What is Marcus Gunn Pupil?
Marcus-Gunn Pupil is quite a rare ophthalmologic condition in which each eye behaves differently to light being shown on them. It is actually a sign that something is just not right with either the optic nerve or the retina. It is a variant of what is medically termed as Relative Afferent Pupillary Defect. This is defined as a finding on ophthalmologic examination in people where there is asymmetry in the reaction of the pupils when bright light is shown on the eyes .
The best way to identify a Marcus-Gunn Pupil is by conducting what is known as a Swinging Light Test. This examination involves testing the patient in a dimly lit room by asking to fixate on a single object placed distantly. Following this, a light is shown on one eye directly for a couple of seconds and then the other. Under normal circumstances, both pupils should constrict symmetrically. In cases of Marcus-Gunn Pupil, however, there is a marked difference in how the eye behaves when light is shown. In fact, instead of constricting, the pupils actually dilate on exposure to light .
The same finding is observed on repeated testing. A finding of a Marcus-Gunn Pupil can be an indicator of an ophthalmic condition that requires treatment even though Marcus-Gunn Pupil in itself does not pose a health risk.
What Causes Marcus Gunn Pupil?
Marcus-Gunn Pupil as stated is normally an indicator of an underlying ophthalmologic condition. These conditions include :
- Optic neuropathy resulting in vision loss
- Inflammation of the optic nerve with resultant damage medically termed as optic neuritis
- Severe cases of glaucoma in which the optic nerve gets damaged
- Trauma to the eye or the head which causes damage to the optic nerve
- Vascular malformation resulting in compression of the optic nerve
- Gradual degeneration of the optic nerve
- Infection of the optic nerve
- Retinal detachment which is quite a serious condition and requires surgical correction
- Macular degeneration
- Infection of the retina
- People with lazy eye also tend to display a Marcus-Gunn Pupil 
What are the Symptoms of Marcus Gunn Pupil?
Marcus-Gunn Pupil tends to cause visual disruptions either in one eye or both eyes. This symptom will have to be checked by the ophthalmologist as to whether it is caused by Marcus-Gunn Pupil or some other condition affecting the eye like cataracts. Other than this, there are no other specific symptoms which may indicate the presence of a Marcus-Gunn Pupil .
How to Diagnose Marcus Gunn Pupil?
Diagnosing Marcus-Gunn Pupil is quite easy. It is the way that the pupils react when light is shown directly on the eyes is what confirmatively gives the diagnosis of a Marcus-Gunn Pupil. If the eyes are functioning normally, then both eyes will constrict symmetrically to light. However, if instead of constricting the pupils dilate asymmetrically then the diagnosis of Marcus-Gunn Pupil is confirmed .
How Is Marcus Gunn Pupil Treated?
Since Marcus-Gunn Pupil is not a medical condition but is rather a symptom of another ophthalmologic issue. Thus a cause needs to be identified and treatment for that should be initiated to treat Marcus-Gunn Pupil. The treatment for the conditions that cause Marcus-Gunn Pupil include:
Optic Neuritis: This in most cases resolves on its own but at times requires steroid treatment to calm down the inflammation. To treat the pain, over the counter pain killers are adequate .
Severe Glaucoma: Laser treatment is considered to be the best for treating severe glaucoma. However, the underlying cause needs to be identified first and treated. In some cases, people may need to have surgery for treatment of glaucoma .
Optic Nerve Tumor: The treatment for this condition is determined after diagnosing as to whether the tumor is benign or malignant. If the tumor is malignant, the patient will require radiation along with chemotherapy for treatment of the condition and associated Marcus-Gunn Pupil .
Optic Atrophy: This condition does not have a cure and so treatment is focused mainly on stopping the progression of the disease .
Retinal Detachment: This usually happens when the retina gets detached from the back portion of the eyes. It is a condition that normally requires surgical correction.
However, this may not always be possible due to scar tissue formation and also the success rate of this procedure is about 90%. If the procedure fails, then there may be complete loss of vision in the affected eye .
Macular Degeneration: As of now, there is no cure for this condition and treatment is mainly aimed at slowing down the progression of the disease .
Retinal Infection: This s usually treated with medications that are given either orally or through an IV line. Laser treatments have also shown some effectiveness in treating retinal infections .
Lazy Eye: The primary mode of treatment of this condition is by way of eyedrops, glasses, and eye patches .