Anisocoria
The medical term for unequal size of pupils is anisocoria. 1
Definition and Clinical Significance
Anisocoria refers to asymmetric pupillary diameter between the two eyes. 2, 3 This condition is identified during pupillary examination when pupils are assessed for size, symmetry, and shape. 1
Clinical Threshold for Pathology
Anisocoria greater than 1 millimeter may indicate a pathological process and warrants further evaluation. 1, 4 Differences of 1 mm or less are often physiologic and benign. 3
Important Pathological Causes to Consider
When anisocoria exceeds 1 mm, the following conditions should be considered:
- Horner syndrome - characterized by miosis (smaller pupil), ptosis, and anhidrosis 1
- Adie tonic pupil - typically presents with a dilated, poorly reactive pupil 1
- Pupil-involving third-cranial-nerve palsy - presents with dilated pupil, often with ptosis or ophthalmoplegia 1, 5
- Pharmacological causes - including topical mydriatics or anticholinergic medications 5, 2
- Traumatic sphincter damage - from direct ocular injury 1
- Acute angle-closure glaucoma - pupils become mid-dilated and non-reactive 5
Clinical Pitfall to Avoid
Do not confuse anisocoria (difference in pupil size) with relative afferent pupillary defect (RAPD), as these represent different pathophysiologic processes. 4, 6 RAPD refers to a difference in pupillary response to light between the two eyes and indicates optic nerve or retinal pathology, whereas anisocoria simply describes unequal pupil sizes. 4
Examination Considerations
Pupillary evaluation may be challenging in certain populations due to hippus (physiologic pupillary oscillations), poorly maintained fixation, and rapid changes in accommodative status, particularly in infants and children. 1, 4 In older adults, pupils may measure 2-3 mm even in dim lighting due to age-related miosis, which is a normal finding. 4, 6