Discussion

This report describes our experience with a previously healthy 12-year-old boy presenting after resuscitated cardiac arrest due to ventricular fibrillation, and initial evaluation found a myocardial bridge of the left anterior descending artery, raising concern for an acute coronary event. However, subsequent focused cardiac testing confirmed the bridge was unlikely to have caused coronary insufficiency and ischaemic arrhythmias, and the transient troponin elevation at presentation was attributable to systemic hypotension during the cardiac arrest.^{Reference Agusala, Khera, Cheeran, Mody, Reddy and Link6} An implantable cardioverter defibrillator was placed for secondary prevention, subsequent genetic testing revealed a MYH7 variant of unknown significance, and he has been clinically stable on nadolol/mexiletine for 10 months after hospital discharge.

The presentation of sudden cardiac arrest in a healthy child presents special diagnostic challenges, and management is directed at the likely underlying aetiology to prevent and/or treat possible future events. In our patient, a review of event chronology and personal and family medical history did not suggest an acquired or heritable cardiac channelopathy or cardiomyopathy. His physical exam and laboratory testing, serial 12 lead electrocardiograms, and echocardiogram also did not help to establish a diagnosis. Initial cardiac catheterisation with coronary angiography suggested a myocardial bridge involving the left anterior descending coronary artery. Genetic testing was requested, but the results were weeks away.

The finding of this bridge posed a clinical conundrum with respect to causation of this patient’s cardiac arrest. Whereas most myocardial bridges are usually considered benign, specific anatomical characteristics and the presence of hypertrophic cardiomyopathy influence the likelihood that it could cause symptoms/events.^{Reference Roberts, Charles and Ang7,Reference Konen, Goitein, Sternik, Eshet, Shemesh and Di Segni8} Myocardial bridges are quite prevalent, with 15–85% reported at autopsy, 0.5–16% by coronary angiography, and 3.5–38.5% by coronary cardiac tomographic angiography.^{Reference Michael and Cheng-Han1} A meta-analysis showed an average prevalence of 33%,^{Reference Roberts, Charles and Ang7} the highest prevalence reported in the autopsy series. A lower prevalence detected in the general population could reflect limitations of imaging modalities, or limited provider interest to closely look for them in asymptomatic patients.

There is limited information on myocardial bridges in children. Maeda et al. reported the largest series of children (n = 14) with structurally normal hearts with symptoms/events attributed to left anterior descending myocardial bridges.^{Reference Maeda, Schnittger and Murphy9} The most common symptom was chest pain, and three patients had syncope/sudden cardiac arrest. Transthoracic echocardiogram demonstrated septal dyskinesia with apical sparing and CT angiography confirmed the presence of a myocardial bridge. Intravascular ultrasound was utilised to document coronary compression at resting heart rates, and stress haemodynamic data revealed abnormal fractional flow reserve (0.59 ± 0.13). All patients had improved symptoms after surgical unroofing.^{Reference Maeda, Schnittger and Murphy9} In a meta-analysis of 115 studies, only 3 were paediatric, and these patients also had hypertrophic cardiomyopathy, a population where myocardial bridges appear to be common.^{Reference Michael and Cheng-Han1,Reference Roberts, Charles and Ang7} When a myocardial bridge is suspected to cause symptoms or events, coronary angiography with intravascular ultrasound and fractional flow reserve more precisely describes haemodynamic details, shedding light on the clinical relevance.^{Reference Ge, Jeremias and Rupp10} In adults, a fractional flow reserve < 0.75 during dobutamine stress is reported to represent clinically significant coronary compression due to myocardial bridges.^{Reference Pijls5} This approach in our patient confirmed the bridge was unlikely to cause coronary insufficiency and ischaemia, and surgical unroofing was therefore not indicated.

Having ruled out the myocardial bridge as the culprit, our patient seemed at risk for future adverse events, perhaps due to a yet elusive cardiac channelopathy/cardiomyopathy. Advances in genetic testing have improved the detection of diverse variants responsible for previously unexplained sudden cardiac arrest in patients; however, there remains no definitive diagnosis in 40–50% of cases.^{Reference Visser, van der Heijden, Doevendans, Loh, Wilde and Hassink11} Contemporary comprehensive genetic screening identified only a variant of unknown significance in the MYH7 gene, also found in two asymptomatic first-degree relatives. This missense variant has been observed in individual(s) with hypertrophic cardiomyopathy, is quite rare in population databases, and is expected to disrupt MYH7 protein function. Therefore, we were concerned our case may represent a so-called pre-structural phase of a cardiomyopathy, with the first manifestation being an arrhythmia.^{Reference Olivotto, Cecchi, Poggesi and Yacoub12} Given the likely predisposition to future events, an implantable cardioverter defibrillator was placed for secondary prevention, and future management will include continuing nadolol and mexiletine, non-invasive imaging at regular intervals to monitor for the development of cardiomyopathy, and potential adjustments to anti-arrhythmic therapy pending clinical course.