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) | Mid-dilated to fully dilated | | Pupil reactivity | **Completely normal** | Sluggish or fixed | | 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 | **Clinical Pearl:** The parasympathetic fibres of CN III travel on the **outer surface** of the nerve and are compressed first by an expanding aneurysm. A PCoA aneurysm classically presents with a painful, pupil-involving CN III palsy. Even partial or sluggish pupillary involvement is sufficient to warrant urgent vascular imaging. The rule "pupil-sparing = microvascular, no imaging" applies **only when the pupil is completely normal in size and reactivity**. ## Management Algorithm - **Pupil completely normal (truly spared) + age >50 + vascular risk factors** → Microvascular aetiology likely → Optimize risk factors, follow-up 6–8 weeks - **Any pupillary abnormality (mid-dilation, sluggish reactivity)** → **Urgent MR angiography (or CT angiography) of the circle of Willis** to exclude PCoA aneurysm In this patient, the mid-dilated, sluggishly reactive pupil represents partial parasympathetic involvement. This is **not** a truly pupil-sparing palsy. Urgent MR angiography is the most appropriate next step to exclude a life-threatening aneurysm. **Mnemonic:** **Any pupil abnormality = Aneurysm until proven otherwise.** [cite: Harrison's Principles of Internal Medicine, 21e, Ch 428; Walsh & Hoyt's Clinical Neuro-Ophthalmology, 6e; American Academy of Ophthalmology guidelines on CN III palsy]