Meningeal Layers
The three meningeal layers protect the brain and spinal cord. The spaces between them are the sites of clinically critical haemorrhages — each with a distinct mechanism, CT appearance, and management.
✦ The Mnemonic
"Delicate Artful Protection — Dura, Arachnoid, Pia"
Dura mater · Arachnoid mater · Pia mater — with epidural, subdural, subarachnoid spaces
Clinical Breakdown
The dura has two layers in the skull: the outer periosteal layer (adherent to skull) and the inner meningeal layer. These separate at the dural sinuses (superior sagittal, transverse, sigmoid). The dura folds inward to form: falx cerebri (between hemispheres), tentorium cerebelli (between cerebrum and cerebellum), and the diaphragma sellae (roof of pituitary fossa).
Extradural haematoma: arterial bleed (usually middle meningeal artery from temporal bone fracture). Classic 'lucid interval': brief loss of consciousness, recovery, then progressive deterioration as the haematoma expands. Biconvex (lens-shaped) on CT — does not cross suture lines. Surgical emergency — burr hole/craniotomy.
Subdural haematoma: venous bleed from torn bridging veins (cortical veins crossing the subdural space). More common in elderly, alcoholics, patients on anticoagulants. Crescent-shaped on CT — follows brain surface and crosses suture lines. Acute (<3 days), subacute (3–21 days), or chronic (>21 days) — chronic SDH appears hypodense on CT.
⭐ Clinical Pearl
Uncal herniation: a supratentorial mass displaces the uncus (medial temporal lobe) through the tentorial notch, compressing CN III (dilated pupil), the posterior cerebral artery (ipsilateral occipital infarct), and the midbrain (Duret haemorrhages — secondary brainstem haemorrhage). The sequence: unilateral pupil dilation → decreasing consciousness → bilateral motor signs → death.