On chest X-ray, a pleural effusion appears as a homogeneous opacity that obscures the costophrenic angle and rises obliquely across the chest wall. This radiographic sign is known as:

Explanation

## Radiographic Signs of Pleural Effusion **Key Point:** The meniscus sign is the classic radiographic appearance of pleural effusion on frontal chest X-ray, characterized by a homogeneous opacity with a curved upper border that rises obliquely from the lateral costophrenic angle toward the mediastinum. ### Meniscus Sign — Pathophysiology When fluid accumulates in the pleural space, gravity causes it to pool dependently. The curved upper border (concave upward) represents the interface between fluid and aerated lung, creating the characteristic meniscus appearance. This is most prominent at the costophrenic angle, where the lung base meets the lateral chest wall. ### Radiographic Features | Feature | Description | | --- | --- | | **Location** | Costophrenic angle (lateral and posterior) | | **Shape** | Curved/concave upper border | | **Opacity** | Homogeneous, water density | | **Blunting** | Obscures the sharp costophrenic angle | | **Oblique rise** | Rises toward mediastinum (higher medially) | **High-Yield:** The meniscus sign requires at least 200–250 mL of pleural fluid to become radiographically visible on upright frontal films. Lateral decubitus views can detect smaller effusions (as little as 50 mL). **Clinical Pearl:** If the effusion is loculated (trapped by adhesions), the meniscus sign may be absent or irregular, and the opacity may not follow gravity-dependent distribution. [cite:Felson's Principles of Chest Roentgenology 3e]