A 68-year-old woman from Mumbai presents with acute onset of left eye pain and blurred vision. On examination, the left pupil is 8 mm and completely unreactive to light, while the right pupil is 3 mm and reacts normally. She has left-sided ptosis and the left eye is abducted and depressed. Fundoscopy shows a normal optic disc bilaterally. What is the most likely diagnosis?

Explanation

## Oculomotor Nerve Palsy with Pupillary Involvement ### Clinical Presentation Analysis **Key Point:** The combination of a dilated, unreactive pupil (8 mm) + ptosis + ophthalmoplegia (abduction and depression of the eye) is pathognomonic for **oculomotor (CN III) nerve palsy**. The pupillary involvement indicates that the parasympathetic fibers in CN III are affected. ### Anatomy of CN III and Pupillary Fibers 1. **Parasympathetic fibers in CN III** originate from the Edinger-Westphal nucleus in the midbrain 2. These fibers travel in the **medial aspect of CN III** (dorsomedial location) 3. They are vulnerable to compression from external structures (aneurysms, tumors, increased ICP) 4. They innervate the iris sphincter (constriction) and ciliary muscle (accommodation) **High-Yield:** **"Pupil-involving CN III palsy"** = external compression (aneurysm, tumor, increased ICP). **"Pupil-sparing CN III palsy"** = microvascular ischemia (diabetes, hypertension, age >50). ### Why the Pupil Is Dilated and Unreactive **Mnemonic:** **CN III palsy** = Loss of **P**arasympathetic fibers → **P**upil dilates and becomes **P**upil-unreactive. - Loss of parasympathetic innervation to iris sphincter - Unopposed sympathetic tone causes dilation - Complete loss of light reflex (no parasympathetic constriction possible) - Pupil size: 8 mm (markedly dilated) ### Associated Signs in This Patient | Sign | Mechanism | Present in This Case | |------|-----------|----------------------| | Ptosis | Loss of CN III to levator palpebrae superioris | Yes (left-sided) | | Ophthalmoplegia | Loss of CN III to extraocular muscles (medial rectus, superior rectus, inferior rectus, inferior oblique) | Yes (abducted and depressed) | | Mydriasis | Loss of parasympathetic to iris sphincter | Yes (8 mm pupil) | | Unreactive pupil | No parasympathetic constriction possible | Yes | | Normal optic disc | Retrobulbar pathology is excluded | Yes | ### Differential Diagnosis ```mermaid flowchart TD A["Dilated, unreactive pupil + Ptosis + Ophthalmoplegia"]:::outcome --> B{"Optic disc normal?"}:::decision B -->|Yes| C{"Pupil involved?"}:::decision B -->|No| D["Retrobulbar pathology<br/>e.g., optic neuritis"]:::outcome C -->|Yes| E["CN III palsy<br/>with pupillary involvement"]:::action C -->|No| F["Pupil-sparing CN III palsy<br/>Microvascular ischemia"]:::action E --> G{"Cause?"}:::decision G -->|Compression| H["Aneurysm, tumor, ICP"]:::urgent G -->|Ischemia| I["Diabetes, HTN, age"]:::outcome ``` ### Why This Is NOT Adie's Tonic Pupil **Warning:** Adie's tonic pupil is a common trap: - Adie's presents with a dilated pupil that reacts *very slowly* (tonic reaction over seconds to minutes) - Light reaction is present, though sluggish - **No ptosis or ophthalmoplegia** - This patient has complete loss of light reaction and associated CN III motor signs ### Why This Is NOT Horner Syndrome Horner syndrome causes: - **Miosis** (small pupil, 1–2 mm), NOT mydriasis - Ptosis (mild, 1–2 mm) - Anhidrosis - No ophthalmoplegia This patient has a **dilated** pupil and **complete ophthalmoplegia**, which excludes Horner. ### Why This Is NOT RAPD RAPD (from retrobulbar optic neuritis) would show: - **Normal optic disc appearance** (true in this case) - **Relative dilation of the affected pupil** (appears dilated only when light swings to it) - **Preserved light reaction** (sluggish but present) - **No ptosis or ophthalmoplegia** This patient has a completely unreactive pupil with motor signs, ruling out RAPD. ### Clinical Pearl: Pupil-Involving vs. Pupil-Sparing CN III Palsy **Pupil-involving CN III palsy** (this case): - Suggests external compression (posterior communicating artery aneurysm is classic) - Requires urgent neuroimaging (CT/MRI) and possibly angiography - Parasympathetic fibers are in the periphery of CN III and are compressed first **Pupil-sparing CN III palsy**: - Suggests microvascular ischemia (diabetes, hypertension, age >50) - Microvascular infarction affects the core of CN III, sparing peripheral parasympathetic fibers - Usually resolves spontaneously in 3–6 months ### Investigation Priorities 1. **Urgent neuroimaging:** MRI brain with contrast or CT angiography to rule out aneurysm 2. **Lumbar puncture:** If imaging is negative and subarachnoid hemorrhage is suspected 3. **Blood glucose, blood pressure:** To assess for microvascular risk factors (though pupil involvement makes ischemia less likely)