Subaortic Stenosis

Subaortic stenosis is fixed obstruction of the left ventricular outflow tract (LVOT) beneath the aortic valve. The usual form is a discrete fibromembranous ridge or collar. Rarer forms are fibromuscular tunnel stenosis and obstruction from accessory mitral tissue.

It makes up about 1% of congenital heart disease. It is usually not present at birth and develops through childhood. Associated lesions are ventricular septal defect, coarctation, interrupted arch, atrioventricular septal defect, bicuspid aortic valve and Shone complex.

Familial cases occur.

Pathophysiology. A steep aortoseptal angle and abnormal LVOT geometry create turbulent flow that stimulates fibrous tissue formation. The membrane causes pressure overload and left ventricular hypertrophy.

The turbulent jet damages the aortic leaflets and causes progressive aortic regurgitation in a large proportion of patients. It also raises endocarditis risk. After resection, the lesion recurs in about 20 to 30%.

• Often asymptomatic in childhood.
• Severe obstruction: exertional dyspnoea, angina, presyncope, syncope, and rarely sudden death.
• Ejection systolic murmur at the left sternal edge radiating to the neck, often with a thrill, and no ejection click.
• Early diastolic murmur of aortic regurgitation in many.
• Normal or slightly slow carotid upstroke.

Red flags.

• Exertional syncope or chest pain
• Peak gradient of 50 mm Hg or more or a rising gradient on serial scans
• Progressive aortic regurgitation
• Left ventricular dysfunction or marked hypertrophy
• Ventricular arrhythmia
• Fever with a new murmur (endocarditis)

• ECG: left ventricular hypertrophy with strain.
• Chest X-ray: normal or prominent left ventricle.
• Transthoracic echocardiography: parasternal long axis and apical three chamber or five chamber views show the membrane or ridge, usually 5 to 10 mm under the valve. Continuous wave Doppler gives the peak and mean gradient. Measure LVOT diameter, left ventricular wall thickness, aortic valve morphology, aortic regurgitation grade and the aortoseptal angle.
• Transoesophageal echocardiography: defines the lesion in adults and before surgery.
• Cardiac MRI or CT: for tunnel forms and associated anomalies.
• Cardiac catheterisation: rarely needed.

Differential diagnosis.

• Valvar aortic stenosis (ejection click, bicuspid or calcified valve)
• Supravalvular aortic stenosis
• Hypertrophic obstructive cardiomyopathy (dynamic gradient, systolic anterior motion, late peaking dagger shaped Doppler signal, changes with Valsalva)
• Sigmoid septum in older adults
• Mid-cavity obstruction
• Accessory mitral valve tissue
• Ventricular septal defect

• No drug treats the obstruction. Avoid dehydration and vasodilators when the gradient is significant. Limit heavy isometric exertion with moderate or severe obstruction.
• Surgery is indicated in adults for a peak Doppler gradient of 50 mm Hg or more or a mean gradient of 30 mm Hg or more, for symptoms, or for left ventricular dysfunction. It is also considered with a lower gradient when aortic regurgitation progresses beyond mild or hypertrophy is marked. In children, earlier resection is considered when the gradient is rising or regurgitation is developing, because the obstruction drives valve damage.
• Operation: resection of the membrane or ridge, usually with septal myectomy to reduce recurrence, via the aortic root. Tunnel type obstruction needs an outflow tract enlargement (Konno type procedure) or a Ross-Konno procedure. Aortic valve replacement for severe aortic regurgitation.
• Complications: complete heart block (about 1 to 3%), iatrogenic ventricular septal defect, mitral and aortic valve injury, and recurrence.
• Balloon dilatation gives poor results and is not used.
• Endocarditis prophylaxis is reserved for a prosthetic valve or material, previous endocarditis, or unrepaired cyanotic disease. Isolated subaortic stenosis does not qualify.

Special groups. Pregnancy is tolerated with mild obstruction. With a peak gradient of 50 mm Hg or more or symptoms, treat before conception.

Monitoring. Echocardiography every 6 to 12 months in children and every 1 to 2 years in stable adults, with the interval shortened when the gradient or regurgitation progresses. After resection, yearly echocardiography for recurrence and regurgitation. ECG for conduction abnormalities after surgery.

Referral and escalation. Refer to a congenital heart disease centre. Urgent when there is syncope, angina, rapid gradient rise, left ventricular dysfunction or endocarditis.