Relative Afferent Pupillary Defect (RAPD) — Swinging Light Test

A clinically accurate animation of a relative afferent pupillary defect (RAPD), demonstrated with the swinging light test. What to look for: • Light in the normal eye → both pupils constrict briskly • Swing to the affected eye → both pupils dilate instead of constricting The defect is 'relative': the affected eye carries a weaker afferent signal, so pupillary constriction is reduced when light swings to that side. Because the efferent pathway to both irises is intact, the pupils always move together — an RAPD causes no anisocoria, which is why it can be missed unless you perform the swinging light test. Common causes: optic neuritis, ischaemic optic neuropathy, severe retinal disease, optic nerve compression. Made by a neurology doctor. For medical students, junior doctors and anyone preparing for OSCEs, PACES, MRCP or USMLE. Educational animation, not medical advice. #neurology #ophthalmology #RAPD #MarcusGunnPupil #medicaleducation #OSCE #MRCP #USMLE