Overview

CSF in the subarachnoid space≈125 mL

Lumbar cisternL2 to S2

The subarachnoid space contains most of the body's CSF (≈125 of ≈150 mL). It communicates with the ventricular system through the median and lateral apertures of the fourth ventricle. Because the arachnoid bridges over irregularities while the pia dips into them, the space widens into cisterns wherever the brain surface curves away from the skull. The main cisterns are:
• the cerebellomedullary cistern (cisterna magna), behind the medulla;
• the pontine (prepontine) cistern, around the basilar artery;
• the interpeduncular cistern, with CN III and the circle of Willis;
• the chiasmatic cistern;
• the quadrigeminal (ambient) cistern, with the great cerebral vein and pineal gland;
• the cistern of the lateral sulcus, with the MCA branches.
All the cerebral arteries and veins, and the proximal cranial nerves, run within it, so a ruptured aneurysm bleeds directly into the CSF.

The space also extends:
• along the optic nerve sheath as far as the back of the eyeball, so raised intracranial pressure is transmitted to the optic disc and causes papilloedema;
• along each spinal nerve root;
• below the conus as the lumbar cistern (L2 to S2), containing the cauda equina and filum terminale.
The lumbar cistern is where CSF is sampled and where spinal anaesthetic is injected.

Location: Between the arachnoid and pia around the whole CNS; largest at the cisterns and the lumbar cistern (L2–S2).

Parts & subdivisions

Cerebellomedullary cistern (cisterna magna)
Largest cistern, between the cerebellum and the back of the medulla; receives CSF from the median aperture.
Pontine and interpeduncular cisterns
In front of the pons and between the cerebral peduncles; contain the basilar artery, circle of Willis and CN III/VI. The floor of the third ventricle is opened into these in endoscopic third ventriculostomy.
Chiasmatic and quadrigeminal (ambient) cisterns
Around the optic chiasm; and around the great cerebral vein and dorsal midbrain.
Lumbar cistern
From the conus (L1–L2) to the end of the dural sac (S2); contains the cauda equina; site of lumbar puncture.
Optic nerve sheath extension
Carries CSF pressure to the optic disc (papilloedema).

Function

Contains the CSF that cushions and buoys the CNS
Pathway for CSF circulation from the fourth ventricle to the arachnoid granulations
Route of the major cerebral arteries and veins
Allows CSF sampling and intrathecal drug delivery (lumbar cistern)

Clinical significance

Subarachnoid haemorrhage

Blood in the basal cisterns and sulci on CT, usually from a ruptured saccular aneurysm, produces a thunderclap headache, meningism and reduced consciousness. If CT is negative after ≈6 h, LP is done to look for xanthochromia (yellow bilirubin pigment appearing ≈12 h after bleeding).

Lumbar puncture

With the patient curled on their side, the needle is inserted at L3/L4 or L4/L5, below the conus. The line joining the iliac crests (Tuffier line) marks ≈L4. The needle passes in turn through skin, subcutaneous fat, supraspinous ligament, interspinous ligament, ligamentum flavum (felt as a "give"), epidural space, dura and arachnoid, and reaches the lumbar cistern. Contraindications are raised ICP with a mass lesion (risk of coning), coagulopathy and local infection.

Compiled from standard anatomy and physiology references — see the Nervous System sources. For education only; not medical advice.