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Ebstein's Anomaly of the Tricuspid Valve: A Wolf(f) in Sheep's Clothing

Ebstein’s anomaly of the tricuspid valve is an uncommon congenital heart defect, comprising <1% of individuals having congenital heart disease. The extent of structural malformation is variable among affected individuals, and there may be associated defects, especially atrial septal defect and right ventricular outflow tract obstruction. Typically, the hemodynamic alterations result from a combination of right ventricular dysfunction, tricuspid regurgitation, and possible rightto-left atrial-level shunting. The gross morphogenesis of Ebstein’s anomaly involves failure of normal delamination of the superficial layer of right ventricular myocardium to form the septal and posterior (mural) leaflets of the tricuspid valve. The annulus fibrosis remains at its usual level, but the hinge-points of the affected leaflets are displaced apically into the right ventricle. Hence, there is some distance between the annulus and leaflet coaptation plane, giving rise to the “atrialized portion” of the right ventricle. Histologically, this portion of the right ventricle shows fibrosis and decreased numbers of myocytes.1,2 Insufficient apoptosis during right ventricular development is the currently favored mechanism to explain Ebstein’s anomaly. The specialized conduction system is primarily or secondarily affected in most Ebstein patients. Histologically, the atrioventricular (AV) node is compressed, the central fibrous body is malformed, and there is fibrosis within the right bundle branch.3 Surprisingly, first degree AV block is only present in 15–35% of patients,4 and is equally related to PR segment prolongation as to P-wave lengthening.5 Even among patients not having preexcitation, the HV interval is usually not prolonged, at least in younger patients. Not surprisingly, right ventricular conduction delay or right bundle branch block is present in 75–80%, and the QRS morphology is either trior polyphasic in V1.4 There may be discontinuity of the central fibrous body and septal atrioventricular annular ring with direct muscular connections between the atrium and ventricle, especially in the region of the septal and posterior (mural) leaflets.6 This accounts for the preponderance of accessory AV connections associated with the inferior half of the tricuspid valve and the tendency toward multiple accessory pathways.7,8 Ebstein patients are of interest to the electrophysiologist due to clinically important tachycardias. Indeed, the pathology of Ebstein’s anomly is the “perfect storm” for reentry: AV