endocardial cushion defect

**Embryology/Pathophysiology:** Endocardial cushions (superior and inferior, plus lateral) form in the AV canal around weeks 4-5 and fuse to divide the common AV canal into the right (tricuspid) and left (mitral) orifices. They also contribute to the membranous interventricular septum, the lower atrial septum (septum primum), and the AV valve leaflets. Failure of fusion yields a spectrum of defects: - **Partial AVSD:** Ostium primum ASD ± cleft anterior mitral leaflet (mitral regurgitation). Two separate AV valve annuli. Ventricular septum is intact. - **Complete AVSD:** Primum ASD + inlet VSD + a single common AV valve (5 leaflets). Large L→R shunt at atrial and ventricular levels, with AV valve regurgitation. - **Transitional/intermediate:** Primum ASD + small restrictive inlet VSD.

**Associations:** Down syndrome (trisomy 21) is the classic association. About 40-50% of children with Down syndrome have CHD, and AVSD is the most common lesion in this group. Also seen in heterotaxy/asplenia syndromes and with some Ellis-van Creveld cases (common atrium).

**Clinical presentation:** - Complete AVSD: Tachypnea, poor feeding, failure to thrive, and heart failure at 2-6 weeks of life, as pulmonary vascular resistance falls and L→R shunt increases. Recurrent pulmonary infections. - Partial AVSD: May be asymptomatic until later childhood or adulthood, with a presentation similar to ASD. - Exam: Loud single S2 or fixed split S2, systolic murmur of AV valve regurgitation (holosystolic at apex), mid-diastolic flow rumble at the LLSB from increased flow across the tricuspid valve, and a hyperactive precordium.

**Diagnosis:** - Echocardiography is the test of choice (defines the defect, valve anatomy, and ventricular balance). - ECG: **Superior (left) axis deviation** (northwest axis) is characteristic, with a prolonged PR interval (first-degree AV block). - CXR: Cardiomegaly and increased pulmonary vascular markings.

**Management:** Medical stabilization of heart failure (diuretics, ACE inhibitors, digoxin, nutrition). Definitive surgical repair (patch closure of defects and valve reconstruction) is typically performed at about 3-6 months for complete AVSD, before irreversible pulmonary hypertension develops. Complications include residual AV valve regurgitation, left ventricular outflow tract obstruction, and heart block.

**Complications if untreated:** Pulmonary hypertension leads to **Eisenmenger syndrome** (shunt reversal to R→L with late cyanosis, clubbing, and polycythemia). Early development occurs in complete AVSD, especially in Down syndrome.

**High-yield pearls:** - Most common congenital heart defect in Down syndrome = AVSD (endocardial cushion defect). - Other Down syndrome-associated lesions: VSD, ASD, PDA. - Primum ASD (AVSD) is distinguished from secundum ASD (most common ASD overall, from a patent foramen ovale or deficient septum secundum). Secundum ASD has a right axis deviation and rSR' pattern, whereas primum ASD has left axis deviation. - Cyanosis is not initially present (acyanotic L→R shunt) until Eisenmenger physiology develops.