Double orifice mitral valve (DOMV) is an uncommon anomaly that was first described by Greenfield in 1876.  Since that time, more than 200 cases have been reported.  This anomaly is characterized by a mitral valve with a single fibrous annulus with 2 orifices that open into the left ventricle (LV), as depicted in the image below.
Subvalvular structures, especially the tensor apparatus, invariably show various degrees of abnormality.  Although double orifice mitral valve may allow normal blood flow between the left atrium and LV, it can substantially obstruct mitral valve inflow or produce mitral valve incompetence.
Recognition of double orifice mitral valve and awareness of the anatomic variations are important to achieve good therapeutic results. Treatment is only necessary if significant mitral stenosis (MS) or mitral regurgitation (MR) is present.
The normal mitral valve consists of a large, central orifice located between a large sail-like anterior leaflet and a small, C-shaped posterior leaflet. In double orifice mitral valve, abnormal tissue divides the orifice into 2 parts.
Eccentric or hole type
This is the most common type, accounting for about 85%, and was seen in the first case reported by Greenfield in 1876.  It is characterized by a larger main orifice and a smaller accessory orifice situated at either the anterolateral or the posteromedial commissure. Other anomalies of the valve apparatus, such as cleft leaflets, accessory papillary muscles, fused papillary muscles, and crossing chordae tendineae, are commonly present. Abnormal chordal rings, with or without subjacent papillary musculature, are a constant finding in this classic type. When the accessory orifice is located at the posteromedial commissure, a common atrioventricular canal is usually present. By comparison, the atrioventricular canal is normally divided if the accessory orifice is located at the anterolateral commissure. 
Central or bridge type
In about 15% of patients with double orifice mitral valve, a central bridge of fibrous or abnormal leaflet tissue connects the 2 leaflets of the mitral valve, dividing the orifice into medial and lateral parts. These 2 openings may be equal or unequal, and the papillary muscles are usually normal, with chordae surrounding each orifice inserting into one papillary muscle. In this type of double orifice mitral valve, dilatation of the posteromedial orifice is feasible by means of balloon valvuloplasty.
Duplicate mitral valve type
This condition involves 2 mitral valve annuli and valves, each with its own set of leaflets, commissures, chordae, and papillary muscles. 
Associated congenital heart defects are common, although double orifice mitral valve can occur as an isolated anomaly. The most common associated lesion is atrioventricular (AV) septal defect (endocardial cushion defect), where it has been found in as many as 4.9-17.9% of cases in necropsy studies. 
In a postmortem series of 27 cases of double orifice mitral valve, Bano-Rodrigo et al concluded that 44% had an intact AV septum and 56% had an AV septal defect; 52% had mitral stenosis or mitral regurgitation. Double orifice mitral valve was always associated with an anomaly of the subvalvular apparatus because, by definition, a separate tensor apparatus is attached to each orifice.
Abnormal structures, including large bridging tissue, bulky abnormal leaflets, fused chordae, or abnormal papillary muscles reduce the effective area of the valve. Abnormalities in the leaflets include thickening, fusion, perforations, restricted movements, and ruptured chordae with flail cusps. Such valves can result in clinically significant degrees of mitral incompetence.
In another published clinical series of 18 patients with double orifice mitral valve and intact AV septum, Das et al found that double orifice mitral valve was most commonly associated with left sided obstructed lesions in 39% of the cases and with ventricular septal defects (VSDs) in 17% of the cases.  In contrast to the above postmortem study, only 11% had significant mitral stenosis or mitral regurgitation. Higher incidence of mitral stenosis and mitral regurgitation noted in the postmortem study may be related to the selective bias because it was performed at a tertiary care specialized center.
Presence of atrial septal defects (ASD) may lead to underestimation of the frequency and the severity of mitral stenosis, which is revealed after the atrial septal defect closure.  Tendinous attachments to both openings distinguishes double orifice mitral valve from acquired lesions, such as perforation or partial fusion of valve leaflets caused by inflammatory lesions, traumatic ruptures, and complications of interventional procedures. 
On occasion, double orifice mitral valve is observed with tetralogy of Fallot, hypoplastic left heart syndrome, persistent left superior vena cava, unroofed coronary sinus, Ebstein anomaly of the tricuspid valve, dysplastic tricuspid valve, double-orifice tricuspid valve, Shone syndrome, parachute mitral valve, flail mitral-valve leaflet, truncus arteriosus, pulmonary stenosis, bicuspid aortic valve, left ventricular noncompaction (isolated and nonisolated types), tricuspid atresia, and corrected transposition. Double orifice mitral valve has also been reported in patients presenting with atrial tachycardia or congenital complete heart block. [9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]
Double orifice mitral valve has also been reported in trisomy 21, de Lange syndrome, Astrom syndrome, Kabuki syndrome, and isovaleric academia.
In a published clinical series of 46 children with double orifice mitral valve, Zalzstein et al reported that mitral regurgitation was most frequent association and was detected by color Doppler in 43% of the patients.  Double orifice mitral valve with normal flow was seen in 37% of the patients, and mitral stenosis was detected in 13% of the patients. Mitral stenosis and regurgitation were found together in 6.5% of the patients. However, only 16% of the patients had severe enough lesions to require surgical intervention.
A reduction in the total area of the mitral valve orifice obstructs its inflow. When mitral stenosis is clinically significant, a diastolic pressure difference builds up between the left atrium and the LV. The rise in left atrial and pulmonary venous pressure leads to exudation of fluid into the interstitium of the lung and increased pulmonary stiffness (the main cause of breathlessness). In severe cases, frank pulmonary edema develops. Persistent pulmonary venous hypertension leads to pulmonary arterial hypertension, a rise in pulmonary vascular resistance and eventually, failure of the right ventricle (RV) with tricuspid regurgitation.
About 25% of patients with double orifice mitral valve present with mitral regurgitation as the dominant hemodynamic abnormality, especially when it is associated with an atrioventricular canal defect. Similar to mitral stenosis, mitral regurgitation causes left atrial and pulmonary venous hypertension. Clinical effects depend on the severity and duration of mitral regurgitation.
LV outflow obstruction from associated subaortic stenosis or coarctation can aggravate the mitral regurgitation. With increasing mitral regurgitation, cardiac output is maintained by increasing LV diastolic volume and hypertrophy. In chronic mitral regurgitation, LV function deteriorates, leading to further worsening of pulmonary venous congestion, pulmonary hypertension, and right heart failure.  Ventricular dysfunction secondary to chronic dilatation is often irreversible even after the primary mitral lesion is corrected. The left atrium tends to markedly enlarge in mitral regurgitation and may compress the left bronchus.
When an ASD with a significant left to right atrial shunt is present, reduction of transmitral flow and left ventricular volume occurs, which may lead to underestimation of the mitral stenosis. 
Double orifice mitral valve was found in 1% of autopsied cases of congenital heart disease (CHD) in the Cardiac Registry of the Children’s Hospital, Boston. 
To the author's' knowledge, no data are available on the incidence of double orifice mitral valve. More than 180 cases of double orifice mitral valve are reported in the literature. Approximately 50% of all cases of double orifice mitral valve are detected during investigation of other CHDs.
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