Which radiological sign describes the concave meniscus-like appearance of a pleural effusion at the lung-fluid interface on frontal chest X-ray?

Explanation

## Radiological Signs of Pleural Effusion on Chest X-ray **Key Point:** The **Meniscus sign** describes the concave, upward-curving (meniscus-like) appearance of pleural fluid at the lung-fluid interface on a frontal chest X-ray. This characteristic concave upper border is caused by fluid rising higher along the lateral chest wall due to capillary forces and pleural pressure gradients. ### Why the Meniscus Sign? **High-Yield:** When a moderate-to-large pleural effusion accumulates, the fluid-air interface on an upright frontal (PA/AP) chest X-ray takes on a concave shape — higher laterally and lower medially — resembling a meniscus. This is the **Meniscus sign** (also called the "meniscoid" or "concave upper border" of pleural effusion). It is the sign that specifically and directly describes the *concave meniscus-like appearance* at the lung-fluid interface. ### Comparison of Key Radiological Signs | Sign | Description | View | | --- | --- | --- | | **Meniscus sign** | Concave upper border of pleural fluid at lung-fluid interface | Frontal (PA/AP) CXR | | Blunting of costophrenic angle | Rounded/obliterated costophrenic angle; earliest sign (~200–300 mL) | Frontal CXR | | Silhouette sign | Loss of normal border between two structures of similar density | Frontal CXR | | Air bronchogram sign | Air-filled bronchi visible within opacified lung | Frontal CXR; seen in consolidation, NOT effusion | **Clinical Pearl:** Blunting of the costophrenic angle is the *earliest* sign of pleural effusion on frontal CXR (requires ~200–300 mL), but it does not describe the concave meniscus-like appearance. The Meniscus sign specifically refers to the concave fluid contour seen as the effusion becomes larger and the fluid-air interface becomes visible above the diaphragm. **Important Distinction:** The Meniscus sign is visible on an upright frontal CXR with moderate-to-large effusions. It should not be confused with the lateral decubitus view, which is used to confirm that fluid is free-flowing (not loculated). **Reference:** Grainger & Allison's Diagnostic Radiology; Felson's Principles of Chest Roentgenology — the meniscus sign is the classic descriptor for the concave upper margin of pleural fluid on frontal chest radiograph. **Mnemonic:** **MENISCUS** = the **M**oon-shaped concave fluid **E**dge **N**oted **I**n pleural effusion on **S**tandard **C**hest **U**pright **S**hot.