Tetralogy of Fallot

 

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Tetralogy of Fallot (TOF) is a congenital heart defect  which is classically understood to involve four anatomical abnormalities of the heart (although only three of them are always present). It is the most common cyanotic  heart defect, and the most common cause of blue baby syndrome.  This is the most common cyanotic congenital cardiac abnormality in children who survive beyond the neonatal period. It comprises 5-10% of all congenital heart disease. It consists of four components:

  1. Pulmonary stenosis.
  2. Ventricular septal defect.
  3. Right ventricular hypertrophy.
  4. Overriding of aorta.

Cause: Its cause is thought to be due to environmental or genetic factors or a combination. It is associated with chromosome 22 deletions and DiGeorge syndrome.

Specific genetic associations include:

It occurs slightly more often in males than in females.Embryology   studies show that it is a result of anterior malalignment of the aorticopulmonary septum.resulting in the clinical combination of a VSD, pulmonary stenosis, and an overriding aorta. Right ventricular hypertrophy develops progressively from resistance to blood flow through the right ventricular outflow tract.

Clinical features:

Symptoms:

  1. Progressive increasing cyanosis from birth (cyanosis may not be present in the new-born).
  2. Exertional dyspnoea.
  3. Characteristic squatting position after exertion.
  4. Easy fatigability.
  5. Fallot’s spells: Attack of deep cyanosis, apnoea & unconsciousness often after feeding or a crying attack.
  6. Growth may be impaired.

Signs:

  1. Central cyanosis commonly present from birth.
  2. Growth retardation.
  3. Clubbing of finger & polycythemia are obvious after 1 year.
  4. Cardiac findings (of pulmonary stenosis)
  • A loud ejection systolic murmur over pulmonary area.
  • A parasternal heave.
  • A thrill in 2nd left parasternal space close to sternum.

Pathophysiology:

Primary four malformations

Tetralogy denotes a four-part phenomenon in various fields, including literature,  and the four parts the syndrome’s name implies are its four  signs. This is not to be confused with the similarly named teratology , a field of medicine concerned with abnormal development and congenital malformations, which thereby includes tetralogy of Fallot as part of its subject matter.

As such, by definition, tetralogy of Fallot involves four  heart  malformations which present together:

Condition Description
A: Pulmonary Infundibular Stenosis A narrowing of the right ventricular outflow tract. It 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. This malformation is infrequently described as sub-pulmonary stenosis or subpulmonary obstruction.
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 tends to increase with age.

 

There is anatomic variation between the hearts of individuals with tetralogy of Fallot. Primarily, the degree of right ventricular outflow tract obstruction varies between patients and generally determines clinical symptoms and disease progression.

Presumably, this arises from an unequal growth of the aorticopulmonary septum. The aorta is too large, thus “overriding,” and this “steals” from the pulmonary artery, which is therefore stenosed. This then prevents ventricular wall closure, therefore VSD, and this increases the pressures on the right side, and so the R ventricle becomes bigger to handle the work.

Additional anomalies

In addition, tetralogy of Fallot may present with other anatomical anomalies, including:

  1. Stenosis of the left pulmonary artery, in 40% of patients
  2. A  bicuspid pulmonary valve, in 60% of patients
  3. Right-sided aortic arch, in 25% of patients
  4. Coronary artery  anomalies, in 10% of patients
  5. A patent foramen ovale  or atrial septal defect,  in which case the syndrome is sometimes called a pentalogy of Fallot
  6. An atrioventricular septal defect.
  7. Partially or totally  anomalous pulmonary venous return.
  8. Forked ribs and scoliosis.

Tetralogy of Fallot with pulmonary atresia (pseudotruncus arteriosus) is a severe variant in which there is complete obstruction (atresia) of the right ventricular outflow tract, causing an absence of the pulmonary trunk during embryonic development. In these individuals, blood shunts completely from the right ventricle to the left where it is pumped only through the aorta. The lungs are perfused via extensive collaterals from the systemic arteries, and sometimes also via the ductus arteriosus.

Diagnosis: Congenital heart defects are now diagnosed with echocardiography, which is quick, involves no radiation, is very specific, and can be done prenatally.

Before more sophisticated techniques became available, chest x-ray was the definitive method of diagnosis. The abnormal “coeur-en-sabot”  (boot-like) appearance of a heart with tetralogy of Fallot is classically visible via chest x-ray, although most infants with tetralogy may not show this finding. Absence of interstitial lung markings secondary to pulmonary oligaemia are another classic finding in tetralogy.

Investigations:

  1. X-ray chest shows:
  • Large right ventricle (Boot shaped heart).
  • A small pulmonary artery.
  • Pulmonary field is oligamic.
  1. ECG: Right ventricular hypertrophy (RVH).
  2. Echo: Demonstrate discontinuity between the aorta and the anterior wall of ventricular septum.
  3. Cardiac catheterization: to evaluate the size & degree of the right ventricular outflow obstruction.

Management:

[A] Medical management:

  1. Treatment of severe cyanotic attacks.
  2. Suppression of Fallot’s spell:
  • Knee-elbow position.
  • Oxygen inhalation.
  1. Adequate intake of iron & avoidance of anaemia.
  2. Avoidance of dehydration.
  3. Injection Propranolol or Morphine (for relaxation of right ventricular outflow obstruction).

[B] Surgical management:

  1. Definitive management: is total correction of the defect by surgical relief of pulmonary stenosis and closure of VSD.
  2. Palliative management: If the child is too small to frial for definitive correction, a block-taussing shunt can be performed by anastomosing one of subclavian artery to a pulmonary artery.

This improves pulmonary blood flow & does not preclude definitive correction at a layer stage.

Prognosis: Untreated, tetralogy of Fallot rapidly results in progressive right ventricular hypertrophy due to the increased resistance on the right ventricle. This progresses to heart failure (dilated cardiomyopathy) which begins in the right heart and often leads to left heart failure. Actuarial survival for untreated tetralogy of Fallot is approximately 75% after the first year of life, 60% by four years, 30% by ten years, and 5% by forty years.

Patients who have undergone total surgical repair of tetralogy of Fallot have improved hemodynamics and often have good to excellent cardiac function after the operation with some to no exercise intolerance (New York Heart Association Class I-II). Surgical success and long-term outcome greatly depend on the particular anatomy of the patient and the surgeon’s skill and experience with this type of repair.

Ninety percent of patients with total repair as infants develop a progressively leaky pulmonary valve later in adulthood. They require, therefore, follow up in specialized Adult Congenital Heart Disease centres. 55% are present.

Complications:

  1. Blue spell (Cyanotic spell).
  2. Cerebral abscess.
  3. CVD due to polycythemia.
  4. Infective endocarditis.
  5. Right heart failure.

Reference:

  1. Davidson’s Principles and practice of Medicine, 21st
  2. Wikipedia the free encyclopedia.

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