A 40-year-old patient is found to have a dilated, unreactive pupil with ptosis and a 'down and out' eye position. CT reveals a posterior communicating artery aneurysm. Why does CN III palsy from a PCA aneurysm characteristically affect the pupil first (pupil-involving palsy)?

• A Pupillary sympathetic fibres are embedded within the CN III fascicles centrally
• B Parasympathetic fibres run in the outer mantle of CN III and are first compressed by external pressure ✓
• C The ciliary ganglion is compressed before CN III exits the cavernous sinus
• D The oculomotor nerve supplies sphincter pupillae via sympathetic fibres superficially

Explanation

The pupilloconstrictor parasympathetic fibres travel in the outer, superficial mantle (pial surface) of CN III, derived from the Edinger-Westphal nucleus. Because aneurysmal compression exerts pressure from outside the nerve, these outer fibres are compressed first, causing pupillary dilation before motor weakness becomes apparent. In contrast, ischaemic CN III palsy (e.g., in diabetic neuropathy) primarily affects the central axons (motor fibres) and typically spares the pupil, because the outer parasympathetic fibres are supplied by the pial vessels and are relatively protected from ischaemia. This anatomical principle is critical for differentiating compressive from ischaemic CN III palsy.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.