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Marcus Gunn Pupil or relative afferent pupillary defect is an important sign that can be tested by swinging light from one eye to another.
This test is very important to assess the optic nerve and the anterior part of visual pathway such as optic chiasm and optic tract.
Optic nerve consists of 1.2 million axons. These axons start in the retina and pass through optic disc to form optic nerve. At optic chiasm, fibers from optic nerves of both eyes will connect. Optic tract starts from optic chiasm and consists of fibers from optic nerve of both eyes. Each optic tract has fibers from both eyes, in contrast to optic nerve which has fibers only from on eye.
When normal eye is stimulated by light, the pupil in this eye will constrict and this is called a direct pupillary reflex and also the pupil in the other eye will constrict and this is called consensual pupillary reflex. Pupil should constrict to a normal size. Once the light stimulator is removed, the pupil in both eyes will return to its normal size.
In relative afferent pupillary reflex test, light is focused in one eye and in response to that; there will be direct and consensual pupillary reflex. The light stimulator then switched to the second eye quickly, the pupil in this eye should be constricted and remain constricted once stimulated by light. This is the normal response.
1- Light focused in right eye, pupil in right and left eye will constrict to the same amount.
2- Light switched to left eye quickly. Pupil will remain constricted in both left and right eye.
1- Light is focused in right eye. Pupil in right and left eyes will constrict to the same amount.
2- Light switched to the left eye quickly. Pupil in both eyes will dilute instead of constriction.
3- Light switched again to right eye quickly. Pupil in both eyes will constrict again.
4- Light switched again to the left eye quickly. Pupil in both eye will dilute instead of constrict.
These results will tell us that the optic nerve in the left eye is weaker than that in the right eye. It doesn’t mean that the optic nerve in the right is healthy and that in the left is not healthy. It compares between both optic nerve and shows that one eye is weaker than the other eye.
In this example there is left relative afferent pupillary defect or left marcus gunn pupil.
1- Before optic disc such as total retinal detachments
2- At or after optic disc. Any diseases that affect optic disc and nerve such as glaucoma, optic neuritis, compressive lesions, autoimmune diseases and ischemic lesions that affect optic nerve, optic chiasm and optic tract.
Watch this video about Relative Afferent Pupillary reflex test
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