What does it mean when one pupil dilates and the other does not?

There isn't enough information to answer with any specificity as more details are required.

In the case where one pupil does not respond to dilating drops there are several possibilities. The iris could be partially stuck to the lens because of prior inflammation or trauma or prior surgery.

The condition of one pupil being larger than the other without having received dilating drops is called anisocoria. The causes of this are too numerous to discuss in detail. Most common is simple anisocoria, which is usually a small difference between the pupil sizes and is a normal variant. If this is a new observation then the ophthalmologist would have to determine if the abnormal pupil is the smaller one or the larger one.

A unilaterally constricted pupil can be caused by local ocular factors such as inflammation or trauma but also may be related to a failure of the dilator muscle due to a neurological condition such as Horner's syndrome. Horner's syndrome can be a benign condition or be caused by a vast array of things such as a tumor in the chest, carotid artery dissection, migraines, trauma and some medications to name just a few. It can also be congenital. Some drops and some systemic medications and drugs may cause a constricted pupil.

A unilaterally dilated pupil can be a benign finding such as a tonic pupil. It can be related to local factors such as a history of eye trauma or surgery. It can also be a result of paralysis of the iris constrictor muscle which may result from many neurological conditions but especially from involvement of the third nerve. If that is the case this can be an emergency resulting from a brain aneurysm. There are also many drugs and systemic medications that can cause a dilated pupil. The main point here is that differences in pupil size, reactivity, or reaction, while often benign, may be a harbinger of a very serious condition or even an emergent life threatening condition. Examination by an ophthalmologist is required.

This question was originally answered on Dec. 10, 2012.