Anisocoria is a physical condition characterized by one pupil being bigger than the other. A variety of disorders can cause this irregularity, including Adie's pupil and Horner syndrome. It also might be caused by damage to the iris sphincter, parasympathetic innervation failure, oculomotor nerve palsy or other causes. The severity of these causes ranges from harmless to potentially fatal.
About 20% of people have mild, physiologic anisocoria. In most cases, the size of the pupil can change at random during the course of the day. Patients who have additional symptoms in combination with irregular pupil size might have a more serious disorder.
Adie's pupil is a condition that usually affects women between the ages of 20 and 40 that is caused by a viral infection in the nerve tissues of the eye. This condition is generally benign and does not cause serious physical damage, and there is no specific treatment for it. On occasion, patients might also suffer from irregular heartbeat, low blood pressure when rising to a standing position, a loss of reflexes and areas of the skin that no longer produce sweat. This condition is called Adie syndrome, and is treated with changes to the patient's diet and lifestyle.
Another condition that can make one pupil bigger than the other is called Horner syndrome, and it is typically accompanied by a slightly droopy eyelid. This disorder usually is a symptom of a more serious problem rather than a disease in itself. Some patients develop Horner syndrome because of a brain injury or stroke. Other possible causes include lung cancer, damage to the spinal cord or damage to the carotid artery. Many of the disorders that cause this condition can be life-threatening.
In the case of Horner syndrome, one pupil appears bigger than the other because the pupil of the affected eye is smaller. Both pupils react to bright light at equal rates. Some patients also stop sweating or perspire less on one side of the face than on the other.
A patient might also have one pupil bigger than the other if he or she experiences damage to the iris sphincter. The normally round iris develops an irregular shape, and the eye might be swollen or inflamed. A medical professional can examine the eye to look for damage to the muscles.
There are several other reasons for having pupils of different sizes, including parasympathetic innervation failure, which is a condition in which the pupil is no longer able to contract normally and remains dilated. A disorder such as third cranial nerve or oculomotor nerve palsy can cause this secondary condition. Migraine headaches, eye infections, exposure to atropine-based medications and prior eye surgeries can also leave one pupil bigger than the other.