Q5 LONG ANSWER 10M ★ High-yield

Describe the meninges and the ventricular system. Explain the formation, circulation and absorption of cerebrospinal fluid and add a note on hydrocephalus.

Anatomy · 1st Professional — Pre-Clinical

The Meninges

Layer	Features
Dura mater	Two layers — endosteal (the periosteum of the skull) and meningeal. They separate to form the venous sinuses, and the meningeal layer forms the falx cerebri, tentorium cerebelli, falx cerebelli and diaphragma sellae
Arachnoid mater	Avascular; separated from the pia by the subarachnoid space containing CSF; sends arachnoid granulations into the sinuses
Pia mater	Closely invests the brain, dipping into every sulcus; highly vascular

Extradural space — a potential space in the skull, but a real space in the vertebral canal, containing fat and the internal vertebral venous plexus — the site of epidural anaesthesia
Subdural space — potential; crossed by the bridging veins

The Ventricular System

The lateral ventricle has an anterior, posterior and inferior horn with a body
The cerebral aqueduct is the narrowest part of the system, and the commonest site of congenital obstruction

Cerebrospinal Fluid

Feature	Value
Total volume	130–150 mL
Rate of formation	0.35 mL/min, about 500 mL/day — replaced 3 to 4 times daily
Site of formation	Choroid plexus of the lateral, third and fourth ventricles
Site of absorption	Arachnoid granulations into the superior sagittal sinus
Pressure	60–150 mm H2O in the lateral position
Glucose	Two-thirds of blood glucose (45–80 mg/dL)
Protein	15–45 mg/dL
Cells	0–5 lymphocytes/mm3

Functions — mechanical protection (buoyancy reduces the effective weight of the brain from 1400 g to about 50 g), maintenance of a constant environment, removal of metabolites, and a route for some hormones

Hydrocephalus

Type	Site of the problem	Examples
Obstructive (non-communicating)	Block within the ventricular system	Aqueduct stenosis, posterior fossa tumour, colloid cyst of the third ventricle
Communicating	Block in the subarachnoid space or at the granulations	After subarachnoid haemorrhage or meningitis
Ex vacuo	Not true hydrocephalus	Ventricles enlarge to fill the space left by cerebral atrophy

The Blood–brain Barrier

Formed by tight junctions between the endothelial cells of the cerebral capillaries, with a continuous basement membrane and the foot processes of astrocytes
Lipid-soluble substances cross readily — anaesthetics, alcohol, nicotine; water-soluble ones need carriers
Absent at the circumventricular organs — area postrema (the vomiting centre chemoreceptor trigger zone), pineal, median eminence, posterior pituitary, subfornical organ — which must sample the blood
Broken down by inflammation and tumour, which is why contrast enhances these lesions on imaging

Herniation Syndromes

Type	What herniates	Consequence
Subfalcine (cingulate)	Cingulate gyrus under the falx	Compresses the anterior cerebral artery → leg weakness
Uncal (transtentorial)	Uncus of the temporal lobe through the tentorial notch	Compresses the third nerve — fixed dilated pupil on the same side; then the midbrain
Central	Diencephalon downward	Progressive loss of consciousness; small reactive pupils
Tonsillar (coning)	Cerebellar tonsils through the foramen magnum	Compresses the medulla — respiratory and cardiac arrest

Cushing reflex — rising intracranial pressure produces hypertension, bradycardia and irregular respiration, a late and ominous triad

Applied Aspects

Lumbar puncture at L3–L4 or L4–L5, below the conus; contraindicated when intracranial pressure is raised with a mass, because of the risk of coning
Extradural haematoma — arterial, from the middle meningeal artery at the pterion; a lucid interval then rapid deterioration; biconvex (lens-shaped) on CT and limited by the sutures
Subdural haematoma — venous, from torn bridging veins; commoner in the elderly and alcoholics with cerebral atrophy; crescentic on CT and crosses sutures
Subarachnoid haemorrhage — usually a ruptured berry aneurysm on the circle of Willis; sudden "worst headache of my life" with neck stiffness; blood in the CSF or xanthochromia
Ventriculoperitoneal shunt diverts CSF in hydrocephalus; its complications are blockage, infection and over-drainage
The blood–brain barrier limits chemotherapy and antibiotic penetration, which is why higher doses or intrathecal routes are used in CNS infection and leukaemia
Normal pressure hydrocephalus — the triad of gait apraxia, urinary incontinence and dementia, with normal opening pressure; reversible by shunting, so it must not be dismissed as ageing
Epidural anaesthesia exploits the true extradural space of the vertebral canal, which does not exist inside the skull
Post-dural puncture headache — from CSF leak; worse on sitting up and relieved by lying flat, and treated with a blood patch
Meningism — neck stiffness with Kernig and Brudzinski signs, from irritation of the lumbar roots as they are stretched across an inflamed subarachnoid space
Tuberculous meningitis is common in India and characteristically affects the basal meninges, obstructing CSF flow and causing communicating hydrocephalus and cranial nerve palsies
The falx and tentorium limit the brain's movement, and it is against their rigid free edges that herniating tissue is damaged
Choroid plexus papilloma is a rare cause of hydrocephalus by overproduction rather than obstruction
CSF rhinorrhoea after a fracture of the cribriform plate risks meningitis; the fluid is identified by its glucose content or by beta-2 transferrin
The subarachnoid space extends around the optic nerve, which is why raised intracranial pressure produces papilloedema and why the disc must be examined before a lumbar puncture

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