endocardial cushion

**Embryology** - Form around weeks 4–5 as the cardiac jelly in the AV canal expands. Superior and inferior (dorsal/ventral) cushions appear first, with lateral cushions forming later. - Formed by endothelial-to-mesenchymal transition (EMT). Endocardial cells migrate into the cardiac jelly and become mesenchyme. Signaling involves TGF-beta, BMP, and Notch. - The superior and inferior cushions grow toward each other and fuse. This divides the common AV canal into the right AV canal (tricuspid) and the left AV canal (mitral).

**Derivatives and contributions** 1. Septum intermedium, which separates the right and left AV orifices. 2. Lower portion of the atrial septum: the cushions fuse with the septum primum and close the ostium primum. 3. Membranous part of the interventricular septum: cushion tissue joins the muscular IV septum and the aorticopulmonary (conotruncal) septum. 4. AV valve leaflets (tricuspid and mitral), together with the myocardium. - Outflow tract (conotruncal) cushions are separate structures. Neural crest cells migrate into them and form the aorticopulmonary septum. Defects here cause persistent truncus arteriosus, transposition of the great vessels, and tetralogy of Fallot.

**Pathology: endocardial cushion defect (AV septal defect, AV canal defect)** - Results from failure of the superior and inferior cushions to fuse. - **Partial AVSD:** ostium primum ASD, often with a cleft anterior mitral leaflet (mitral regurgitation). - **Complete AVSD:** ostium primum ASD + inlet VSD + a single common AV valve. This produces a large left-to-right shunt, pulmonary overcirculation, and early heart failure. Without repair it leads to pulmonary hypertension and Eisenmenger syndrome (reversal to right-to-left shunt, cyanosis, clubbing, late cyanosis). - **Association:** Down syndrome (trisomy 21). About 40–50% of children with Down syndrome have congenital heart disease, and AVSD is the most characteristic lesion. It is also seen with heterotaxy syndromes.

**Clinical features** - Neonatal or infant presentation with poor feeding, tachypnea, failure to thrive, and recurrent respiratory infections. - Exam findings: a systolic murmur (VSD or AV valve regurgitation), a widely fixed split S2 (from the ASD), and a loud P2 if pulmonary hypertension has developed. - Echocardiography is diagnostic and shows loss of the normal offset of the AV valves (a common AV valve). - ECG shows left axis deviation (superior axis), a classic clue. It is atypical for an ASD, which usually has right axis deviation. - Chest x-ray shows cardiomegaly and increased pulmonary vascular markings.

**Management** - Medical therapy for heart failure (diuretics, etc.). - Surgical repair in infancy, usually within the first 3–6 months for complete AVSD, to prevent pulmonary vascular disease.

**High-yield pearls** - Endocardial cushion defect + Down syndrome is a classic board association. - Ostium primum ASD (cushion-related) vs. ostium secundum ASD (septum primum resorption or deficient septum secundum, the most common ASD type). - Think "cushion" when a question describes ASD + VSD + a common AV valve. - Also associated with other trisomies (e.g., trisomy 18 and 13).