Recesses of the Pleura — Costodiaphragmatic and Costomediastinal
Definition
Pleural recesses are expanded regions of the pleural cavity formed where one part of the parietal pleura reflects onto another without the lung filling the angle of reflection. They act as reserve spaces — allowing the lungs to expand into them during deep inspiration.
Recesses are only obvious in expiration. During deep inspiration, the lungs expand into them and they are obliterated.
Costodiaphragmatic Recess
Location: Inferiorly — formed at the junction of the costal pleura and diaphragmatic pleura
Extent:
- Vertically: approximately 5 cm
- Rib levels: lies opposite the 8th–10th ribs along the midaxillary line
Both sides: Present on both right and left sides
Most dependent part: The costodiaphragmatic recesses are the most dependent parts of the pleural cavities — fluid from pleural effusion first collects here.
Access: The costodiaphragmatic recess can be entered through the 9th and 10th intercostal spaces (in midaxillary line) without penetrating the lung in a patient breathing quietly.
Inferior margins compared:
| Structure | MCL | MAL | Lateral border of erector spinae |
|---|---|---|---|
| Inferior border of lung | 6th rib | 8th rib | 10th rib |
| Inferior border of pleura (costodiaphragmatic recess) | 8th rib | 10th rib | 12th rib |
The lung margin is 2 ribs higher than the pleural reflection at all three reference lines.
Costomediastinal Recess
Location: Anteriorly — formed at the junction of the costal pleura and mediastinal pleura, behind the sternum and costal cartilages
Right side: Possibly occupied by the anterior margin of the right lung even during quiet breathing — recess is small
Left side: Large recess — due to the presence of the cardiac notch of the left lung. The left lung does not occupy this space, so the costomediastinal recess is obvious even in quiet breathing
Area of superficial cardiac dullness: The left costomediastinal recess — unoccupied by lung — overlies the pericardium directly. Percussion here gives dullness instead of resonance. This is clinically called the area of superficial cardiac dullness.
Comparison Table
| Feature | Costodiaphragmatic Recess | Costomediastinal Recess |
|---|---|---|
| Location | Inferior — between costal and diaphragmatic pleurae | Anterior — between costal and mediastinal pleurae |
| Extent | ~5 cm vertically; 8th–10th ribs (MAL) | Behind sternum and costal cartilages |
| Both sides? | Yes — right and left | Yes — right and left |
| More prominent side | Equal | Left (due to cardiac notch) |
| Most dependent? | Yes — fluid collects here first | No |
| Clinical relevance | Pleural effusion; paracentesis | Area of superficial cardiac dullness |