Case Presentation

A 27-year-old man with HCM and a 2-year history of palpitations was referred to our institution for further evaluation. The patient did not have a family history of premature sudden death or relatives known to be affected by HCM. Cardiac examination revealed a grade II/VI to III/VI midsystolic murmur at the mesocardium. Electrocardiography showed sinus rhythm and left ventricular (LV) hypertrophy with deep negative T waves in leads V ₃ to V ₆ . Two-dimensional echocardiography showed asymmetric LV hypertrophy, with a maximal wall thickness of 18 mm at the level of the anterior septum. The mitral valve leaflets were extremely elongated, and the anterior leaflet appeared as a free-floating structure in the LV outflow tract in systole ( Figures 1 A and 1 B, Videos 1 and 2 ). Mitral valve regurgitation was mild. A resting peak LV outflow gradient of 20 mm Hg was present under basal conditions and increased to 40 mm Hg after 5 minutes of exercise in the supine position. Transesophageal echocardiography showed a mobile, free-floating membrane-like structure in contiguity with the ventricular side of the anterior mitral valve leaflet, which moved into the LV outflow tract during systole ( Figure 2 ), consistent with the echocardiographic diagnosis of accessory mitral valve tissue. Atenolol 25 mg/day and aspirin 100 mg/day were started. Endocarditis prophylaxis was recommended. At a 3-year follow-up evaluation, the patient was asymptomatic, and clinical and echocardiographic features were unchanged. Genetic screening identified an heterozygous mutation (c.1987C>T, p.Arg663Cys) in the β-myosin heavy-chain gene, previously reported as a cause of HCM.

Two-dimensional echocardiographic parasternal long-axis (A) and apical four-chamber (B) views showing an extremely elongated mitral valve (arrow) , which curls in on itself and toward the interventricular septum in systole. Asymmetric LV hypertrophy is present, with a maximal wall thickness of 18 mm at the level of the interventricular septum.

Transesophageal echocardiogram showing a free-floating, membrane-like structure in contiguity with the ventricular side of the anterior mitral valve (MV) leaflet, which moves into the left ventricular outflow tract during systole. These ultrasound images are consistent with a diagnosis of accessory mitral valve tissue. LA , Left atrium; LV , left ventricle.

Discussion

Accessory mitral valve tissue is a rare congenital cardiac anomaly. The histopathology of this form has not been precisely defined and has been reported as either normal or characterized by mild myxomatous tissue degeneration. In the absence of surgical or necropsy documentation of the gross valve morphology, the diagnosis of accessory mitral valve tissue is based on echocardiographic criteria. On echocardiographic examination, the abnormal mitral valve apparatus may appear either as a mobile, free-floating membrane-like structure contiguous to the ventricular side of the anterior mitral valve leaflet or as a fixed structure anchored to the interventricular septum by a short chordal apparatus and without contiguity with the anterior mitral valve leaflet. However, other intermediate morphologic features have also been described.

Patients with accessory mitral valve tissue anomaly may be asymptomatic but more frequently experience palpitations and fatigue. A cerebrovascular embolic event may occur as the first clinical manifestation of the disease. The mechanisms responsible for this complication remain undefined. However, embolization of platelets or other debris from this highly mobile and often fragile tissue is a likely possibility. Although the risk for embolic events has not been precisely determined, anticoagulant treatment may be advisable in patients with accessory mitral valve tissue and history of embolization. Surgical removal of the accessory mitral valve tissue has been reported in selected patients. However, cardiac surgery is not indicated on a routine basis in patients with this rare congenital anomaly and otherwise normal cardiac anatomy.

The major interest of the clinical case we report is the coexistence of genetically proven HCM and congenital accessory mitral valve tissue anomaly. In patients with HCM, elongated and redundant mitral valve leaflets are a relatively common morphologic feature. However, in the patient with HCM we describe, the mobile, free-floating membrane-like structure identified by echocardiography in contiguity with the anterior mitral valve leaflet had extreme morphologic characteristics consistent with the present diagnostic echocardiographic criteria for accessory mitral valve tissue anomaly. Because of the reported occurrence of embolic events in this rare congenital valve abnormality, an increased risk for embolization should be taken into account in patients with both HCM and accessory mitral valve tissue anomaly.