A 58-year-old man presents to the ophthalmology clinic with a 3-week history of progressive blurred vision and eye pain. On examination, his right pupil is 6 mm and reacts sluggishly to light, while the left pupil is 3 mm and reacts briskly. When a torch is swung from the left eye to the right eye (swinging flashlight test), the right pupil dilates paradoxically. Visual acuity in the right eye is 6/60 and in the left eye is 6/6. Fundoscopy reveals optic disc swelling and retinal hemorrhages in the right eye. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Relative Afferent Pupillary Defect (RAPD) ### Key Clinical Features **Key Point:** The swinging flashlight test demonstrating paradoxical pupil dilation of the right eye (when light is directed to it) is pathognomonic for RAPD. This occurs because the right eye has reduced afferent pupillary input due to optic nerve pathology. **High-Yield:** RAPD is detected when: 1. Light directed to the affected eye → pupil dilates (paradoxical) 2. Light directed to the normal eye → both pupils constrict normally 3. This indicates the affected eye is sending less light signal to the pupillary center ### Mechanism of RAPD The pupillary light reflex depends on intact afferent (optic nerve) and efferent (parasympathetic) pathways. In optic neuritis: - Inflammation of the optic nerve reduces light transmission - The right eye's afferent signal is diminished - When light hits the right eye, the brain perceives less light, so both pupils dilate - When light hits the left eye, the brain perceives normal light, so both pupils constrict ### Clinical Correlation with Optic Neuritis | Feature | Finding in This Case | |---------|----------------------| | **Onset** | Acute (3 weeks) | | **Pain** | Yes (retrobulbar or with eye movement) | | **Vision loss** | Marked (6/60 vs 6/6) | | **Fundoscopy** | Disc swelling + hemorrhages | | **Pupil size** | Right larger (due to reduced afferent input) | | **Swinging flashlight test** | Paradoxical dilation of right pupil | **Clinical Pearl:** Optic neuritis is the most common cause of RAPD in younger patients and is often the first manifestation of multiple sclerosis in 15–20% of cases. MRI brain and spine should be obtained to assess for demyelinating lesions. ### Why the Pupil Appears Larger The right pupil (6 mm) is larger than the left (3 mm) because: - Reduced afferent input from the damaged optic nerve - The pupil cannot constrict as effectively to light - At baseline (in dim light), the pupil remains more dilated **Mnemonic: RAPD = Right Afferent Pupillary Defect** - **R**elative (not absolute; pupil still reacts, but sluggishly) - **A**fferent (problem is in the sensory pathway — optic nerve) - **P**upillary (affects the pupil reflex) - **D**efect (reduced light perception) ### Differential Considerations **Key Point:** RAPD is an afferent defect; it is NOT due to parasympathetic (efferent) dysfunction. The efferent pathway is intact, which is why both pupils still react (albeit differently depending on which eye receives light).