Mitral Stenosis, Supravalvular Ring 

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.

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 4 congenital heart defects: supravalvar mitral ring, parachute mitral valve, subvalvar aortic stenosis, and aortic coarctation. 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 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.

Frequency

International

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

No specific race predilection has been reported.

Sex

Supravalvular ring has no specific sex predilection.

Age

No specific age predilection is noted.