Mitral valve and papillary muscle abnormalities are common in hypertrophic cardiomyopathy (HCM) patients and contribute to systolic anterior motion (SAM) and left ventricular outflow tract (LVOT) obstruction (1,2). The necessary conditions for SAM are anterior position of the mitral valve in the left ventricular (LV) cavity, LV flow striking the posterior aspect of the mitral leaflets, and leaflet slack that permits increased mobility (3,4). Without leaflet slack, the mitral leaflets will remain in their more or less central position in the LV, regardless of the hemodynamic force compelling them forward. Among the most common abnormalities observed are elongated mitral leaflets, producing an anterior leaflet residual leaflet tip that protrudes past the coaptation plane into the LV ejection flow (5). The normal leaflet is held in place by the LV-left atrial pressure difference; in contrast, the diaphanous residual leaflet is bounded only by LV flow, and LV currents easily sweep it toward the septum in early systole. In this issue of the Journal, Ferrazzi et al. (6) present observations in a subset of young HCM patients that have a developmental abnormality, consisting of a band of muscular tissue in the intervalvular fibrosa between the base of the mitral valve and the aortic valve. This novel observation may explain leaflet elongation in some young patients. Other investigators have previously described immunohistopathological findings in the residual mitral leaflet tip resected at the time of surgery that indicate a nonmuscular developmental abnormality in this portion of the leaflet (7).

Another common anomaly is an anteriorly positioned head of the anterolateral papillary muscle or its chordae that pre-position the valve into the outflow tract by tenting the mitral leaflets anteriorly. This also reduces the posteriorly directed traction and restraint on the mitral leaflets that act to prevent SAM. Indeed, SAM is best thought of as the result of a dynamic disequilibrium between the restraining forces of the subvalvular apparatus and the displacing anteriorly directed pushing force of flow (8). Patients with LVOT obstruction who have no or only modest thickening of the septum invariably have abnormalities of their mitral valves that predispose to SAM.

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