Third-degree or complete atrioventricular block (CAVB) is seen either in the fetal life or any time after birth with complete atrioventricular dissociation and bradycardia and is called congenital heart block to differentiate it from acquired third-degree heart block. It can occur in the fetal life due to maternal disease or due to a congenital heart defect in the fetus and can manifest at any given time before or after birth. CAVB can occur in a structurally normal heart (isolated CAVB) or in association with congenital heart disease (complex CAVB with congenital heart defects). (See Etiology.)
More recently, it has been recognized that the association and prognosis of CAVB differ depending on whether the block is identified in the fetus, newborn, or older child.
Isolated CAVB occurs in the absence of other congenital heart defects. It usually is seen in association with certain autoimmune antibodies in the mother that cross the placenta and damage the atrioventricular (AV) node of the fetus. The mother can be completely asymptomatic in presence of these autoimmune antibodies or may have a diagnosis of an autoimmune disorder (eg, systemic lupus erythematosus, Sjögren syndrome). (See Etiology, Clinical, and Workup.)
Isolated CAVB can also occur due to myocarditis and rare hereditary conditions, such as storage disorders (eg, Hurler syndrome, Hunter syndrome). Often, no etiology is found for an isolated CAVB.
CAVB can also be seen with certain congenital heart defects, most often complex defects, such as heterotaxy with accompanying AV canal defects and L-transposition of the great arteries.
CAVB with structural heart disease is considered to be caused by failure of the AV conduction system to develop during heart development. This may be a result of increased distance between the AV node and the ventricular conduction tissues, as when associated with structural congenital heart disease or damage related to the passage of maternal autoantibodies.
Isolated CAVB has been described since 1901. In some cases, since the early 1970s, it has been associated with the presence of maternal connective-tissue disease.
Autoimmune CAVB is presumed to be caused by injury from the placental passage of maternal anti-Ro and anti-La (or related) antibodies, which are present in more than 90% of mothers during pregnancy or at the time of delivery. The mothers may or may not have a diagnosis of an autoimmune disease. Once these autoantibodies have developed, they can be detected in the mother lifelong, although the titers may vary. These autoantibodies damage the AV conduction tissue possibly by inflammation or direct ion channel interaction in the early stage and later by fibrosis. [1, 2, 3] Further studies should be undertaken to elucidate the molecular pathways involved in the development of CAVB. 
CAVB occurs in as many as 5% of children born to mothers with anti-Ro antibody, which can be seen with subclinical or clinical maternal lupus erythematosus, maternal Sjögren syndrome, or other maternal autoimmune diseases.  After birth, the children may present with varying degrees of heart block, including CAVB, cardiomyopathy, and other manifestations of neonatal lupus syndrome.  However, the majority of infants who are born to these mothers do not manifest AV block. Owing to the high incidence of anti-Ro/SSA and anti-p200 antibodies in female patients with connective tissue diseases, screening for congenital heart block-associated autoantibodies during pregnancy may be a strong consideration. 
Many times, no clear etiology is determined for isolated CAVB. Rarely, it can occur as a result of myocarditis, infiltrative disease, or cardiomyopathy. Hereditary diseases such as Hurler cardiomyopathy and Hunter cardiomyopathy may be associated with CAVB.
Complex CAVB is associated with congenital heart defects that have structural abnormality of the conduction system. These heart defects are usually complex, such as L-transposition of great arteries.
The majority of the congenital CAVB cases are autoimmune AV blocks.  Autoimmune AV block occurs in approximately 1 per 14,000-20,000 live births. However, because significant fetal loss is thought to result from this disease, the true incidence of the disease (per conception) may be significantly higher. Structural congenital heart block is also rare, but with a higher proportion of fetal loss. The prevalence of isolated CAVB may be slightly higher in females than in males.
The prognosis in isolated complete CAVB is relatively good but may be influenced by the patient's age at presentation. Patients presenting as fetuses or at birth have significantly higher morbidity and mortality rates than do patients presenting later in childhood. 
Fetal bradyarrhythmia associated with congenital heart defects has a poor prognosis.  Patients with L-transposition of the great arteries and other complex structural cardiac defects have a worse prognosis unless detected and treated early. 
According to a long-term follow-up study by Michaelsson and colleagues, adults with complete CAVB who did not receive pacing systems had a poorer prognosis than did those with pacing because of multiple complications related to their disease.  Therefore, in the adolescent who has not yet developed indications for pacing (an unusual case), recommendations for a pacing system should be considered, regardless of symptoms or underlying escape rate.
Morbidity and mortality
The fetal mortality rate of isolated CAVB may be as much as 30-50%. Patients who are diagnosed and treated in the neonatal period have a survival rate of 94%, and patients who are diagnosed and treated in childhood have a survival rate of 100%.
Risk factors for death in patients with isolated CAVB include fetal diagnosis, very low heart rate, low birth weight, premature gestation, male gender, hydrops fetalis, endocardial fibroelastosis, and diminished ventricular function.
Hydrops fetalis is the risk factor for patients with structural heart disease and CAVB. Fetal and newborn mortality rates in congenital heart block with structural heart disease remain high, even if effective pacing is used.
Long-term potential complications in all patients include development of ventricular dilatation and dysfunction. Children with autoimmune CAVB may also have frequent ectopy and primary or secondary long QT syndrome. Patients without a pacemaker may develop AV valve regurgitation, atrial rhythm disorders, thromboembolism, congestive failure, or sudden death.  Patients with a pacemaker may develop pacing system–related complications, including lead fracture, mal-sensing, and pacing system infections. Replacement of pacemakers is required at intervals and can lead to complications from the procedure. Congenital complete heart block is an increasingly recognized cause of fetal loss. (See Treatment.)
Parents who are at risk of having a child with CAVB must be informed that this disease is easily identifiable and relatively easily treated after birth. The stigma of pacing as a therapy associated with elderly persons should be avoided. Parents should recognize that their affected offspring are likely to receive and benefit from pacing therapy at some point during childhood but that pacemaker therapy is intentionally deferred until indications are present to preserve lifelong access for pacing systems.
Patients and their families should be instructed to avoid medications that can cause AV block (eg, calcium channel blockers, beta blockers).
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