What is the diagnosis and classification of fetal pleural effusion in a pregnant woman?

Fetal Pleural Effusion: Diagnosis and Classification

Fetal pleural effusion is diagnosed by ultrasound and classified based on laterality (unilateral vs. bilateral), presence of hydrops fetalis, and underlying etiology, with primary (idiopathic) effusions distinguished from secondary causes such as chromosomal abnormalities, infections, cardiac anomalies, and thoracic masses. 1, 2

Diagnostic Approach

Initial Ultrasound Evaluation

• Characterize the effusion by documenting whether it is unilateral or bilateral, measuring the size, and assessing for mediastinal shift 2, 3
• Evaluate for hydrops fetalis, defined as fluid accumulation in two or more body compartments (ascites, pleural effusion, pericardial effusion, skin edema, or polyhydramnios) 1
• Assess for structural abnormalities including cardiac defects, thoracic masses (congenital pulmonary airway malformation, bronchopulmonary sequestration), diaphragmatic hernia, and skeletal dysplasias 2

Comprehensive Diagnostic Workup

• Fetal echocardiography to exclude cardiac arrhythmias, structural heart disease, and assess cardiac function 2, 3
• Maternal testing including blood group and antibody screen, hemoglobinopathy screening, and serologic testing for congenital infections (parvovirus B19, cytomegalovirus, toxoplasmosis) 2
• Invasive testing with amniocentesis for karyotype, chromosomal microarray, and viral cultures on amniotic fluid 2
• Thoracentesis for pleural fluid analysis (cell count, biochemistry, karyotype, viral studies) when etiology remains unclear 2

Classification System

By Etiology

Primary (idiopathic) pleural effusions account for a significant proportion of cases where no underlying cause is identified after comprehensive evaluation 2, 3

Secondary pleural effusions are associated with:

• Chromosomal abnormalities (particularly trisomy 21, Turner syndrome, trisomy 18) 1
• Cardiac causes including arrhythmias and structural heart disease 1
• Fetal anemia from parvovirus B19 infection or fetomaternal hemorrhage 1
• Thoracic masses such as congenital pulmonary airway malformation or bronchopulmonary sequestration 1
• Chylothorax (lymphatic obstruction) 1
• Twin-twin transfusion syndrome or twin-anemia polycythemia sequence in multiple gestations 1

By Severity and Progression

Resolved effusions demonstrate spontaneous resolution on serial ultrasound examinations, occurring in approximately 27% of primary pleural effusions 3

Stable effusions remain unchanged in size without development of hydrops or mediastinal shift, also occurring in approximately 27% of cases 3

Progressive effusions are characterized by increasing size, new onset of hydrops fetalis, or impaired cardiac function on fetal echocardiography, occurring in approximately 40% of cases 3

By Presence of Hydrops

Non-hydropic effusions have the best prognosis with survival rates of 60-93% 4, 5

Hydropic effusions carry significantly worse prognosis, though survival improves to 44-86% with successful intervention when hydrops resolves after thoracoamniotic shunting 4, 5

Clinical Implications

Risk Stratification

• Small, apparently stable effusions require close surveillance as they can progress rapidly to severe hydrops 6
• Bilateral effusions are present in approximately 50% of primary pleural effusion cases and may indicate higher risk 3
• Mediastinal shift indicates significant mass effect requiring consideration for intervention 2

Prognostic Factors

The prognosis depends critically on:

• Underlying etiology, with treatable causes (cardiac arrhythmias, parvovirus infection) having better outcomes than chromosomal abnormalities 1
• Presence and reversibility of hydrops, as persistent hydrops after intervention carries 48% survival compared to 86% when hydrops resolves 5
• Gestational age at diagnosis, with detection before 24 weeks associated with higher rates of aneuploidy and extremely poor survival 1
• Isolated chylothorax has mortality as low as 6%, but increases to two-thirds mortality when associated anomalies are present 1

Common Pitfalls

• Failing to perform comprehensive evaluation before labeling an effusion as "primary" may miss treatable secondary causes 2
• Underestimating progression risk in small effusions can lead to delayed intervention when rapid deterioration occurs 6
• Not distinguishing between primary and secondary effusions affects both prognosis counseling and management decisions 2, 3