A 52-year-old man presents to the emergency department following a fall from a height. CT head shows a crescent-shaped hematoma that does not cross the midline. Which radiological feature best distinguishes this finding from a subdural hematoma?

Explanation

## Distinguishing Subdural from Extradural Hematoma ### Key Radiological Features **Key Point:** The stem describes a **crescent-shaped hematoma that does not cross the midline** — this is a **subdural hematoma (SDH)**. The question asks which feature best distinguishes *this finding* (SDH) from an extradural hematoma (EDH). The correct distinguishing feature of SDH is that it **follows the contour of the brain surface and respects dural attachments** (e.g., the falx cerebri prevents midline crossing). ### Why Option B is Correct A subdural hematoma lies between the dura mater and the arachnoid mater. Because it is not bounded by suture lines (unlike EDH), it spreads freely along the brain surface, conforming to its curvature. However, it **does** respect major dural reflections such as the falx cerebri, which is why it does not cross the midline. This combination — following brain contours yet respecting dural attachments — is the hallmark distinguishing feature of SDH on CT. ### Morphological Comparison | Feature | Extradural Hematoma | Subdural Hematoma | | --- | --- | --- | | **Shape** | Lens-shaped (biconvex) | Crescent-shaped | | **Margins** | Sharp, well-demarcated | Ill-defined, follows brain surface | | **Location** | Between skull and dura mater | Between dura and arachnoid | | **Crosses suture lines** | No (confined by dural attachments) | Yes | | **Crosses midline** | No | No (falx acts as barrier) | | **Typical vessel** | Middle meningeal artery | Cortical bridging veins | ### Why Other Options Are Wrong - **Option A:** Crossing the midline and extending across both hemispheres is NOT a feature of SDH — the falx prevents this. This option is incorrect. - **Option C:** A lens-shaped (biconvex) appearance with sharp margins is the hallmark of **extradural hematoma**, not SDH. This is the opposite of what the stem describes. - **Option D:** The space between the pia mater and brain parenchyma is the **subarachnoid space** — SDH is located between the dura and arachnoid, not between pia and brain. This is anatomically incorrect for SDH. ### Anatomical Basis **High-Yield:** SDH results from tearing of cortical bridging veins traversing the subdural space. The blood collects between the dura and arachnoid, spreading freely over the hemisphere but stopped at the falx and tentorium (dural reflections). EDH, by contrast, is bounded by suture lines where the dura is firmly adherent to the skull, producing the classic biconvex shape. **Clinical Pearl:** The "lucid interval" is more classic for EDH (arterial bleeding from middle meningeal artery). Acute SDH often presents with immediate neurological deficit due to associated parenchymal injury. Chronic SDH may present insidiously in elderly patients on anticoagulants. [cite: Robbins & Cotran Pathologic Basis of Disease, 10e, Ch 28; Osborn's Brain, 3e]