## Deep Sulcus Sign in Pneumothorax **Key Point:** The deep sulcus sign is a radiological indicator of pneumothorax, particularly useful for detecting small or subtle pneumothoraces on frontal chest radiographs. ### Anatomical Basis The costophrenic angle (sulcus) becomes abnormally lucent and extends deeper into the hemidiaphragm when air accumulates in the pleural space. This occurs because: 1. Free air in the pleural cavity collects in the most dependent area (costophrenic recess) 2. The air-filled pleural space appears radiolucent compared to normal lung tissue 3. The diaphragm appears depressed and the sulcus deepened ### Clinical Significance - Most sensitive sign for **supine or semi-upright patients** (ICU patients, trauma patients) - More reliable than the classic "lung edge" sign in supine positioning - Can detect pneumothorax when the visceral pleural line is not visible - Particularly valuable in trauma and critical care settings ### Radiographic Features | Feature | Description | |---------|-------------| | **Appearance** | Hyperlucency extending from costophrenic angle upward | | **Location** | Basilar region, typically unilateral | | **Associated finding** | Depressed hemidiaphragm | | **Best seen on** | Supine or semi-upright CXR | **High-Yield:** The deep sulcus sign is often the ONLY radiological clue to pneumothorax in supine patients and should be actively sought in trauma and ICU settings. **Clinical Pearl:** When you see an unusually lucent costophrenic angle on a supine film, think pneumothorax before assuming it is normal anatomy or artifact. 
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