A 42-year-old woman from Delhi presents with a 6-month history of progressive ptosis and diplopia. On examination, the right pupil is 4 mm and does not react to light, but constricts normally when she focuses on a near object. The left pupil is 3 mm and reacts normally to both light and accommodation. Extraocular movements reveal right-sided ophthalmoplegia affecting CN III. MRI brain and orbitalimaging are normal. What is the most likely diagnosis?

Explanation

## Diagnosis: Adie Tonic Pupil with Concurrent CN III Nerve Palsy ### Key Clinical Findings **Key Point:** The right pupil is **dilated (4 mm)**, shows **light-near dissociation** (no reaction to light, but constricts to near), and is associated with **right-sided CN III ophthalmoplegia and ptosis** — this combination is characteristic of **Adie tonic pupil with concurrent CN III palsy**, not Argyll Robertson pupil. ### Why NOT Argyll Robertson Pupil? The classic Argyll Robertson (AR) pupil is: - **Small (miotic, 2–3 mm)**, not dilated - **Bilateral** in the vast majority of cases - Associated with tertiary syphilis affecting the **pretectal nucleus** - Does NOT cause CN III palsy directly (AR pupils are miotic and do not involve the CN III efferent pathway) In this vignette, the right pupil is **4 mm (dilated)** — this is incompatible with a classic Argyll Robertson pupil, which is characteristically small. This is a critical distinguishing feature. ### Adie Tonic Pupil: Mechanism Adie tonic pupil results from **postganglionic parasympathetic denervation** of the ciliary ganglion (often post-viral or idiopathic): 1. **Light reflex pathway is damaged** → pupil does NOT react to light (or reacts very slowly/tonically) 2. **Near reflex pathway is relatively PRESERVED** → pupil constricts with accommodation (near-light dissociation) 3. The pupil is typically **dilated** due to loss of parasympathetic tone 4. **Denervation supersensitivity** to dilute pilocarpine (0.1%) is pathognomonic ### Why CN III Palsy Co-exists Adie syndrome (Holmes-Adie syndrome) involves dysfunction of the **autonomic nervous system**. Concurrent CN III palsy can occur due to: - Shared postganglionic parasympathetic involvement - Orbital or cavernous sinus pathology affecting both the ciliary ganglion and CN III - Idiopathic or post-infectious autonomic neuropathy The **normal MRI** supports a peripheral/postganglionic etiology rather than a compressive or central lesion. ### Comparison Table | Feature | Adie Tonic Pupil | Argyll Robertson Pupil | |---------|-----------------|----------------------| | Pupil size | **Dilated (large)** | **Small (miotic)** | | Light reflex | Absent/tonic | Absent | | Near reflex | Present | Present | | Laterality | Usually unilateral | Usually bilateral | | Cause | Ciliary ganglion damage | Neurosyphilis (pretectal) | | Pilocarpine 0.1% | **Constricts** (supersensitivity) | No response | ### Why Other Options Are Wrong - **B (Dorsal midbrain/Parinaud syndrome):** Causes light-near dissociation but with **bilateral** involvement, upgaze palsy, convergence-retraction nystagmus, and lid retraction — not unilateral CN III palsy - **C (Argyll Robertson/syphilis):** AR pupils are **small and bilateral**; a 4 mm dilated pupil is incompatible - **D (PCA aneurysm):** Would cause **acute** presentation with a painful, dilated, fixed pupil; 6-month progressive course and normal MRI argue strongly against this **Clinical Pearl:** Always check pupil **size** when differentiating Adie from Argyll Robertson: *"Adie = big, AR = small."* Dilute pilocarpine (0.1%) test confirms Adie tonic pupil by demonstrating denervation supersensitivity. (Reference: Walsh & Hoyt's Clinical Neuro-Ophthalmology; Kanski's Clinical Ophthalmology) **High-Yield:** Adie tonic pupil + areflexia = Holmes-Adie syndrome. The key distinguishing feature from AR pupil is the **dilated** (not miotic) pupil and **unilateral** presentation in a young to middle-aged woman.