---
title: Marcus gunn pupil
description: "A Marcus Gunn pupil, or relative afferent pupillary defect RAPD, indicates asymmetric optic nerve or severe retinal disease. It is detected using the swinging flashlight test. ## Features and causes The affected eye has reduced afferent input. - When light…"
url: "https://bluefinvision.com/knowledge-base/topics/conditions-diseases/marcus-gunn-pupil/"
updated: "2025-12-17"
category: "Conditions & Diseases"
---

# Marcus gunn pupil

A Marcus Gunn pupil, or relative afferent pupillary defect RAPD, indicates asymmetric optic nerve or severe retinal disease. It is detected using the swinging flashlight test.

## Features and causes

The affected eye has reduced afferent input.

- When light is moved from the normal to the affected eye, both pupils paradoxically dilate
- Seen in optic neuritis, ischaemic optic neuropathy, retinal detachment, or large macular lesions
- Does not occur in equal bilateral disease or media opacities alone
- Helps localise pathology to the afferent visual pathway

## Management

The RAPD itself is a sign, not a diagnosis.

- Thorough evaluation of optic nerve and retina to identify cause
- Neuroimaging or systemic work-up when indicated, for example in suspected demyelination
- Monitoring of RAPD over time as a marker of disease progression or recovery
- Visual rehabilitation if permanent optic nerve damage has occurred