Bicuspid aortic valve

Bicuspid aortic valve (BAV) refers to a spectrum of deformed aortic valves with two functional leaflets or cusps which are often unequal in size.

They are most often congenital while an acquired bicuspid valve occurs when there is fibrous fusion between the right and left cusps of a pre-existing trileaflet aortic valve.

A congenitial biscuspid aortic valve is considered to be one of the most common causes of isolated aortic stenosis ⁴. It is considered a major cause of aortic valve disease in young adults.

Epidemiology

The estimated incidence of a congenital bicuspid valve in the general population is thought to be ~2%. They may be more common in males.

Pathology

BAV refers to a spectrum of deformed aortic valves with two functional leaflets or cusps which are often unequal in size. Only two cusps, commissures and sinuses are seen in the less common "pure" BAV subtype. The more common form of BAV occurs in a valve with three cusps with underdevelopment of a commissure and fusion of two adjacent cusps to form a raphe ⁷. Over time, the abnormal stress across the valve leads to calcification, usually in adulthood.

Associations

• dilatation of the proximal ascending aorta: most frequent association ⁹
• Turner syndrome: 10-12 % of women with Turner syndrome ¹¹
• coarctation of the aorta: approximately 70% (range 50-85%) of coarctations are thought to associated with bicuspid aortic valves ²
• left sided lesions, e.g. hypoplastic left heart
• other congenital lesions, e.g. atrial and ventricular septal defects, patent ductus arteriosus
• autosomal dominant polycystic kidney disease
• intracranial aneurysm
• hypoplastic left heart syndrome ⁹

Radiographic features

Plain radiograph

The usefulness of plain chest radiographs in the detection of a bicuspid valve is considered to be rather poor. Occasionally the presence of a single calcified raphe at the expected site of the bicuspid valve, which is best seen on lateral view as aortic valve is superimposed on spine on frontal radiographs, as well as post stenotic dilatation of the ascending aorta may suggest a biscuspid valve ⁴.

Echocardiography

While echocardiography is the standard diagnostic procedure for the evaluation of patients with valvular disease, differentiation of bicuspid valve from other types of calcific aortic stenosis can sometimes be difficult ^4-5.

At the time of initial writing, an echocardiogram for the detection of a bicuspid aortic valve is thought to carry ⁶:

• sensitivity: 76%
• specificity: 60%
• positive predictive value: 68%
• negative predictive value: 95%

CT

At the time of writing, CT has a much higher reported detection rate for bicuspid valves and include ⁶:

• sensitivity: 94%
• specificity: 100%
• positive predictive value: 100%
• negative predictive value: 97%

Characteristic “fish-mouth” shape of the open valve in systole is noted on ECG-gated cardiac CT ⁷.

There is often earlier leaflet calcification.

MRI

Cardiac MRI has the advantage of demonstrating the dynamic motion of the bicuspid valve when heavily calcified valves make echocardiography difficult to interpret. Furthermore, MRI can provide non-invasive assessment of the ascending aortic diameter and the presence of a coarctation in a single study.

Treatment and prognosis

A minority of patients develop aortic regurgitation as a young adult but the majority have progressive valve stenosis, with only about 1% maintaining normal valve function over their lifetime ¹⁰.

High blood pressure should be controlled.

Symptomatic children have very little calcification, and are treated by balloon valvuloplasty. Also, insertion of a valve in a child is not advisable, as the child will continue to grow.

In adults, aortic valve replacement is performed, and occurs in a younger age group than in patients with tri-leaftlet valve stenosis. Aortic root replacement is also required in 30% of patients undergoing valve replacement ⁹.

Complications

Recognised complications include:

• aortic stenosis
  • this is secondary to leaflet calcification which occurs earlier (around age 40) than in patients with tri-leaflet aortic valves
  • turbulent flow across the valve predisposes to leaflet calcification
• aortic insufficiency
  • in children: incompetence develops secondary to redundant valve leaflets, endocarditis, or following balloon valvuloplasty
  • in adults: dilatation of the ascending aorta can lead to regurgitation
• ascending aortic aneurysm
  • an aortopathy is present irrespective of the valve function; aortic dilatation (and dissection) is due to abnormal media; as such, BAV can be considered a disease of both the valve and ascending aorta, and this should be a consideration in surgical decision making
• aortic dissection: secondary to aortopathy and abnormal media
• endocarditis: due to turbulent flow