Observing that one pupil is larger than the other can be an unsettling experience. This difference in pupil size is known as anisocoria. While it can be a sign of an underlying health issue, it is often a harmless and common variation.
What is Physiologic Anisocoria?
Physiologic anisocoria, also called simple anisocoria, is the most common cause of pupils of unequal size, affecting up to 20% of the general population. It is considered a normal, benign variation and is not associated with any underlying disease. The defining feature of this condition is a minimal difference in pupil diameter, typically 1 millimeter or less, that remains relatively constant regardless of lighting conditions.
This condition is often present from birth or develops early in life, and its exact cause is not fully understood. It is thought to be related to minor, transient differences in the nerve signals that control pupil size. For many individuals, physiologic anisocoria is a lifelong trait that has no impact on vision or eye health. It does not have a predilection for either sex or a particular age group.
The stability of the pupillary difference over time is a key indicator of its benign nature. Because it is asymptomatic and does not affect vision, physiologic anisocoria does not require any treatment or monitoring. Its prevalence means that a noticeable but slight difference in pupil size is a normal finding in a significant portion of the population.
Signs of Pathologic Anisocoria
While physiologic anisocoria is harmless, certain signs indicate that unequal pupil sizes may stem from an underlying medical condition, known as pathologic anisocoria. The sudden onset of a noticeable difference in pupil size is a primary warning sign that warrants medical evaluation. A new or rapidly changing inequality can signal an active problem.
The presence of other symptoms alongside anisocoria is an important indicator of a potential problem. If the anisocoria becomes more pronounced in either bright or dim light, it helps doctors identify which pupil is abnormal. Medical evaluation is warranted if the change in pupil size is accompanied by:
- A droopy eyelid (ptosis)
- Vision disturbances, including double vision (diplopia) or blurred vision
- Pain in the eye or a headache
- A pupil that does not react to bright light by constricting
- A recent head or eye injury
How Doctors Diagnose the Cause
To determine the cause of unequal pupils, an ophthalmologist will conduct a thorough evaluation. The process begins with a review of the patient’s medical history, including any recent injuries or new medications, and a physical examination of the eyes. This exam includes assessing visual acuity, eye movements, and the position of the eyelids.
A key part of the diagnosis involves observing the pupils in both bright and dim lighting. This test helps determine if the size difference remains constant, which is characteristic of physiologic anisocoria, or if it changes depending on the light level. Anisocoria that is more pronounced in the dark suggests an issue with the smaller pupil’s ability to dilate, while a greater difference in bright light points to a problem with the larger pupil’s ability to constrict.
In cases where the onset is uncertain, doctors may ask to see old photographs of the patient to establish a timeline. This can help determine if the anisocoria is a long-standing, benign condition or a recent development. For more complex cases, pharmacologic testing may be used. This involves administering specific eye drops that help differentiate between potential causes by observing how each pupil reacts.