A 52-year-old man from Mumbai presents with acute onset diplopia, ptosis, and pupillary dilation on the right side. On examination, the right eye is adducted and depressed. Which cranial nerve is most commonly affected by ischemic stroke in this clinical scenario?

Explanation

## Oculomotor Nerve (CN III) — Most Common Ischemic Cranial Neuropathy ### Clinical Presentation Analysis **Key Point:** The combination of **ptosis, pupillary dilation, and impaired eye adduction** is pathognomonic for oculomotor nerve (CN III) palsy. This is the most common cranial nerve affected by ischemic stroke. ### Anatomy of CN III and Its Vulnerabilities | Feature | Detail | |---------|--------| | **Nuclei location** | Midbrain (interpeduncular fossa) | | **Fascicular course** | Passes through cerebral peduncle; vulnerable to Weber's syndrome | | **Subarachnoid course** | Longest intracranial course (80 mm); crosses cavernous sinus | | **Blood supply** | Penetrating branches from PCA and SCA; watershed zone | | **Vascular territory** | Supplied by perforating branches — susceptible to small-vessel disease | **Clinical Pearl:** CN III has the longest intracranial course of any cranial nerve, making it the most vulnerable to ischemic injury, particularly in patients with diabetes, hypertension, and age >50 years. ### CN III Palsy: Motor and Parasympathetic Components **High-Yield:** CN III innervates: 1. **Somatic motor** (via fascicles in nerve trunk): - Medial rectus (adduction) - Superior rectus (elevation) - Inferior rectus (depression) - Inferior oblique (elevation + abduction) - Levator palpebrae superioris (eyelid elevation) 2. **Parasympathetic** (preganglionic fibers in dorsomedial fascicles): - Pupillary constrictor (via ciliary ganglion) - Ciliary muscle (accommodation) ### Clinical Features of CN III Palsy | Sign | Mechanism | |------|----------| | **Ptosis** | Loss of levator palpebrae function | | **Pupillary dilation** | Loss of parasympathetic pupillary constriction | | **"Down and out" eye** | Unopposed action of CN IV (superior oblique) and CN VI (lateral rectus) | | **Diplopia** | Vertical and horizontal misalignment | | **Loss of accommodation** | Ciliary muscle paralysis | ### Ischemic vs. Compressive CN III Palsy | Feature | Ischemic (Microvascular) | Compressive (Tumor/Aneurysm) | |---------|--------------------------|-----------------------------| | **Pupil involvement** | Typically spared (pupil-sparing CN III) | Dilated pupil (early sign) | | **Onset** | Sudden | Gradual | | **Progression** | Maximal at onset | Progressive | | **Associated findings** | Diabetes, HTN, age >50 | Headache, mass effect | | **MRI/CTA** | Normal or small infarct | Lesion visible | | **Prognosis** | Good recovery (weeks–months) | Depends on etiology | **Mnemonic: CN III PALSY** — **C**ranial nerve **III**, **P**upil dilated, **A**dduction lost, **L**evator weak (ptosis), **S**uperior rectus weak, **Y**—down and out eye ### Why CN III is Most Commonly Affected by Stroke 1. **Longest intracranial course**: 80 mm from midbrain to cavernous sinus — maximum exposure to ischemia. 2. **Penetrating artery territory**: Supplied by small perforating branches from PCA and SCA, which are prone to occlusion in small-vessel disease. 3. **Watershed vulnerability**: The nerve passes through areas with tenuous collateral circulation. 4. **Epidemiology**: CN III ischemic palsy accounts for ~50% of all acute cranial nerve palsies in stroke patients. ### Differential Diagnosis of Acute Ophthalmoplegia ```mermaid flowchart TD A[Acute ophthalmoplegia]:::outcome --> B{Pupil involvement?}:::decision B -->|Dilated pupil| C[CN III palsy]:::action B -->|Normal pupil| D[Pupil-sparing CN III]:::action C --> E{Sudden onset + risk factors?}:::decision E -->|Yes: age, DM, HTN| F[Ischemic stroke - MRI]:::action E -->|No: gradual + headache| G[Compressive lesion - CTA/MRI]:::urgent D --> H{Down and out eye?}:::decision H -->|Yes| I[CN III involvement]:::outcome H -->|No| J{Lateral rectus weakness?}:::decision J -->|Yes| K[CN VI palsy]:::outcome J -->|No| L[CN IV palsy]:::outcome ``` ### Management of Ischemic CN III Palsy 1. **Neuroimaging**: MRI brain with DWI/PWI to confirm acute ischemia; CTA head/neck to exclude aneurysm if pupil dilated. 2. **Stroke protocol**: Thrombolysis (if within 4.5 hours) or thrombectomy (if large vessel occlusion). 3. **Risk factor modification**: Aspirin, statin, blood pressure control, glycemic control. 4. **Eye care**: Lubricating drops, eye taping, protective glasses to prevent corneal ulceration. 5. **Prognosis**: Most ischemic CN III palsies recover within 3–6 months with supportive care. **High-Yield:** The presence of pupillary dilation in CN III palsy suggests ischemic etiology (microvascular stroke in elderly/diabetic patients) rather than compressive lesion, which typically presents with gradual onset and other mass effect signs.