When the strength of the sensory detection from one eye is different from the other, it will cause the degree of papillary constriction with light shining on one pupil will be less than with the light shining onto the other eye.
The above diagram shows a normal person and a person who has Marcus Gunn pupil. A normal eye will constrict when swing light test was carried out while a Marucs Gunn pupil will not constrict.
The causes include optic neuritis (Multiple Sclerosis – MS) and retinal injuries. Not only have that, compression of the optic nerve in the orbit also caused an afferent papillary defect. However injuries of the optic pathways that are distal to the lateral geniculate will not produce an afferent papillary defect.
There are several causes of this abnormal papillary condition. They include central retinal artery occlusion, central retinal vein occlusion, optic atrophy, marked retinal detachment, anterior ischemic optic neuropathy, branch retinal vein occlusion and asymmetric primary open angle glaucoma.
Treatment is not usually necessary for this condition but if required surgery with bilateral levator excision and frontalis brow suspension may be used.
Here is a video which can fully explain the Marcus Gunn's pupil defect.
Sources taken from:
http://www.opt.indiana.edu/riley/HomePage/Pupil_Abnormal/1_Saint_Pupil_Abnormal.html
http://rad.usuhs.edu/medpix/parent.php3?mode=single&recnum=273
http://en.wikipedia.org/wiki/Marcus_Gunn_phenomenon#Treatment
http://umed.med.utah.edu/neuronet/lectures/2002/Basics%20in%20Neuro-Ophthalmology.htm
