Twin-to-Twin Transfusion Syndrome (TTTS): Effects on the Fetal Heart

Cardiac changes in Twin-to-Twin Transfusion Syndrome occur as a result of unbalanced blood flow associated with placental vascular anastomoses. In the recipient fetus, cardiomegaly and ventricular hypertrophy due to volume overload may be observed, while decreased cardiac output related to hypovolemia may occur in the donor fetus.

Signs of cardiovascular overload in the recipient fetus are characterized by myocardial wall thickening, tricuspid regurgitation, and pulmonary hypertension due to increased blood volume. In advanced cases, hydrops fetalis may develop, significantly increasing the risk of perinatal mortality.

Cardiac dysfunction in the donor fetus is associated with low cardiac output and growth restriction due to decreased circulating volume. Hypotension and oligohydramnios frequently accompany this condition. Prolonged hypoperfusion may negatively affect organ development and myocardial functions.

Cardiac evaluation with fetal echocardiography is critically important in determining the prognosis in Twin-to-Twin Transfusion Syndrome. Ventricular functions, valvular insufficiencies, and flow patterns are examined in detail to perform staging and ensure appropriate treatment planning.

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Hemodynamic Basis of Twin-to-Twin Transfusion Syndrome

The pathophysiology of TTTS is based on arterio-venous (AV), arterio-arterial (AA), and veno-venous (VV) anastomoses present in the placenta. Especially when there is net blood flow from one fetus to the other through deep arteriovenous connections, a chronic volume imbalance develops between the donor and recipient fetuses.

While the donor fetus develops hypovolemia (decreased blood volume) and consequently oligohydramnios (reduced amniotic fluid), the recipient fetus experiences hypervolemia (increased blood volume) and polyhydramnios (increased amniotic fluid). This chronic volume load leads to structural and functional changes particularly in the cardiac system of the recipient fetus.

Mechanism of Cardiac Overload in the Recipient Fetus

The recipient fetus is continuously exposed to increased preload (the amount of blood returning to the heart) and increased afterload (the resistance encountered while the heart pumps blood). This condition may cause adaptive but eventually pathological changes in the cardiovascular system.

Increased intravascular volume may lead to dilation of the cardiac chambers. The right ventricle is affected more prominently by the volume load because it is dominant in fetal circulation. Prolonged volume overload may predispose to the development of myocardial dysfunction.

During this process, the fetal heart initially attempts to adapt through compensatory mechanisms; however, signs of heart failure may appear in later stages.

Structural Cardiac Changes

The most commonly observed cardiac changes in the recipient fetus with TTTS include:

Cardiomegaly (heart enlargement)
Ventricular hypertrophy (thickening of the heart muscle)
Atrioventricular valve insufficiency (especially tricuspid regurgitation)
Pulmonary valve stenosis or right ventricular outflow tract obstruction

Increased pressure and volume load may lead to hypertrophy in the ventricular walls. Increased right ventricular wall thickness is particularly notable. Tricuspid valve insufficiency may develop due to increased right ventricular pressure and can be detected by Doppler during fetal echocardiography.

In some cases, functional stenosis may develop at the level of the pulmonary valve. This condition is associated with increased pressure load in the right ventricular outflow tract and may rarely progress to permanent structural cardiac pathology.

Functional Cardiac Disorders

In addition to structural changes, myocardial function may also be affected. The following findings may be observed on fetal echocardiography:

Systolic and diastolic dysfunction
Increased myocardial performance index (MPI)
Abnormalities in ductus venosus flow

Diastolic dysfunction means impairment in the filling of the heart during the relaxation phase. This condition is associated with increased ventricular stiffness and pressure load. In advanced stages, fetal hydrops (widespread fluid accumulation in fetal tissues) may develop; this picture suggests severe heart failure.

Cardiac Changes in the Donor Fetus

In the donor fetus, a different type of cardiac adaptation related to hypovolemia is generally observed. Due to decreased preload, the cardiac chambers may appear smaller. However, marked cardiomegaly or severe structural heart changes are rarer in the donor fetus compared to the recipient fetus.

Nevertheless, chronic hypoperfusion (decreased blood flow to tissues) is associated with fetal growth restriction and reduced organ perfusion. This condition may indirectly affect cardiac function.

Cardiac Evaluation in the Diagnostic Process

TTTS is generally diagnosed by obstetric ultrasound. However, fetal echocardiography is the primary examination method for evaluating cardiac effects.

In fetal cardiac evaluation:

Sizes of the cardiac chambers
Ventricular wall thicknesses
Valve functions (especially tricuspid and mitral valves)
Doppler flow patterns
Myocardial performance index

are examined in detail.

This evaluation plays an important role in determining the stage of the syndrome and predicting prognosis. For example, while Quintero staging is based on amniotic fluid and bladder findings, the presence of cardiac dysfunction may suggest increased clinical risk.

Course of Cardiac Changes After Treatment

Today, the most commonly applied treatment method for TTTS is fetoscopic laser ablation of placental anastomoses. This procedure aims to stop the pathological blood transfer between the fetuses.

After successful laser treatment, many cardiac changes observed in the recipient fetus may regress over time. In particular, some functional disorders may be reversible. However, valvular anomalies or permanent cardiac sequelae may be observed in some cases.

Therefore, prenatal and postnatal pediatric cardiology evaluation is important. The degree of cardiac recovery may differ in each case and requires individualized follow-up.

Importance of Cardiac Findings for Families

For families diagnosed with TTTS, one of the most concerning issues is the risk of permanent heart damage. The presence of cardiac changes does not always mean permanent structural disease. In many cases, cardiac functions may improve once the hemodynamic load is eliminated.

However, the risk may increase when advanced dysfunction, hydrops, or severe valvular pathology is detected. Therefore, the multidisciplinary approach of perinatology, fetal cardiology, and neonatology specialists is of great importance.

Prof. Dr. İbrahim Cansaran TANIDIR

Prof. Dr. İbrahim Cansaran Tanıdır is an internationally recognized specialist with nearly 20 years of clinical, academic, and interventional experience in the fields of congenital heart diseases, pediatric interventional cardiology, hybrid procedures, and radiation-reduced / fluoroscopy-free catheterization techniques.

To date, he has performed more than 10,000 pediatric cardiac catheterization procedures both in Turkey and abroad, and has pioneered advanced interventional and hybrid applications in complex structural heart diseases in our country.

His academic productivity represents a scientifically strong authority with 110 SCI/SCIE publications, 1,440 citations, and an H-index of 18 (Google Scholar, 2025).