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
