Trabecular (muscular) ventricular septal defect (VSD) is the second most common type of VSD, occurring in 5-20% of most series. Trabecular muscular VSDs are divided into separate distinct regional groups, including midmuscular, apical, anterior, and posterior.
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. (See Epidemiology.)
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). (See Prognosis.)
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.
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 one 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.)
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. (See Etiology.)
Complications of VSDs may include the following (see Clinical and Workup):
Congestive heart failure
Double-chambered right ventricle
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 CHF.