Neuroanatomy
This domain is examined through applied questions: which structure is compressed, which vessel is at risk along a corridor, where the craniotomy goes. You must move fluently between surface landmarks, cisternal anatomy and cross-sectional images.
Cranial nerves that decide answers
- CN III emerges between the posterior cerebral and superior cerebellar arteries, crosses the interpeduncular cistern, then runs in the lateral wall of the cavernous sinus before dividing above the superior orbital fissure. It is therefore the first nerve compressed in uncal herniation and by PCoA aneurysms.
- CN IV is the only nerve to exit dorsally and to cross the midline; it has the longest intracranial course.
- CN VI pierces the clivus, passes Dorello canal and lies within the sinus, lateral to the ICA. It is the only nerve inside the sinus rather than in its wall, so it fails early in cavernous sinus disease.
- Foramina: rotundum, V2; ovale, V3; spinosum, middle meningeal artery; jugular, IX, X, XI with the sigmoid sinus; hypoglossal canal, XII.
Supratentorial and spinal anatomy
- The genu and most of the anterior and posterior limbs of the internal capsule are perforated by lenticulostriate branches of the MCA, Charcot artery of cerebral haemorrhage; the posterior third and retrolenticular part are supplied by the anterior choroidal artery.
- Eloquent cortex: Broca, pars opercularis and triangularis of the inferior frontal gyrus; Wernicke, posterior superior temporal gyrus.
- Both dorsal columns and the spinothalamic tract are laminated with sacral fibres lateral and cervical fibres medial, so an extramedullary cervical lesion loses sacral sensation first.
Worked example
A 42-year-old has sudden left ptosis, a fixed dilated left pupil and an eye deviated down and out, with normal acuity. Ptosis, mydriasis and a down-and-out globe make a complete CN III palsy; the pupil is involved because superficial parasympathetic fibres are compressed first. If the right arm is also weak, remember the Kernohan notch: the left uncus compresses the right cerebral peduncle, giving false-localising ipsilateral weakness. A PCoA aneurysm is the classic cause.
Traps
- Placing the third nerve inside the cavernous sinus rather than in its wall.
- Confusing PCA and PCoA relationships.
- Reversing sensory lamination in the cord.
- Forgetting that a pupil-sparing third nerve palsy in a diabetic is usually microvascular, not compressive.