Background
Perimembranous ventricular septal defects (VSDs) are located in the left ventricle outflow tract beneath the aortic valve. They are the most common VSD subtype in the United States, occurring in 75-80% of cases. Defects may extend into adjacent portions of the ventricular septum. When tissue forms on the right ventricular septal surface (often thought to be tricuspid valvular in origin), it is termed an aneurysm of the membranous septum. Such tissue serves as a mechanism of restriction or spontaneous closure. The defect may be partially or completely occluded by the septal leaflet of the tricuspid valve. (See Epidemiology, Prognosis, and Treatment.)
Normal closure of the ventricular septum occurs through multiple concurrent embryologic mechanisms that help to close the septum’s membranous portion: (1) downward growth of the conotruncal ridges forming the outlet septum, (2) growth of the endocardial cushions forming the inlet septum, and (3) growth of the muscular septum forming the apical and midmuscular portions of the septum.
Ventricular septal defects (VSDs) occur when any portion of the ventricular septum does not correctly form or if any of components do not appropriately grow together. The ventricular septum is complete by 6 weeks' gestation. VSDs are typically classified according to the location of the defect in 1 of the 4 ventricular components: the inlet septum, trabecular septum, outlet/infundibular septum, or membranous septum. This article specifically addresses defects in the trabecular muscular septum. (See Etiology.)
Small VSDs (defined as VSD dimension less than half the size of the aortic annulus diameter) are usually isolated defects with otherwise normal cardiac anatomy and function. Large VSDs (defined as defect size equal to or greater than the diameter of the aortic annulus) typically have left heart dilatation and pulmonary artery hypertension with normal left ventricular systolic function. (See Workup.)
Perimembranous VSD is caused by failure of the endocardial cushions, the conotruncal ridges, and the muscular septum to fuse at a single point in space.
Hemodynamic effects of VSD
Independent of the type of ventricular septal defect (VSD), the hemodynamic significance of the VSD is determined by two factors: the size of the defect and the resistance to flow out of the right ventricle, including the pulmonary vascular resistance (PVR) and anatomic right ventricular outflow obstruction.
In small to moderate VSDs, left-to-right shunting is primarily limited by the size of the defect. Conversely, in large VSDs without right ventricular outflow obstruction, the left-to-right shunting is determined by the relative degree of PVR and systemic vascular resistance.
Because PVR is high at birth and does not reach its nadir until age 6-8 weeks, the development of significant left-to-right shunting and pulmonary overcirculation, often termed congestive heart failure (CHF), can be delayed until the second or third month of life. Additional cardiac lesions that increase left-to-right shunting (eg, atrial septal defect, patent ductus arteriosus, right heart obstructive lesions) may predispose patients to earlier development of CHF. Noncardiac abnormalities, including prematurity, infection, anemia, and other congenital anomalies, also may predispose infants to significant symptoms of heart failure.
Additional congenital heart lesions (eg, muscular right ventricular outflow tract obstruction, pulmonary valve stenosis, pulmonary venous obstruction, persistent elevation of PVR, mitral stenosis) can restrict shunting, possibly leading to right-to-left shunting at the VSD, depending on the ultimate resistance balance between the systemic and the total right-sided resistances.
Complications
Complications may include the following (see Prognosis):
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CHF
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Bacterial endocarditis primarily for restrictive defects
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Eisenmenger syndrome
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Aortic valve insufficiency if there is evidence of leaflet prolapse
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Subaortic stenosis
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Double-chambered right ventricle
Patient education
Advise the patient and/or his or her parents regarding the importance of good oral hygiene. Subacute bacterial endocarditis prophylaxis for unrepaired ventricular septal defects is not recommended. Educate them concerning signs and symptoms of CHF.
For patient education information, see the Heart Health Center, as well as Congestive Heart Failure and Ventricular Septal Defect.
Etiology
Perimembranous ventricular septal defects (VSDs) have a multifactorial etiology and are predominantly the result of spontaneous abnormalities in development. The precise etiology of muscular septal defect formation is unknown. However, the proposed mechanisms are many. Muscular defects may occur because of a lack of merging in the walls of the trabecular septum or because of excessive resorption of muscular tissue during ventricular growth and remodeling.
No significant correlation between the cause of VSDs and the age of the mother or the birth order of the child is observed.
VSDs are the most common congenital heart lesion associated with chromosomal anomalies and syndromes. VSDs are especially common in patients with trisomy 13, trisomy 18, and trisomy 21. However, nearly 95% of VSDs are not associated with chromosomal abnormalities. Noncardiac conditions associated with VSD include prematurity.
Regular maternal cannabis use slightly increases the incidence of VSD. [1] The use of selective serotonin reuptake inhibitors (SSRIs) during early pregnancy also slightly increases the incidence of VSD. [2]
Epidemiology
Occurrence in the United States
Without regard to type, ventricular septal defect (VSD) is the most common congenital heart defect, with an incidence between 1.5 and 4.2 cases for every 1000 live-term infants. VSD is more common in premature infants with an incidence of 4.5-7 cases for every 1000 liveborn infants.
Clinically significant VSD that requires medical or surgical management accounts for only 15% of such defects (0.35-0.50 cases for every 1000 live births). When viewing congenital heart disease in total, solitary VSD cases account for 20-40% of congenital heart disease. Perimembranous VSD is the most common type, accounting for as many as 50% of VSD cases identified in most surgical or autopsy series.
Race-, sex-, and age-related demographics
Inheritance patterns of different VSDs vary widely by race. Perimembranous VSD has no known race predilection. Defects located in a subpulmonary position, such as supracristal defects, are more common in the Asian population. VSDs are slightly more common in females than in males.
Most restrictive perimembranous VSDs present clinically in the neonatal period secondary to a murmur. These defects, especially the smaller defects, are not typically suspected at birth and may not be identified by auscultation until PVR begins to fall in the first few days to weeks of life. Large perimembranous VSDs may not present until patients are aged 6-8 weeks, when decreased PVR allows significant left-to-right shunting and clinical signs and symptoms of CHF. If the PVR does not decrease normally, then the clinical presentation may be delayed further since pulmonary overcirculation does not occur. VSDs may present soon after birth if they are associated with significant additional congenital heart lesions, or if they occur with an associated chromosomal anomaly or syndrome.
Prognosis
Children with small-to-moderate sized ventricular septal defects (VSDs) have an excellent prognosis; infants and children with large VSDs have a good prognosis. Optimal medical management, with appropriate timing of surgical intervention, has the best outcome.
Morbidity and mortality
Morbidity and mortality are influenced by the number and size of VSDs, the degree of left-to-right shunting, the presence of associated congenital heart defects, the presence of associated noncardiac defects and syndromes, and age at repair of VSD.
Perimembranous VSDs may spontaneously decrease in size and eventually close. (This often occurs with a small defect.) Closure rates as high as 50% have been reported in some series, but continued follow-up care is warranted until documented VSD closure occurs.
Although patients with a small VSD have an excellent prognosis, small perimembranous VSDs may lead to the development of aortic insufficiency.
For patients with moderate-sized VSD, defects may allow the development of voluminous left-to-right shunting in the first few months of life as PVR falls. Failure of medical management, with persistent evidence of CHF, is the primary indication for surgical closure of moderate-sized defects. Fewer than 25% of moderate-sized defects require surgical closure.
For patients with large VSDs, surgical repair is indicated at any time during the neonatal period if the infant fails to grow appropriately despite optimal medical management. Elective surgical closure of large VSDs should be planned as soon as feasible, especially if a genetic syndrome is present, to prevent development of irreversible pulmonary vascular obstructive disease (ie, Eisenmenger syndrome).
