A patient presents with a dilated pupil that does not react to light or accommodation. The pupil is fixed in mid-dilation. Which nerve lesion is most likely responsible?

Explanation

## Fixed Mid-Dilated Pupil: CN III Parasympathetic Lesion ### Clinical Presentation A **fixed, mid-dilated pupil** (4–6 mm) that does not react to light or accommodation indicates **complete loss of parasympathetic innervation** to the iris sphincter muscle. ### Anatomical Basis **Key Point:** The parasympathetic fibers of the **oculomotor nerve (CN III)** are responsible for pupillary constriction. Damage anywhere along the CN III parasympathetic pathway results in a fixed, dilated pupil. ### CN III Parasympathetic Pathway 1. Preganglionic fibers originate from the **Edinger-Westphal nucleus** (midbrain) 2. Travel with CN III through the **cavernous sinus** and **orbital apex** 3. Synapse in the **ciliary ganglion** 4. Postganglionic fibers travel via **short ciliary nerves** to innervate: - **Iris sphincter muscle** (pupillary constriction) - **Ciliary muscle** (accommodation) ### Why Mid-Dilation? The pupil is **not maximally dilated** (8–9 mm) because: - Sympathetic innervation to the **iris dilator muscle** remains intact - The pupil settles at the balance point between unopposed sympathetic tone and absent parasympathetic tone - This is the **resting state** of the iris without parasympathetic input ### Differential: Fixed Pupil Etiologies | Pupil Finding | Cause | Light Reflex | Accommodation | | --- | --- | --- | --- | | Mid-dilated, fixed | CN III parasympathetic lesion | Absent | Absent | | Maximally dilated, fixed | Sympathomimetic toxin (cocaine, amphetamine) or acute angle closure | Absent | Absent | | Pinpoint, fixed | Pontine hemorrhage or opioid toxicity | Absent | Absent | | Argyll Robertson | Dorsal midbrain lesion | Absent | Present | **High-Yield:** In **acute CN III palsy**, the pupil may be dilated and fixed, often accompanied by **ptosis** and **ophthalmoplegia** (down-and-out eye position). **Mnemonic:** **"CN III = Constriction"** — CN III parasympathetic fibers = pupil constriction. Loss = fixed dilated pupil.