Third, fourth and sixth cranial nerve palsies
Description
The three nerves
| Supplies | Palsy: eye position | Diplopia worst | Head posture | |
|---|---|---|---|---|
| III oculomotor | SR, IR, MR, IO, levator, parasympathetic to sphincter pupillae + ciliary | Down and out, ptosis, +/- dilated pupil | All directions except abduction | Variable |
| IV trochlear | Superior oblique (intorsion, depression in adduction) | Hypertropia, extorsion | Down and in (reading, stairs) | Head tilt away from lesion |
| VI abducens | Lateral rectus | Esotropia, fails to abduct | Looking toward the lesion, distance | Face turn toward lesion |
- SO4, LR6, all the rest 3 (superior oblique IV, lateral rectus VI)
Anatomical quirks that get examined
- IV - only cranial nerve to decussate and to exit dorsally; longest intracranial course -> most vulnerable to closed head injury; bilateral IV palsy after trauma is classic
- VI - longest intradural course, runs over the petrous ridge -> false localising sign of raised ICP
- III - pupillomotor fibres run superficially and dorsomedially
- Compression hits the pupil first; ischaemia (vasa nervorum, core) spares it
- This single fact drives the whole acute assessment
Parkinson's law of the third nerve
- Painful + pupil-involving III palsy = posterior communicating artery aneurysm until proven otherwise
6 more sections, plus exam facts
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