A 28-year-old man is brought to the emergency department after a motor vehicle collision with loss of consciousness at the scene. He regains consciousness and appears alert, conversing normally with paramedics during transport. Upon arrival, he suddenly becomes drowsy and develops a fixed, dilated right pupil. Non-contrast CT head is performed urgently. The structure marked **A** in the diagram shows a biconvex, hyperdense collection limited by suture lines overlying the right temporal region with an associated skull fracture. Which of the following best explains the mechanism of formation of this lesion?

Explanation

## Why option 1 is right The structure marked **A** — a biconvex, lens-shaped, hyperdense collection limited by suture lines — is the pathognomonic CT appearance of an epidural hematoma (EDH). The classic mechanism is laceration of the middle meningeal artery (MMA) by a fracture of the squamous temporal bone at the pterion (the thinnest part of the skull). Arterial pressure rapidly dissects the dura mater off the inner table of the bone in the potential epidural space. This expansion is halted by the tight adherence of dura to bone at suture lines (coronal, sagittal, lambdoid), creating the characteristic lens-shaped boundary. The patient's lucid interval (initial loss of consciousness, then apparent recovery, followed by sudden deterioration with fixed pupil and herniation signs) is the clinical hallmark of EDH. [Harrison 21e Ch 444] ## Why each distractor is wrong - **Option 2**: Bridging vein tears cause subdural hematoma (SDH), which is venous and expands slowly. Crucially, SDH crosses suture lines because it lies between dura and arachnoid, not bounded by sutures. The crescent shape crossing sutures is the hallmark of SDH, not the lens-shape respecting sutures seen here. - **Option 3**: Intraparenchymal hemorrhage (traumatic intracerebral hematoma) would appear as a hyperdense collection within brain tissue with surrounding edema, not as an extra-axial collection bounded by sutures. This corresponds to structure **D**, not **A**. - **Option 4**: Subarachnoid hemorrhage produces diffuse hyperdensity in the basal cisterns and sulci, not a focal lens-shaped collection. This corresponds to structure **C** and is not limited by suture lines. **High-Yield:** Lens-shaped + respects sutures = EDH (arterial, MMA, pterion fracture); crescent + crosses sutures = SDH (venous, bridging veins). [cite: Harrison 21e Ch 444 — Traumatic Brain Injury]