Anisocoria means that the pupils of the eyes are not equal in size. The pupil may appear to open and close but it is really the iris (the colored part of the eye) that is the prime mover; the pupil is merely the absence of iris. The size of the pupil determines how much light is let into the eye. With anisocoria, the larger pupil lets more light enter the eye and there may, therefore, be photophobia (light sensitivity) on that side. Anisocoria raises the suspicion of a space occupying lesion (tumor or blood clot) on one side of the brain, especially if the subject has an impaired sensorium. In the emergency room the most commonly encountered conditon associated with anisocoria is subdural hematoma, a true medial/surgical emergency.