Tetralogy of Fallot

Definition
Consists of 4 congenital heart defects:
A- subpulmonary stenosis
B- overriding aorta (dextroposition of the aorta such that it is positioned towards the right side or middle of the heart, which receives blood flow from both ventricles)
C- ventricular septal defect
D- right ventricle hypertrophy

Epidemiology
most common cyanotic congenital heart disease after 1 year of age
10% of all congenital heart disease
occurs slightly more often in males than in females
if an atrial septal defect is also present (10%) - it is called pentalogy of Fallot

Aetiology
associated with 22q11 chromosomal microdeletion, thymic hypoplasia (diGeorge Syndrome)
Specific genetic association with JAG1, NKX2-5, ZFPM2, VEGF
Embryology studies- amterior malalignment of the articopulmonary septum

Pathophysiology

Condition	Description
A:Pulmonary stenosis	A narrowing of the right ventricular outflow tract and can occur at the pulmonary valve (valvular stenosis) or just below the pulmonary valve (infundibular stenosis). Infundibular pulmonic stenosis is mostly caused by overgrowth of the heart muscle wall (hypertrophy of the septoparietal trabeculae),however the events leading to the formation of the overriding aorta are also believed to be a cause. The pulmonic stenosis is the major cause of the malformations, with the other associated malformations acting as compensatory mechanisms to the pulmonic stenosis.[ The degree of stenosis varies between individuals with TOF, and is the primary determinant of symptoms and severity.
B:Overriding aorta	An aortic valve with biventricular connection, that is, it is situated above the ventricular septal defect and connected to both the right and the left ventricle. The degree to which the aorta is attached to the right ventricle is referred to as its degree of "override." The aortic root can be displaced toward the front (anteriorly) or directly above the septal defect, but it is always abnormally located to the right of the root of the pulmonary artery. The degree of override is quite variable, with 5-95% of the valve being connected to the right ventricle.
C:ventricular septal defect(VSD)	A hole between the two bottom chambers (ventricles) of the heart. The defect is centered around the most superior aspect of the ventricular septum (the outlet septum), and in the majority of cases is single and large. In some cases thickening of the septum (septal hypertrophy) can narrow the margins of the defect.
D: Right ventricular hypertrophy	The right ventricle is more muscular than normal, causing a characteristic boot-shaped (coeur-en-sabot) appearance as seen by chest X-ray. Due to the misarrangement of the external ventricular septum, the right ventricular wall increases in size to deal with the increased obstruction to the right outflow tract. This feature is now generally agreed to be a secondary anomaly, as the level of hypertrophy generally increases with age.

Presentation
Harsh systolic crescendo-decrescendo murmur along the left sternal border
 - radiates to left upper sternal border
 - critical obstruction/atresia may exhibit little/no murmur
Cyanosis secondary to right-to-left shunting
CHF (dyspnoea, poor feeding/growth, profuse prespiration, recurent resp. infection)
Hypercynotic (Tet) spell - intense cyanosis & hyperpnea due to decreased pulmonary blood flow, resulting in a greater right-to-left shunt to the aorta
- commonest from 2-9 months
- children assume a squatting position

DDx
Cyanotic congenital heart defects with pulmonary outflow obstruction
Ebstein anomaly
Eisenmonger syndrome
Tranposition of the great arteries

Investigation
Clinical examination
ECG - R-axis deviation & RVH
CXR- clasic boot-shaped cardiac sillhouette with a prominent right ventricle & concavity of the right ventricular outflow tract & main pulmonary artery, a R-sided aortic arch & large ascending aorta
Echo+doppler
Cardiac MRI
Cardiac catheterisation - define peripheral pulmonary artery size, coronary artery distribution, magnitude of the shunt

Management

In severe obstruction-	start prostaglandin E1 in the neonatal period- to maintain PDA
Corrective surgery	before 1 year of age unless significant hypoplastic pulmonary arteries If the pulmonary arteries are too hypoplastic to allow primary repair perform palliation (modified Blalock-Taussig shunt followed by more definitive repair within 6-12 months) In patients who are candidates for complete surgical repair, palliative surgery can be performed- to increase pulmonary blood flow and decrease cyanosis. Potts procedure: anastomosis of descending aorta to the left pulmonary artery Blalock-Taussig: anastomosis of subclavian to the pulmonary artery
VSD	Transvemtricular or transatrial repair of the ventriclular septal defect
“Tet” spells	Treat with supplemental oxygen and morphine, and place the child in a knee-to-chest position
Follow up	Monitor with Echo or MRI for evidence of heart failure & progression of pulmonary regurgitation

Prognosis
Majority survive intervention with good long-term cardiac function
Pt with significant pulmonary artery hypoplasia- poorer long-term right ventricular function
Pulmonary regurg- common following repair, but well tolerated, some pt may require pulmonary valve replacement
Pt with Fallot tetralogy - increased risk of endocarditis, cerebral  abscess, stroke