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(Unequal Pupils)

by Kathryn Colby, MD, PhD

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


The most common cause of anisocoria is

  • Physiologic (present in about 20% of people)

See Some Common Causes of Anisocoria for other causes of anisocoria.

Some Common Causes of Anisocoria


Suggestive Findings

Adie tonic pupil (idiopathic impaired constriction)

Pupils that respond more to accommodation than to light; delayed dilation after constriction

Argyll Robertson pupil (due to syphilis)

Pupils that respond more to accommodation than to light; possibly findings suggesting syphilis

Congenital iris defects

Associated ocular abnormalities, chromosomal disorder, nonocular congenital defects, chronicity

Drugs (eg, scopolamine patch; animal flea collars or sprays, organophosphates, or aerosolized ipratropium if they contact the eye; cycloplegic, mydriatic, clonidine, or apraclonidine eye drops)

History of use or exposure

Horner syndrome (eg, congenital, traumatic, postsurgical, due to migraine or lung tumors)

Ptosis, anhidrosis, delayed dilation after constriction, features of causative disorder

Iris or other ocular dysfunction after surgery


Physiologic anisocoria

Chronicity, absence of symptoms or associated findings, difference of < 1 mm (usually < 0.4 mm) between pupil sizes, normal pupillary light responses

Third cranial nerve palsy (eg, due to aneurysm or tumor)

Impaired extraocular movements, ptosis

Traumatic mydriasis

History or evidence of trauma

Many disorders are accompanied by anisocoria due to iris or neurologic dysfunction but usually manifest with other, more bothersome symptoms (eg, uveitis, optic neuritis, stroke, subarachnoid hemorrhage, acute angle-closure glaucoma).


The goal of evaluation is to elucidate the physiologic mechanism of anisocoria. By identifying certain mechanisms (eg, Horner syndrome, 3rd cranial nerve palsy), clinicians can diagnose the occasional serious occult disorder (eg, tumor, aneurysm) manifesting with anisocoria.


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

Review of systems seeks symptoms that may suggest a cause, such as birth defects or chromosomal abnormalities (congenital defects); droopy eyelid, cough, chest pain, or dyspnea (Horner syndrome); genital lesions, adenopathy, rashes, or fever (syphilis); and headaches or other neurologic symptoms (Horner syndrome or 3rd cranial nerve palsy).

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. Common causes include 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).

If the difference in pupillary sizes is greater in light, the larger pupil is abnormal. If extraocular movements are impaired, particularly with ptosis, 3rd cranial nerve palsy 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.


Testing is usually unnecessary but is indicated for clinically suspected disorders. Patients with Horner syndrome or 3rd cranial nerve palsy usually require brain MRI or CT.


Treatment of anisocoria is unnecessary.

Key Points

  • Physiologic anisocoria is very common and causes < 1 mm of difference between the pupils in size.

  • Examining the pupils in light and dark and inspecting an old photograph or the driver’s license of the patient provide a great deal of diagnostic information.

  • Serious disorders should be considered in patients with Horner syndrome or 3rd cranial nerve palsy.

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