That only one eye dilates while the other does not is called anisocoria. There are numerous causes of anisocoria. Simple anisocoria is most common, and it means that there is slight difference in the size of the two pupils. This is not an abnormal variant. The unilaterally constricted pupil is likely to be the result of local ocular factors like inflammation, trauma and a failure of dilator muscle caused by neurological condition such as Horner syndrome. This syndrome is benign and results from conditions such as tumor in the chest, carotid artery dissection, migraines and trauma. Horner syndrome can be passed from parents to offspring. Some kinds of eye drops and certain systematic medications can lead to constricted pupil. I suggest you to see a doctor, for it may be a sign of a life threatening condition even though it is usually benign.
