Background
Supravalvar mitral ring is a rare congenital heart defect of surgical importance. The condition characterized by an abnormal ridge of connective tissue on the atrial side of the mitral valve. Often circumferential in shape, the supravalvar ring may encroach on the orifice of the mitral valve and may adhere to the leaflets of the valve and restrict their movements. Although a supravalvar mitral ring may allow for normal hemodynamic flow from the left atrium to the left ventricle (LV), it often substantially obstructs mitral valve inflow.
Two subtypes of this anomaly have been described, a "supramitral" variant and an "intramitral" variant. The supramitral type is a fibrous shelflike membrane located just above the mitral valve annulus but inferior to the left atrial appendage. The membrane is distinct from and not adherent to the mitral valve leaflets. This variant typically has an otherwise normal mitral valve apparatus. The intramitral variant is a membrane located within the mitral tunnel, closely adherent to the valve leaflets. This subtype is associated with a high incidence of mitral valve abnormalities, including restricted mobility of the mitral leaflets, reduced chordal length, reduced interpapillary muscle distance, single papillary muscle, or hypoplastic mitral annulus.
The intramitral type is also frequently part of the Shone complex, in which multiple levels of left heart obstruction are present, including aortic arch hypoplasia or coarctation, aortic valve stenosis, or subaortic stenosis.
Supravalvar mitral stenosis can develop as an acquired lesion late after mitral annuloplasty to repair mitral regurgitation.
Patient education
Educate the patient and family about the importance of regular medical treatment, of periodic medical review, of restricting heavy physical exertion, of the need for antibiotic prophylaxis during dental and surgical procedures, and of the need to promptly attend to all infections.
Pathophysiology
The physiologic effect of a supravalvular mitral ring depends on the degree to which the membrane obstructs mitral valve inflow as well as the associated anatomic and functional impairment of the mitral valve. The supramitral ring may initially be incomplete and eccentric, allowing for unobstructed flow through the mitral valve. However, turbulence can cause a progressive increase in the supravalvar membrane or ridge, worsening mitral inflow obstruction. The same mechanism is responsible for the acquired variety of supravalvar mitral stenosis that occurs after mitral annuloplasty for repair of mitral regurgitation.
The intramitral variant may also be eccentric or circumferential in nature. This variety may be difficult to detect because the membrane often adheres to mitral valve leaflets. Adhesion to the valve may impair opening of the leaflets, and this impairment may be the main mechanism of mitral valve inflow obstruction in some patients.
Supravalvar mitral ring rarely occurs as an isolated defect; other congenital heart defects are also present in 90% of patients. The mitral valve itself is often abnormal and stenotic at the valvar or subvalvar level; fusion of leaflets, a small valve orifice, and abnormal papillary muscles are common abnormalities. Shone complex is a combination of four congenital heart defects: supravalvar mitral ring, parachute mitral valve, subvalvar aortic stenosis, and aortic coarctation. [1, 2] Other common associated lesions in patients with supravalvar mitral ring include ventricular septal defect (VSD), patent ductus arteriosus (PDA), atrioventricular (AV) canal defect, and tetralogy of Fallot.
Uncommonly associated defects include atrial septal defect, cor triatriatum, left superior vena cava, unroofed coronary sinus, partial anomalous pulmonary venous drainage, pulmonary venous obstruction, double-orifice mitral valve, and Wolff-Parkinson-White syndrome. Lesions such as transposition of the great arteries, AV discordance, and double outlet right ventricle are occasionally complicated by a supravalvar left AV valvular ring.
Obstruction to mitral inflow results from reduced area of the mitral valve orifice. When clinically significant, a diastolic pressure difference occurs between the left atrium and the LV. Left atrial and pulmonary venous pressures increase, leading to exudation of fluid into the pulmonary interstitium, which increases lung stiffness. Breathlessness and tachypnea are secondary to the interstitial edema and diminished pulmonary compliance. In severe cases, frank pulmonary edema can occur. An associated atrial septal defect may decompress the left atrium, reducing or masking the severity of the mitral-valve obstruction. Associated lesions, such as VSD or PDA, which increase LV output, exacerbate the manifestations of mitral-inflow obstruction. In the converse, a supravalvar mitral ring may be difficult to detect in conditions with diminished pulmonary blood flow, such as cyanotic tetralogy of Fallot.
Persistently elevated pulmonary venous hypertension leads to pulmonary arterial hypertension, a rise in pulmonary vascular resistance, and eventual failure of the right ventricle. Tricuspid regurgitation commonly accompanies right heart failure from pulmonary hypertension.
Epidemiology
International data
In clinical series of patients with congenital heart disease (CHD), the reported incidence rate of supravalvar mitral ring is 0.2-0.4%, but it is as much as 8% in patients with congenital mitral valve disease. The incidence in autopsy series of patients with congenital heart disease is reported to be 0.6-1.5%. In most patients, the supravalvar mitral ring is detected during investigation for other CHD or mitral valve disease.
Race-, sex-, age-related demographics
No specific race predilection has been reported for supravalvular ring. No specific sex or age predilection is noted.
Prognosis
Among patients with congenital mitral stenosis, those with supravalvar mitral ring have a relatively good prognosis. Complete surgical excision is feasible and usually provides lasting relief. The presence of a normal underlying mitral valve and absence of other major cardiac lesions are associated with a better surgical outcome.
The prognosis is poor in patients who require resection at an early age. Recurrent supravalvar mitral stenosis is a risk in survivors, probably because of continuing turbulence across the small left ventricular (LV) inflow tract.
In patients with Shone complex, thickened mitral valve leaflets, shortened chordae, subvalvar abnormalities, left ventricular outflow obstruction, and aortic incompetence were associated with rapid progression of hemodynamic abnormalities and poor prognosis. [3]
Complications
Possible complications of supravalvar mitral ring include pulmonary edema, pulmonary arterial hypertension, atrial arrhythmia, left atrial thrombus, embolic episodes, recurrent pulmonary infections, and infective endocarditis.
Cerebral venous thrombosis has been described in infants with supravalvar mitral ring.
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Supravalvular Ring Mitral Stenosis. Parasternal long-axis echocardiographic image showing a supra mitral variant of supravalvular mitral stenosis. A discrete membrane is visualized superior to but distinct from the mitral valve. The mitral valve appears otherwise normal.
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Supravalvular Ring Mitral Stenosis. Parasternal long-axis color flow image showing a supra mitral variant of supravalvular mitral stenosis. Turbulence is noted at the level of the supra mitral membrane. In this case, Doppler interrogation revealed only mild obstruction.
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Supravalvular Ring Mitral Stenosis. Apical four-chamber echocardiographic image showing an intramitral variant of supravalvular mitral stenosis. A membrane is visualized that is closely adherent to the mitral valve leaflets, restricting leaflet mobility.
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Supravalvular Ring Mitral Stenosis. Apical four-chamber color flow echocardiographic image showing an intramitral variant of supravalvular mitral stenosis. Color flow imaging demonstrates severe mitral valve stenosis.
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Supravalvular Ring Mitral Stenosis. Continuous wave Doppler interrogation of the mitral valve in a patient with supravalvular mitral stenosis demonstrates severe stenosis with a mean gradient of 25 mmHg.
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Supravalvular Ring Mitral Stenosis. Simultaneous recording of pressures in the pulmonary artery wedge (PAW) position and the left ventricle (LV) shows a large gradient in diastole across the mitral valve. PAW pressure is markedly elevated.
