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AN56.1-2 | Meninges & CSF — Practice Quiz
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The falx cerebri is a fold of dura mater that separates the two cerebral hemispheres. Its anterior attachment is to the:
Correct! The falx cerebri attaches anteriorly to the crista galli (a superior projection of the ethmoid bone) and posteriorly to the internal occipital protuberance.
Falx cerebri: crista galli (anterior) → internal occipital protuberance (posterior). Superior sagittal sinus runs in its upper border; inferior sagittal sinus in its lower border. Straight sinus at its posterior junction with tentorium.
Incorrect. Falx cerebri = crista galli (anterior) + internal occipital protuberance (posterior). The cribriform plate is horizontal and lies beside the crista galli but is not the attachment.
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A patient with a rapidly expanding right extradural haematoma develops a fixed dilated right pupil. This is due to compression of which structure at the tentorial notch?
Correct! Uncal herniation through the tentorial notch compresses the ipsilateral CN III. The parasympathetic pupilloconstrictor fibres run on the outside of CN III and are compressed first → dilated fixed pupil (blown pupil). A sign of impending coning.
Herniation syndromes: Uncal (transtentorial) = ipsilateral CN III palsy + contralateral hemiplegia (cerebral peduncle compression). Central herniation = bilateral signs, Cheyne-Stokes breathing. Cerebellar tonsillar herniation through foramen magnum = compression of medullary respiratory centres → death.
Incorrect. Tentorial notch (uncal) herniation: medial temporal lobe (uncus) herniates → compresses ipsilateral CN III → dilated fixed pupil. CN VI palsy (lateral rectus) causes different signs.
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Which of the following nerves travels within the substance of the lateral wall of the cavernous sinus (NOT free within the sinus)?
Correct! CN III, CN IV, CN V1, and CN V2 travel in the lateral wall of the cavernous sinus (embedded in the wall). CN VI and the internal carotid artery are free within the sinus cavity itself.
Cavernous sinus mnemonic (lateral wall, superior to inferior): "O T T M A" = Oculomotor (III), Trochlear (IV), ophthalmic (V1), Maxillary (V2). Abducens (VI) and ICA are medial within the sinus cavity. Cavernous sinus thrombosis (from facial furuncle spreading via facial/ophthalmic veins) affects all these nerves → multiple cranial nerve palsies + proptosis.
Incorrect. Lateral wall (superior to inferior): CN III, CN IV, CN V1, CN V2. Free within the sinus: CN VI + ICA. This matters because ICA-cavernous fistula affects CN VI most prominently (it is unprotected inside the sinus).
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The majority of cerebrospinal fluid is produced by the:
Correct! The choroid plexus — specialised ependymal cells overlying capillary tufts — produces 500 mL/day of CSF. The lateral ventricles have the largest choroid plexus and produce the most CSF.
CSF facts: 500 mL produced/day; only 150 mL present at any time (rapid turnover). Choroid plexus papilloma = tumour of choroid plexus → CSF overproduction → communicating hydrocephalus in children. Acetazolamide reduces CSF production (carbonic anhydrase inhibitor) — used in pseudotumour cerebri.
Incorrect. CSF production = choroid plexus (primarily lateral ventricles). Arachnoid granulations ABSORB CSF. Ependymal cells lining the aqueduct do not produce significant CSF.
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A 2-year-old child has enlarging head circumference. MRI shows dilated lateral and 3rd ventricles with a normal-sized 4th ventricle. The most likely site of obstruction is the:
Correct! Dilation of lateral + 3rd ventricles with NORMAL 4th ventricle = obstruction at the cerebral aqueduct of Sylvius (between 3rd and 4th ventricles). This is the most common cause of congenital obstructive hydrocephalus.
Localise the block from the imaging: (1) Lateral + 3rd dilated, 4th normal = aqueduct stenosis. (2) All four ventricles dilated = communicating hydrocephalus (absorption block). (3) One lateral ventricle dilated = foramen of Monro block (colloid cyst). Aqueduct stenosis is the most common congenital cause → VP shunt is the treatment.
Incorrect. The key is the 4th ventricle is NORMAL → obstruction is ABOVE it, at the aqueduct. If Magendie/Luschka were blocked, the 4th ventricle would also dilate.
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Lumbar puncture is routinely performed at the L3–L4 interspace in adults. The main reason this level is safe (rather than L1–L2) is that:
Correct! The spinal cord (conus medullaris) ends at L1–L2 in adults. Below this, the lumbar cistern contains only cauda equina nerve roots, which move away from the needle. Dura extends to S2.
LP landmarks: Tuffier's line = line connecting the tops of the iliac crests = L4 vertebra (or L3–L4 interspace). Used to identify the LP level clinically. In neonates, conus medullaris is at L3 → LP at L4–L5 in neonates. Spinal anaesthesia similarly uses L3–L4 or L4–L5.
Incorrect. The safe LP level is because the spinal cord ends at L1–L2. The dura extends to S2 (not L3). The epidural space size is not the safety criterion.
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Cerebrospinal fluid is primarily absorbed into the venous system through which structure?
Correct! Arachnoid granulations (Pacchionian bodies) are tufted projections of arachnoid mater into the superior sagittal sinus. They function as one-way pressure valves, allowing CSF absorption when CSF pressure > venous pressure.
Normal pressure hydrocephalus (NPH): arachnoid granulation dysfunction in elderly → impaired absorption → enlarged ventricles but normal LP pressure. Classic triad: gait ataxia, urinary incontinence, dementia (Wet, Wobbly, Wacky). Responds to VP shunting.
Incorrect. CSF absorption = arachnoid granulations → superior sagittal sinus. Choroid plexus PRODUCES CSF; it does not absorb it.
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A 25-year-old man sustains a blow to the right temporal region. CT shows a biconvex (lens-shaped) hyperdense collection between the skull and brain. The source of bleeding is most likely:
Correct! Extradural haematoma (EDH) = biconvex shape = arterial bleeding from the middle meningeal artery (MMA) after temporal bone fracture at the pterion. The dura is stripped from the skull. Classic "lucid interval" followed by rapid deterioration.
EDH: Biconvex, temporal region, temporal bone fracture, MMA laceration, lucid interval → rapid deterioration. SDH: Crescent-shaped, crosses suture lines, bridging vein rupture, in elderly/alcoholics (cerebral atrophy stretches bridging veins). EDH is a surgical emergency — burr hole/craniotomy.
Incorrect. Biconvex (lens-shaped) = extradural haematoma = arterial (MMA). Crescent-shaped = subdural haematoma = venous (bridging veins). Temporal blow + temporal bone fracture = MMA laceration.
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The denticulate ligaments of the spinal cord are lateral projections of which meningeal layer?
Correct! Denticulate ligaments are lateral projections of the pia mater that attach to the inner surface of the dura. They suspend the spinal cord within the subarachnoid space, preventing excessive movement.
Pia mater derivatives at the spinal cord: (1) Denticulate ligaments — lateral, tether cord to dura. (2) Filum terminale — inferior; pia stretching from conus medullaris → coccyx (with dural sleeve at S2 = coccygeal ligament). Both support the spinal cord. Tethered cord syndrome = filum terminale is too short/thick → pulls on conus → neurological deficits.
Incorrect. Denticulate ligaments = pia mater → dura. 21 pairs, running along the length of the cord. They are surgically useful as landmarks in spinal surgery.
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A patient develops severe positional headache (worse on sitting/standing, relieved by lying down) 24 hours after a lumbar puncture. The mechanism is:
Correct! Post-LP headache = low CSF pressure (intracranial hypotension) from ongoing CSF leak through the dural hole. Upright posture worsens it (gravity reduces CSF volume around brain) → traction on pain-sensitive dural structures and bridging veins.
Post-LP headache prevention: use thin (25G) atraumatic (Whitacre) needles. Treatment: lie flat, hydration, caffeine (vasoconstriction). Epidural blood patch = 15–20 mL autologous blood injected epidurally at LP site → clot seals the CSF leak → instant relief. Success rate >90%.
Incorrect. Post-LP headache = positional, from low pressure due to CSF leak. Treat: bed rest, oral hydration, caffeine. If persistent: epidural blood patch (patient's own blood seals the hole).
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