Introduction

Tricuspid atresia 

Tricuspid atresia (TA) is a rarely complex congenital heart disease (CHD) characterised by cyanosis during the neonatal period. TA occurs due to a congenital defect that results in the absence of the morphologic tricuspid valve, lack of flow across it, and, hence, lack of cellular transduction between the right atrium and ventricle. TA occurs in equal prevalence in males and females and accounts for 1% of all cases of CHD. TA is classified based on several factors, such as the presence of pulmonary obstruction, cardiac complications, and anatomical changes.^1,2,3,4

Diagnosis 

The diagnosis and management of TA should be performed as soon as possible to avoid more complications which cause variable fatal rates among TA patients, knowing that TA demonstrates high mortality rates. Advancements in technology play an essential role in the diagnosis of TA cases via antenatal ultrasound and fetal echocardiogram; it has been concluded that echo-doppler assessments are useful in the evaluation of abnormal anatomic features, and cardiac defects.^1,2

Physical Examination and Imaging

Overview

The clinical presentation usually relies on the severity of pulmonary obstruction, the presence of ventricular septal defect (VSD), and the condition of the great arteries. The average gestational age at diagnosis is approximately 22 weeks, although the earliest can be 11 weeks. According to a large-scale study, the incidence of prenatally diagnosed TA ranges from 0.2 to 0.9 per 10000. Moreover, the diagnosed cases are observed to increase over time.²

Anatomic features of TA

Tricuspid atresia (TA) classification depends on pulmonary vascular characteristics, valve morphology, and relevant cardiac abnormalities. Thus, the resulting categories are based mainly on clinical features in terms of defects and great artery relationships: normal great arteries, d-transposition, other malpositions, and truncus arteriosus, which belong to Type I, II, III, and IV respectively. Further, classified based on pulmonary artery anatomy.¹

The atretic tricuspid valve can be predominantly muscular accounting for 89 % of cases. The others could be unguarded with muscular shelf, atrioventricular canal type, Ebstein’s, valvular, or membranous. 

Anatomically, there is right atrial enlargement with an atretic tricuspid valve in the form of fibrous thickening. There is a patent foramen ovale (PFO) or atrial septal defect (ASD) as a requirement for survival, while the left atrium and left ventricle are hypertrophied and enlarged. The right ventricle is hypoplastic and inadequate for pulmonary circulation. 

In the majority of patients, there is a ventricular septal defect (VSD) that may be of mixed type and causes subpulmonary or subaortic obstruction. Obstruction of pulmonary blood supply is based on the great artery configuration, more commonly at the level of VSD in normally related great arteries and subvalvular in transposition. Pulmonary atresias are reliant on a patent ductus arteriosus (PDA) or collateral circuits to sustain circulation. Other concomitant defects, such as coarctation of the aorta and left superior vena cava persistency, occur in 30% and influence management.¹

Physical examination

Neonates commonly present with cyanosis, particularly post-ductal closure. The clinical presentation is as follows:

Patients with pulmonary obstruction

• Reduced right ventricular impulse
• Continuous murmur (heard due to the blood flow between two areas of different pressure)
• Thrill: a vibration or buzzing-like sound felt on the chest while doing a physical examination under the fingertips. May occur in restrictive VSD (a hole in the ventricular septum causes abnormal blood flow between the right and left ventricles) or severe pulmonary stenosis (PS), which is a narrowing of the pulmonary artery
• Nail Clubbing: the thickening of the fingertips and nails due to prolonged low oxygen levels. It develops particularly in older and chronic patients
• Central cyanosis: a bluish discoloration of the skin due to inadequate oxygen levels in the blood. 
• Systolic ejection murmur associated with PS: this heart murmur occurs during systole while the heart pumps blood out
• Holosystolic murmur: indicates VSD, this heart murmur is of the same intensity during the entire systolic phases from S1 to S2, caused by blood flowing from a high-pressure to a low-pressure chamber²

Patients without pulmonary obstruction 

• Tachycardia (elevated heartbeats)
• Hepatomegaly (also known as enlarged liver) in patients with elevated blood flow in the lungs (pulmonary plethora)
• Tachypnea (rapid breathing)²

Imaging

• X-ray: The undiagnosed cases of critical congenital heart diseases can be revealed by clinicians during neonatal screening if they detect a heart murmur or low oxygen levels (hypoxemia). A diminished blood flow to the lungs (pulmonary oligemia) is shown via a chest X-ray as well as the enlarged right heart border, suggesting right atrial dilation
• Electrocardiogram (ECG): demonstrates a left superior QRS axis (-30 to -90) in TA cases. Additionally, ECG plays a significant role in revealing signs of left ventricular hypertrophy as well as weak right ventricular signals
• Echocardiography: acts as a diagnostic tool for TA. The three showing features via the two-dimensional echocardiography are the left ventricle larger than the right, unequal sizes of the ventricular chamber, and the absence of the tricuspid valve

On 2D echocardiography, the tricuspid valve is designated by dense bands of echoes which represent the muscular types, indicating TA. The best-seen characteristics that demonstrate these findings are apical and subcostal four-chamber views. However, 2D echocardiography can detect other rare types of the disorder as well, such as unguarded tricuspid valve with a muscular shelf, and membranous, valvular, Ebstein’s, atrioventricular septal defect (AVSD).¹

Through evaluating certain features, echocardiography can differentiate between these disorders (the atrioventricular canal type of tricuspid atresia from the classic muscular tricuspid atresia cases) according to the following:

• Membranous type: demonstrates a thin membrane instead
• Muscular Type: at the tricuspid valve site, it shows a thick band of echoes
• AVSD type: enlarged atrioventricular valve leaflet impairs the pathway from the right atrium to the right ventricle, besides, the heart mid-junction is malformed

Therefore, echocardiography has a significant role in terms of diagnosis and management of TA as the third most persistent cyanotic congenital heart disease. Echocardiography efficiently detects the atretic tricuspid valve, asses left ventricular (LV) function, ventricular size, and relevant abnormalities like ventricular septal defects (VSD), atrial septal defects (ASD), and pulmonary outflow anomalies. 

However, in post-palliative procedures (e.g., Fontan surgeries, and aortopulmonary shunts), echo-doppler is important in measuring the outcomes, follow-up the case, and detecting any arisen complications. Hence, the echo-doppler tool is necessary in both the diagnosis and management of long-term outcomes. Colour flow Doppler confirms the absence or lack of flow across the tricuspid valve.²

Cardiac catheterization is not often required for diagnosis in this case. However, it might become a necessary diagnostic tool, particularly a balloon atrial septostomy, to use in cases where there is inadequate blood flow from the right atrium to the left atrium. enetic testing may become a requirement in this case l, as it is linked to genetic abnormalities such as trisomies, 22q11, and VACTERL syndrome.

Summary

• Tricuspid atresia (TA) is a complex and uncommon birth heart defect that presents bluish skin colour in the neonatal period
• Diagnosis and treatment of TA must be done as early as possible to prevent any additional complications
• Technical advances help in the diagnosis of TA cases through antenatal ultrasound and fetal echocardiogram; echo-doppler evaluations are helpful to evaluate anomalous anatomic structures, and cardiac anomalies
• The clinical picture tends to vary with the extent of pulmonary obstruction, the presence of a ventricular septal defect (VSD), and the status of the great arteries. The mean gestational age at diagnosis is around 22 weeks
• Tricuspid atresia (TA) classification is founded on the type of pulmonary vascularisation, valve morphology, and associated cardiac anomalies. Infants typically have cyanosis, especially following ductus arteriosus closure
• The clinical presentation in pulmonary obstruction patients differs from non-pulmonary obstruction patients. Pulmonary obstruction patients present with diminished right ventricular impulse, continuous murmur, thrill and others. However non-pulmonary obstruction patients present with tachycardia, tachypnea, and hepatomegaly
• Diagnosis relies on imaging, a diminished blood flow to the lungs (pulmonary oligemia) is shown via a chest X-ray, while the electrocardiogram (ECG) demonstrates a critical observation which is a left superior QRS axis (-30 to -90)
• On 2D echocardiography, the tricuspid valve is designated by dense bands of echoes which represent the muscular types, indicating TA
• The colour flow Doppler ensures the absence of flow across the tricuspid valve
• Cardiac catheterisation is seldom necessary for diagnostic purposes
• Genetic testing would be required since TA is related to genetic disorders