Relative afferent pupillary defect:
Also known as the Marcus-Gunn pupil, a relative afferent pupillary defect is found by the ‘swinging light test’.
It is caused by a lesion anterior to the optic chiasm i.e. optic nerve or retina.
Causes:
Retina: detachment
Optic nerve: Optic Atrophy, Optic neuritis (e.g. multiple sclerosis)
Pathway of pupillary light reflex:
Afferent: retina → optic nerve → lateral geniculate body → midbrain
Efferent: Edinger-Westphal nucleus (midbrain) → oculomotor nerve.
Examination of the pupils using a light shone alternately in each eye reveals that when the light is shone in the right eye both pupils constrict but when the light source immediately moves to the left eye both eyes appear to dilate.>> Left optic neuritis.
See this Video of RAPD (Link): https://youtu.be/1FMdUZyHfLM
This is the ‘swinging light test’ and reveals a relative afferent pupillary defect. As there is a defect in the afferent nerve on the left side the pupils constrict less than normal, giving the impression of dilation.
In young female age, multiple sclerosis causing optic neuritis is the likely underlying diagnosis.
Optic neuritis typically causes a dull ache in the region of the eye which is aggravated by movement.