Question:hard
The earliest feature of third cranial nerve involvement in a patient with diabetes mellitus is:
The earliest feature of third cranial nerve involvement in a patient with diabetes mellitus is:
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Diabetic third-nerve palsy is classically pupil-sparing.
Updated On: Jun 24, 2026
- Normal light reflex
- Abnormal light reflex
- Normal light and accommodation reflex
- Abnormal light and accommodation reflex
Show Solution
The Correct Option is A
Solution and Explanation
Step 1: Map the third nerve. The oculomotor nerve moves the eye, lifts the lid, and through its parasympathetic component controls pupil constriction and the focusing reflex of accommodation.
Step 2: Recall the blood supply pattern. The pupil-controlling fibres sit superficially and are fed by surface (pial) vessels, whereas the motor core depends on the central vasa nervorum.
Step 3: Apply diabetic ischaemia. Diabetes produces a microvascular, ischaemic injury that strikes the central fibres first while leaving the well-perfused outer pupillary fibres intact. Hence the pupil and its light reflex stay normal in the early stage, the classic pupil-sparing palsy of diabetics over 50.
Step 4: Reject compressive patterns. A blown pupil with an abnormal light reflex signals external compression, for example a posterior communicating artery aneurysm, which is the opposite of the diabetic picture, so the abnormal-reflex options are wrong.
\[\boxed{\text{Normal light reflex}}\]
Step 2: Recall the blood supply pattern. The pupil-controlling fibres sit superficially and are fed by surface (pial) vessels, whereas the motor core depends on the central vasa nervorum.
Step 3: Apply diabetic ischaemia. Diabetes produces a microvascular, ischaemic injury that strikes the central fibres first while leaving the well-perfused outer pupillary fibres intact. Hence the pupil and its light reflex stay normal in the early stage, the classic pupil-sparing palsy of diabetics over 50.
Step 4: Reject compressive patterns. A blown pupil with an abnormal light reflex signals external compression, for example a posterior communicating artery aneurysm, which is the opposite of the diabetic picture, so the abnormal-reflex options are wrong.
\[\boxed{\text{Normal light reflex}}\]
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