The ultrasound findings are demonstrated in the following images, videos and figures.

Images 3-5 and Video 1, 2 show a four-chamber view of the heart. The heart is enlarged with dilated ventricles, predominantly affecting the right side. The interventricular septum (IVS) of the heart has an abnormal appearance with a central cavity and separation of the left and right walls. A defect can be seen in the right wall close to the apex. During the cardiac cycle, blood flows in and out of the abnormal interventricular septal cavity through the defect in the right wall (depicted by white curved arrows in Images 3-5). Blood flowing into the septum dissects its walls, and the left wall of the septum bulges into the left ventricle, obstructing the outflow tract. These structural changes are responsible for worsening cardiac function with diminished ejection fraction and dilated cardiomyopathy.

Image 6 caption: Diagram demonstrating the dissecting aneurysm of the interventricular septum seen in our case. There is a defect in the right wall of the septum which allows blood flow into the interventricular septum. (RV–right ventricle; LV–left ventricle; IVS–interventricular septum; RA–right atrium; LA–left atrium; Ao–aorta)

Discussion

Congenital ventricular aneurysms are broad-based bulges in the ventricular wall with noncontractile, or dyskinetic myocardium [1]. While in the postnatal period, a ventricular aneurysm can be acquired as a complication of surgical procedures [2], trauma [3], Kawasaki disease [4], infection [4], or following a myocardial infarction [6], congenital aneurysms have no clear causative factor.  In contrast, a congenital ventricular diverticulum is a fingerlike pouch with a narrow connection to the ventricle that contains epicardium, myocardium, and endocardium on histology, and contracts either normally or near normally [1]. On prenatal imaging, it can be difficult to distinguish between ventricular aneurysms and diverticula [1]. Given their rarity, the prevalence of ventricular diverticula and aneurysms is difficult to estimate. A study reviewing 12,924 consecutive autopsies in children found 750 cases with congenital cardiac defects and only 3 cases of ventricular diverticula (23 in 100,000 cases) [7]. A more recent publication in 12,271 consecutive adult patients undergoing coronary angiography demonstrated a prevalence of 76 in 10,000 cases of congenital left ventricular aneurysm and diverticula [8]. The etiology of congenital ventricular aneurysms is not known, though may be related to prenatal ischemia leading to aneurysm formation [9], connective tissue disorders in patients with Ehlers-Danlos syndrome [10], or a genetic defect in mesenchymal cell migration in patients with familial occurrence [11,12].

Congenital ventricular aneurysms are more commonly located in the apex and anterolateral wall of the ventricle [8], and are rarely found in the interventricular septum. In ventricular septal aneurysms, the abnormal tissue can involve the membranous or muscular portion of the septum [13]. In cases involving the membranous portion of the ventricular septum, aneurysms may represent a mechanism for spontaneous closure of ventricular septal defects [14].

Congenital aneurysms of the muscular intraventricular septum are extremely rare. There have been 22 cases reported in the literature (Table 1) [11,13,15-29], 10 of which were diagnosed prenatally. Approximately 75% (16) of cases had associated cardiac anomalies, most commonly ventricular septal defect (7) located either in the perimembranous septum or in the aneurysm, atrial septal defect (5), and right heart lesions (5) such as pulmonary atresia with tricuspid valve stenosis and right ventricular aneurysm. Patients rarely presented with extracardiac anomalies. While 55% (12) of patients were asymptomatic, rhythm disturbances such as first-degree atrioventricular block, right bundle branch block, and Wolf-Parkinson-White occurred in 18% (4), and symptoms of congestive heart failure occurred in another 18% (4). Seven cases were treated surgically, though only 3 cases required plication or resection of the aneurysm, and the remainder underwent surgical treatment of concomitant cardiac structural defects. Survival was approximately 90% with one termination of pregnancy and one death. Follow up extended anywhere between 3 months and 12 years.

In patients with congenital muscular interventricular septal defect, there have been several familial cases, suggesting an autosomal dominant inheritance with variable expressivity [11,27,28]. Of the 22 cases reported in the literature, 32% (7) mentioned a family history of heart disease. Development of a muscular interventricular septal defect may be a result of a genetic mutation in mesenchymal cell migration from the atrioventricular and conotruncal regions to the cephalic portion of the developing interventricular septum [11,12]. Alternatively, a muscular septal aneurysm may be a result of ischemia or embolism [22]. In our literature review, 13 cases documented the coronary artery anatomy, 9 of which were normal, 3 showed right ventricular sinusoids associated with pulmonary atresia, and one showed abnormal circumflex artery distribution.

Our case more specifically represents a dissecting aneurysm of the interventricular septum. There are several reports in adults with a prevalence of 1.6 to 19.5 in 100,000 cases [30, 31]. While acquired lesions are a result of myocardial infarction, cardiac surgery, or endocarditis [30], an aneurysm of the sinus of Valsalva is a common etiology of an interventricular septal dissection [30, 31]. The mechanism is thought to be a result of compression of the coronary artery by the aneurysm of the coronary sinus of Valsalva leading to ventricular septal ischemia [32]. In cases with dissecting interventricular aneurysm, approximately 90% had an aneurysm that originated in the right coronary sinus of Valsalva [33].

This is the first case of dissecting aneurysm of the interventricular septum identified prenatally. Six cases have been reported in children, five of which were in adolescents [33-38]. These all originated from a right sinus of Valsalva aneurysm except for one in which there was no communication found between the dissecting interventricular aneurysm and the sinus of Valsalva (38). Similarly, two cases documented by Wu et al had normal sinuses of Valsalva, suggesting there may be other causes of this condition [31]. In our case, we hypothesize that a partial rupture of the interventricular aneurysm may lead to bleeding into the interventricular septum with dissection of the interstitium. The diagonal course of the defect through the interventricular septum may be a predisposing factor for the dissection (Image 8-9, Video 3-4).

While adult and adolescent patients with interventricular septal dissection may have significant anatomic and hemodynamic derangements leading to arrhythmias or congestive heart failure and necessitating surgery [30, 31, 33-37], there is no data to understand the natural history in fetal and neonatal life. Aneurysms of the membranous portion of the ventricular septum can be associated with complications such as cerebral infarction secondary to embolism [39], and right ventricular outflow obstruction [40]. In our literature review, muscular aneurysms can lead to conduction abnormalities [15, 17, 22, 24], and congestive heart failure [18, 23, 25, 28], though several cases documented improvement of the interventricular septal findings on serial echocardiography likely due to ventricular remodeling [22, 24, 29]. This was the case in our patient with a favorable outcome of the pregnancy and delivery, and gradual improvement of the cardiac findings. Due to lack of data, counselling patients can be a challenge, though lack of cardiac failure, such as fetal hydrops, or pericardial effusion could be a favorable predictive sign.

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