Question:medium
The earliest feature of third cranial nerve involvement in a diabetes mellitus patient is:
The earliest feature of third cranial nerve involvement in a diabetes mellitus patient is:
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Diabetic third nerve palsy is classically pupil-sparing.
Updated On: Jun 23, 2026
- Normal light reflex
- Abnormal light reflex
- Normal light and accommodation reflex
- Abnormal light and accommodation reflex
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The Correct Option is A
Solution and Explanation
Step 1: Key to this question is the anatomy of the third nerve: motor fibres lie centrally, while the parasympathetic pupil-constricting fibres travel superficially around the periphery. Step 2: Diabetes causes a microvascular (vasa nervorum) infarct that damages the central core of the nerve first, sparing the well-perfused outer pupillary fibres. This is the classic pupil-sparing third nerve palsy. Step 3: Clinically, the patient develops ptosis and an eye that is depressed and abducted, yet the pupil reacts normally to light early in the course. Step 4: Hence the earliest sign is a preserved (normal) light reflex; loss of the light reflex points toward external compression rather than diabetic ischaemia. \[\boxed{\text{Normal light reflex (pupil sparing)}}\]
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