A 58-year-old man with a 10-year history of diabetes mellitus presents with progressive vision loss and difficulty reading. On examination, his pupils are mid-dilated (4 mm) and completely unreactive to light bilaterally, though accommodation is preserved. Visual acuity is 6/18 in both eyes. What is the most common cause of this pupillary abnormality in diabetic patients?

Explanation

## Most Common Cause of Pupillary Dysfunction in Diabetes **Key Point:** Diabetic autonomic neuropathy is the most common cause of pupillary abnormalities in longstanding diabetes mellitus, affecting the parasympathetic nerve fibres that control pupillary constriction. ### Pathophysiology Diabetic autonomic neuropathy causes selective damage to: - Parasympathetic fibres in the ciliary nerves (CN III branches) - Preganglionic and postganglionic parasympathetic neurons - Results in mid-dilated, unreactive pupils with preserved accommodation (as accommodation fibres are relatively spared initially) ### Clinical Features of Diabetic Pupillary Neuropathy | Feature | Diabetic Autonomic Neuropathy | Third Nerve Palsy | Adie's Tonic Pupil | |---------|-------------------------------|-------------------|--------------------| | **Pupil size** | Mid-dilated (4–5 mm) | Dilated (5–6 mm) | Dilated (5–6 mm) | | **Light reflex** | Absent or sluggish | Absent | Absent initially | | **Accommodation** | Preserved (relatively) | Affected | Affected | | **Associated findings** | Autonomic symptoms | Ptosis, ophthalmoplegia | Areflexia, pain | | **Prevalence in DM** | Common (10–20%) | Rare (0.5%) | Unrelated to DM | | **Bilaterality** | Usually bilateral | Unilateral | Usually unilateral | **High-Yield:** In a diabetic patient with bilateral mid-dilated unreactive pupils and preserved accommodation, think diabetic autonomic neuropathy first — it is 20–40 times more common than third nerve palsy in this population. ### Why Accommodation is Preserved Accommodation fibres are located in the inner layers of CN III and are relatively protected from ischaemic damage in early autonomic neuropathy, whereas pupillary parasympathetic fibres (outer layers) are more vulnerable to hyperglycaemic and oxidative stress. **Clinical Pearl:** The presence of bilateral pupillary involvement in a diabetic patient is a red flag for systemic autonomic neuropathy — screen for orthostatic hypotension, gastroparesis, and cardiac autonomic dysfunction. ### Differential Diagnosis in This Case - **Third nerve palsy:** Would present with ptosis, ophthalmoplegia, and unilateral involvement — not bilateral as in this case. - **Adie's tonic pupil:** Typically unilateral, young to middle-aged patients, associated with areflexia — not related to diabetes. - **Posterior communicating artery aneurysm:** Would cause acute onset, unilateral dilated pupil with ophthalmoplegia — not bilateral or chronic. [cite:Harrison 21e Ch 428]