Meninges and Subarachnoid Cisterns
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Direct answer
Three membranes — dura mater, arachnoid mater and pia mater — enclose the brain, and the spaces between them define the three classic intracranial collections: blood between bone and dura is extradural, between dura and arachnoid is subdural, and within the subarachnoid space is subarachnoid. Where the arachnoid bridges across concavities of the brain's base, the subarachnoid space widens into cisterns — the cerebellomedullary (cisterna magna), pontine, interpeduncular (basal), chiasmatic, ambient, quadrigeminal and lumbar cisterns — each named for the structures it cradles. Arachnoid granulations on the vault sinuses return CSF to venous blood, and their failure is one root of hydrocephalus.
What you must remember
- Dura mater: an outer periosteal layer (endosteum of the skull) and an inner meningeal layer, fused except where they split to enclose the venous sinuses; the meningeal layer folds into the falx cerebri, tentorium cerebelli, falx cerebelli and diaphragma sellae.
- Extradural haematoma: rupture of the middle meningeal artery (or its groove) after temporal head injury — biconvex, lens-shaped clot on CT that does not cross cranial sutures, with the classical lucid interval.
- Subdural haematoma: torn bridging veins passing from cortical surface to dural sinuses — crescentic clot that does cross sutures, common in the elderly and in alcoholics after trivial falls, chronic forms enhancing after weeks.
- Subarachnoid haemorrhage: ruptured berry aneurysm in most cases — thunderclap headache, meningism, blood in the cisterns on CT and xanthochromia in CSF after some hours.
- Arachnoid: avascular membrane applied to the dura; its granulations (Pacchionian bodies) herniate into the superior sagittal sinus and return CSF to blood; the subdural "space" is only a potential fluid-filled cleft.
- Pia mater: vascular, intimately clothing the brain and dipping into sulci; modified as the denticulate ligament and filum terminale in the spinal canal; the pia and arachnoid together are the leptomeninges.
- Cisterna magna: the largest cistern, between cerebellum and medulla, below the fourth ventricle foramina; the target of cisternal puncture in infants.
- Basal cisterns: the interconnected pontine, interpeduncular, chiasmatic and circummesencephalic (ambient) cisterns around the brainstem — effacement on CT signals raised pressure and herniation risk, and their anatomy guides safe endoscopic corridors.
Telling the three blood collections apart by their space
A man struck at cricket becomes drowsy after a clear half-hour; a haemodialysis patient slips over weeks into confusion; a hypertensive smoker is struck by the worst headache of his life — three different spaces, one method. The extradural clot is arterial: the middle meningeal branch of the maxillary artery grooves the inner temporal squama and tears with linear fractures; being under the periosteal dura, which is suture-bound, the clot cannot cross sutures, so CT shows a lens-shaped density confined by the coronal suture. The subdural clot is venous: bridging veins have no support once the brain atrophies, so the elderly fall trivially and tear them; the clot lies free over the convexity, crossing sutures but not the dural folds, hence its sickle shape along the hemisphere. The subarachnoid bleed is neither subdural nor epidural but into the CSF itself: an aneurysm of the circle of Willis ruptures into the basal cisterns, CT shows blood filling the interpeduncular and ambient cisterns within hours, and lumbar puncture later shows uniformly blood-stained CSF with xanthochromia by about twelve hours — the timing that separates true haemorrhage from a traumatic tap. Each diagnosis is an anatomy question wearing a clinical gown.
Where students slip
Students call the subdural space a real space; it is a potential cleft — the arachnoid is held to the dura only by a film of fluid, which is why it shears so easily in the elderly. The second error is the cistern inventory: the interpeduncular cistern contains the oculomotor nerves, and an aneurysm of the posterior communicating artery lying within it explains a painful third nerve palsy — an anatomy-to-pathology link examiners reward. Third, remember the arachnoid granulations are on the venous sinuses of the vault only, mainly the superior sagittal; CSF absorption, not production, is their job, and papilloedema with normal ventricles can follow their blockage.
Frequently asked questions
Which membrane gives rise to the dural folds and venous sinuses?
The inner meningeal layer of the dura forms the falx cerebri, tentorium cerebelli, falx cerebelli and diaphragma sellae; the split between this layer and the outer periosteal layer creates the dural venous sinuses.
What are arachnoid granulations and what do they do?
Tufted herniations of arachnoid, mainly into the superior sagittal sinus and its lateral lacunae, through which CSF passes into venous blood. They are the principal site of CSF absorption and may print pits on the inner skull table.
Name the major subarachnoid cisterns and one clinical use of each.
The cerebellomedullary (cisterna magna — cisternal puncture), pontine (vertebral artery and aneurysm surgery), interpeduncular (oculomotor nerve and posterior communicating aneurysm), chiasmatic (trans-sphenoidal landmarks), quadrigeminal (pineal region surgery) and lumbar cistern (lumbar puncture). Effacement of the basal cisterns on CT indicates impending herniation.
Why does a subdural haematoma cross sutures but an extradural haematoma not?
The extradural clot lies beneath the periosteal dura, which is firmly attached at sutures, limiting its spread. The subdural collection lies inside the dural sleeve, free to track over the whole convexity, stopping only at the dural folds.
Which vessels cause extradural and subdural haematomas?
The middle meningeal artery (with its accompanying veins) classically causes extradural haemorrhage after temporal fracture; torn bridging cerebral veins entering the superior sagittal sinus cause subdural haemorrhage.