The first point to distinguish is if the abnormal eye is the dilated one or if it is the other eye which may have a smaller pupil. This is done by checking the difference in the pupil size good room lighting and then in dim room illumination.
For a dilated pupil on one side which does not react well to light, there are a variety of causes.
1. Inadvertent use of eye drops which can dilate the eyes.
2. Damage to the nerve that cause the pupil to constrict. This nerve that constricts the pupil is associated with cranial nerve 3 which also works to move your eye up, down, and in towards your nose. If you are not having double vision
, it is not likely that you are having problems moving that eye.
Things which can cause damage to cranial nerve 3 are tumors, inflammation, or aneurysms. The MRI was likely done to ensure none of these would be causes of your dilated pupil. From what you have described, this doesn't appear to be the case.
3. Another possibility could be what is called an Adie's tonic pupil. This condition is thought to be caused by a virus with damages the ciliary ganglion, another collection of nerves behind the eyes. Unlike the situation I discussed in #2, motility of the eye is not affected. This condition is generally not serious. Testing for this condition is done with careful examination to look for the movements of the iris
. In these patients, the pupil will get smaller with dilute pilocarpine. In normal people, dilute pilocarpine has no effect.
From what you have described, #3 is the most likely scenario. However, conditions in example #2 are the most serious and therefore need to be excluded -- either by exam or with MRI. http://emedicine.medscape.com/article/1158571-overview
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