8.2 Fetal Cardiomyopathies & Myocardial Dysfunction

Introduction to Fetal Cardiomyopathies

Fetal cardiomyopathies refer to primary diseases of the myocardium that result in structural and functional abnormalities of the heart, independent of congenital structural heart defects, primary valve disease, or severe arrhythmias. In the fetal period, these conditions can manifest as significant hemodynamic compromise, potentially progressing to heart failure and hydrops fetalis. The diagnosis of fetal cardiomyopathy relies on a comprehensive sonographic assessment of myocardial thickness, chamber size, and biventricular function (both systolic and diastolic). The three primary morphologic classifications are hypertrophic, dilated, and restrictive cardiomyopathies. A unique entity often seen in the fetal and neonatal period, endocardial fibroelastosis (EFE), will also be discussed. Identifying the underlying etiology is crucial, as it dictates counseling, potential in utero therapy, and postnatal management.

Fetal Cardiomyopathies Summary & Comparison

Hypertrophic Cardiomyopathy (HCM)

Hypertrophic cardiomyopathy in the fetus is characterized by inappropriate and excessive thickening of the ventricular myocardium, often disproportionately affecting the interventricular septum (asymmetric septal hypertrophy). The hallmark is increased wall thickness in the absence of a hemodynamic cause like severe outflow tract obstruction (e.g., severe aortic or pulmonary stenosis, though HCM itself can cause secondary obstruction).

The most common etiology for fetal hypertrophic cardiomyopathy is maternal diabetes mellitus, particularly poorly controlled gestational or pre-existing diabetes. The pathophysiology involves fetal hyperglycemia secondary to maternal hyperglycemia. This triggers fetal hyperinsulinemia. Insulin acts as a primary growth factor in the fetus, leading to increased protein and lipid synthesis and subsequent macrosomia. In the heart, this manifests as myocardial hypertrophy, most prominently in the interventricular septum due to a high concentration of insulin receptors in this region.

Sonographically, the interventricular septum appears disproportionately thick, often >5 mm in the third trimester. Bi-ventricular hypertrophy can also occur. The hypertrophy can lead to significant complications. First, the thickened walls are non-compliant, leading to severe diastolic dysfunction and elevated filling pressures. Second, profound septal hypertrophy can narrow the left ventricular outflow tract (LVOT) or right ventricular outflow tract (RVOT), creating a dynamic, functional obstruction. This obstruction can be exacerbated by hypercontractility. In severe cases of maternal diabetes, this can lead to congestive heart failure and hydrops. Prognostically, hypertrophy related to maternal diabetes is generally transient and resolves spontaneously within weeks to months after birth as the hyperinsulinemic state subsides.

Other causes of fetal HCM are less common but often carry a worse prognosis. These include familial hypertrophic cardiomyopathy (often sarcomeric protein gene mutations) and metabolic/storage disorders (e.g., Pompe disease - a glycogen storage disease). In Pompe disease, the hypertrophy is typically global and massive, often presenting earlier in gestation, and carries a very poor prognosis without early enzyme replacement therapy postnataly.

Dilated Cardiomyopathy (DCM)

Dilated cardiomyopathy is characterized by ventricular dilation and impaired systolic function (reduced contractility). It can affect the left ventricle, right ventricle, or both. The cardinal sonographic features are cardiomegaly (increased cardiothoracic ratio), spherical dilatation of the ventricles, and subjectively or objectively reduced myocardial contractility. The ejection fraction and fractional shortening are decreased. Secondary atrioventricular valve regurgitation (mitral or tricuspid regurgitation) is common due to annular dilation and papillary muscle dysfunction.

The etiologies of fetal DCM are diverse. Infectious causes, specifically viral myocarditis, are a primary consideration. Parvovirus B19, Cytomegalovirus (CMV), Coxsackievirus, and Rubella can all cross the placenta and directly infect the fetal myocardium, causing inflammation, necrosis, and subsequent dilation. Parvovirus B19 is particularly notable as it also causes fetal anemia (by suppressing erythropoiesis), leading to a combined hit of primary myocardial dysfunction and high-output failure, often resulting in severe hydrops.

Another critical cause of a DCM phenotype is Twin-Twin Transfusion Syndrome (TTTS). In TTTS, the recipient twin experiences severe volume overload and increased afterload (due to vasoactive mediators transferred from the donor). This initially leads to ventricular hypertrophy, but over time, the myocardium decompensates, resulting in marked cardiomegaly, ventricular dilation, severe systolic dysfunction, and significant AV valve regurgitation. This progressive cardiac failure is a key component of staging TTTS (Quintero staging) and dictates the need for interventions like fetoscopic laser photocoagulation.

Primary genetic or familial dilated cardiomyopathies can also present in utero, though less commonly than acquired causes. The prognosis for fetal DCM depends on the etiology. Viral myocarditis can sometimes resolve, while progressive dilation with hydrops carries a high mortality rate.

Restrictive Cardiomyopathy (RCM)

Restrictive cardiomyopathy is the least common of the major cardiomyopathies in the fetus. It is characterized by severe diastolic dysfunction with restrictive filling physiology. The myocardial walls are excessively stiff and non-compliant, severely limiting diastolic filling.

Echocardiographically, the hallmark of RCM is biatrial enlargement with normal or near-normal ventricular size and wall thickness. Because the ventricles cannot fill properly, atrial pressures rise precipitously, causing the atria to dilate. Systolic function (ejection fraction) is typically preserved, at least in the early stages. Doppler interrogation of the AV valves (mitral and tricuspid inflow) is crucial for diagnosis, demonstrating a restrictive filling pattern characterized by a very tall E wave (early rapid filling due to high atrial pressure) and a very small or absent A wave (reduced atrial contraction efficiency against a stiff ventricle), resulting in an elevated E/A ratio. Venous Doppler (ductus venosus, IVC) often shows marked reverse flow during atrial systole, reflecting the high right-sided filling pressures.

The etiology of fetal RCM is often idiopathic or associated with genetic mutations (e.g., sarcomeric proteins, desminopathies). It can also be a late consequence of severe, prolonged fetal distress or endocardial pathology. The prognosis for fetal restrictive cardiomyopathy is generally extremely poor. The severe diastolic dysfunction rapidly leads to elevated central venous pressure, intractable heart failure, and hydrops. Postnatal survival usually requires cardiac transplantation.

Endocardial Fibroelastosis (EFE)

Endocardial fibroelastosis (EFE) is a specific pathological condition characterized by a diffuse, thick layer of collagen and elastin proliferating in the endocardium, giving it a striking "porcelain" or bright white appearance. In the fetus, EFE is most commonly secondary to severe left-sided obstructive lesions.

EFE is classically associated with critical aortic stenosis or evolving Hypoplastic Left Heart Syndrome (HLHS). The underlying mechanism is believed to be related to markedly elevated intra-ventricular pressures. When the aortic valve is severely stenotic, the left ventricle must generate suprasystemic pressures to eject blood. This leads to subendocardial ischemia, which triggers a fibrotic and elastotic response.

Sonographically, EFE presents as a distinct, highly echogenic (bright) lining of the endocardium, most often in the left ventricle. The affected ventricle is typically globular, thick-walled, and exhibits severely depressed contractility. The bright endocardium may extend to the papillary muscles and chordae tendineae, contributing to severe mitral regurgitation. The presence of EFE is a poor prognostic sign for achieving a two-ventricle circulation postnatally. A left ventricle with extensive EFE is generally non-functional and unable to support systemic circulation, often pushing the management pathway towards single-ventricle palliation (e.g., Norwood procedure).

Primary EFE, not associated with structural defects, can rarely occur and is sometimes associated with maternal autoantibodies (anti-Ro/SSA) or viral infections. In these cases, the entire heart might be involved, leading to a restrictive/dilated phenotype. Identification of EFE mandates careful counseling regarding the poor functional prognosis of the affected ventricle.