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The straddling mitral valve: morphological observations and clinical implications.

The morphological features of 23 patients with straddling or overriding mitral valve are presented. Levocardia was present in 20 of 23; visceroatrial situs solitisu in 20 of 23, with 3 patients, 2 with asplenia and 1 with polysplenia, having visceral heterotaxia. A concordant D-ventricular loop was present in the 20 patients with visceroatrial situs solitus. Six of these had double outlet right ventricle; 2 had asplenia syndrome; 1 had D-transposition of the great arteries, ventricular defect and pulmonary atresia; 1 with tricuspid atresia and double outlet-outlet chamber; 1 with polysplenia syndrome; and 12 had endocardial cushion defect with marked underdevelopment of the left ventricle, and normally related great arteries. Left ventricular size was related to the amount of mitral valve (or left-sided component of a common atrioventricular valve) connected to it. In those patients in whom little effective mitral orifice was connected to the left ventricle, the left ventricle was diminutive. Endocardial fibroelastosis of the left ventricle was noted in only a single patient. Six of the 7 patients with double outlet right ventricle (including one with double outlet bulbus) had subpulmonary obstruction, and in one of these, this was related in part to the straddling mitral valve. In 1 patient with double outlet right ventricle, there was a double orifice mitral valve, and it was the accessory mitral orifice that straddled. The diagnosis of overriding mitral valve should be suspected in any patient with significant conotruncal anomalies and underdeveloped left ventricle, especially the patient with double outlet right ventricle, and in the patient with endocardial cushion defect, hypoplasia of the left ventricle, and obstructive anomalies of the aortic arch. In certain patients, selective left atriography, left ventriculography, and single and two dimensional echocardiography may be diagnostic of this condition.