Thursday, February 2, 2023

Pediatric Tricuspid Atresia

Background

Tricuspid atresia may be defined as congenital absence or agenesis of the tricuspid valve.
It is the third most common cyanotic congenital heart defect; the other 2 frequently observed cyanotic congenital cardiac anomalies are transposition of the great arteries and tetralogy of Fallot. Tricuspid atresia is the most common cause of cyanosis with left ventricular hypertrophy.

Although some authors state that Holmes (1824) or Kuhne (1906) first described tricuspid atresia,
Rashkind’s methodical and thorough historical review indicates that Kreysig (1817) reported the first case in 1817.
An 1812 report by the editors of the London Medical Review (1812) appears to fit the description of tricuspid atresia, but they did not use this specific term.

Terminology

Little more than 3 decades ago, the terminology for this defect (eg, tricuspid atresia, univentricular heart, univentricular atrioventricular connection) was intensely debated.
This debate was summarized in a 1990 issue of The American Journal of Cardiology,
in which Rao offered strong evidence and argued on the basis of data that Bharati and Lev,
Wenink and Ottenkamp,
Gessner,
and Rao gathered support for tricuspid atresia as the correct and logical term to describe this well-characterized pathologic and clinical condition.

Embryology

The atrioventricular valves develop shortly after the atrioventricular canal divides. The tricuspid valve leaflets have several origins. The septal leaflet of the tricuspid valve mostly develops from the inferior endocardial cushion with a small contribution from the superior cushion. The anterior and posterior tricuspid valve leaflets develop by undermining of a skirt of ventricular muscle tissue. The process of undermining extends until the atrioventricular valve junction is reached. Resorption of the muscle tissue produces normal-appearing valve leaflets and chordae tendineae.
Fusion of developing valve leaflet components results in stenosis (partial fusion) or atresia (complete fusion) of the valve.

Whether a muscular type of tricuspid atresia develops or whether well-formed but fused tricuspid-valve leaflets develop depends on the stage of development when the embryologic aberration takes place.
The classic muscular form of tricuspid atresia develops if the embryologic insult occurs early in gestation, and fused valve leaflets occur if the embryologic abnormality occurs slightly later than this in gestation. If the valve fusion is incomplete, stenosis of the tricuspid valve develops.

The pathologic, clinical, and electrocardiographic features of tricuspid stenosis and atresia are similar.
Therefore, the fact that isolated congenital tricuspid stenosis belongs to the group of tricuspid atresia defects and that their embryologic developments are similar is no surprise. Thus, the tricuspid valve stenosis, tricuspid atresia with well-formed but fused valve leaflets, and the muscular type of tricuspid atresia represent a spectrum of morphologic abnormalities.

Anatomy

The pathologic anatomy of tricuspid atresia is best understood by reviewing variations in valvar morphology.

The most common type of tricuspid atresia is muscular (see the image below).
It is characterized by a dimple or a localized fibrous thickening in the floor of the right atrium at the expected site of the tricuspid valve. The muscular variety accounts for 89% of cases.

Cardiac specimen from a patient with the muscular

Cardiac specimen from a patient with the muscular type of tricuspid atresia. The right atrium was opened by cutting through the right atrial appendage (RAA). Note the dimple (arrow) in the floor of the right atrium with muscle fibers radiating around it. An atrial septal defect (ASD) is also shown. From Rao PS, Levy JM, Nikicicz E, Gilbert-Barness EF. Tricuspid atresia: association with persistent truncus arteriosus. Am Heart J 1991, 122:829, with permission.

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In the membranous type (6.6%), the atrioventricular portion of the membranous septum forms the floor of the right atrium at the expected location of the tricuspid valve. This particular type appears to be associated with absent pulmonary valve leaflets.

Minute valvar cusps are fused together in the valvar type (1%).

In the Ebstein type (2.6%), fusion of the tricuspid valve leaflets occurs; attachment is displaced downward, and plastering of the leaflets to the right ventricular wall occurs.
This variant is rare but well documented.

The atrioventricular canal type is extremely rare (0.2%). In this type, a leaflet of the common atrioventricular valve seals off the only entrance into the right ventricle.

In the final type, unguarded with a muscular shelf (0.6%), the atrioventricular junction is unguarded, but the inlet component of the morphologic right ventricle is separated from its outlet by a muscular shelf.

The right atrium is enlarged and hypertrophied. An interatrial communication is necessary for survival. This communication most commonly is a stretched patent foramen ovale. Sometimes, an ostium secundum or an ostium primum atrial septal defect (ASD) is present. In rare cases, the patent foramen ovale is obstructive and may form an aneurysm of the fossa ovalis, which is sometimes large enough to produce mitral inflow obstruction. The left atrium may be enlarged, especially when the pulmonary blood flow is increased. The mitral valve is morphologically normal; it is rarely incompetent and has a large orifice. The left ventricle is enlarged and hypertrophied but often morphologically normal.

The ventricular septal defect (VSD) is usually small; however, it can be large, or several VSDs may be present.
The ventricular septum is rarely intact. When present, the VSD may be conoventricular or perimembranous in type (inferior to the septal band), it may be of conal septal malalignment type (between the limbs of the septal band), or it may be of the muscular or atrioventricular canal type.
Muscular VSDs are the most common defects and are usually restrictive; they produce subpulmonary stenosis in patients with normally related great arteries and simulate subaortic obstruction in patients with transposition of the great arteries.

The right ventricle is small and hypoplastic, and its size largely depends on the anatomic type.
In patients with a large VSD or transposition of the great arteries, the size of the right ventricle may be larger, but, even in these patients, the right ventricle is smaller than normal. In patients with pulmonary atresia and normally related great arteries, the right ventricle is small and may escape detection. However, it is a true right ventricle in most patients; it is composed of a sharply demarcated infundibulum with septal and parietal bands and a sinus with trabeculae, which may communicate with the left ventricle by means of a VSD. By definition, the inflow region is absent, although papillary muscles may occasionally be present.

The great artery relationship is variable and forms the basis of a major classification and will be described in the next section. Obstruction to the pulmonary outflow tract is present in most cases of tricuspid atresia and is used in the scheme of classification. The aorta is either normal or slightly larger than normal. In 30% of patients, various associated cardiac defects are present; aortic coarctation and persistent left superior vena cava are particularly notable.

Associated cardiac defects in tricuspid atresia outlined below.

Defects that form the basis for classification are as follows:

D-Transposition of the great arteries

L-Transposition of the great arteries

Double outlet right ventricle

Double outlet left ventricle

Other malpositions of the great arteries

Truncus arteriosus

Defects that may need attention before or during palliative or total surgical correction are as follows:

Absent pulmonary valve

Aneurysm of the atrial septum

Anomalous origin of the coronary arteries from the pulmonary artery

Anomalous origin of the left subclavian artery

Anomalous origin of the right subclavian artery

Aortopulmonary fistula

Coarctation of the aorta

Common atrium

Cor triatriatum dexter

Coronary sinus atrial septal defect

Double aortic arch

Double-outlet left atrium

Hemitruncus

Hypoplastic ascending aorta and/or aortic atresia

Ostium primum ASD

Parchment right ventricle

Patent ductus arteriosus

Persistent left superior vena cava

Right aortic arch

Subaortic stenosis

Total anomalous pulmonary venous connection

Tubular hypoplasia of the aortic arch

Valvar aortic stenosis

Other associated defects are as follows:

Juxtaposition of the atrial appendages

Anomalous entry of coronary sinus into the left atrium

Classification

Tricuspid atresia is classified according to the morphology of the valve,
the radiographic appearance of pulmonary vascular markings,
and the associated cardiac defects.

Van Praagh and associates (1971) initially proposed a classification based on the morphology of the atretic tricuspid valve.
He and others later modified and expanded the classification, as described in Tricuspid Atresia.
All other morphologic types are described above in the Anatomy section. For pathologic, echocardiographic, and angiographic examples, particularly the rare anatomic types, the interested reader is referred to Tricuspid Atresia
and the Atlas of Heart Disease: Congenital Heart Disease.

Astley and associates (1953) proposed a classification based on pulmonary vascular markings on a chest radiograph: Group A are cases with decreased pulmonary vascular markings, and group B are those with increased pulmonary vascular markings.
Dick et al (1975) added a third group, group C, to describe cases with a transition from increased to decreased pulmonary vascular markings.
This type of classification has some clinical value, although a more precise definition than these can often be made by using noninvasive 2-dimensional (2D) and Doppler echocardiography.

In 1906, Kuhne first proposed a classification based on great-artery relationships,
which Edwards and Burchell expanded in 1949.
Keith, Rowe, and Vlad popularized this classification in 1967.
Other investigators have offered various other classifications. These are reviewed in detail in the American Heart Journal
and Tricuspid Atresia.
Although these classifications are generally good, their exclusion of some variations in great-artery relationships and the lack of consistency in subgroups are problematic. Therefore, the following comprehensive-yet-unified classification was proposed
:

The principle grouping continues to be based on the following interrelationships of the great arteries:

Type I – Normally related great arteries

Type II – D-Transposition of the great arteries

Type III – Great artery positional abnormalities other than D-transposition of the great arteries: (1) Subtype 1 involves L-transposition of the great arteries, (2) subtype 2 involves double outlet right ventricle, (3) subtype 3 involves double outlet left ventricle, (4) subtype 4 involves D-malposition of the great arteries (anatomically corrected malposition), and (5) subtype 5 involves L-malposition of the great arteries (anatomically corrected malposition)

Type IV – Persistent truncus arteriosus

All types and subtypes are subdivided into the following subgroups:

Subgroup a – Pulmonary atresia

Subgroup b – Pulmonary stenosis or hypoplasia

Subgroup c – No pulmonary stenosis (normal pulmonary arteries)

After the above categorization, the status of the ventricular septum (intact or VSD) and the presence of other associated malformations are described.

This unified classification includes all the previously described abnormalities in the positions of the great arteries and can be further expanded if new variations are revealed. This classification maintains uniformity of the subgroups and preserves the basic principles of classification that Kuhne, Edwards and Burchell, and Keith, Rowe, and Vlad devised.

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