Myocardial bridging and sudden death in hypertrophic cardiomyopathy: Salome drops another veil.

Hypertrophic cardiomyopathy (HCM) is a heterogeneous disease at the clinical, phenotypic, and molecular level. In spite of considerable progress in understanding the disease, many issues continue to be shrouded in mystery, with very slow and often incomplete shedding of the veils, reminiscent of Salome’s dance. These issues include the unpredictable occurrence of sudden cardiac death, the clinical relevance of coronary artery bridging, and their possible relationship. 6 To this day, HCM is among the most common causes of sudden cardiac death in the young, and strategies aimed at improving risk stratification remain less than satisfactory, largely due to the capricious course and individual heterogeneity of the disease. Several potential predictors of risk have been identified, each with very low positive predictive accuracy, that may be important in some patients but have little relevance in others. Symptoms and signs of myocardial ischaemia are often evident in patients with HCM, classically in the absence of epicardial coronary artery disease, and reflect the interplay of structural and functional small-vessel abnormalities, reduced arteriolar density, fibrosis, and elevated left ventricular (LV) end-diastolic pressures. –9 Occasionally, angina on effort in children or young adults with HCM may be associated with an intramural course of the left anterior descending (LAD) artery, a condition also referred to as myocardial bridging. 6 In the most severe instances, the artery is embedded in the myocardium for most of its course, and is more appropriately referred to as tunnelled LAD. Of note, fibrous bands overlying the LAD may also be found in the context of large myocardial bridges. In consecutive series, systolic compression of an epicardial coronary branch as a result of bridging is seen on angiography in 15% of patients with HCM, and its prevalence is independent of the degree of septal hypertrophy. – 5 Myocardial bridging in adults with HCM is often a benign condition, and in one large cohort it was found to have no impact on overall outcome. However, some reports have suggested a distinct association between myocardial bridging and severe symptoms, ventricular arrhythmias, and sudden death in children with HCM. As yet, the issue remains controversial. From a pathophysiological standpoint, LAD tunnelling may precipitate severe regional ischaemia on effort, when the adrenergic drive enhances systolic compression of the artery to a sufficient degree to disturb blood flow. Indeed, impairment of flow due to coronary bridging is not limited to the duration of systole, but extends well into the diastolic phase critical to myocardial perfusion. Thus, it is not surprising that the recurrence of regional ischaemia upon the complex HCM substrate, with its corollary of hypertrophy, disarray, microvascular dysfunction, and fibrosis, –9 may precipitate malignant arrhythmias. As is often the case with HCM, however, all these elements are of uncertain value in the assessment of individual patients at risk. The study presented by Basso et al. represents an important contribution to our understanding of the relationship between myocardial bridging and sudden death. In their report, based on a large series of autopsied hearts, myocardial bridges were more commonly found in deceased HCM patients, irrespective of the cause of death, compared with non-HCM hearts with or without LV hypertrophy. Specifically, deeply embedded LAD segments (with bridges measuring 2 mm in thickness) occurred in 23% of HCM patients, vs. 5% in the control groups. According to a recent hypothesis, the HCM disease process could have a profound influence on the coronary circulation during development. In the embryonic heart, following a process called epithelial– mesenchymal transformation, pluripotent epicardium-derived cells (EPDCs) migrate diffusely into the myocardium and differentiate into diverse cell types, including the smooth muscle cells and