A 58-year-old man presents to the ophthalmology clinic with a 3-week history of progressive vision loss in the left eye and headaches. On examination, the left pupil is 5 mm and reacts sluggishly to light, while the right pupil is 3 mm and reacts briskly. When a light is swung from the right eye to the left eye, the left pupil paradoxically dilates. Visual acuity is 6/60 in the left eye and 6/6 in the right eye. Fundoscopy reveals optic disc swelling on the left. MRI brain shows a 2 cm mass in the left optic nerve. What is the most likely explanation for the pupillary findings?

Explanation

## Diagnosis: Relative Afferent Pupillary Defect (RAPD) ### Clinical Context This patient has a left optic nerve mass causing severe optic nerve damage with profound vision loss (6/60) and optic disc swelling. The key finding is the **paradoxical dilation of the left pupil when light is swung to it** — this is the hallmark of RAPD. ### Mechanism of RAPD **Key Point:** RAPD occurs when there is asymmetric afferent (sensory) pupillary pathway damage. The affected eye receives less light signal to the pretectal nucleus, so both pupils receive a weaker parasympathetic drive when light is directed to the damaged eye. 1. When light shines on the **normal right eye** → both pupils constrict (normal consensual reflex) 2. When light swings to the **damaged left eye** → less signal reaches the brainstem → both pupils receive weaker parasympathetic input → **both pupils dilate relative to their previous state** ### Why the Left Pupil Appears to "Dilate" The left pupil does not truly dilate (it does not become larger than baseline). Rather, it **fails to constrict as much as the right pupil** when light is directed to it. This relative failure of constriction is perceived as dilation and is diagnostic of RAPD. ### Supporting Features in This Case - **Severe unilateral vision loss** (6/60 vs 6/6) — indicates significant afferent pathway damage - **Optic disc swelling** — confirms left optic nerve pathology - **Structural lesion on MRI** — mass compressing/infiltrating the optic nerve - **Sluggish pupillary light reflex on the left** — consistent with reduced afferent input **High-Yield:** RAPD is the most sensitive sign of asymmetric optic nerve or retinal disease. It can be present even when the direct pupillary light reflex appears relatively preserved, because the efferent (parasympathetic) pathway is intact — only the afferent pathway is damaged. ### Testing for RAPD The **swinging flashlight test** (as described in this case) is the gold standard: - Swing light from normal eye to affected eye - Affected eye's pupil will dilate (or fail to constrict) when light is directed to it - This is pathognomonic for RAPD **Clinical Pearl:** RAPD can occur with any unilateral lesion of the optic nerve, optic chiasm, or retina anterior to the lateral geniculate nucleus. Common causes include optic neuritis, optic nerve compression (tumor, aneurysm), retinal detachment, and advanced glaucoma.