Human Anatomy
Meninges and Cerebrospinal Fluid
Meninges and cerebrospinal fluid for MBBS: dural folds, meningeal haemorrhage, CSF secretion and circulation, hydrocephalus and lumbar puncture, mapped to NMC codes AN56.1 and AN56.2.
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Meninges and Cerebrospinal Fluid
Meninges and cerebrospinal fluid for MBBS: dural folds, meningeal haemorrhage, CSF secretion and circulation, hydrocephalus and lumbar puncture, mapped to NMC codes AN56.1 and AN56.2.
This chapter covers the layered coverings of the brain and spinal cord and the production, circulation and absorption of cerebrospinal fluid. It provides the anatomical basis for intracranial haemorrhage, hydrocephalus and safe lumbar puncture.
High-yield: Meninges and Cerebrospinal Fluid
- The three cranial meninges, from outside in, are the dura mater, the arachnoid mater and the pia mater.
- The cranial dura has two layers, an outer endosteal layer and an inner meningeal layer, which separate to enclose the dural venous sinuses.
- The falx cerebri, tentorium cerebelli, falx cerebelli and diaphragma sellae are folds of the meningeal dura.
- The middle meningeal artery, a branch of the maxillary artery, supplies most of the cranial dura and is torn in extradural haemorrhage.
- An extradural haematoma is classically due to middle meningeal artery rupture at the pterion and produces a lens-shaped, biconvex collection.
- A subdural haematoma results from tearing of the cerebral bridging veins and forms a crescent-shaped collection that crosses suture lines.
- Subarachnoid haemorrhage, often from a ruptured berry aneurysm of the circle of Willis, fills the subarachnoid space with blood.
- Cerebrospinal fluid is secreted by the choroid plexuses of the lateral, third and fourth ventricles, at roughly 500 millilitres per day.
- The total volume of cerebrospinal fluid is about 150 millilitres, so the whole volume is turned over several times daily.
- Fluid flows from the lateral ventricles through the interventricular foramina to the third ventricle, then the cerebral aqueduct to the fourth ventricle.
- It leaves the fourth ventricle through the median aperture of Magendie and the two lateral apertures of Luschka into the subarachnoid space.
- Cerebrospinal fluid is reabsorbed into the superior sagittal sinus through the arachnoid granulations.
- Blockage of flow gives obstructive hydrocephalus; failure of absorption gives communicating hydrocephalus.
- Lumbar puncture is done between the third and fourth or fourth and fifth lumbar spines, safely below the end of the spinal cord.
- The pia mater closely invests the brain and spinal cord and forms the denticulate ligaments and the filum terminale.
Intracranial haemorrhage at a glance
- **Extradural:** Middle meningeal artery torn at the pterion. Biconvex lens shape on scan, does not cross sutures. Lucid interval then deterioration.
- **Subdural:** Bridging cerebral veins torn. Crescent shape, crosses sutures. Common in the elderly and after minor trauma.
- **Subarachnoid:** Ruptured berry aneurysm of the circle of Willis. Thunderclap headache; blood in the subarachnoid space.
- **CSF circulation:** Choroid plexus makes it; lateral to third to aqueduct to fourth ventricle; out via Magendie and Luschka; absorbed at arachnoid granulations.
NMC competencies in this chapter
- **AN56.1:** Layers, extent and modifications of the meninges
- **AN56.2:** Circulation of cerebrospinal fluid and its applied anatomy
Frequently Asked Questions
How do extradural and subdural haematomas differ anatomically?
An extradural haematoma is arterial, from the middle meningeal artery at the pterion, and appears biconvex without crossing suture lines. A subdural haematoma is venous, from torn bridging veins, and appears crescent-shaped and can cross sutures.
Where is cerebrospinal fluid made and absorbed?
It is secreted by the choroid plexuses of the ventricles and absorbed into the superior sagittal sinus through the arachnoid granulations. About 150 millilitres circulates and is renewed several times a day.
Why is lumbar puncture done below the second lumbar vertebra?
The spinal cord ends at about the lower border of the first lumbar vertebra in the adult, so a needle placed at the third-fourth or fourth-fifth lumbar space enters the cerebrospinal fluid without risking the cord.
What causes hydrocephalus?
Obstructive hydrocephalus follows a block in the ventricular pathway, such as aqueduct stenosis. Communicating hydrocephalus follows impaired absorption at the arachnoid granulations, for example after subarachnoid haemorrhage or meningitis.
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