Anisocoria Overview

    • Difference in pupil size between the two eyes as viewed in dim illumination
    • May be physiologic or pathologic
    • Causes of pathologic anisocoria
      • Horner syndrome
      • Dorsal (tectal) midbrain lesion
      • Preganglionic third nerve palsy
      • Ciliary ganglion or ciliary nerve lesion (tonic [Adie] pupil)
      • Iris sphincter damage
      • Topical exposure to anticholinergic or sympathomimetic agent
    • If both pupils constrict normally to light
      • Instill apraclonidine 0.5% into each eye; if reexamination 30 minutes later shows that the formerly smaller pupil has become the larger pupil (“reversal of anisocoria”), diagnose Horner syndrome
      • In children under age 2 years, instill cocaine 10% (to avoid cardiopulmonary toxicity of apraclonidine) into each eye; if reexamination 30 minutes later shows residual anisocoria greater than 1mm, diagnose Horner syndrome
    • If at least one pupil fails to constrict normally to light, look for the following features
      • Ocular ductional deficits or ptosis ipsilateral to the abnormal pupil, which suggests third nerve palsy
      • Upgaze abnormality with or without light-near dissociation and lid retraction, which suggests a dorsal (tectal) midbrain lesion
      • Slow or segmental pupil constriction to a target held within 3 inches of eye, slow redilatation when eyes shift to viewing a distant target, and impaired constriction to light with preserved constriction to a near target (“light-near dissociation”), which suggest tonic (Adie) pupil
      • Iris deformities, sphincter tears, synechiae, or transillumination defects, which suggest iris muscle damage
      • Small pupils with light-near dissociation and no tonic features, which suggest Argyll Robertson pupils
      • Failure of the affected pupil to constrict to topical 1% pilocarpine, which suggests pharmacologic blockade
    • Tip: the only important cause of isolated anisocoria is Horner syndrome, which can be present even if there is no ptosis on the side of the smaller pupil
    • If you diagnose Horner syndrome as the only abnormality, perform CT/CTA or MRI/MRA of the neck and chest, looking for cervical carotid artery dissection or other neck/chest lesions, including tumor
    • If you diagnose Horner syndrome accompanied by other pertinent localizing abnormalities, target the imaging based on the suspected localization of the lesion
    • Tip: if the Horner syndrome is acute and accompanied by neck or face pain, imaging aimed at carotid artery dissection is urgent, as the lesion can cause stroke
    • If one pupil does not constrict normally to light, exclude other features of a third nerve palsy, which would mandate prompt brain imaging to rule out aneurysm
    • If both pupils fail to constrict normally to light, exclude other features of dorsal midbrain syndrome, especially impaired upgaze
    • If at least one pupil does not constrict normally to light, and there are no other pertinent abnormalities, consider these 4 explanations
      • Tonic (Adie) pupil
      • Topical exposure to a pharmacologic agent that is blocking iris sphincter action or activating the iris dilator
      • Traumatic, inflammatory, or dysplastic iris pathology
      • Argyll Robertson pupil
    • Tip: if one pupil does not constrict normally to light, and there are no other pertinent findings, the abnormality is ALWAYS extracranial; brain imaging is NOT INDICATED
    • Tip: anisocoria can occur episodically as a benign autonomic dysregulatory phenomenon sometimes associated with migraine, seizure, or trigeminal autonomic cephalalgia; if it persists beyond 24 hours, consider exposure to a topical pharmacologic agent
    • Try this flowchart approach as a way to cover this topic

    Pupil Disorders

    Anisocoria Overview Physiologic Anisocoria Horner Syndrome Tonic (Adie) Pupil Argyll Robertson Pupil Pharmacologic Mydriasis Episodic Mydriasis Iridoplegic Mydriasis Mydriasis of Third Nerve Palsy Tectal Pupils