Extradural versus Subdural Haematoma
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Direct answer
An extradural (epidural) haematoma collects between bone and dura, usually from a torn middle meningeal artery at the pterion, and appears on CT as a biconvex, lens-shaped mass that respects cranial sutures. A subdural haematoma collects beneath the dura from torn bridging cortical veins, appears as a crescentic mass that crosses sutures but stops at the falx, and favours the elderly, the alcoholic and the anticoagulated. Surgery is driven by thickness, midline shift and the patient's condition — classically evacuation for an extradural above about 30 mL and an acute subdural thicker than about 10 mm or shifting the midline more than 5 mm.
What you must remember
- Extradural: temporal fracture over the pterion, arterial bleeding, rapid progression, classically a lucid interval before deterioration; biconvex on CT, limited by sutural dural attachments so it does not cross sutures (a venous posterior fossa or frontal extradural may cross the midline by stripping dural sinuses).
- Subdural: bridging-vein rupture in cortical atrophy, falls from standing, alcohol and anticoagulants; crescentic on CT, crosses suture lines, respects the midline, may be bilateral and interhemispheric.
- CT density tells the age: acute subdural is hyperdense, subacute isodense (look for displaced cortex and effaced sulci), chronic hypodense; mixed density suggests acute-on-chronic rebleed.
- Surgical thresholds in common use: extradural volume above roughly 30 mL whatever the GCS; acute subdural thicker than 10 mm or midline shift above 5 mm regardless of GCS.
- Acute subdural is evacuated by craniotomy; chronic subdural by burr-hole drainage, with recurrence managed by re-drainage, and subdural drains after burr holes reduce recurrence.
- Extradural in the posterior fossa and in children without fracture (vascular-splitting skull) are recognised exceptions to the classic picture; always reverse anticoagulation.
- Both may coexist with underlying contusions — the subdural is often just the marker of a more severely injured brain, which explains its worse prognosis.
Common confusion
Shape mnemonics get reversed under pressure: lens-shaped (lentiform, biconvex) is extradural because sutures fence it in; crescent (concave inner margin) is subdural because it spreads freely over the hemisphere. The lucid interval is over-read as typical of all extradurals when it occurs in a minority, and candidates forget that a chronic subdural in an elderly patient may present with fluctuating confusion and no history of trauma — the "great imposter" of geriatric neurosurgery.
Exam-focused takeaway
This pairing is tested by numbers, shapes and populations: a young man with a temporal fracture who talks then deteriorates; an elderly anticoagulated patient with confusion and a crescentic isodense collection; an image asking biconvex versus crescentic with the sutures visible. Expect threshold questions (30 mL, 10 mm, 5 mm) and next-step questions where reversing anticoagulation, urgent CT or theatre wins over observation. The prognosis contrast — extradural with a good outcome if evacuated promptly versus subdural reflecting underlying brain injury — is a recurring viva line.
Frequently asked questions
Why is an extradural haematoma biconvex on CT?
Dural attachments at cranial sutures tether the collection, producing a lens shape, whereas the subdural space allows a crescentic spread across sutures.
What are the surgical thresholds for evacuation?
Classically extradural volume above about 30 mL regardless of GCS, and acute subdural thicker than 10 mm or with midline shift above 5 mm regardless of GCS.
How does a chronic subdural present and how is it treated?
Often with fluctuating headaches, confusion or focal deficit weeks after trivial or forgotten trauma, treated by burr-hole drainage, with drains reducing recurrence.
Who gets subdural rather than extradural haematoma?
Elderly, atrophic, alcoholic and anticoagulated brains with bridging-vein rupture after low-energy falls; young temporal fractures favour arterial extradural collections.
What does mixed density in a subdural mean?
An acute rebleed into a chronic collection — acute-on-chronic subdural haematoma.