anisocoria

Anisocoria is the condition of unequal pupil sizes. In many healthy people, small differences in pupil size are normal, typically up to about 0.4–0.6 millimeters, and may vary with lighting and focusing. Determining the pattern of the difference by comparing pupil size in bright and dim light and assessing the pupillary light reflex helps distinguish benign physiologic anisocoria from an underlying disorder.

Assessment focuses on relative pupil size under different lighting and on the light and near responses, plus signs such as ptosis or extraocular movement abnormalities. If anisocoria is greater in dim light, the affected pupil tends not to dilate well, suggesting a sympathetic defect (as in Horner syndrome). If anisocoria is greater in bright light, the problem is with constriction, suggesting a parasympathetic lesion such as a third nerve palsy or pharmacologic dilation. A careful neurologic and ocular examination is essential to identify accompanying findings like ptosis, ophthalmoplegia, or facial anhidrosis.

Common causes include physiologic (benign) anisocoria; Horner syndrome (ptosis, miosis, possible anhidrosis); Adie tonic pupil (slow, irregular constriction with light, preserved near response); third nerve palsy with pupil involvement (dilated pupil, ptosis, impaired movements; urgent evaluation for compressive lesions such as aneurysm); and pharmacologic mydriasis from eye drops or systemic medications. Trauma and iris injury can also produce unequal pupils.

Management depends on the cause. Physiologic anisocoria requires no treatment. Acute onset anisocoria with neurologic signs or a new dilated pupil warrants urgent medical evaluation and often imaging to exclude life-threatening etiologies.