*Department Obstetrics Gynecology, Rambam Medical Center, Haifa, Israel; **Femicare, Center of prenatal ultrasonographic diagnostics, Kollarova 17/A, 036 01 Martin, Slovak Republic; ***Centro Médico Recoletas. Valladolid. Spain
Posting Dates: May 1 - May 14, 2022
Presented here are 4 different cases. What is the cardiac defect in common in all these cases?
We present 4 different cases of Gerbode defect: The 1st is isolated and noted at 25 weeks gestation, the 2nd is associated with tricuspid dysplasia and identified at 15 weeks gestation, the 3rd is associated with ventricular septal defect and diagnosed at 15 weeks gestation, and finally the 4th case is associated with Ebstein's anomaly at 16 weeks of gestation.
The interventricular septum is a musculomembranous structure that separates the right ventricular chamber from the left ventricular chamber. It extends from the apex of the heart to unite with the atrial septum at the level of the atrioventricular valves (endocardial cushions). It is predominantly made up of a small thin membranous septum and a larger muscular septum. The muscular septal region can be subdivided into the inlet, the outlet, and the trabecular regions.
A ventricular septal defect (VSD) is defined as an opening or hole in the interventricular septum. Usually, it causes a communication between the right and the left ventricles, allowing blood to shunt between the two ventricles. Most ventricular septal defects occur in isolation, but up to 30% may accompany other structural cardiac abnormalities . Isolated defects are the most commonly recognized cardiac defects, accounting for 25% to 30% of all cardiac defects in live-born infants and only 5% to 7% of prenatally diagnosed congenital heart diseases .
Although several classification systems for ventricular septal defects have been proposed , typically are reported based on their anatomic locations on the septum: perimembranous VSD (also called infracristal or conoventricular), inlet VSD (posterior or atrioventricular septum type), outlet VSD (supracristal, subpulmonic, subarterial or doubly committed), and muscular or trabecular VSD. In contrast to the findings of postnatal studies, muscular defects are more common than perimembranous ventricular septal defects in the fetus .
The membranous septum is a very small area divided by the septal leaflet of the tricuspid valve such that a portion contacts with the right atrium (since the tricuspid valve attaches to the membranous septum about 1 cm apical to the attachment of the mitral valve), while the other portion is interventricular and is in continuity with all other components of the ventricular septum.
Perimembranous defects include defects of the membranous septum alone, but most involve more than just the membranous septum, and are therefore termed perimembranous. Van Praagh  prefers the term paramembranous to perimembranous because these defects are beside the membranous septum and confluent with it, not surrounding the membranous septum on all sides.
A defect not included in any of the more usual classifications (classic anatomic nomenclature, nomenclature system advocated by Robert Anderson, and Van Praagh’s nomenclature system) is the Gerbode defect. The International Paediatric and Congenital Cardiac Code (IPCCC) defines it as a deficiency of the atrioventricular component of the membranous septum permitting shunting of blood from the left ventricle to the right atrium . It does not present any other components that may be commonly seen in association with spectrum of disorders associated with atrioventricular septal defects such as a common atrioventricular valve or interatrial or interventricular communications .
In cases where a deficiency of tricuspid valvar tissue permits ventriculo-atrial shunting subsequent to initial interventricular shunting (left ventricle to right ventricle to right atrium) could be considered a ventricular septal defect .
Although the first pathologic description of this defect was made by Thurnam in 1838 in an autopsy report  and the first surgical correction of the same was performed by Kirby at the Hospital of the University of Pennsylvania in 1957 , it was not fully characterized until the publication of Gerbode et al in 1958 . In their description of five patients, four had a perimembranous ventricular septal defect with an associated deficiency of the septal leaflet of the tricuspid valve and the fifth patient exhibited a congenital deficiency of the atrioventricular component of the membranous septum (the true Gerbode defect). The congenital form of this very rare communication anomaly accounts for less than 1% of all congenital heart disease .
Gerbode et al already described initially two types of communication between the left ventricle and the right atrium . This defect has been classified in different ways over time . Riemenschneider and Moss classified the defects into two types: direct (or true, through the membranous septum) and indirect (or false, via a perimembranous ventricular septal defect with a defect in the tricuspid valve) . Silbiger et al modified the terminology of these two types of defects to describe the position of the same in relation to the tricuspid valve . Approximately one third of such defects occur in the atrioventricular septum and are known as supravalvular defects and the remaining two thirds occur between the ventricles and are known as infravalvular defects. Sakakibara and Konno further elaborated the classification to add to supravalvular (Type I) and infravalvular (Type II) types a third type that is the combination of both defects (valvular type, Type III) . These are demonstrated in the following drawings.
In recent years the acquired left ventricle-right atrium shunts are much more common than congenital because they have more predisposing risk factors, such as infective endocarditis, trauma, cardiac surgical procedures, and myocardial infarction . Because of this, the current incidences of the three types (I to III) are 76%, 16% and 8%, respectively . The most common congenital forms are infravalvular defects, which may have different tricuspid valve abnormalities, including clefts or perforations of the septal leaflet, deformity or adherence of valve tissue to the margins of the septal defect and widening of the anteroseptal commissure .
The left ventricular-to-right atrial shunt is of high velocity due to the large pressure gradient that exists between these cardiac chambers. Unlike other ventricular septal defects, Gerbode defect results in a special hemodynamic situation, since this communication can cause volume overload in the right atrium and, subsequently, in the right ventricle. The absence of tricuspid regurgitation distinguishes the true defect from the indirect forms. There is only one published case of Gerbode defect diagnosed prenatally , a septal defect between the left ventricle and the atrialized portion of the right ventricle in a fetus with Ebstein anomaly. Small defects can be managed conservatively, but most authors advise surgical repair by patch closure through the right atrium.
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