A 26-year-old man with type 1 diabetes of 12 years duration develops sudden diplopia. Examination shows ptosis and impaired adduction and elevation of the right eye. The right pupil reacts normally to light and accommodation. What is the most likely explanation?
- A Ischaemia of the lateral rectus supplied by the abducens nucleus
- B Compression of the third nerve by a posterior communicating artery aneurysm
- C Infarction of the third cranial nerve from microangiopathy of the vasa nervorum ✓
- D Demyelinating lesion in the midbrain affecting the oculomotor fascicles
Correct answer: C. Infarction of the third cranial nerve from microangiopathy of the vasa nervorum
Explanation
Diabetic cranial mononeuropathy classically affects the third nerve with pupillary sparing because the parasympathetic pupilloconstrictor fibres travel in the peripheral rim of the nerve, supplied by pial vessels, while the central fibres are supplied by vasa nervorum vulnerable to diabetic microangiopathy. C compressive aneurysm characteristically causes a fixed dilated pupil, making option B the key distractor to reject. Painful ophthalmoplegia with spontaneous recovery over weeks is typical of the diabetic form.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.