Anisocoria is the condition defined by a measurable difference in the diameter of the two pupils (unequal pupil sizes). While noticing this phenomenon can cause immediate concern, it is often a common and harmless finding. Nevertheless, anisocoria can sometimes signal a serious underlying health problem, especially if it appears suddenly or is accompanied by other physical changes.
Telling the Difference Between Normal and Abnormal
Pupil size is controlled by the autonomic nervous system. The parasympathetic nervous system causes the pupil to constrict (become smaller), while the sympathetic nervous system causes it to dilate (become larger). Anisocoria results from an imbalance or disruption in one of these two opposing systems affecting a single eye.
The most common form is physiological anisocoria, affecting approximately 20% of the population. This is considered a normal variation, distinguished by a small difference (usually less than one millimeter) that remains constant regardless of lighting. The pupils react normally to light, and this condition poses no threat to vision or overall health.
Pathological anisocoria suggests a disease process, and the difference in pupil size changes with light exposure. If the anisocoria is more pronounced in bright light, the larger pupil cannot constrict, indicating a failure of the parasympathetic (constricting) pathway. Conversely, if the difference is greater in dim light, the smaller pupil fails to dilate, pointing to a disruption in the sympathetic (dilating) pathway. This observation helps clinicians narrow down the potential cause.
Common Causes of Unequal Pupils
Causes of unequal pupils can be localized to the eye or involve temporary external factors. Pharmacological anisocoria occurs when an eye is exposed to chemical agents, such as eye drops used during routine examinations to purposely dilate the pupil. Accidental exposure can also occur if a foreign substance, like medication residue, is inadvertently rubbed into one eye.
Minor localized trauma can cause mechanical anisocoria. This happens when the iris muscles, which control pupil size, are directly damaged, or when inflammation occurs. Conditions like iritis (inflammation of the iris) can cause the pupil to become irregular or fixed due to swelling.
Another common, non-urgent cause is an Adie tonic pupil, which presents as a larger pupil showing delayed constriction to light. This condition is often benign and may be an isolated finding, sometimes developing after a viral infection. Although a form of pathological anisocoria, it is usually not life-threatening and may only cause minor light sensitivity.
Urgent Warning Signs
When anisocoria is accompanied by other specific symptoms, it may indicate a serious neurological event requiring immediate medical attention. A sudden onset of unequal pupils is a major red flag, especially if paired with a severe headache or stiff neck, which can signal a bleed or mass in the brain. The presence of a drooping eyelid (ptosis) alongside anisocoria suggests damage to a cranial nerve.
Horner syndrome is indicated by a small, unresponsive pupil and ptosis, resulting from a disruption to the sympathetic nerve pathway. This nerve damage can be caused by a stroke, a tumor near the lung, or a carotid artery dissection in the neck. The affected side may also show reduced or absent sweating on the face (anhidrosis).
If the abnormal pupil is the larger one, accompanied by ptosis and inability to move the eye in certain directions, a third nerve palsy is suspected. The third cranial nerve controls eye muscles, including the one that constricts the pupil. Compression of this nerve, possibly by a brain aneurysm, is a medical emergency due to the risk of rupture. Acute angle-closure glaucoma is another urgent possibility, characterized by sudden, severe eye pain, redness, and a fixed, mid-dilated pupil due to rapidly increased intraocular pressure.
Diagnosis and Treatment
Diagnosis begins with a detailed patient history, focusing on the timing of onset, accompanying symptoms, and recent medication use. A physician performs an eye examination, observing the pupils in bright and dim light to identify which pupil is abnormal. If the difference is greater in the dark, the smaller pupil suggests sympathetic system involvement.
If the size difference is greater in the light, the larger pupil is abnormal, pointing to a problem with the parasympathetic system. Specialized testing may involve applying dilute pharmacological eye drops, such as pilocarpine, which help localize nerve damage based on pupil reaction. If a serious neurological cause is suspected, immediate imaging tests like a CT scan or MRI of the head and neck are necessary to look for tumors, aneurysms, or dissections.
Treatment for anisocoria depends entirely on the underlying diagnosis. If the cause is physiological or a benign condition like Adie tonic pupil, no specific treatment is necessary, and the condition is observed. When unequal pupils are a symptom of a serious condition, such as a brain aneurysm or a tumor, the focus shifts to treating that specific cause, often requiring urgent intervention like surgery or targeted medication.