What are the clinical presentation, diagnostic work‑up, and management recommendations for sub‑aortic membrane?

Subaortic Membrane: Clinical Presentation, Diagnosis, and Management

Surgical resection is recommended for adults with subaortic stenosis when the maximum gradient is ≥50 mm Hg, or when gradients are lower but accompanied by symptoms, LV systolic dysfunction, or progressive aortic regurgitation. 1

Clinical Presentation

Typical Symptoms and Physical Findings

• Crescendo-decrescendo systolic murmur heard at the apex and left parasternal border, with inconsistent carotid transmission and no ejection click (distinguishing it from valvular aortic stenosis) 1
• Symptomatic patients present with dyspnea, chest pain, syncope, fatigability, or palpitations, typically when peak gradients exceed 85 mm Hg 2, 3
• High-frequency early diastolic murmur along the left sternal border indicates associated aortic regurgitation, which occurs in >50% of patients 1
• A precordial thrill may be palpable in some patients 1

Natural History and Complications

• The condition is progressive, with increasing risk of moderate-to-severe aortic regurgitation once peak gradients reach ≥50 mm Hg 1
• Untreated patients face risks of ventricular dysfunction, infective endocarditis, and sudden cardiac death 1
• Male predominance of 2:1, with prevalence of 6.5% among adults with congenital heart disease 1
• May be familial (as in Shone syndrome) or associated with other congenital defects including VSDs, AVSDs, or conotruncal anomalies 1, 4

Diagnostic Work-Up

Initial Evaluation

• Transthoracic 2D echocardiography with Doppler is the diagnostic method of choice to characterize LV outflow anatomy, measure gradients, assess aortic valve abnormalities, quantify aortic regurgitation, evaluate mitral valve involvement, and assess LV hypertrophy and function 1, 5
• TEE provides valuable anatomic detail preoperatively and intraoperatively, particularly when transthoracic windows are suboptimal 1
• 3D echocardiography is particularly helpful for demonstrating complex LV outflow anatomy 1

Electrocardiogram and Chest X-Ray

• ECG may be normal with mild disease or show LV hypertrophy with secondary repolarization abnormalities in significant obstruction 1
• Chest x-ray is typically normal unless significant AR develops, causing LV dilatation 1

Cardiac Catheterization

• Noninvasive imaging is usually sufficient; catheterization is reserved for cases with associated lesions or when noninvasive data are inconclusive 1
• Accurate gradient measurement requires end-hole or micromanometer-tipped catheters 1
• LV angiography is often unreliable for visualizing discrete subaortic membranes 1

Exercise Testing

• Stress testing may be reasonable to assess exercise capacity, stress-induced arrhythmias, and ischemia when indications for intervention are otherwise equivocal 1, 5

Diagnostic Pitfalls

• Subaortic membrane findings may be subtle on TTE without optimal acoustic windows perpendicular to the membrane 1
• Can be confused with hypertrophic obstructive cardiomyopathy when severe LV hypertrophy obscures the membrane 1
• Gradient severity may be underestimated or overestimated in the presence of associated VSDs, depending on VSD location relative to the obstruction 1

Management Recommendations

Definitive Surgical Indications (Class I)

Symptomatic patients:

• Maximum gradient ≥50 mm Hg with attributable symptoms requires surgical intervention 1, 5

Patients with ventricular dysfunction:

• Maximum gradient <50 mm Hg with heart failure, ischemic symptoms, and/or LV systolic dysfunction attributable to subaortic stenosis requires surgery 1, 5
• This applies even when depressed LV function prevents manifestation of high resting gradients 1

Progressive aortic regurgitation:

• Peak gradient <50 mm Hg with progressive AR and LV end-systolic diameter ≥50 mm OR LV ejection fraction <55% requires surgical intervention 1, 5

Preventive Surgery Consideration (Class IIb)

• Asymptomatic patients with at least mild AR and maximum gradient ≥50 mm Hg may be considered for surgery to prevent AR progression 1, 5
• Surgery may be considered with mean gradient ≥30 mm Hg in the presence of LV hypertrophy, planned pregnancy, or planned participation in strenuous/competitive sports 1, 5

Contraindication to Surgery (Class III)

• Surgery is not recommended for patients with trivial LVOT obstruction or trivial-to-mild AR 1

Medical Therapy

• No specific medical therapy exists for subaortic stenosis itself 1
• Endocarditis prophylaxis is indicated only when there is a prior history of endocarditis 1
• Patients with moderate-to-severe stenosis should be strongly counseled against competitive athletics and strenuous isometric exercise due to sudden cardiac death risk 1

Surgical Approach and Risks

Standard Technique

• Circumferential resection of the fibrous ring through a transaortic approach with resection of the muscular base along the left septal surface 5
• Routine septal myectomy appears to reduce recurrence risk 6, 5
• Fibromuscular or tunnel-type obstruction requires more aggressive septal resection 5
• Severe long-segment LVOT obstruction may require a Konno procedure with extensive patch augmentation 5
• Concomitant aortic valve repair should be performed when moderate or severe AR is present 5

Operative Risks

• Complete heart block occurs in 10-15% of cases 1, 5
• Other risks include injury to aortic or mitral valves and creation of iatrogenic VSD 5
• Recurrence occurs in approximately 20% over 10 years, particularly when initial resection was performed in childhood 1, 5, 2

Follow-Up Protocol

Surveillance Intervals by Physiological Stage

• Stage A/B (mild disease): ACHD cardiologist every 24 months, ECG and TTE every 24 months 1
• Stage C (moderate disease): ACHD cardiologist every 6-12 months, ECG and TTE every 12 months 1
• Stage D (severe disease): ACHD cardiologist every 3-6 months, ECG and TTE every 12 months 1

Postoperative Monitoring

• All patients require lifelong cardiology follow-up by an adult congenital heart disease specialist 5
• Postoperative surveillance must detect late restenosis, progressive AR despite resection, arrhythmias, and heart block 5
• Unoperated asymptomatic adults require yearly monitoring for increasing obstruction, development or progression of AR, and LV systolic/diastolic function 5