A 68-year-old man with chronic alcoholism and on warfarin for atrial fibrillation presents to the emergency department following a minor fall from standing height. CT head (non-contrast) shows a hyperdense crescent-shaped extra-axial collection marked **A** that crosses multiple suture lines but does not cross the midline. Which of the following best explains the anatomical basis for this imaging pattern?

Explanation

## Why Option 1 is correct The structure marked **A** is an acute subdural hematoma (SDH), which is a collection of blood between the dura mater and arachnoid mater. The crescent shape crossing sutures but NOT crossing the midline is pathognomonic for SDH. This pattern arises because: (1) SDH results from tearing of bridging veins that traverse the subdural space to drain into the dural venous sinuses; (2) venous bleeding is low-pressure and spreads diffusely along the dural surface; (3) unlike epidural hematomas (EDH), SDH is NOT constrained by dural attachments at suture lines, so it crosses them freely; (4) however, it CANNOT cross the midline because the falx cerebri acts as an anatomical barrier. The patient's age, chronic alcohol use (causing brain atrophy and stretching of bridging veins), and anticoagulation (warfarin) are classic risk factors for SDH from trivial trauma. (Harrison 21e Ch 434; Greenberg Handbook of Neurosurgery 9e) ## Why each distractor is wrong - **Option 2**: This describes an epidural hematoma (EDH), which lies between skull and dura, arises from middle meningeal artery injury, presents as a biconvex/lentiform collection, DOES NOT cross sutures (constrained by dural attachments), but CAN cross the midline. The crescent shape crossing sutures rules out EDH. - **Option 3**: This incorrectly places the collection within the subdural space (redundant phrasing) and attributes it to cortical arteries with a biconvex pattern—features of EDH, not SDH. SDH is venous (bridging veins), not arterial, and is crescent-shaped, not biconvex. - **Option 4**: This places the collection in the subarachnoid space (between arachnoid and pia), which is incorrect for SDH. Subarachnoid hemorrhage arises from rupture of circle of Willis aneurysms or cortical vessels and has a different clinical presentation and imaging pattern. The constraint by both sutures and falx is also anatomically incorrect for any subdural process. **High-Yield:** SDH = crescent, crosses sutures, does NOT cross midline (falx limits), venous (bridging veins), elderly + trivial trauma. EDH = biconvex, does NOT cross sutures (dural attachments), CROSSES midline, arterial (middle meningeal), lucid interval classic. [cite: Harrison 21e Ch 434; Greenberg Handbook of Neurosurgery 9e]