The accessory mitral orifice is a rare congenital malformation characterized by the presence of a membranous structure or abnormal valvular tissue that partially or completely divides the mitral valve orifice, resulting in the formation of a second (accessory) passage between the left atrium and ventricle. Depending on the extent and location of the membrane, this condition may cause variable obstruction to mitral flow, mimicking a picture of mitral stenosis. Although it can occur as an isolated anomaly, it is more frequently associated with other congenital heart defects, particularly partial or complete atrioventricular canal defects. In clinically significant cases, the condition is dominated by symptoms of pulmonary congestion and left-sided heart failure, whereas in mild forms the finding may be incidental.
The etiology of the accessory mitral orifice lies in an anomaly of embryological development of the endocardial cushion and atrioventricular valvular tissue between the fifth and eighth week of gestation. During this phase, the mitral valve leaflets are formed through delamination of mesenchymal tissue and the shaping of a single mitral orifice. A defect in tissue reorganization may lead to the persistence of a fibrous or membranous structure at the line of coaptation, resulting in the formation of a partial septum dividing the orifice into two distinct channels. The result is a partial duplication of the mitral orifice, which may take the form of a “net”, “curtain”, or “diaphragm”-like structure.
Risk factors for this malformation are primarily genetic or syndromic in nature. The most frequent associations include:
No acquired environmental factors or specific teratogenic exposures have been described as causative in the absence of a genetic predisposition.
From a pathogenic perspective, the accessory structure may obstruct diastolic flow between the left atrium and ventricle, increasing left atrial pressure and reducing ventricular filling. Depending on its extent and mobility, the membrane may act as a fixed diaphragm or as a mobile leaflet that impedes flow during diastole. In some forms, a mobile accessory flap may produce a valve-like effect, worsening the pressure gradient during exertion or conditions of elevated heart rate.
The resulting pathophysiology is similar to that of congenital mitral stenosis. Chronic elevation of left atrial pressure is transmitted to the pulmonary circulation, leading to capillary congestion, thickening of vascular walls, and subsequent post-capillary pulmonary hypertension. Although the left ventricle is structurally normal, it may present with relative hypovolemia and reduced compliance, especially in the presence of compensatory hypertrophy. Over time, increased right ventricular afterload and left atrial dilation predispose to supraventricular arrhythmias and the development of biventricular heart failure.
The severity of the clinical presentation depends on multiple factors: the size and location of the accessory orifice, the rigidity of the membrane, the presence of other structural anomalies, and the adaptive response of the cardiac chambers. In more severe forms, the condition may be clinically indistinguishable from true mitral stenosis; in partial or fenestrated forms, progression may be insidious, with diagnosis delayed until adolescence or adulthood.
The clinical picture of an accessory mitral orifice is extremely variable and reflects the degree of obstruction caused by the accessory structure, as well as the possible association with other cardiac malformations. In many cases—particularly those with a double fenestrated or thin, non-rigid membrane—the condition may remain asymptomatic for years and be discovered incidentally during echocardiography performed for other reasons. However, in more significant forms, where the accessory tissue substantially obstructs diastolic flow, the clinical picture overlaps with that of congenital mitral stenosis.
In neonates and infants, onset may be early and severe. Obstruction to left ventricular filling leads to increased left atrial and pulmonary capillary pressure, with symptoms of respiratory congestion and failure.
The most common clinical signs in this age group include:
In older children or adolescents, symptoms may begin more slowly and subtly, presenting as reduced exercise tolerance, progressive dyspnea, and fatigue. As the disease progresses, orthopnea, tachycardia, and—in advanced cases—peripheral edema and systemic circulatory disturbances may appear, indicating biventricular failure. In the presence of significant pulmonary hypertension, relative cyanosis may also be observed during intense physical activity.
On physical examination, a diastolic murmur of medium or low frequency is often present, audible at the apex and accentuated in the left lateral decubitus, resembling that of rheumatic mitral stenosis. The murmur may be preceded by an opening snap, indicative of tension in the accessory tissue during the early filling phase. In some cases, a third ventricular sound (S3) is also noted. In patients with pulmonary hypertension, the second heart sound may be accentuated in the pulmonary area, and bibasilar rales due to capillary congestion may be present.
A distinctive, though not always present, feature is the variability of the murmur with heart rate and loading conditions: in some forms, the accessory membrane behaves like a mobile valve, increasing the gradient during tachycardia or exertion. This makes clinical evaluation particularly important during dynamic testing or stress echocardiography.
In patients with associated anomalies—particularly partial atrioventricular canal—the symptoms may be modulated by concurrent mitral regurgitation, left-to-right atrial shunt, and the interaction of various anatomical abnormalities. In such cases, the accessory orifice may exacerbate an already complex hemodynamic situation and contribute to early-onset heart failure.
Finally, in asymptomatic individuals, the first evidence of the malformation may arise in adulthood as an isolated echocardiographic finding or as an unrecognized cause of pulmonary hypertension, atrial arrhythmias, or left ventricular dysfunction. In such contexts, an accurate and retrospective diagnosis is often needed to explain a long history of nonspecific symptoms.
The diagnosis of an accessory mitral orifice requires a combination of clinical acumen and high-resolution cardiac imaging, as symptoms may be subtle and the abnormal structure difficult to visualize with first-line investigations. Clinical suspicion arises in the presence of signs and symptoms consistent with mitral stenosis in a young patient, without a history of rheumatic fever or endocarditis. A diastolic apical murmur, especially in pediatric age or in a syndromic context, should raise the hypothesis of a congenital mitral orifice anomaly.
The electrocardiogram is often normal in mild forms; it may show broad P waves from left atrial overload or signs of right ventricular hypertrophy in cases with pulmonary hypertension.
The chest X-ray may reveal cardiomegaly with left atrial enlargement and pulmonary vascular congestion, especially in cases with significant obstruction.
Transthoracic echocardiography (TTE) is the cornerstone of diagnosis. In experienced hands, it allows identification of a membrane or abnormal structure partially crossing the mitral orifice. The most useful views are the parasternal long axis and apical four-chamber views, where the membrane may appear as a fibrous bridge or curtain-like structure dividing the mitral passage. Doppler flow often reveals an increased transvalvular gradient during diastole, although assessment of the degree of obstruction may be affected by artifacts or irregular flow direction.
In patients with suboptimal acoustic windows or when the anomaly is not clearly defined on TTE, transesophageal echocardiography (TEE) is indicated, as it provides a direct and high-resolution view of the mitral valve apparatus. TEE confirms the presence, extent, mobility, and location of the accessory membrane, evaluates its continuity with the valvular leaflets, and rules out associated anomalies. It is also crucial for surgical planning, as it defines the relationship between the accessory structure and the chordae tendineae, papillary muscles, and interatrial septum.
In selected cases, particularly for three-dimensional morphological assessment or in adult patients, 3D echocardiography can be useful, enabling volumetric reconstruction of the mitral orifice and accessory membrane, facilitating the identification of fenestrations and precise measurement of the two orifices.
Cardiac magnetic resonance (CMR) has a more limited role but may be useful for comprehensive volumetric assessment of the heart, biventricular function, and mitral diastolic flow in complex cases or when echocardiographic findings are inconclusive.
The differential diagnosis must include other forms of congenital mitral stenosis, such as double-orifice mitral valve (a true duplication of the valvular plane), supravalvular mitral membranes (typical of Shone’s syndrome), valvular dysplasias with accessory commissures, and the presence of anomalous chordae mimicking an intracavitary septum. It is also essential to rule out acquired forms, such as rheumatic stenosis, which typically present with fused commissures, calcifications, and altered leaflet morphology.
An accurate and early diagnosis is crucial to guide the therapeutic strategy and prevent progression to fixed pulmonary hypertension, left ventricular dysfunction, or structural arrhythmias. For this reason, the integration of clinical suspicion, advanced echocardiographic imaging, and complete morphological assessment is the cornerstone of the diagnostic management of the accessory mitral orifice.
The treatment of the accessory mitral orifice essentially depends on the degree of functional obstruction caused by the abnormal structure, the presence of clinical symptoms, and the overall echocardiographic findings. In asymptomatic patients, with well-preserved mitral flow and no signs of atrial overload or pulmonary hypertension, a conservative approach is possible, based on periodic echocardiographic evaluations and annual clinical follow-up.
However, in the presence of a mean transvalvular gradient ≥ 5 mmHg at rest, signs of pulmonary congestion, left atrial dilation, or symptoms suggestive of heart failure, early surgical correction is indicated.
Surgical intervention is the definitive treatment in symptomatic patients or those with significant obstruction.
The indication is particularly strong when the following are present:
The procedure is performed under cardiopulmonary bypass via a left atrial approach. It consists of the complete resection of the accessory membrane and reconstruction of the mitral valvular plane, with possible commissuroplasty or leaflet repair if anatomical distortion is present. In cases associated with atrioventricular canal defects, a combined procedure is often necessary, including septal defect closure and correction of the leaflet coaptation mechanism.
Postoperative prognosis is excellent in patients undergoing complete and early resection. In most cases, there is definitive resolution of the mitral gradient, normalization of left atrial pressure, and immediate clinical improvement. Left ventricular function remains preserved and, in patients operated on before the development of fixed pulmonary hypertension, a complete regression of congestive signs is usually observed.
Long-term follow-up requires serial echocardiographic monitoring to assess mitral orifice patency, the possible presence of residual regurgitation (less common than residual stenosis), and the overall function of the cardiac chambers. In patients operated on during childhood, somatic growth can alter mitral geometry, requiring particular attention during puberty and adulthood.
Positive prognostic factors include: early intervention before the onset of pulmonary hypertension, absence of complex associated valvular anomalies, good native valvular tissue quality, and complete surgical resection. Conversely, delayed or incomplete intervention, or the presence of complex genetic syndromes with multiple anomalies, may negatively influence outcomes and necessitate closer follow-up.
Overall, in properly treated patients, long-term survival and quality of life are excellent, with a very low risk of reoperation and full return to normal daily and school activities.
Complications of the accessory mitral orifice may result from either the natural progression of the untreated malformation or from surgical correction outcomes. Although the overall incidence is low, the severity of such events depends strictly on the timeliness of diagnosis, the precision of the preoperative echocardiographic assessment, and the technical adequacy of the procedure.
In the natural course of the disease, the main complication is progressive obstruction to mitral flow. Even when the structure is initially flexible or fenestrated, patient growth can lead to a relative increase in obstruction, with gradual worsening of the transvalvular gradient. The result is chronic left atrial pressure overload, which may lead to post-capillary pulmonary hypertension and, in advanced cases, right ventricular dysfunction. Left atrial dilation also provides a substrate for supraventricular arrhythmias such as atrial fibrillation or multifocal atrial tachycardia, which further aggravate the hemodynamic picture and increase thromboembolic risk.
Other potential complications include reduced cardiac output in response to exertion, development of signs of congestive heart failure (hepatomegaly, edema, fatigue), and growth retardation in pediatric patients. These manifestations are more frequently observed in untreated patients or in those diagnosed late.
Postoperative complications, though rare in centers with advanced cardiac surgical expertise, must be carefully monitored. The most relevant include:
Rarely, in cases where the membrane is tightly adherent to the interatrial septum or in the presence of complex anatomical variants, injury to the atrioventricular conduction system may occur, leading to varying degrees of heart block. These situations require prolonged electrocardiographic monitoring and, in selected cases, pacemaker implantation.
In the long term, one of the main concerns is the possibility of recurrence of stenosis, related to the formation of reactive fibrous tissue or disproportionate growth of the residual valvular apparatus. This event is rare but justifies the importance of a structured and regular echocardiographic follow-up.
Overall, the risk of complications can be significantly reduced through timely diagnosis, detailed preoperative echocardiographic evaluation, and meticulous surgical resection. In patients treated at an early age and with favorable anatomy, long-term survival is excellent, with good functional recovery and a low incidence of adverse events.