Muscular Ventricular Septal Defect

Trabecular (muscular) ventricular septal defect (VSD) is the second most common type of VSD, occurring in 15-20% of most series. ^[1] Trabecular muscular VSDs are divided into separate distinct regional groups, including midmuscular, inlet, apical, anterior, and posterior. ^[2] Midmuscular is the most common subtype of muscular VSD. Defects occurring centrally or along the margin of the interventricular septum and free wall are termed anterior VSDs.

Normal closure of the ventricular septum occurs through multiple concurrent embryologic mechanisms that help to close the membranous portion of the septum: (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.

The ventricular septum is complete by 6 weeks' gestation. VSDs occur when any portion of the ventricular septum does not correctly form or if any of components do not appropriately grow together. VSDs are typically classified according to the location of the defect in one of the four ventricular components: the inlet septum, trabecular septum, outlet/infundibular septum, or membranous septum. This article specifically addresses defects in the trabecular muscular septum.

When multiple muscular VSDs occur with a very large communication between the ventricles, it is also known as "Swiss cheese" VSD. Frequently, spontaneous closure of small muscular VSDs occurs in the first 2 years of life (usually by age 6 mo). ^{[3, 4]}

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.

Patient education

Advise the patient and/or his or her parents regarding the risks of bacterial endocarditis and the importance of oral hygiene. Educate them concerning signs and symptoms of congestive heart failure.

Independent of the type of ventricular septal defect (VSD), the hemodynamic significance of a 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) may predispose patients to earlier development of CHF. Noncardiac abnormalities, including prematurity, infection, anemia, or 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 trans-VSD flow, depending on the ultimate resistance balance between the systemic and the total right-sided resistances.

Muscular ventricular septal defects (VSDs) have a multifactorial etiology and are predominantly the result of spontaneous abnormalities in development. ^[5] No correlation with maternal age or birth order is observed.

VSD is the most common congenital heart lesion in most chromosomal anomalies and syndromes. VSD is especially common in patients with trisomy 13, trisomy 18, and trisomy 21. In addition, there are numerous single-gene deletion syndromes associated with VSDs. However, the majority of VSDs (>95%) are not associated with chromosomal abnormalities. ^[6]

Noncardiac conditions associated with VSD include prematurity, syndromes, and chromosomal anomalies. Regular maternal cannabis slightly increases the incidence of VSD, as does the use of selective serotonin reuptake inhibitors (SSRIs) during early pregnancy. ^{[7, 8]}

Ventricular septal defects (VSDs) are slightly more common in females than in males. Large muscular VSDs may not present until age 6-8 weeks, when decreased pulmonary vascular resistance (PVR) allows significant left-to-right shunting and the appearance of clinical signs and symptoms of congestive heart failure (CHF). Most muscular VSDs present clinically in the neonatal period. Typically, these defects, especially the smaller ones, are not suspected at birth and may not be identified by auscultation until the PVR begins to fall in the first few days to weeks of life.

The VSD may manifest soon after birth if it is associated with significant additional congenital heart lesions or if it occurs with an associated chromosomal anomaly or syndrome.

Occurrence in the United States

Without regard to type, VSD is the most common congenital heart defect in the first 3 decades of life, with an incidence between 1.5-4.2 cases for every 1000 live term infants. ^[3]  One study described an incidence of muscular VSD of 2.7 cases per 1000 live births. ^[9]

VSD is more common in premature infants, with an incidence of 4.5-7 cases for every 1000 live births. ^[3] 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% of congenital heart disease.

Muscular VSD is the second most common type of VSD, accounting for as many as 15-20% of VSD cases. ^[1]

International occurrence

The prevalence of VSD worldwide is relatively constant. However, the type of VSD that predominates in a region widely varies. In the United States, perimembranous VSDs are most common. In Asia, subaortic VSDs (outlet type) are most common. ^[10]

Children with small to moderate-sized ventricular septal defects (VSDs) have an excellent prognosis. Infants and children with large VSDs have a good overall prognosis. Optimal medical management with appropriate timing of surgical intervention in these patients has the best outcome. However, patients with multiple muscular VSDs have a more variable prognosis.

Muscular VSDs may spontaneously decrease in size and eventually close. Small muscular VSDs have the greatest likelihood of spontaneous closure, with closure rates approaching 80-90% by age 2 years. Muscular defects in these patients decrease in size due to growth of the ventricular myocardium, which fills in the defect. One study that used fetal echocardiography showed that 33% of all defects closed in utero, 44% of defects spontaneously closed within the first postnatal year, and 23% of defects did not close. ^[11]

Morbidity and mortality

Morbidity and mortality associated with VSDs are influenced by the number and size of the defects, the degree of left-to-right shunting, the presence of associated congenital heart defects, the presence of associated noncardiac defects and syndromes, and the patient's age at repair of the VSD.

For patients with large muscular VSDs, surgical repair is indicated at any time during the first year of life if the infant fails to grow appropriately despite optimal medical management. Surgical risk and mortality for patients with large VSDs are higher in the first 2 months of life (10-20%) than after age 6 months (1-2%), although these figures are currently decreasing. Elective surgical closure of large VSDs should be planned within the first year of life to prevent development of irreversible pulmonary vascular obstructive disease (ie, Eisenmenger syndrome).

Multiple muscular VSDs, also known as Swiss cheese ventricular septum, are significantly more complex. Patients often show early signs of congestive heart failure (CHF). This lesion may require staged surgical palliation.

Complications

Complications of VSDs may include the following:

• Congestive heart failure

• Bacterial endocarditis

• Eisenmenger syndrome

• Aortic insufficiency

• Subaortic stenosis

• Double-chambered right ventricle