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Congenital Heart Disease - Pathology (MBBS)

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Definition & Incidence

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Etiology & Pathogenesis

• Prematurity
• Family history (genetic)
• Maternal conditions: diabetes, hypertension, obesity, phenylketonuria, thyroid disorders, connective tissue disorders
• Maternal drugs: phenytoin, retinoic acid, alcohol, smoking
• Assisted reproductive technology (IVF)
• Genetic/chromosomal associations: Trisomy 21 (Down - AV septal defect, VSD), Trisomy 18, Trisomy 13, Turner syndrome (coarctation of aorta)
• In utero infections: rubella (classically - PDA, pulmonary artery stenosis)

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Classification (Hemodynamic Basis)

1. Left-to-Right Shunts (Acyanotic - initially)

2. Right-to-Left Shunts (Cyanotic - "blue babies")

3. Obstructive Lesions (no shunt)

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Group 1: Left-to-Right Shunts

• Increased pulmonary blood flow → pulmonary hypertension
• Right ventricular hypertrophy → right heart failure
• Eventually shunt reversal (right-to-left) → late-onset cyanosis = Eisenmenger syndrome (irreversible)

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Atrial Septal Defect (ASD)

• Secundum ASD (90%) - deficient septum secundum formation at the center of the atrial septum; not associated with other anomalies
• Primum ASD (5%) - adjacent to AV valves; associated with AV valve abnormalities and VSD
• Sinus venosus defect (5%) - near entrance of superior vena cava; may be associated with anomalous pulmonary venous return

• Usually asymptomatic until adulthood (most common CHD diagnosed in adults, since ASDs rarely close spontaneously)
• Left-to-right shunt → pulmonary flow 2-8x normal
• Murmur due to increased flow through pulmonary valve
• Well tolerated; irreversible pulmonary hypertension is unusual
• Risk of paradoxical embolization, atrial arrhythmias

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Ventricular Septal Defect (VSD)

• Most common CHD overall (42%)
• ~90% are in the membranous (perimembranous) portion of the interventricular septum
• Remaining are muscular VSDs
• Most small VSDs close spontaneously in childhood (this is why ASD - not VSD - is the most common defect found in adults)

• Small VSDs: asymptomatic; may produce a loud systolic murmur ("maladie de Roger")
• Large VSDs: left-to-right shunt → volume/pressure overload of pulmonary circulation
• Progresses faster to Eisenmenger syndrome than ASD (due to higher flow volumes and pressures)
• Small/medium VSDs: risk of infective endocarditis (jet lesion damages right ventricular endothelium)
• Early surgical correction is indicated for large VSDs

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Patent Ductus Arteriosus (PDA)

• The ductus arteriosus connects the left pulmonary artery to the aorta just distal to the left subclavian artery
• In fetal life: diverts blood from pulmonary artery to aorta (bypassing unoxygenated lungs)
• Normally closes within 1-2 days of birth in response to: ↑ arterial oxygenation, ↓ pulmonary vascular resistance, ↓ prostaglandin E2
• Becomes the ligamentum arteriosum after closure
• Delayed/absent closure occurs with hypoxia (respiratory distress syndrome, other heart disease)
• Accounts for ~7% of CHD; 90% are isolated defects

• Harsh "machinery-like" murmur (continuous, throughout systole and diastole)
• High-pressure left-to-right shunt
• Small PDA: often asymptomatic
• Large PDA: Eisenmenger syndrome, congestive heart failure
• Risk of infective endocarditis
• Treatment: Isolated PDAs should be closed early (pharmacologically with indomethacin in premature infants - blocks prostaglandin synthesis; surgical ligation in others)

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Group 2: Right-to-Left Shunts (Cyanotic CHD)

• Clubbing (hypertrophic osteoarthropathy)
• Polycythemia (erythrocytosis due to chronic hypoxemia)
• Paradoxical embolization (venous thrombus → systemic arterial circulation → stroke, etc.)

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Tetralogy of Fallot (TOF)

1. Ventricular septal defect (large, membranous)
2. Obstruction of right ventricular outflow (subpulmonic stenosis - variable severity)
3. Overriding of the aorta over the VSD
4. Right ventricular hypertrophy (secondary to obstruction)

• Heart is enlarged and "boot-shaped" (coeur en sabot) on X-ray - due to RV hypertrophy
• Proximal aorta is dilated; pulmonary trunk is hypoplastic
• RV wall markedly hypertrophied (may exceed LV thickness)
• Large VSD near the membranous portion of the interventricular septum

• Cyanosis from birth (if severe pulmonary stenosis) or after infancy
• Severity of symptoms depends on degree of right ventricular outflow obstruction
• If pulmonary stenosis is mild → may not be cyanotic initially ("pink tet")
• "Tet spells" (hypercyanotic spells): paroxysmal episodes of severe cyanosis; child characteristically squats to relieve symptoms (squatting ↑ systemic vascular resistance → reduces right-to-left shunt)
• Polycythemia, clubbing
• Right-sided aortic arch in ~25% of cases
• Pulmonary atresia may occur as the extreme form
• Prognosis without surgery: poor; most die in childhood
• Surgical repair is possible and yields good long-term outcomes

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Transposition of the Great Arteries (TGA)

• Aorta arises from the right ventricle and the pulmonary artery arises from the left ventricle - complete reversal
• Results from failure of the aortopulmonary septum to spiral during development
• Creates two separate, parallel circuits (systemic and pulmonary) that are not interconnected - incompatible with life unless a communication exists
• Survival depends on a mixing defect - PFO, ASD, VSD, or PDA must be present

• Severe cyanosis from birth
• Right ventricle becomes the systemic ventricle → right ventricular hypertrophy
• Without intervention, most infants die within months
• Balloon atrial septostomy (Rashkind procedure) is performed emergently to create/enlarge ASD
• Definitive: arterial switch operation (Jatene procedure) in the neonatal period

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Group 3: Obstructive Lesions

Coarctation of the Aorta

• Narrowing (stenosis) of the aorta, most often near the ligamentum arteriosum (juxtaductal position)
• Association: Turner syndrome (45,X)
• Two classic forms:

• Hypertension in upper extremities; hypotension in lower extremities
• Weak/absent femoral pulses
• Intercostal "notching" on chest X-ray - due to collateral circulation through intercostal arteries (dilated and tortuous)
• "3 sign" on chest X-ray (indentation of aorta at coarctation site)
• Headache, epistaxis (hypertension in upper body)
• Claudication in lower limbs
• Risk of: aortic rupture, infective endocarditis, intracranial hemorrhage (from berry aneurysms - associated with bicuspid aortic valve)

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Eisenmenger Syndrome

• Chronic high-flow/high-pressure pulmonary circulation → irreversible pulmonary hypertension → pulmonary vascular resistance exceeds systemic vascular resistance → shunt reversal (right-to-left) → late cyanosis
• Once established, the structural defects are no longer surgically correctable
• This is the main reason for early intervention in large left-to-right shunts

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Summary Table - Key Exam Points

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High-Yield Associations for Exams

• VSD = most common CHD overall
• ASD = most common CHD in adults
• TOF = most common cyanotic CHD; "boot-shaped heart"
• TGA = most common cyanotic CHD presenting on day 1 of life (severe cyanosis at birth)
• Coarctation = associated with Turner syndrome and bicuspid aortic valve
• Rubella → PDA, pulmonary artery stenosis
• Down syndrome (Trisomy 21) → AV septal defect, VSD
• PDA closure → indomethacin (premature infants); kept open with prostaglandins (PGE1) in duct-dependent lesions like TGA/pulmonary atresia
• Eisenmenger syndrome = irreversible pulmonary hypertension from uncorrected L→R shunt → R→L shunt → late cyanosis

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