In a patient with acute third nerve palsy, which pupillary finding would indicate involvement of parasympathetic fibres?

Explanation

## Third Nerve Palsy and Pupillary Involvement ### Anatomy of Parasympathetic Fibres in CN III - Parasympathetic preganglionic fibres originate from the **Edinger-Westphal nucleus** in the midbrain - These fibres run in the **peripheral (outer) portion** of the third nerve - They are vulnerable to **compression** (e.g., from aneurysm, tumour) - They innervate the **pupillary sphincter** and **ciliary muscle** ### Pupillary Response in CN III Palsy with Parasympathetic Involvement **Key Point:** When parasympathetic fibres are affected, the pupil becomes **dilated and fixed** — it does not react to light or accommodation. | Feature | Normal | CN III Palsy (with parasympathetic involvement) | |---------|--------|-----------------------------------------------| | Pupil size | 3–4 mm | Dilated (5–6 mm or larger) | | Light reflex | Present | Absent | | Accommodation reflex | Present | Absent | | Associated findings | — | Ptosis, ophthalmoplegia, "down and out" eye | ### Clinical Significance **High-Yield:** A **dilated, fixed pupil in CN III palsy suggests a compressive lesion** (e.g., posterior communicating artery aneurysm, tumour) rather than ischaemic palsy. Ischaemic CN III palsy typically **spares the pupil** because the parasympathetic fibres are in the periphery and are less affected by microvascular ischaemia. **Mnemonic:** **"Pupil-sparing CN III = Ischaemic; Pupil-involving CN III = Compressive"** **Clinical Pearl:** In a patient with acute CN III palsy and a dilated pupil, urgent imaging (MRI or CT angiography) is mandatory to exclude a compressive lesion such as an aneurysm, which requires emergency intervention.