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AN63.1-3 | Ventricular System & Special sensory pathways — Practice Quiz
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The foramen of Monro (interventricular foramen) connects:
Correct! Foramen of Monro = each lateral ventricle → 3rd ventricle. Two foramina, one from each lateral ventricle. Colloid cyst at the foramen of Monro can cause sudden obstructive hydrocephalus (one or both lateral ventricles blocked).
Colloid cyst: benign, at the foramen of Monro; can cause sudden "ball-valve" obstruction → acute obstructive hydrocephalus → sudden-onset thunderclap headache → can cause sudden death. MRI: round lesion at foramen of Monro, hyperintense on T1. Treatment: neuroendoscopic removal.
Incorrect. Foramen of Monro = lateral → 3rd ventricle. Aqueduct of Sylvius = 3rd → 4th. Foramina of Magendie/Luschka = 4th ventricle → subarachnoid.
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The floor of the 4th ventricle (rhomboid fossa) is formed by the posterior surface of:
Correct! The rhomboid fossa (4th ventricle floor) is formed by: upper half = posterior surface of pons; lower half = posterior surface of open (rostral) medulla. Key structures visible on the floor: facial colliculus, median eminence, hypoglossal triangle, vagal triangle, sulcus limitans, locus coeruleus.
Structures on the 4th ventricle floor are clinically significant: facial colliculus (CN VI + VII genu), hypoglossal triangle (CN XII), vagal triangle (dorsal motor nucleus X). Surgical access to 4th ventricle (for medulloblastoma removal) must avoid these structures to prevent CN palsies postoperatively.
Incorrect. 4th ventricle floor = pons (upper) + open medulla (lower). The ROOF is formed by the superior medullary velum + inferior medullary velum + the cerebellar nodule.
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In Dandy-Walker malformation, all four ventricles are dilated because:
Correct! Dandy-Walker = failure of foramina of Magendie and Luschka to open → CSF cannot exit the 4th ventricle → 4th ventricle expands into a large cyst + upward CSF pressure expands lateral + 3rd ventricles → all four dilate. Cerebellar vermis is hypoplastic/absent.
Key imaging distinction: Aqueductal stenosis = lateral + 3rd dilated, 4th NORMAL. Dandy-Walker = ALL four dilated + large 4th ventricle cyst + high tentorium + cerebellar vermis agenesis. Dandy-Walker is also associated with corpus callosum agenesis, occipital encephalocele.
Incorrect. Dandy-Walker = foraminal block (Magendie/Luschka fail to open) → all four ventricles dilate. Aqueductal stenosis → only lateral + 3rd dilate, 4th normal. Different CSF blockade level.
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A 30-year-old woman presents with progressive visual field loss. Perimetry shows loss of temporal fields in both eyes (bitemporal hemianopia). The most likely site of the lesion is:
Correct! Bitemporal hemianopia (both temporal fields lost) = lesion at the OPTIC CHIASM. At the chiasm, nasal retinal fibres (carrying temporal visual field information) from both eyes cross. Chiasm compression cuts these crossing fibres bilaterally → both temporal fields lost.
Optic chiasm compression: most commonly from ABOVE = craniopharyngioma (common in Indian children + adults) or pituitary adenoma (below). Craniopharyngioma calcifies → seen on skull X-ray in children. Both compress the chiasm → bitemporal hemianopia. MRI is gold standard.
Incorrect. Bitemporal hemianopia = optic chiasm (nasal fibres cross here). Optic nerve = monocular defect. Optic tract or beyond = homonymous hemianopia (same field, both eyes).
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Homonymous hemianopia with macular sparing (central vision preserved) typically indicates an infarct in the:
Correct! Macular sparing in occipital cortex infarcts occurs because the macular (central) visual cortex at the posterior pole of the occipital lobe has dual blood supply from BOTH PCA and MCA. A PCA territory infarct spares the macular cortex (still supplied by MCA), giving homonymous hemianopia with macular sparing.
Clinical test for macular sparing: ask patient to look at the centre of a page. They should be able to see words around the centre even if half the page is invisible (hemianopia). Macular sparing preserved = they can still read (macular cortex intact). Important for driving and daily function assessment in Indian stroke rehabilitation.
Incorrect. Macular sparing = occipital cortex lesion. Optic tract lesions do NOT have macular sparing (they cause complete hemianopia with no field at any eccentricity). Macular sparing is a hallmark of occipital (PCA territory) lesions.
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The optic radiation fibres that form Meyer's loop sweep through the temporal lobe. Damage to these fibres causes contralateral:
Correct! Meyer's loop carries fibres from the INFERIOR part of the retina (representing the SUPERIOR visual field). Temporal lobe damage (Meyer's loop) → contralateral SUPERIOR quadrantanopia — loss of the upper part of the contralateral visual field (pie in the sky).
Mnemonic for quadrantanopias: Temporal lobe (Meyer's loop) = T for Top (superior quadrantanopia). Parietal lobe = P for Pete fell to the floor (inferior quadrantanopia). Right temporal lobe epilepsy surgery (temporal lobectomy for TLE) carries risk of left superior quadrantanopia.
Incorrect. Meyer's loop = inferior retinal fibres = superior visual field. Temporal damage = superior quadrantanopia. Parietal optic radiation = superior retinal fibres = inferior visual field → parietal lesion = inferior quadrantanopia.
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Which special sensory pathway is unique in that it reaches the primary cortex WITHOUT first relaying in the thalamus?
Correct! The olfactory pathway is the ONLY sensory pathway that does NOT relay in the thalamus before reaching primary cortex. Olfactory nerve → olfactory bulb → olfactory tract → DIRECTLY to piriform cortex (uncus), amygdala, entorhinal cortex. All other sensory pathways relay in the thalamus first.
Clinical relevance of olfaction-limbic connection: smell triggers strong memories and emotions (Proust phenomenon) because olfactory signals go directly to the amygdala and hippocampus without thalamic filtering. TLE (temporal lobe epilepsy) often begins with an olfactory aura (uncinate fits = uncus seizures from piriform cortex): "burning rubber" or "strange smell" before generalisation.
Incorrect. ONLY olfaction bypasses the thalamus. Vision → LGN. Hearing → MGN. Taste → VPM. The evolutionary reason: olfaction is phylogenetically the oldest sense and was hardwired to limbic structures before the thalamic relay system evolved.
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Heschl's gyri (primary auditory cortex, areas 41 and 42) are located in the:
Correct! Primary auditory cortex = Heschl's transverse temporal gyri (areas 41/42) in the superior temporal gyrus, buried in the lateral sulcus (Sylvian fissure). Wernicke's area (area 22) is the POSTERIOR part of the superior temporal gyrus (speech comprehension).
Bilateral Heschl's gyrus lesions are needed for cortical deafness (very rare, requires bilateral MCA/superior temporal strokes). Unilateral lesion causes mild hearing difficulty (bilateral cortical representation). Wernicke's area (adjacent to Heschl's, area 22) = speech comprehension; its damage causes Wernicke's aphasia (fluent + poor comprehension).
Incorrect. Heschl's gyri = primary auditory cortex = superior temporal gyrus, buried in the Sylvian fissure. Calcarine = visual (area 17). Postcentral = somatosensory. Inferior frontal = Broca's.
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A 5-year-old child has obstructive hydrocephalus from aqueductal stenosis (lateral and 3rd ventricles dilated, 4th ventricle normal). The most appropriate surgical treatment that avoids implanting foreign hardware is:
Correct! ETV (endoscopic third ventriculostomy) creates a fenestration in the 3rd ventricle floor → CSF flows directly into the subarachnoid space, bypassing the stenosed aqueduct. No hardware implanted. Preferred for aqueductal stenosis in older children. Success rate ~70–80% in India.
ETV failure rate in infants < 6 months is high (choroid plexus cauterisation + ETV = CPC-ETV improves outcomes in infants). VP shunt complications: shunt infection, obstruction, overdrainage, slit ventricle syndrome. India is one of the highest-volume centres for ETV due to high prevalence of congenital and post-infective hydrocephalus.
Incorrect. ETV = no hardware, ideal for aqueductal stenosis in older children. VP shunt = hardware (valve + tubing) that requires revision as the child grows. Lumbar-peritoneal shunt = only for communicating hydrocephalus.
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Which part of the lateral ventricle extends into the temporal lobe and contains choroid plexus?
Correct! The inferior horn of the lateral ventricle extends into the temporal lobe (following the temporal lobe as it curves downward and anteriorly). The choroid plexus is present in the inferior horn and the body. The anterior horn and posterior horn do NOT have choroid plexus.
Temporal horn dilation is an early MRI sign of medial temporal lobe atrophy in Alzheimer's dementia — the hippocampus (which forms the medial wall of the inferior horn) shrinks → inferior horn dilates. The choroid plexus of the inferior horn is continuous with that of the body; the glomus (calcified) of the choroid plexus in the atrium is a normal MRI finding and should not be confused with a tumour.
Incorrect. Inferior horn = temporal lobe extension; has choroid plexus. Anterior horn = frontal (no choroid plexus). Posterior horn = occipital (no choroid plexus). Body = parietal (has choroid plexus in its floor).
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