A patient has complete left CN III palsy with a blown pupil and ptosis. On attempted right gaze, the left eye cannot adduct. This reflects damage at which level?
- A Left frontal eye field
- B Left CN III nucleus in the midbrain
- C Left CN III nerve in the subarachnoid space (e.g., posterior communicating artery aneurysm) ✓
- D Left medial longitudinal fasciculus
Correct answer: C. Left CN III nerve in the subarachnoid space (e.g., posterior communicating artery aneurysm)
Explanation
A complete CN III palsy with pupil involvement (mydriasis) and ptosis strongly suggests extrinsic compression of the CN III nerve in the subarachnoid space, classically by a posterior communicating artery aneurysm. The pupillary fibres run on the outer surface of CN III and are most vulnerable to compressive lesions. In contrast, ischaemic CN III palsies (e.g., diabetic) typically spare the pupil because the ischaemia affects the central core fibres while the superficial pupillomotor fibres survive. MLF lesion causes internuclear ophthalmoplegia without ptosis or pupil change.
Reference: BD Chaurasia's Human Anatomy, 8th ed.
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Written and medically reviewed by the StethoPrep medical team.