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Anisocoria

Key Points ⚡

  • Anisocoria: unequal pupil sizes; can be physiological or pathological.
  • Physiological anisocoria affects up to 20% of people, usually <1mm difference, consistent in light and dark.
  • Pathological anisocoria varies with lighting conditions and requires careful assessment with light/dark tests and accommodation.
  • Anisocoria greater in dark: abnormal pupil fails to dilate (miotic pupil), often due to sympathetic disruption (e.g., Horner's syndrome). Other causes: Argyll-Robertson pupil, trauma, pharmacological agents (pilocarpine, organophosphates).
  • Horner’s syndrome triad: miosis, anhidrosis, ptosis; causes include central lesions (stroke, MS), preganglionic (Pancoast tumour, thyroid mass), postganglionic (carotid artery dissection). Diagnosis confirmed with hydroxyamphetamine test.
  • Anisocoria greater in light: abnormal pupil fails to constrict (mydriatic pupil), often due to parasympathetic disruption. Causes: Adie’s pupil, oculomotor nerve palsy, pharmacological agents (atropine, tropicamide).
  • Adie’s pupil: benign tonic pupil with poor light response but preserved accommodation; constricts with diluted pilocarpine.
  • Oculomotor nerve palsy: ‘down and out’ eye, ptosis, possible mydriasis; pupil-sparing in microvascular causes (diabetes), non-sparing in compressive lesions (PCOM aneurysm).
  • Diagnosis aided by pilocarpine testing: diluted pilocarpine constricts Adie’s pupil; undiluted pilocarpine constricts oculomotor palsy pupil; no constriction with pharmacologic dilation.
  • Other causes include acute angle-closure glaucoma (fixed mid-dilated pupil) and unilateral dilated pupil from subdural hematoma.

Introduction

Anisocoria is the term for unequal pupil sizes, which may be benign (physiological) or indicate serious pathology requiring urgent diagnosis and management.

Physiological Anisocoria

  • Prevalence up to 20%, cause unknown.
  • Difference <1mm, consistent in light and dark conditions.

Pathological Anisocoria

  • Requires assessment of pupil size in both light and dark.
  • Test accommodation and pupillary light reflex.

Anisocoria Greater in Dark (Miotic Pupil)

  • Due to failure to dilate, indicating sympathetic pathway disruption.
  • Causes:
  • Horner’s syndrome (miosis, ptosis, anhidrosis).
  • Argyll-Robertson pupil (neurosyphilis).
  • Mechanical trauma or surgery.
  • Pharmacological (pilocarpine, organophosphates).

Horner’s Syndrome

  • Triad: miosis, anhidrosis, ptosis (Muller’s muscle).
  • Causes:
  • Central: stroke, multiple sclerosis, tumors.
  • Preganglionic: Pancoast tumor, thyroid mass, cervical rib.
  • Postganglionic: carotid artery dissection, cavernous sinus thrombosis, cluster headache.
  • Urgent MRI neck if acute Horner’s with neck pain/dysphagia to exclude carotid dissection.
  • Confirm diagnosis with 1% hydroxyamphetamine: dilates pupil if preganglionic, no dilation if postganglionic lesion.

Anisocoria Greater in Light (Mydriatic Pupil)

  • Due to failure to constrict, indicating parasympathetic pathway disruption.
  • Causes:
  • Adie’s pupil: benign tonic pupil, poor light response, better accommodation; constricts with diluted pilocarpine.
  • Oculomotor nerve palsy: ‘down and out’ eye, ptosis, mydriasis; pupil-sparing in diabetic/ischaemic; non-sparing in compressive lesions like PCOM aneurysm.
  • Pharmacological agents: atropine, tropicamide, MDMA.

Diagnosis

  • Pilocarpine test:
  • Diluted pilocarpine → constriction in Adie’s pupil.
  • Undiluted pilocarpine → constriction in oculomotor palsy.
  • No constriction → pharmacological dilation.

Other Causes

  • Fixed mid-dilated pupil: acute angle-closure glaucoma.
  • Unilateral dilated pupil: ipsilateral subdural hematoma.

References

  • EyeWiki, 2021
  • Lam BL et al., Am J Ophthalmol, 1987
  • Bye L et al., Basic Sciences for Ophthalmology, 2013
  • Gross JR et al., Curr Opin Ophthalmol, 2016
  • Payne W et al., 2021
  • Acute Angle-Closure Glaucoma
  • Age-related Macular Degeneration (ARMD)
  • Amblyopia
  • Ametropia
  • Anterior Uveitis (Iritis)