One of my patients today had a difference in pupil size. The right pupil was about 1 mm larger than the left. She is 13 years old and a new patient to my practice. There are no signs of anterior eye disease nor a history of trauma and no other neurological signs-oculo-motility, eye alignment, visual fields and pupil reactions are all normal. The difference in pupil size does not change when I decrease and increase the light in my examination room. She has not noticed the difference in pupil size and does not know if it is new. Should I be worried?

The key clinical decision that needs to be made here is: Is this physiological or pathological anisocoria? Anisocoria is a condition characterized by unequal pupil sizes. It is common, and causes vary from benign physiologic anisocoria to potentially life-threatening emergencies. Clinical evaluation is important for the appropriate diagnosis and management of the underlying cause.

If the difference in pupil size remains constant under variable light conditions, it is very likely that the anisocoria is physiological-normal for that person and not a sign of disease. If the difference in pupil size increases or decreases under variable light conditions then it has a pathological cause-a physical disease.

The main characteristic that distinguishes physiological anisocoria is an increase of pupil size with lower light or reduced illumination, such that the pupils differ in size between the two eyes. The presence of physiologic anisocoria has been estimated at 20% of the normal population, so some degree of pupil difference may be expected in at least 1 in 5 clinic patients.

When detected during childhood, without any other symptoms and when other disorders can be discounted through clinical testing, it can be considered a developmental or genetic phenomenon.

In this case, the size of the pupil difference, the lack of other neurological signs and no change in the pupil difference with changing light conditions means that your patient is one of the 1 in 5 who have physiological anisocoria. The patient should be informed of this and advised to return if there are any changes in pupil size and/or any other new eye signs or symptoms.