A patient presents with a right-sided dilated pupil that is fixed to light but shows normal accommodation. Which nerve is most likely affected?

Explanation

## Efferent Pupillary Defect: Parasympathetic Fiber Involvement ### Clinical Presentation Analysis The patient has a **right-sided dilated pupil that is fixed to light but shows normal accommodation**. This dissociation must be carefully analyzed: - **Light reflex pathway:** Disrupted (pupil fixed/non-reactive to light) - **Accommodation pathway:** Intact (normal accommodation) - **Pupil size:** Dilated (mydriasis) This pattern — dilated pupil, absent light reflex, **preserved accommodation** — is the hallmark of a lesion in the **efferent parasympathetic fibers of CN III** (specifically the postganglionic fibers), most classically seen in **Adie's tonic pupil** or selective damage to the parasympathetic fibers of the oculomotor nerve. **Key Point:** The parasympathetic fibers of CN III subserve BOTH the light reflex AND accommodation via the ciliary ganglion. However, in partial or selective lesions of the postganglionic parasympathetic fibers (e.g., Adie's tonic pupil), the light reflex is lost while accommodation may be relatively preserved (or slow/tonic). The stem describes exactly this efferent parasympathetic pattern. ### Why NOT CN II (Option D)? An afferent (CN II) defect causes a **relative afferent pupillary defect (RAPD / Marcus Gunn pupil)**. In a true CN II afferent defect: - The **ipsilateral pupil is NOT necessarily dilated at rest** — both pupils may appear equal in size - The defect is detected by the **swinging flashlight test** (paradoxical dilation when light swings to the affected eye) - The pupil **can still react** consensually when light is shone in the normal eye - A fixed, dilated pupil at rest is NOT the typical presentation of a pure CN II afferent defect A **fixed, dilated pupil** at rest indicates an **efferent** problem (CN III parasympathetic pathway), not an afferent one. ### Anatomical Basis ``` Light → Retina → CN II (afferent) → Pretectal nucleus → Edinger-Westphal nucleus → CN III parasympathetic fibers (preganglionic) → Ciliary ganglion → Short ciliary nerves (postganglionic) → Sphincter pupillae (light reflex) + Ciliary muscle (accommodation) ``` When the **preganglionic or postganglionic parasympathetic fibers of CN III** are selectively damaged, the sphincter pupillae is denervated → **fixed dilated pupil**. The accommodation pathway may be partially spared in selective/partial lesions. ### Differential Diagnosis: Fixed Dilated Pupil | Lesion Site | Light Reflex | Accommodation | Pupil Size | |---|---|---|---| | **CN III parasympathetic fibers** | Fixed/absent | Normal or slow (tonic) | Dilated ← **This question** | | **CN II (afferent defect / RAPD)** | Relative defect (swinging light test) | Normal | Normal size at rest | | **Edinger-Westphal nucleus** | Fixed | Fixed | Dilated | | **Ciliary ganglion (Adie's)** | Fixed | Slow/tonic (eventually preserved) | Dilated | **High-Yield:** A **fixed, dilated pupil with preserved accommodation** points to the **efferent parasympathetic limb of CN III** — specifically the fibers that preferentially innervate the sphincter pupillae. This is the classic description of Adie's tonic pupil or selective CN III parasympathetic fiber damage. **Clinical Pearl (KD Tripathi / Kanski):** In Adie's tonic pupil, postganglionic parasympathetic fibers to the sphincter pupillae are damaged more than those to the ciliary muscle, explaining why light reflex is lost but accommodation (ciliary muscle) is relatively preserved. The correct answer is the **right oculomotor nerve (CN III) parasympathetic fibers**.