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  • Author: Eric R Eggenberger, MS, DO, FAAN; Chief Editor: Edsel Ing, MD, FRCSC  more...
Updated: Jun 16, 2016


Anisocoria, or unequal pupil sizes, is a common condition. The varied causes have implications ranging from life-threatening to completely benign, and a clinically guided history and examination is the first step in establishing a diagnosis.



Pupil size depends on the effects of the autonomic nervous system and the iris muscle, and numerous pathophysiological processes can cause anisocoria.

From the autonomic nerve standpoint, the parasympathetic system constricts the iris, while the anatomically distinct sympathetic channels dilate the iris. The sympathetic system begins in the hypothalamus, descends through the brain stem (including the lateral medulla) and into the cervical cord to synapse in the ciliospinal center of Budge-Waller at the C8-T1 level. The second-order neuron then exits the C8-T1 nerve root, travels over the lung apex, and ascends to the superior cervical ganglia with the carotid artery. The third-order neuron leaves the superior cervical ganglia to ascend as a plexus around the internal carotid artery through the cavernous sinus, where fibers destined for the pupil dilator and the Mueller muscle of the eyelid travel with the trigeminal nerve through the superior orbital fissure to their orbital targets. Fibers destined to modulate sweating of the face travel with the external carotid artery.

The parasympathetic fibers begin in the Edinger-Westphal subnucleus of cranial nerve III in the midbrain. Parasympathetic fibers travel with the oculomotor (cranial III) nerve, traverse the cavernous sinus, and enter the orbit via the superior orbital fissure to synapse in the ciliary ganglia. The short ciliary nerves then innervate the iris sphincter and muscles of accommodation. 




United States

Anisocoria is common, although no overall prevalence statistics are available. The incidence and prevalence data for anisocoria depend on the specific pathophysiology and population. The presence of physiologic anisocoria has been estimated at 20% of the normal population, so some degree of pupil difference may be expected in at least 1 in 5 clinic patients.


Mortality and morbidity rates associated with anisocoria depend entirely upon the specific pathophysiology.

Several causes of anisocoria are life threatening, including Horner syndrome due to carotid dissection or third nerve palsy due to an aneurysm or uncal herniation.

Other causes of anisocoria are completely benign (eg, simple or physiologic anisocoria), although the unnecessary evaluation of these disorders may produce morbidity inadvertently.

Contributor Information and Disclosures

Eric R Eggenberger, MS, DO, FAAN Professor, Vice-Chairman, Department of Neurology and Ophthalmology, Colleges of Osteopathic Medicine and Human Medicine, Michigan State University; Director of Michigan State University Ocular Motility Laboratory; Director of National Multiple Sclerosis Society Clinic, Michigan State University College of Human Medicine

Eric R Eggenberger, MS, DO, FAAN is a member of the following medical societies: American Academy of Neurology, American Academy of Ophthalmology, American Osteopathic Association, North American Neuro-Ophthalmology Society

Disclosure: Serve(d) as a speaker or a member of a speakers bureau for: Biogen; Genzyme; Novartis; Teva <br/>Received research grant from: Biogen; Genzyme; Novartis<br/>Received consulting fee from Biogen for consulting; Received consulting fee from Teva for consulting; Received consulting fee from Acorda for consulting; Received grant/research funds from Novartis for independent contractor; Received honoraria from Genentech for speaking and teaching; Received honoraria from Genzyme for speaking and teaching.

Specialty Editor Board

Francisco Talavera, PharmD, PhD Adjunct Assistant Professor, University of Nebraska Medical Center College of Pharmacy; Editor-in-Chief, Medscape Drug Reference

Disclosure: Received salary from Medscape for employment. for: Medscape.

Jane W Chan, MD Professor of Neurology/Neuro-ophthalmology, Department of Medicine, Division of Neurology, University of Nevada School of Medicine

Jane W Chan, MD is a member of the following medical societies: American Academy of Neurology, American Academy of Ophthalmology, American Medical Association, North American Neuro-Ophthalmology Society, Phi Beta Kappa

Disclosure: Nothing to disclose.

Chief Editor

Edsel Ing, MD, FRCSC Associate Professor, Department of Ophthalmology and Vision Sciences, University of Toronto Faculty of Medicine; Consulting Staff, Hospital for Sick Children and Sunnybrook Hospital

Edsel Ing, MD, FRCSC is a member of the following medical societies: American Academy of Ophthalmology, American Association for Pediatric Ophthalmology and Strabismus, American Society of Ophthalmic Plastic and Reconstructive Surgery, Royal College of Physicians and Surgeons of Canada, Canadian Ophthalmological Society, North American Neuro-Ophthalmology Society, Canadian Society of Oculoplastic Surgery, European Society of Ophthalmic Plastic and Reconstructive Surgery, Canadian Medical Association, Ontario Medical Association, Statistical Society of Canada, Chinese Canadian Medical Society

Disclosure: Nothing to disclose.

Additional Contributors

James Goodwin, MD Associate Professor, Departments of Neurology and Ophthalmology, University of Illinois College of Medicine; Director, Neuro-Ophthalmology Service, University of Illinois Eye and Ear Infirmary

James Goodwin, MD is a member of the following medical societies: American Academy of Neurology, Illinois State Medical Society, North American Neuro-Ophthalmology Society, Royal Society of Medicine

Disclosure: Nothing to disclose.

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Flowchart to assist in the diagnosis of anisocoria (modified with permission from Thompson and Pilley)
Horner syndrome secondary to carotid dissection. Note that degree of anisocoria is relatively mild in room light. Also, see the image below of the same patient.
Horner syndrome due to carotid dissection. Note the increase in degree of anisocoria under dark conditions.
Typical pupil in third nerve palsy, with mydriasis. Note the inability to adduct the right eye. This patient has a skull-based meningioma that is compressing the right third nerve. At rest, complete ptosis is present in the right eye; however, lid elevation with attempted adduction of the right eye is noted, which is consistent with aberrant regeneration.
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