Anatomic abnormalities of the mitral valve (MV) are common among patients with hypertrophic obstructive cardiomyopathy (HOCM), and contribute to subvalvular left ventricular (LV) outflow obstruction. This review summarizes these abnormalities and surgical options for their relief. The following are 10 points to remember:

1. Genesis of MV abnormalities in HOCM. Leaflet elongation is observed among patients with an HCM-associated mutation but who have not yet developed LV hypertrophy, suggesting that mitral leaflet elongation is a primary phenotypic expression of HCM, and not acquired due to leaflet stretch from systolic anterior motion (SAM) of the MV.
2. Historical recognition of MV abnormalities in HCM. The initial report that LV outflow obstruction in HOCM was caused by SAM and mid-septal contact of the MV was in 1969. Anterior displacement of papillary muscles within the LV was noted in 1974. In 1991, direct insertion of an anomalous anteriorly displaced papillary muscle into the middle of the anterior mitral leaflet (without intervening chordae) was described. In 1992-1993, the importance of mitral leaflet elongation was appreciated.
3. Elongated anterior and posterior mitral leaflets. A large majority of patients with HCM have elongated anterior and posterior mitral leaflets. Elongated leaflets extend into the LV cavity well above the plane of the mitral annulus, and a residual portion of the anterior leaflet extends past the point of coaptation. Bounded only by the LV and thus freely moving with LV flow, the residual leaflet contacts the septum and contributes to LV outflow obstruction.
4. Surgical approaches to address anterior leaflet elongation. Surgical options include vertical plication of the anterior leaflet (plication parallel to the long axis of the valve), and horizontal plication (plication perpendicular to the long axis of the valve); the authors prefer horizontal plication. Partial anterior leaflet excision may be used when excessive residual leaflet length contributes to obstruction. Finally, a stiff pericardial patch can be inserted in the anterior mitral leaflet, leading to stiffening of the leaflet in an attempt to decrease leaflet billowing.
5. Anterior and basilar displacement of the anterolateral papillary muscle. Anterior displacement of the papillary muscles results in an anterior position of the mitral valve coaptation plane within the LV cavity, predisposing to SAM. This is sometimes accompanied by abnormal muscular connections between the papillary muscle head and the anterolateral wall (inserting into or near the A1 scallop of the anterior mitral leaflet).
6. Insertion of the anterolateral papillary muscle directly into the mid-anterior mitral leaflet. Without intervening chordae, apposition of the papillary muscle and the septum may cause LV outflow obstruction.
7. Anterior mitral tenting. Anomalous anterior papillary muscles; or fibrotic, retracted secondary mitral chordae also may lift and tent the MV anteriorly, pre-positioning it in the LV outflow stream, and contributing to SAM and LV outflow obstruction.
8. Elongated posterior mitral leaflet with SAM. Although an uncommon cause of mitral–septal contact, isolated elongation of the posterior mitral leaflet also can cause SAM.
9. Mitral leaflet or annular calcification. Calcification of the mitral leaflets or annulus can occur among patients with SAM. A clue that calcification or fibrosis, rather than SAM, is the cause of mitral regurgitation comes from an anterior or mid-directed regurgitant jet (rather than the usual posteriorly directed jet caused by SAM). Mitral tissue manipulation is less feasible in this setting, and mitral intervention might require valve replacement rather than repair.
10. Surgical referral. Both the 2011 American College of Cardiology Foundation/American Heart Association and the 2014 European Society of Cardiology guidelines addressing the diagnosis and management of HOCM recognize abnormalities of the MV among patients with HOCM, and suggest that surgical intervention (rather than septal alcohol ablation) should be used when present. However, judging how much muscle to resect during myectomy is challenging; the authors suggest that, given the experience required to master the surgical techniques to treat HOCM, patients resistant to maximum pharmacotherapy should be referred to an HCM reference center.

Keywords: Cardiac Surgical Procedures, Cardiomyopathy, Hypertrophic, Heart Failure, Heart Valve Diseases, Hypertrophy, Left Ventricular, Mitral Valve Insufficiency, Papillary Muscles, Systole

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