## Clinical Diagnosis This patient presents with a **third cranial nerve (CN III) palsy** with pupillary involvement: - Dilated, poorly reactive pupil → parasympathetic fibre involvement (pupillary fibres travel on the periphery of CN III) - Impaired adduction, elevation, depression → motor fibre involvement (medial rectus, superior rectus, inferior rectus) - Preserved accommodation initially (though will deteriorate) is a subtle clue that this is acute **Key Point:** A dilated pupil with CN III palsy is a **medical emergency** until posterior communicating artery (PCA) aneurysm is excluded. This is the most common cause of acute pupil-involving CN III palsy in adults. ## Why Urgent Neuroimaging? **High-Yield:** The combination of acute-onset CN III palsy WITH pupillary involvement mandates **urgent vascular imaging** to rule out compressive lesion (aneurysm, tumour, or other mass). This is different from pupil-sparing CN III palsy (microvascular), which can be managed conservatively. **Clinical Pearl:** PCA aneurysms compress CN III as it exits the brainstem, affecting peripheral parasympathetic fibres first (hence pupil involvement before motor signs in some cases). The headache is a red flag for subarachnoid haemorrhage risk. ## Management Algorithm ```mermaid flowchart TD A[CN III palsy + dilated pupil]:::outcome --> B{Pupil involved?}:::decision B -->|Yes| C[URGENT imaging]:::urgent C --> D[CT head + CTA/MRA circle of Willis]:::action D --> E{Aneurysm found?}:::decision E -->|Yes| F[Neurosurgery consultation, coil/clip]:::action E -->|No| G[MRI brain to exclude mass/demyelination]:::action B -->|No - pupil sparing| H[Microvascular CN III palsy]:::outcome H --> I[Observe, manage vascular risk factors]:::action ``` **Tip:** CT is faster for acute bleed detection; MRA/CTA are complementary. In a real emergency, CT + CTA can be done in <10 minutes. [cite:Harrison 21e Ch 428] 
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