In a patient with a relative afferent pupillary defect (RAPD), which of the following mechanisms is responsible for the paradoxical dilation of the affected pupil when light is swung to it?

Explanation

## Relative Afferent Pupillary Defect (RAPD): Mechanism of Paradoxical Dilation ### Definition **Key Point:** RAPD (Marcus Gunn pupil) occurs when one eye has reduced afferent input due to optic nerve or severe retinal disease. When light is swung to the affected eye, it appears to **dilate** (paradoxical dilation) because the consensual response is weaker than the direct response. ### The Physiology of RAPD ```mermaid flowchart TD A[Light stimulus to normal eye]:::action --> B[Strong afferent signal to pretectal nucleus]:::action B --> C[Bilateral pupil constriction]:::outcome D[Light stimulus to affected eye]:::action --> E[Weak afferent signal to pretectal nucleus]:::action E --> F[Weak bilateral pupil constriction]:::outcome G[When light swings to affected eye] --> H{Comparison of responses}:::decision H -->|Weak direct + weak consensual| I[Net result: apparent dilation]:::outcome ``` ### Why the Pupil Appears to Dilate 1. **Normal eye in light:** Strong afferent input → bilateral constriction 2. **Affected eye in light:** Weak afferent input → weaker bilateral constriction 3. **Swing test:** When light moves from normal to affected eye: - The **consensual constriction** (from the normal eye's response) is stronger than the **direct response** of the affected eye - Net effect: pupil appears to dilate (actually, it constricts less than expected) **High-Yield:** The pupil is NOT actually dilating — it is constricting less than when light was on the normal eye. This relative difference creates the illusion of dilation. ### Causes of RAPD | Category | Condition | |----------|----------| | **Optic nerve** | Optic neuritis, optic atrophy, glaucoma, ischemic optic neuropathy | | **Retina** | Severe retinal detachment, branch retinal artery occlusion, extensive retinitis | | **Chiasm/Tract** | Chiasmal compression (pituitary adenoma), optic tract lesion | | **NOT in pupillary pathway** | Cataract, corneal opacity, refractive error (no RAPD) | ### Clinical Pearl **RAPD indicates afferent (sensory) pathway disease, NOT efferent (motor) pathway disease.** The iris sphincter muscle and parasympathetic innervation are normal; the problem is reduced light input to the brain. ### Mnemonic **"RAPD = Reduced Afferent = Relative dilation"** — the affected pupil appears to dilate because the afferent input is reduced, making the consensual response relatively stronger than the direct response. [cite:Neuro-Ophthalmology by Wall & Olitsky Ch 4]