An Unusual Mechanism of Closure of Muscular Ventricular Septal Defects

Ventricular septal defects (VSDs) are the most common congenital heart defects. Most of the small or moderate size (<6 mm) muscular VSDs close spontaneously within the first two years of life. The usual mechanism of spontaneous closure involves muscular tissue encroachment with superimposed fibrosis or primary fibrous tissue formation around the margins of the defect. We describe an unusual mechanism of spontaneous closure of a muscular VSD.


Introduction
Ventricular septal defects are the most common congenital heart defects (1) and are classi ed by location into muscular, membranous, supracristal, or inlet. Spontaneous closure of a VSD depends on multiple factors including site, size, and age of the patient. e mechanism of closure depends on the location of the VSD. We describe a case of an unusual mechanism of spontaneous closure of a muscular VSD.

Case Report
A two-month-old healthy female infant was referred to pediatric cardiology for evaluation of a heart murmur. Except for a grade 3/6 holosystolic murmur, the rest of the cardiac examination was unremarkable. An echocardiogram (ECHO) showed a 4 mm midmuscular VSD with a left-to-right shunt. e patient remained asymptomatic but did not follow up with cardiology for three years. Cardiac examination was unremarkable, and the murmur was no longer appreciated. A repeat ECHO revealed a 4 mm midmuscular VSD measured from the left ventricular (LV) side.
e color Doppler showed two laminar ows with opposing directions across the interventricular septum (IVS). ere was no left-to-right shunt communicating with the right ventricular (RV) cavity. Close observation of the color Doppler showed a continuous loop starting from the LV, across the VSD continuing through the RV trabeculations, and back to the LV without ever communicating with the RV cavity ( Figures 1 and 2). A cardiac magnetic resonance imaging (MRI) con rmed that postulation and showed the small muscular VSD tract covered by hypertrophied trabeculations on the RV side (Figures 3  and 4). e MRI also demonstrated normal biventricular systolic function and a pulmonary to the systemic blood ow (Q p : Q s ) ratio of 1 : 1.

Discussion
VSDs can be isolated or associated with other congenital heart defects [1]. e incidence of VSDs ranges between 1.5 and 4.2 cases for every 1000 live term infants [2]. ey are more common in premature infants, with an incidence of 4.5-7 cases for every 1000 live births. Membranous VSD is the most common type of VSD (60-70%). Muscular VSD is the second most common type, accounting for as many as 20-30% of cases identi ed in most surgical or autopsy series. e spontaneous closure of VSDs depends on the age/gender of the patient and the size and site of the defect [1]. Most of the VSDs close spontaneously within the rst two years of life [3,4]. Afterwards, the chance of spontaneous closure diminishes remarkably. Muscular and membranous VSDs with diameters less than 6 mm have the best chance of spontaneous closure [5,6]. Supracristal and inlet VSDs usually do not close spontaneously.
Several studies have discussed the di erent mechanisms of VSD closure. Membranous VSDs close by adherence of tricuspid valve lea ets, creating an aneurysm that closes the defect [1,7,8]. It has been suggested that the spontaneous closure of muscular VSDs could be due to muscular encroachment of the septal defect along with superimposed brosis or by brous tissue formation around the margins leading to apposition of the edges of the defect [9].
We describe a case of an unusual mechanism of spontaneous closure of a midmuscular VSD. e VSD in our patient appears to have closed from the RV side secondary to trabecular hypertrophy but remains open from the LV side.
is leads to an unusual ow pattern which could initially be misdiagnosed as bidirectional shunting by the color Doppler ECHO. However, the laminar nature of the ow, lack of its communication with the RV cavity, and the absence of symptoms made the possibility of a bidirectional shunt unlikely. Hence, a cardiac MRI was obtained to better delineate the anatomy. Based on the color Doppler and MRI images, we assume that the ow forms a continuous loop starting from the LV, through the VSD and the RV trabeculations, and back to the LV without ever communicating with the RV cavity. ough rare, it is important to be aware of this unusual method of closure of muscular VSDs which will prevent unnecessary investigations and anxiety.

Conclusion
Multiple mechanisms have been proposed for the closure of muscular VSDs, and almost all of them include formation of brous tissue leading to closure of the defect. is case highlights an unusual method of spontaneous closure of a muscular VSD leading to an unusual color Doppler ow.
is ow pattern could be mistaken for bidirectional shunting. However, its laminar nature and lack of communication with the RV cavity make this possibility unlikely.     Case Reports in Pediatrics