In a patient with a relative afferent pupillary defect (RAPD), which pupil will dilate when the swinging flashlight test is performed?

Explanation

## Relative Afferent Pupillary Defect (RAPD): Swinging Flashlight Test **Key Point:** In RAPD, the **affected pupil (on the side of the optic nerve lesion) dilates** when the light is swung to it during the swinging flashlight test. This occurs because the affected eye sends a weaker afferent signal to the pupillary light reflex center. ### Mechanism of RAPD 1. **Normal eye**: Light → strong afferent signal → bilateral pupillary constriction 2. **Affected eye**: Light → weak afferent signal → inadequate bilateral constriction 3. **When light swings to affected eye**: Both pupils dilate because the weak afferent input cannot maintain constriction ### Swinging Flashlight Test Procedure ```mermaid flowchart TD A[Shine light on normal eye]:::action --> B[Both pupils constrict]:::outcome B --> C[Swing light to affected eye]:::action C --> D[Both pupils dilate]:::outcome D --> E[RAPD present]:::outcome ``` **High-Yield:** The **affected pupil dilates paradoxically** when light is directed to it — this is the hallmark finding of RAPD. ### Causes of RAPD | Category | Examples | | --- | --- | | **Retinal** | Central retinal artery occlusion, branch retinal artery occlusion, retinal detachment | | **Optic nerve** | Optic neuritis, optic atrophy, glaucoma, optic nerve hypoplasia | | **Anterior visual pathway** | Amblyopia (severe), corneal scarring, cataract (dense) | **Mnemonic:** **"RAPD = Afferent Defect"** — the defect is in the **afferent (sensory) pathway**, not the efferent (motor) pathway. The pupils themselves are normal; the problem is the signal they receive. **Clinical Pearl:** RAPD indicates a lesion anterior to the optic chiasm (unilateral optic nerve or retinal pathology). Chiasmal and retrochiasmal lesions do NOT cause RAPD because both eyes' afferent pathways are equally affected. **Warning:** Do not confuse RAPD with **anisocoria** (unequal pupil size). RAPD is about the **reactivity** of pupils to light, not their baseline size. A patient can have RAPD with equal-sized pupils. [cite:Harrison 21e Ch 379]