What are the causes of hydrops fetalis?

Causes of Hydrops Fetalis

Cardiovascular abnormalities are the leading single cause of nonimmune hydrops fetalis, accounting for 17-35% of all cases, followed by chromosomal abnormalities (7-16%) and hematologic disorders (4-12%). 1, 2

Primary Etiologic Categories

Cardiovascular Causes (17-35%)

Cardiac etiologies represent the most common identifiable cause across all gestational ages. 2 This category includes:

• Structural heart defects, particularly right-heart lesions such as endocardial cushion defects and heterotaxy syndromes, which cause hydrops through increased central venous pressure and impaired diastolic ventricular filling 1
• Fetal arrhythmias, including supraventricular tachycardia and atrial flutter (treatable with transplacental antiarrhythmic therapy) 1
• Congenital heart block from transplacental passage of anti-Ro/SSA or anti-La/SSB antibodies in maternal autoimmune disease 1
• Cardiomyopathy, cardiac tumors, and vascular malformations 1

The prognosis for cardiac structural abnormalities is extremely poor, with combined fetal and infant mortality of 92%. 1

Chromosomal Abnormalities (7-16%)

Aneuploidy is the most common cause when hydrops is identified early in gestation. 1 Key chromosomal causes include:

• Turner syndrome (45,X), associated with 50-80% of cystic hygromas due to lymphatic dysplasia and lack of communication between lymphatic and venous drainage 1
• Trisomy 21 (Down syndrome), which may cause hydrops through associated cardiac malformations 1
• Trisomies 13,18, and triploidy 1

Chromosomal abnormalities confer an extremely poor prognosis with very high rates of intrauterine fetal death. 3

Hematologic Disorders (4-12%)

Fetal anemia from various causes produces hydrops through high-output cardiac failure and hypoxia. 1 Major hematologic etiologies include:

• Alpha thalassemia, accounting for 28-55% of hydrops in Southeast Asian populations but only ~10% in other populations 1, 2
• Parvovirus B19 infection causing red cell aplasia 1
• Fetomaternal hemorrhage 1
• Inherited hemoglobinopathies and red cell aplasia 1

Parents can be screened for alpha thalassemia carrier status by mean cell volume <80 fL. 1

Infectious Causes (5-7%)

Infections produce hydrops through anemia, anoxia, endothelial cell damage, and increased capillary permeability. 1 Key pathogens include:

• Parvovirus B19 1
• Cytomegalovirus 1
• Toxoplasmosis 1

Thoracic Abnormalities (6%)

Thoracic lesions cause hydrops through vena caval obstruction or increased intrathoracic pressure impairing venous return. 1 These include:

• Congenital cystic adenomatoid malformation 4
• Pulmonary sequestration 4
• Fetal pleural effusions, hydrothorax, and chylothorax 1, 4

Twin-Twin Transfusion Syndrome (3-10%)

Hypervolemia and increased central venous pressure in the recipient twin produce hydrops. 1

Additional Causes

• Urinary tract abnormalities (2-3%): urinary ascites and nephrotic syndrome with hypoproteinemia 1
• Gastrointestinal abnormalities (0.5-4%): obstruction of venous return and protein loss 1
• Lymphatic dysplasia (5-6%): impaired venous return 1
• Tumors including chorioangiomas (2-3%): anemia, high-output cardiac failure, hypoproteinemia 1
• Skeletal dysplasias (3-4%): hepatomegaly, hypoproteinemia, impaired venous return 1
• Inborn errors of metabolism (1-2%): visceromegaly, decreased erythropoiesis, hypoproteinemia 1

Idiopathic Cases (15-25%)

A cause can be identified in approximately 60% of cases prenatally and 85% when postnatal evaluation is included. 1 The proportion of idiopathic cases has decreased with more thorough diagnostic evaluation. 1

Common Pathophysiologic Mechanisms

The underlying pathophysiology involves an imbalance in fetal fluid regulation between vascular and interstitial spaces. 5 Specific mechanisms include:

• Increased central venous pressure from cardiac structural defects or hypervolemia 1
• High-output cardiac failure from severe anemia or arteriovenous malformations 1
• Hypoproteinemia from nephrotic syndrome, protein-losing enteropathy, or hepatic dysfunction 1
• Lymphatic obstruction or dysplasia impairing venous return 1
• Increased capillary permeability from infection or endothelial damage 1

Clinical Pitfalls

• Do not assume immune hydrops without performing an indirect Coombs test, as >90% of hydrops cases are now nonimmune in origin 6, 5, 7
• Always obtain fetal karyotype/chromosomal microarray regardless of whether structural anomalies are identified, as chromosomal abnormalities may be the sole etiology 1, 3
• Screen parents for alpha thalassemia carrier status (MCV <80 fL) in appropriate ethnic populations, as this is a highly prevalent but often overlooked cause 1
• Perform comprehensive fetal echocardiography in every case, given that cardiovascular causes are the leading etiology and may be subtle 3, 2