A 72-year-old man with chronic alcoholism presents with progressive confusion over 3 weeks. CT head shows a crescent-shaped hypodense collection over the convexity that crosses the midline. Which anatomical structure forms the inner boundary of this hematoma?

Explanation

## Anatomical Boundaries of Subdural Hematoma **Key Point:** Understanding the anatomical spaces is critical to identifying which structure forms the boundary of a subdural hematoma. ### Meningeal Layers (Superficial to Deep) ``` Skull ↓ Dura mater (outermost, thick, tough) ↓ Subdural space (potential space; contains bridging veins) ↓ Arachnoid mater (delicate, avascular) ↓ Subarachnoid space (contains CSF, major vessels) ↓ Pia mater (innermost, adheres to brain) ↓ Brain tissue ``` ### Subdural Hematoma Location - **Definition:** Blood collection between dura mater (outer) and arachnoid mater (inner) - **Inner boundary:** The arachnoid mater forms the inner boundary - **Outer boundary:** The dura mater forms the outer boundary - **Mechanism:** Tearing of bridging veins that cross the subdural space causes bleeding **High-Yield:** The subdural space is a potential space between dura and arachnoid; subdural hematomas fill this space and can expand freely without anatomical constraints. ### Clinical Correlation - **Chronic subdural (as in this case):** Slow accumulation of blood over weeks to months; common in elderly and chronic alcoholics due to brain atrophy and fragile bridging veins - **Crescent shape crossing midline:** Characteristic of subdural because arachnoid does not attach at sutures or falx, allowing blood to spread - **Hypodense on CT:** Chronic subdurals become hypodense as blood breaks down and is diluted by CSF **Clinical Pearl:** The arachnoid is avascular and delicate; it does not bleed itself but is pushed inward by the expanding hematoma. The inner boundary you see on imaging is the arachnoid being compressed inward.