Merck Manual

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(Unequal Pupils)


Christopher J. Brady

, MD, Wilmer Eye Institute, Retina Division, Johns Hopkins University School of Medicine

Reviewed/Revised May 2021 | Modified Sep 2022
Topic Resources

Anisocoria is unequal pupil sizes. Anisocoria itself does not cause symptoms.

Etiology of Anisocoria

The most common cause of anisocoria is

  • Physiologic (present in about 20% of people): The difference between pupil sizes in physiologic anisocoria is typically about 1 mm.


Evaluation of Anisocoria


History of present illness includes the presence, nature, and duration of symptoms. Any history of head or ocular trauma is noted.

Past medical history includes known ocular disorders and surgeries and exposure to drugs.

Physical examination

Pupillary size and light responses should be examined in lighted and dark rooms. Accommodation and extraocular movements should be tested. Ocular structures are inspected by using a slit lamp or other magnification to identify structural abnormalities and ptosis. Other ocular symptoms are evaluated by eye examination as clinically indicated. An old photograph of the patient or the patient’s driver’s license should be examined (under magnification if possible) to see whether anisocoria was present previously.

Red flags

The following findings are of particular concern:

  • Ptosis

  • Anhidrosis

  • Pupils that respond more to accommodation than light

  • Impaired extraocular movements

Interpretation of findings

If the difference in size is greater in the dark, the smaller pupil is abnormal (because the pupil should dilate in the dark to let in more light). Common causes include Horner syndrome Horner Syndrome Horner syndrome is ptosis, miosis, and anhidrosis due to dysfunction of cervical sympathetic output. (See also Overview of the Autonomic Nervous System.) Horner syndrome results when the cervical... read more Horner Syndrome and physiologic anisocoria. An ophthalmologist can differentiate them because the small pupil in Horner syndrome does not dilate after instillation of an ocular dilating drop (eg, 10% cocaine). In physiologic anisocoria, the difference in pupil size may also be equal in light and dark.

If the difference in pupillary sizes is greater in light, the larger pupil is abnormal (because the pupil should constrict in the light to let in less light). If extraocular movements are impaired, particularly with ptosis, 3rd cranial nerve palsy Third Cranial (Oculomotor) Nerve Disorders Third cranial nerve disorders can impair ocular motility, pupillary function, or both. Symptoms and signs include diplopia, ptosis, and paresis of eye adduction and of upward and downward gaze... read more is likely. If extraocular movements are intact, an ophthalmologist can further differentiate among causes by instilling a drop of a pupillary constrictor (eg, 0.1% pilocarpine). If the large pupil constricts, the cause is probably Adie tonic pupil; if the large pupil does not constrict, the cause is probably drugs or structural (eg, traumatic, surgical) damage to the iris.


Treatment of Anisocoria

Key Points

Drugs Mentioned In This Article

Drug Name Select Trade
Adsorbocarpine, Akarpine, Isopto Carpine, Ocu-Carpine, Pilocar, Pilopine HS, Salagen, Vuity
NOTE: This is the Professional Version. CONSUMERS: View Consumer Version
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