A 32-year-old woman presents with acute-onset right eye pain, blurred vision, and photophobia. On examination, visual acuity is 6/12 in the right eye. The right pupil is 2 mm and reacts sluggishly to light, while the left pupil is 4 mm and reacts briskly. When a light is swung from the left eye to the right eye, both pupils dilate. Fundoscopy reveals mild optic disc swelling and perivascular sheathing. What is the most likely diagnosis?

Explanation

## Diagnosis: Relative Afferent Pupillary Defect (RAPD) in Optic Neuritis ### Clinical Presentation The patient demonstrates: 1. **Acute optic neuritis** — pain, vision loss, photophobia, disc swelling, perivascular sheathing 2. **RAPD** — sluggish pupil on affected side, pupil dilation when light swings to affected eye 3. **Asymmetric pupillary response** — right pupil smaller and slower than left ### Understanding RAPD (Afferent Pupillary Defect) **Key Point:** RAPD occurs when there is **asymmetric damage to the afferent (sensory) pathway** of the pupillary light reflex — the optic nerve and retina. The efferent (parasympathetic motor) pathway remains intact. ### Mechanism of RAPD ```mermaid flowchart TD A[Light stimulus]:::action --> B{Afferent pathway intact?}:::decision B -->|Left eye: YES| C[Signal reaches pretectal nucleus]:::action B -->|Right eye: DAMAGED| D[Weak/absent signal reaches pretectal nucleus]:::action C --> E[Both pupils constrict normally]:::outcome D --> F[Weak parasympathetic output]:::outcome G[Swinging light test] --> H{Light on affected eye?}:::decision H -->|YES| I[Weak afferent input → pupils dilate]:::outcome H -->|NO| J[Strong afferent input → pupils constrict]:::outcome ``` ### Swinging Light Test (Marcus Gunn Test) **Clinical Pearl:** The **swinging light test** is the gold standard for detecting RAPD: - **Normal response:** Pupils constrict when light shines on either eye - **RAPD present:** When light swings to the affected eye, **both pupils dilate** (paradoxical dilation) because the afferent signal is weak - This patient shows **pupils dilating** when light swings to the right eye → **right RAPD** ### Causes of RAPD | Cause | Mechanism | Associated Findings | |-------|-----------|---------------------| | **Optic neuritis** | Optic nerve inflammation | Pain, disc swelling, perivascular sheathing, young female, demyelinating | | **Retinal detachment** | Photoreceptor loss | Visual field defect, floaters, flashing lights | | **Branch retinal artery occlusion** | Retinal ischemia | Sudden vision loss, altitudinal field defect | | **Optic atrophy** | Chronic optic nerve damage | Pale disc, gradual vision loss | | **Glaucoma** | Optic nerve cupping | Elevated IOP, visual field loss | **High-Yield:** RAPD is **always ipsilateral to the eye with optic nerve/retinal pathology**. It indicates afferent pathway damage, not pupillary motor dysfunction. ### Why This Patient Has RAPD - **Optic neuritis** (demyelinating disease) → optic nerve inflammation - Reduced light signal from right eye → weak parasympathetic drive - **Swinging light test positive** → paradoxical dilation of both pupils when light on right eye - Disc swelling and perivascular sheathing confirm optic nerve involvement **Mnemonic: RAPD = Retinal/Afferent Pathway Defect** — The **afferent** (sensory) side is damaged, not the **efferent** (motor) side.