Congenital heart block (CHB) is a condition present at birth where the electrical signals that control the heartbeat are partially or completely blocked as they travel from the atria (upper chambers of the heart) to the ventricles (lower chambers). This disruption in electrical conduction causes the heart to beat abnormally slowly or irregularly, which can affect the heart’s ability to pump blood effectively. Understanding what causes congenital heart block involves exploring a combination of genetic, immunological, and developmental factors that interfere with the normal formation and function of the heart’s electrical system during fetal development.
At the core of congenital heart block is a problem with the atrioventricular (AV) node or the conduction pathways that connect the atria and ventricles. The AV node acts like a gatekeeper, regulating the electrical impulses that pass from the atria to the ventricles. When this node or the surrounding conduction tissue is damaged or malformed, the impulses are delayed or completely blocked, leading to heart block.
One major cause of congenital heart block is related to **maternal autoimmune diseases**, particularly when the mother has antibodies against specific proteins called SSA/Ro and SSB/La. These antibodies can cross the placenta during pregnancy and attack the fetal heart tissue, especially the conduction system. This immune-mediated injury leads to inflammation and scarring (fibrosis) of the AV node and surrounding conduction tissue, disrupting the electrical signals. This form of congenital heart block is often called **neonatal lupus-associated heart block** because it is linked to maternal lupus or similar autoimmune conditions. It typically develops between the 16th and 24th weeks of gestation and can be detected by fetal echocardiography. The damage caused by these antibodies is usually irreversible, which means the heart block is permanent and may require pacemaker implantation after birth.
Besides autoimmune causes, **genetic factors** play a significant role in congenital heart block. The heart’s development is a highly complex process controlled by numerous genes that regulate the formation of cardiac structures and the conduction system. Mutations or disruptions in these genes can lead to structural heart defects and abnormalities in the electrical conduction pathways. For example, mutations affecting transcription factors and signaling molecules that guide heart development can result in malformations or functional impairments of the AV node or His-Purkinje system. These genetic disruptions may occur in isolation or as part of broader congenital heart defects, which sometimes accompany heart block.
In some cases, congenital heart block is associated with **structural heart defects** such as atrioventricular septal defects or malformations of the conduction system itself. These defects can physically interrupt the normal pathways through which electrical impulses travel, causing conduction delays or blocks. Structural abnormalities may arise from errors during the embryological development of the heart, where the intricate processes of cell migration, differentiation, and tissue remodeling are disturbed. Such disturbances can be due to genetic mutations, environmental factors, or a combination of both.
Environmental and maternal factors can also contribute to the risk of congenital heart block. Exposure to certain infections, medications, or toxins during pregnancy may interfere with fetal heart development or trigger immune responses that damage the conduction system. However, these causes are less common compared to autoimmune and genetic origins.
The electrical conduction system of the heart relies on specialized cardiac muscle cells that are excitable, meaning they can generate and propagate electrical impulses. These cells maintain a delicate balance of ions like sodium, potassium, and calcium across their membranes, which creates the electrical signals necessary for coordinated heartbeats. Any damage or malformation in these cells or their ion channels can impair conduction. In congenital heart block, the damage to the AV node or conduction fibers disrupts this ion exchange and electrical propagation, leading to slowed or blocked impulses.
In summary, congenital heart block arises primarily from immune-mediated injury due to maternal autoantibodies
