclear antibody, antithrombin levels, prothrombin and Factor V Leiden genotype, was normal. The patient was started on low-dose acetylsalicylic acid. His pain improved over the next four days. Serial blood work revealed a rapid decline in troponin I. ST wave elevations recovered; however, Q waves persisted in the lateral leads. A repeat echocardiogram revealed continued left ventricular hypokinesis and an ejection fraction of 48% (normal ≥55%). The patient was discharged home on low-dose ramipril. Two weeks later, he underwent coronary angiography, which demonstrated normal coronary arteries. Six weeks later, a repeat echocardiogram was completely normal. DMD is a progressive dystrophinopathy that affects one in 3600 to 6000 males (1). It is caused by mutations in the dystrophin gene on chromosome Xp21 (2). Loss of dystrophin protein results in increased susceptibility of myocytes to damage, necrosis and replacement with fibrous noncontractile tissue (2). Boys with DMD present with progressive muscle weakness and eventually become wheelchair bound (2). Cardiomyocytes are affected, resulting in dilated cardiomyopathy and arrhythmias (2). Corticosteroid therapy extends longevity into the fourth decade of life and delays the onset of cardiac complications (1). The differential diagnosis of acute upper back pain in a boy with DMD is broad. The most common etiology is vertebral fractures due to corticosteroid-associated osteoporosis and reduced ambulation. Another common cause is community-acquired and aspiration pneumonia. Restricted mobility poses a risk for constipation, pressure ulcers and thrombosis, while muscle weakness contributes to scoliosis. Additional causes include osteomyelitis, esophagitis, urolithiasis and pyelonephritis. Traumatic causes, such as pneumothorax and herniated disks, should be considered. A rare but important cause in boys with DMD, who have dilated cardiomyopathy, is pulmonary embolus. Finally, many DMD patients experience chronic pain. The onset of cardiac involvement in boys with DMD occurs early in childhood, precedes clinical echocardiographic changes and presents with specific alterations on ECG. Children with DMD are at risk for arrhythmias, dilated cardiomyopathy with a predisposition to heart failure, thromboembolic events due to the prothromobotic effect of muscle degeneration and the underlying cardiac dysfunction, as well as hypertension secondary to chronic corticosteroid therapy (2). Current guidelines for cardiac supervision in DMD suggest that cardiac care should begin at the time of diagnosis, with biannual cardiac examinations until 10 years of age followed by annual examinations thereafter or at the onset of cardiac symptoms (2). Acute myocardial dysfunction, as observed in the current case, is a rare presentation in children with DMD. The case demonstrates the importance for health care providers to maintain a high index of suspicion for cardiac pathology in boys with DMD who present with acute changes in their clinical status. Cardiac abnormalities should be considered when evaluating a child with DMD and new-onset back pain. Clinical investigations could include chest x-ray, ECG and echocardiogram. It is important to compare cardiac results with previous investigations because they may also have been abnormal. If cardiac biomarkers are measured, it should be noted that new-generation troponins are cardiac specific, whereas creatine kinase-MB may be elevated due to dystrophic skeletal muscle. Although rare, children with DMD may present with acute myocardial damage and dysfunction, which requires appropriate management. Routine cardiovascular supervision in patients with DMD is of paramount importance. The authors thank the patient and his family for allowing them to share and learn from their experiences.

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