A 35-year-old man presents with a dilated pupil on the right side that does not react to light but constricts on accommodation. On the left side, the pupil is normal. Which finding best distinguishes Adie's tonic pupil from third nerve palsy affecting the right eye?

## Distinguishing Adie's Tonic Pupil from Third Nerve Palsy ### Clinical Presentation Comparison | Feature | Adie's Tonic Pupil | Third Nerve Palsy | |---------|-------------------|-------------------| | **Pupil size** | Dilated (mydriasis) | Dilated (mydriasis) | | **Light reflex** | Absent or sluggish | Absent | | **Accommodation** | Constricts slowly (tonic) | Normal constriction | | **Ptosis** | Absent | Present (complete) | | **Ophthalmoplegia** | Absent | Present (CN III palsy) | | **Extraocular movements** | Normal | Impaired ("down and out" position) | | **Iris appearance on slit lamp** | **Segmental/vermiform sphincter contraction** | Smooth, uniformly dilated iris | ### Key Point: **The pathognomonic discriminator for Adie's tonic pupil is segmental iris sphincter contraction (vermiform movements) seen on slit lamp examination.** This finding is unique to Adie's pupil and is not present in third nerve palsy. It results from aberrant regeneration of postganglionic parasympathetic fibers from the ciliary ganglion, where fibers originally destined for the ciliary muscle re-innervate only portions of the iris sphincter — producing characteristic sector-by-sector, worm-like contractions visible only on slit lamp. ### Why Option B (Ptosis and Ophthalmoplegia) is Insufficient as the BEST Discriminator: While ptosis and ophthalmoplegia are indeed features of third nerve palsy and absent in Adie's pupil, the question asks what **best distinguishes** Adie's tonic pupil — i.e., a finding that is **specific to Adie's** rather than merely absent in it. Option B describes features of CN III palsy, not a positive finding of Adie's. Option D (segmental iris sphincter contraction) is a **positive, pathognomonic sign of Adie's** that cannot be seen in third nerve palsy under any circumstances. ### Clinical Pearl (Kanski's Clinical Ophthalmology): **Adie's tonic pupil** is a postganglionic parasympathetic denervation syndrome of the ciliary ganglion. The hallmark slit lamp finding is **segmental palsy of the iris sphincter** — only certain sectors contract to near stimuli, producing the characteristic vermiform (worm-like) iris movements. This is the single most specific finding that distinguishes Adie's from all other causes of a dilated, poorly reactive pupil, including third nerve palsy. ### High-Yield: - **Adie's pupil:** Dilated + light-near dissociation + **segmental iris contraction on slit lamp** + supersensitivity to 0.1% pilocarpine (denervation supersensitivity) - **Third nerve palsy:** Dilated + ptosis + ophthalmoplegia ("down and out") + NO segmental iris contraction - **Mnemonic:** "ADIE = Aberrant re-innervation → Divided iris sectors" ### Tip: When a dilated pupil with light-near dissociation is encountered, slit lamp examination for segmental iris sphincter contraction is the single most specific bedside test to confirm Adie's tonic pupil (Kanski's Clinical Ophthalmology, 9th ed.; Walsh & Hoyt's Clinical Neuro-Ophthalmology).