Double Inlet Single Ventricle

Double inlet single ventricle is a severe congenital heart disease where a functionally univentricular heart exists. 
It is a form of univentricular atrioventricular connection in which the two atria connect totally or predominantly to a single ventricle, the dominant ventricle, which maintains systemic and pulmonary circulation. The dominant ventricle defines the large ventricle receiving major atrioventricular (AV) valve inflow. Often, a rudimentary ventricle can be identified which does not connect with the AV valves or only minimally connects with them. This rudimentary ventricle is called an outlet chamber if one or both great arteries arise from it. 
The spectrum of this cardiopathy is broad, and there may be differences in ventricular  morphology, the ventricle-arterial ratio and the size of the outflow tracts. (1-4)

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It is a rare congenital cardiac anomaly with an incidence of 0.05 to 0.1 per 1000 live births. It accounts for 1% of all congenital cardiac anomalies and is present in 4% of neonates with congenital cardiac disease. (1,3,5)

There are two theories concerning the pathogenesis:
The first states that the single ventricle is a consequence of a failure in the correct alignment between the infundibular outflow septum and the rest of the interventricular septum. 
The second establishes that the infundibular septum has a different embryonic origin to the rest of the septum, and, hence, the single ventricle forms as a result of a failure in the development of the posterior interventricular septum. (6,7)

In general, the aetiology is thought to be multifactorial. Even though maternal diabetes and heart diseases have been associated, many other genetic and environmental factors contributing to the development of this condition remain unknown at this time (3). 

The morphology of the dominant ventricle (left, right or indeterminate) must be established through the muscular trabeculation pattern. In most cases, it is possible to identify a rudimentary ventricle that connects minimally with the dominant ventricle through a defect in the infundibular septum known as bulboventricular foramen.
Usually, there are two well-differentiated atrioventricular (AV) valves, although it is also possible that there are different degrees of stenosis or hypoplasia of one of the AV valves.  
It is crucial to determine the type of ventriculo-arterial connection, which may be concordant or discordant, and there may be a variable degree of obstruction in the outflow tracts. In most cases, the great arteries are transposed, the aorta arises from the accessory right ventricle, and the pulmonary artery comes off the main left ventricle chamber. The great vessels are normally related in 30% of cases.
Based on the dominant and rudimentary chamber's morphology, location and trabeculation pattern, the following forms of single ventricle can be recognized.:

●    (1) Double inlet left ventricle - a dominant ventricle of left ventricular morphology, exhibiting fine trabeculations. An accessory chamber is present corresponding to a remnant of the right ventricle that is  usually located anterosuperiorly  at some distance from the crux of the heart. It is the most frequent type and accounts for 80% of cases. In the most frequent subtype, the accessory chamber is found on the left of the dominant ventricle and there is transposition of the great vessels.

●    (2) Double inlet right ventricle - a dominant ventricle of right ventricular morphology, which exhibits coarse trabeculations. There is an accessory chamber which is a remnant of the left ventricle that is usually located posteriorly and closely related to the crux of the heart, which is difficult to identify. In this type, a common AV valve is more frequently present. This type is less frequent.

●    (3) Double inlet indeterminate ventricle - has a ventricular chamber of indeterminate morphology. There is usually no accessory chamber and the existence of a common AV valve is more frequent (5,7,8).

There is a rare association between this cardiopathy and extracardiac anomalies and chromosomal abnormalities, especially if both AV valves are symmetrical. 
The presence of double inlet right ventricle with single valve is associated with heterotaxy syndrome and venous return anomalies. (2)

The diagnosis of double inlet single ventricle can be made by 2D ultrasound in the four-chamber view. 
-An abnormal four-chamber view shows a single ventricle of increased size (dominant ventricle) that connects totally or mostly with the two atria through two AV valves (typically two symmetric valves) and an absent interventricular septum at the usual position. Most often, the dominant ventricle is the left ventricle, presenting a fine muscular trabeculation and the absence of the moderator band at the apex. The two AV valves may be similar or different in size. Therefore, it is important to identify the septum primum and the foramen ovale to assess the atria and AV valves and their relationship with the ventricle.
- The accessory chamber is visualised in a cranial section of the four-chamber view.
It is also important to obtain the ventricular outflow view, due to the frequency of discordance in the ventricular-arterial connections and the varying degrees of arterial obstruction. The size of the vessel leaving the accessory chamber will be determined by the size of the bulboventricular foramen. (1,7,8)

The differential diagnoses of double inlet single ventricle must be made with Univentricular cardiopathies and septal defects.These differentials include:
●    Hypoplastic left heart syndrome: the most common form of presentation includes mitral and aortic valve atresia. It is characterised by the presence of a severely hypoplastic left ventricle, which is absent in the most severe cases. A reverse flow in the aortic isthmus and transverse aortic arch can be seen.  The main difference with double inlet ventricle is that there is no blood flow through the mitral valve and there is only a connection between the right atrium and the right ventricle. The great vessels are normally positioned.

●    Tricuspid atresia: in most cases, it is associated with the existence of a ventricular septal defect. The size of the right ventricle depends on the size of the ventricular septal defect. In tricuspid atresia, there is no communication between the right atrium and the right ventricle, and the right atrioventricular junction is visualised as fibrous tissue. Each atrium is related to a different ventricle.

•    Unbalanced atrioventricular septal defect: in complete atrioventricular septal defect, there is a common atrioventricular valve and a ventricular septal defect. When the atrioventricular septal defect is unbalanced there is a ventricular size disproportion.

●    Mitral atresia with ventricular septal defect: In mitral atresia, there is no communication between the left atrium and the left ventricle. The size of the left ventricle will depend on the size of the septal defect.

●    Pulmonary atresia with intact ventricular septum (PA-IVS): In PA-IVS type I (competent tricuspid valve) the four-chamber view shows a hypertrophic and hypokinetic RV with a variable degree of underdevelopment. The tricuspid valve is usually small with a narrow valvular annulus, poor leaflet mobility, and severely limited opening.

●    Criss-cross heart: the atrioventricular connection is impaired with crossed ventricular inflow streams. The ventricles are in a supero-inferior position with the interventricular septum in a horizontal arrangement.

●    Large ventricular septal defect: When the interventricular septal defect is very wide, it may appear that there is no septum at all. (7,8)
 

The main diagnostic clue for double inlet single ventricle is the presence of a large ventricle in the absence of the interventricular septum in the four-chamber view.

Prenatal diagnosis of a double inlet single ventricle should include a detailed fetal ultrasound to rule out the presence of other associated cardiac or extracardiac anomalies. 
Although the risk of associated chromosomal abnormalities is low, the possibility of invasive fetal cytogenetic testing should be considered. Parents should be offered invasive diagnostic testing to rule out associated chromosomal or genetic abnormalities.

The anatomical spectrum is very diverse; hence the symptomatology in the neonatal stage may change widely. However, the newborn usually has no symptoms in the absence of hypoplasia or significant obstruction of the AV valves and/or outflow tracts.
The long-term prognosis is poor since these patients require   serial palliative surgeries of the Fontan type. Despite major surgical advances over the past several decades, functionally univentricular CHD remains associated with high morbidity and mortality, with heart transplantation representing the only definitive treatment. The type of surgical repair will depend on the disposition of the great vessels and whether there is an obstruction in any of them. These babies with a functionally single ventricle are candidates for single ventricle/Fontan repair and are generally managed by staged total cavo-pulmonary connection in three stages. The prevalence of inter-stage mortality is high (5–15%). The 10-year survival rate of infants with a morphological single left ventricle (LV) (tricuspid atresia and double-inlet left ventricle) is estimated to be between 70% and 80% based on the case series published previously. (3, 8, 9)

A detailed anatomical survey should be performed to assess whether this disease is isolated or associated with other anomalies, and, in addition, the risk of chromosomal defects should be assessed. 
Prenatal counselling should consider that the future neonatal circulation will be univentricular and, therefore, have a worse prognosis.
During follow-up, serial echocardiograms should be performed every four weeks to assess valvular insufficiency, outflow obstruction and rhythm abnormalities. In continuing pregnancies, this disease does not modify the usual obstetric management except in the presence of hydrops or heart failure. There is no indication for elective Cesarean section in the presence of a double-inlet single ventricle. 
Delivery should be planned in a tertiary centre to receive optimal postnatal management with early interventions, including prostaglandin administration and availability of neonatology, cardiology, and pediatric cardiac surgery. (1)

1.    GIDVANI M, RAMIN K, GESSFORD E, AGUILERA M, GIACOBBE L, SIVANANDAM S. PRENATAL DIAGNOSIS AND OUTCOME OF FETUSES WITH DOUBLE-INLET LEFT VENTRICLE. AJP REP. 2011 DEC;1(2):123-8. DOI: 10.1055/S-0031-1293515. EPUB 2011 NOV 7. PMID: 23705101; PMCID: PMC3653524.

2.    SHIRAISHI H, SILVERMAN NH. ECHOCARDIOGRAPHIC SPECTRUM OF DOUBLE INLET VENTRICLE: EVALUATION OF THE INTERVENTRICULAR COMMUNICATION. J AM COLL CARDIOL. 1990 MAY;15(6):1401-8. DOI: 10.1016/S0735-1097(10)80031-2. PMID: 2329242.

3.    PAIGE SL, YANG W, PRIEST JR, BOTTO LD, SHAW GM, COLLINS RT 2ND; NATIONAL BIRTH DEFECTS PREVENTION STUDY. RISK FACTORS ASSOCIATED WITH THE DEVELOPMENT OF DOUBLE-INLET VENTRICLE CONGENITAL HEART DISEASE. BIRTH DEFECTS RES. 2019 JUL 1;111(11):640-648. DOI: 10.1002/BDR2.1501. EPUB 2019 MAR 28. PMID: 30920163.

4.    MUÑOZ-CASTELLAÑOS L, ESPINOLA-ZAVALETA N, KEIRNS C. ANATOMOECHOCARDIOGRAPHIC CORRELATION DOUBLE INLET LEFT VENTRICLE. J AM SOC ECHOCARDIOGR. 2005 MAR;18(3):237-43. DOI: 10.1016/J.ECHO.2004.11.009. PMID: 15746713.

5.    ANDRADE P, SANTOS D, MOREIRA M, ALMEIDA A. DOUBLE-INLET SINGLE VENTRICLE WITH MALPOSED GREAT ARTERIES. ARQ BRAS CARDIOL. 2019 OCT 10;113(3):444-446. DOI: 10.5935/ABC.20190160. PMID: 31621786; PMCID: PMC6882393.

6.    MONIOTTE S, BARREA C. FUNTIONALLY UNIVENTRICULAR HEART. EN:ECHOCARDIOGRAPHY IN PEDIATRIC AND CONGENITAL HEART DISEASE:FROM FETUS TO ADULT. EDS:LAI W,MERTENS L, COHEN M, GEVA T.WILLEY-BLSCKWELL. 2009:459-73

7.    GALINDO. GRATACÓS. MARTÍNEZ. CARDIOLOGÍA FETAL. ECOCARDIOGRAFÍA. ED. MARBÁN. 2015 

8.    RAO PS. SINGLE VENTRICLE-A COMPREHENSIVE REVIEW. CHILDREN (BASEL). 2021 MAY 24;8(6):441. DOI: 10.3390/CHILDREN8060441. PMID: 34073809; PMCID: PMC8225092.

9.    BEROUKHIM RS, GAUVREAU K, BENAVIDEZ OJ, BAIRD CW, LAFRANCHI T, TWORETZKY W. PERINATAL OUTCOME AFTER PRENATAL DIAGNOSIS OF SINGLE-VENTRICLE CARDIAC DEFECTS. ULTRASOUND OBSTET GYNECOL. 2015 JUN;45(6):657-6DOIDOI: 10.1002/UOG.14634. EPUB 2015 APR 23. PMID: 25042627.
 

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