Coarctation of the Aorta - Causes & Pathology

Definition

Types / Forms

1. Preductal ("Infantile") Form

• Narrowing is proximal to the ductus arteriosus (between the left subclavian artery and the ductus)
• Associated with a patent ductus arteriosus (PDA)
• Characterized by tubular hypoplasia of the aortic arch
• The PDA is the main source of (deoxygenated) blood delivered to the distal aorta
• Pulmonary trunk is dilated; right ventricle is hypertrophied (it now perfuses the lower body through the PDA)

2. Postductal ("Adult") Form

• A discrete ridge-like infolding of the aorta adjacent to the closed ductus arteriosus (ligamentum arteriosum)
• More common form
• The constricted segment is made up of smooth muscle and elastic fibers from the aortic media
• Proximal to the coarctation: aortic arch and branch vessels are dilated
• Left ventricle is hypertrophied from chronic pressure overload

Note: In ~90% of cases, the coarctation is directly opposite the ductus arteriosus (juxtaductal position).

Fig. Coarctation of the aorta - left shows preductal form with PDA; right shows postductal form without PDA (Robbins, Cotran & Kumar Pathologic Basis of Disease)

Embryological Causes (Three Main Theories)

1. Ductal muscle theory: During formation of the aortic arch, smooth muscle tissue from the ductus arteriosus is incorporated into the aortic wall. When the ductus constricts at birth, the ductal muscle within the aorta also constricts, producing the coarctation.
2. Abnormal involution theory: Abnormal involution (regression) of a small segment of the left dorsal aorta occurs. This stenotic segment later migrates cranially with the left subclavian artery, forming the coarctation.
3. Aortic isthmus theory: During fetal life, the segment of the aortic arch between the left subclavian artery and the ductus arteriosus is normally narrow because it carries very little blood (most flow is via the ductus). After birth, when the ductus closes, this narrow isthmus should enlarge. If the narrowing persists, it becomes a coarctation.

Genetic / Syndromic Causes

Pathophysiology / Hemodynamic Consequences

Clinical Features Summary

• Preductal (infantile): Presents at birth or early neonatal period with lower-body cyanosis and cardiac failure. Critical - most die without intervention. Prostaglandin E2 given to keep PDA open.
• Postductal (adult): Often asymptomatic until adulthood. Classic findings:
  • Upper extremity hypertension
  • Weak/absent femoral pulses
  • Radiofemoral delay
  • Rib notching on CXR
  • Systolic murmur (+ sometimes thrill)

• Robbins, Cotran & Kumar - Pathologic Basis of Disease, Ch. 12 (Cardiovascular)
• Robbins & Kumar Basic Pathology, Ch. 9
• The Developing Human - Clinically Oriented Embryology, Ch. 13

Diagnostic Findings and Imaging in Coarctation of the Aorta

1. Clinical Diagnostic Findings

Blood Pressure - The Key Clinical Clue

• Systolic BP gradient ≥20 mmHg between the right arm and lower limb indicates significant coarctation
• Upper extremity hypertension with weak/absent femoral pulses and lower extremity hypotension
• Brachial-femoral delay - femoral pulse arrives later than the brachial pulse
• Note: Left arm BP may be misleadingly low if the left subclavian artery was involved in or sacrificed during repair

Physical Examination Signs

Neonatal/Infant Presentation

• Shock, severe metabolic acidosis, oliguria, diminished distal pulses after ductal closure
• Irritability, tachypnea, poor feeding
• Differential cyanosis (preductal form): lower body cyanosis, upper body pink

2. ECG Findings

• Left ventricular hypertrophy (LVH) - due to chronic pressure overload (postductal form)
• Left atrial enlargement
• LV strain pattern - in neonates with severe obstruction
• RV hypertrophy - in preductal form (RV perfuses lower body via PDA)

3. Chest X-Ray (CXR) Findings

"Figure 3" Sign (most specific CXR sign)

• Formed by the silhouette of the dilated aortic arch/left subclavian artery (pre-stenotic dilatation) above and the post-stenotic dilatation of the descending aorta below, with the coarctation notch in between
• Creates the shape of the number "3" on the left mediastinal border

Rib Notching (Roesler's sign)

• Most characteristic plain film finding in adults
• Pressure erosion of the inferior surfaces of ribs 3-9 (usually bilateral but asymmetric)
• Caused by enlarged, tortuous intercostal collateral arteries
• Ribs 1-2 are spared (intercostals here arise from the costocervical trunk, proximal to the coarctation)
• Takes several years to develop - absent in infants and young children
• Can be unilateral if one subclavian artery arises distal to the coarctation

Other CXR Features

• Cardiomegaly (especially in neonates and older adults)
• Pulmonary venous congestion (in neonates with heart failure)
• Prominent ascending aorta (especially with associated bicuspid aortic valve)
• Poorly visible aortic knuckle
• Enlarged internal mammary artery shadow behind sternum (lateral view)

Fig. (A) CXR showing rib notching (red arrows) at 3rd-5th ribs and the "figure 3" mediastinal silhouette (blue dashed line, yellow asterisks). (B) CMR contrast-enhanced angiography confirming coarctation with collaterals - Braunwald's Heart Disease, 15th Ed.

Fig. CXR showing bilateral rib notching - Grainger & Allison's Diagnostic Radiology

4. Echocardiography

• Usually the primary confirmatory modality, especially in neonates and infants
• 2D echo: Demonstrates narrowing of the aorta at the coarctation site
• Color Doppler: Turbulent flow in the descending aorta at the coarctation site; aliasing jet
• Continuous-wave (CW) Doppler: Elevated peak velocity across coarctation (>3.5 m/sec suggests significant obstruction); diastolic tail - flow persists throughout diastole, indicating severe coarctation
• Suprasternal notch view: Shows the double envelope (proximal aortic flow + high-velocity coarctation jet)
• Modified Bernoulli equation applied to peak velocity estimates pressure gradient
• Also assesses: LV function, LV mass/hypertrophy, bicuspid aortic valve, other associated defects
• Limitation: Can be technically difficult in older children and adults (poor acoustic window)

Fig. (A) Color Doppler showing turbulent flow + CW Doppler with diastolic tail; (B) CMR showing complex coarctation with hypoplastic arch; (C) CT 3D showing coarctation with large collaterals - Braunwald's Heart Disease

5. MRI / Cardiac MRI (CMR) - Modality of Choice

What MRI Provides:

• Morphological: T1-weighted spin-echo shows the whole aorta, major branches, and larger collateral vessels
• Functional: Cine phase-contrast imaging visualizes turbulence through the stenosis
• Gradient quantification: Phase-contrast MRI quantifies the pressure gradient across the coarctation
• Collateral flow assessment: Measures flow in proximal vs. descending aorta; the increment = collateral contribution
• 3D gadolinium-enhanced MRA: Provides high anatomical detail - shows length of narrowing, arch involvement, post-stenotic dilatation
• LV assessment: LV mass (indirect marker of increased afterload), function, and volumes
• Aortic root: Detects dilatation associated with bicuspid valve, aortic regurgitation/stenosis

Key Imaging Goals on CMR (Grainger & Allison):

1. Location and degree of stenosis + length of coarctation segment
2. Aortic root involvement and post-stenotic dilatation
3. Aortic valve assessment (often bicuspid)
4. Delineation of collateral vessels
5. Head and neck vessel anatomy
6. Ventricular function, volume, and LV mass
7. Post-repair: re-coarctation or pseudoaneurysm

6. CT Angiography (CTA)

• Also provides excellent anatomical detail of the coarctation, arch, and collaterals
• Used sparingly in children due to radiation dose
• Strongest role: Follow-up of stent-graft placement - CT better than MRI when metal artefact from stent hampers MR visualisation (assesses in-stent restenosis, stent fracture)
• Detects complications: restenosis, aneurysm, collateral supply
• The role of CT vs. MRI is still debated; MRI preferred when no stent is present

7. Cardiac Catheterization / Angiography

• Historically the gold standard but now rarely needed for diagnosis
• Current role: Reserved for intervention (balloon angioplasty, stent placement) or investigation of associated cardiac abnormalities
• Can cross the coarctation from the femoral artery (or brachial artery if femoral approach fails)
• Multiple angiographic views needed due to asymmetry of stenosis
• Invasive peak-to-peak gradient ≥20 mmHg = threshold for intervention in adults

8. Fetal Diagnosis

• Coarctation can be suspected on fetal echocardiography (disproportion between left and right heart structures), though it can be difficult to confirm prenatally as the aortic isthmus normally carries reduced flow in fetal life

Summary Table: Investigations in CoA

• Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine, 12th Ed., Ch. 82
• Grainger & Allison's Diagnostic Radiology, Ch. 17
• Mulholland and Greenfield's Surgery, Ch. 81
• Robbins, Cotran & Kumar - Pathologic Basis of Disease, Ch. 12

Coarctation of the Aorta - Clinical Imaging Gallery

Image 1: CXR - Rib Notching (Subtle)

• Grainger & Allison's Diagnostic Radiology

Image 2: CXR + "Figure 3" Sign + MRI Angiography

• Rib notching (red arrows) at ribs 3-5 bilaterally
• "Figure 3" sign (blue dashed outline, yellow asterisks) = pre-stenotic dilatation of distal arch/dilated left subclavian artery + post-stenotic dilatation of descending aorta

• Braunwald's Heart Disease, 15th Ed.

Image 3: CXR - Bilateral Rib Notching (Pronounced)

• Bilateral rib notching on the inferior margins of multiple ribs
• Poorly visible aortic knuckle (the coarctation flattens the normal aortic prominence)
• Grainger & Allison's Diagnostic Radiology

Image 4: Multi-Modal Panel - CXR + MRI + 3D CTA + Doppler Echo

• Grainger & Allison's Diagnostic Radiology

Image 5: Echocardiography - Suprasternal Arch View + Color Doppler

• (A) 2D image of the aortic arch - shows the arch anatomy
• (B) Color flow Doppler overlay - turbulent aliased flow (mosaic pattern of red/yellow/blue) at the coarctation site in the descending aorta
• Fuster and Hurst's The Heart, 15th Ed.

Summary of What to Look for on CXR