Unequal pupil size, medically called anisocoria, occurs when one pupil is noticeably larger or smaller than the other. It can reflect harmless variations or point to eye or nervous system conditions. Below is general information about possible causes, red flags that require urgent review, and which specialists are commonly involved.
Common causes
Anisocoria has many possible origins. Mild differences may be physiological and long-standing. Other causes include nerve injury affecting pupil control, eye trauma, intraocular inflammation, recent use of certain eye drops or topical agents, and neurological conditions that affect the pathways controlling pupil size.
Signs that need urgent assessment
The context matters: sudden onset or worsening anisocoria with other symptoms may require emergency evaluation. Seek immediate medical attention if anisocoria appears with sudden vision loss, double vision, severe headache, facial weakness, speech changes, altered consciousness or marked imbalance. These may signal a neurological emergency.
How clinicians evaluate it
Initial assessment focuses on history and eye examination: pupil sizes in light and dark, reaction to light and near focus, and eye movements. Clinicians may then involve ophthalmology, neurology or emergency medicine. Further testing can include imaging or specialist neurological assessment when indicated.
Monitoring at home and what to note
If symptoms are stable and not severe, keeping a simple record can help clinicians: when the change began, which eye is affected, how the pupils react to light, and any other symptoms such as headaches or vision changes. Do not delay emergency care when red-flag signs occur.