A 68-year-old woman with a 10-year history of diabetes mellitus type 2 presents with sudden-onset right eye pain and blurred vision. On examination, the right pupil is mid-dilated (5 mm), reacts sluggishly to light, and accommodation is intact. Extraocular movements show impaired adduction and elevation. The left eye is normal. What is the most appropriate next step in management?

Explanation

## Clinical Diagnosis This patient has **partial CN III palsy with pupillary involvement** — a neurological emergency until proven otherwise: - Mid-dilated (5 mm) pupil with **sluggish** (not brisk/normal) light reaction → parasympathetic fibres are **partially involved** - Impaired adduction and elevation → motor fibre involvement - Acute onset with pain - Risk factors: diabetes (age 68) **Key Point:** Although diabetes is a classic cause of microvascular CN III palsy, the **pupil is NOT truly spared** in this case — it is mid-dilated and sluggishly reactive. Any degree of pupillary involvement in an acute CN III palsy mandates urgent exclusion of a posterior communicating artery (PCoA) aneurysm, regardless of vascular risk factors. ## Why Urgent MR Angiography? **High-Yield:** The critical distinction in CN III palsy management: | Feature | Pupil-Sparing (Microvascular) | Pupil-Involving (Compressive) | |---------|---|---| | Pupil size | Normal (≤4 mm) | Dilated (≥5 mm) | | Pupil reactivity | **Fully preserved** | Fixed or sluggish | | Aetiology | Microvascular ischaemia | Aneurysm (PCoA), mass, herniation | | Imaging needed? | No (if truly spared + vascular RF) | **YES — urgent MRA/CTA** | | Recovery | 3–6 months spontaneous | Depends on cause | **True pupil-sparing** means the pupil is **completely normal in size and reactivity**. A mid-dilated, sluggishly reactive pupil represents **partial pupillary involvement** and must be treated as compressive (aneurysmal) until proven otherwise. ## Pathophysiology The parasympathetic fibres of CN III travel on the **outer surface** of the nerve and are compressed first by an expanding aneurysm (e.g., PCoA aneurysm). In microvascular ischaemia, the central motor fibres are preferentially affected while the peripheral parasympathetic fibres are relatively spared — but this sparing must be **complete**, not partial. A PCoA aneurysm causing partial CN III palsy may rupture imminently, causing subarachnoid haemorrhage with catastrophic outcomes. MR angiography (or CT angiography) of the circle of Willis is the investigation of choice to exclude this. **Clinical Pearl (Harrison 21e, Ch 428; Walsh & Hoyt Clinical Neuro-Ophthalmology):** "Pupil-sparing" is defined as a **completely normal pupil** in the context of complete or near-complete CN III palsy. Any pupillary abnormality — even sluggish reactivity or mild dilation — warrants urgent vascular imaging. The "safe" rule: when in doubt, image. ## Management Algorithm - **Pupil completely normal + complete motor palsy + age >50 + vascular RF** → Microvascular; optimize risk factors, review 6–8 weeks - **Any pupillary involvement (dilation, sluggish reactivity)** → **Urgent MRA circle of Willis** to exclude PCoA aneurysm - If MRA negative and clinical picture consistent with microvascular disease → conservative management **Mnemonic:** **PISA** = Pupil Involved → Suspect Aneurysm → Image urgently. [cite: Harrison 21e Ch 428; Walsh & Hoyt Clinical Neuro-Ophthalmology 6e Ch 18; UpToDate "Third cranial nerve (oculomotor nerve) palsy in adults"]