A 32-year-old woman presents with a dilated right pupil that does not react to light but constricts normally to accommodation. On examination, the right eye shows normal extraocular movements and intact corneal reflex. Which single feature best distinguishes an Argyll Robertson pupil from this patient's presentation?

Explanation

## Distinguishing Argyll Robertson Pupil from Other Light-Near Dissociation ### Clinical Presentation Comparison | Feature | Argyll Robertson Pupil | Tonic (Holmes-Adie) Pupil | | --- | --- | --- | | **Pupil size** | Small (miotic), irregular | Large (mydriatic), regular | | **Light reaction** | Absent or sluggish | Absent or very slow | | **Near reaction** | Brisk, prompt | Slow, redilation delayed | | **Accommodation** | Affected | Affected | | **Bilaterality** | Usually bilateral | Often unilateral | | **Etiology** | Neurosyphilis (dorsal midbrain) | Postganglionic parasympathetic denervation | | **Associated findings** | Argyll Robertson sign (light-near dissociation) | Segmental iris constriction, hyporeflexia | **Key Point:** Both conditions show light-near dissociation, but the **size and morphology of the pupil** differ fundamentally. Argyll Robertson pupils are characteristically **small and irregular**, whereas tonic pupils (as in this case) are **dilated and regular**. ### Pathophysiology **Argyll Robertson Pupil:** - Dorsal midbrain involvement affecting pretectal nucleus and Edinger-Westphal nucleus - Associated with neurosyphilis (tertiary syphilis) - Pupils are small because parasympathetic tone is relatively preserved - Light pathway damaged; near pathway intact **Tonic (Holmes-Adie) Pupil:** - Postganglionic parasympathetic denervation at ciliary ganglion level - Benign, idiopathic condition (may follow viral illness) - Pupil is dilated because parasympathetic innervation is lost - Both light and near pathways affected, but near response recovers slowly **High-Yield:** The **pupil size** is the single best clinical discriminator: - **Small + light-near dissociation** = Argyll Robertson (syphilis) - **Large + light-near dissociation** = Tonic pupil (benign) ### Clinical Pearl **Mnemonic — "ARP" (Argyll Robertson Pupil):** - **A**lways small - **R**eacts to accommodation - **P**upil irregular **Mnemonic — "TONIC" (Holmes-Adie):** - **T**onic (slow, sustained constriction to near) - **O**ften unilateral - **N**ormal light reflex absent - **I**rregular segmental constriction - **C**iliary ganglion involvement ### Why This Matters The patient in the stem has a **dilated pupil with light-near dissociation**, which is characteristic of a tonic pupil, NOT an Argyll Robertson pupil. The key discriminator is pupil size and morphology, not the presence of light-near dissociation alone.