Anisocoria is an inequality of pupil size. This may be physiological (said to occur in up to 15% of the population), in which case the inequality is usually mild and does not vary with degree of ambient illumination; or pathological, with many possible causes.

  • Structural:
    • Ocular infection, trauma, inflammation, surgery
  • Neurological:
    • Anisocoria greater in dim light or darkness suggests sympathetic innervation defect (darkness stimulates dilatation of normal pupil). Affected pupil constricted (miosis; oculosympathetic paresis):
      • Horner’s syndrome
        Argyll Robertson pupil
        Cluster headache
    • Anisocoria greater in bright light/less in dim light suggests defect in parasympathetic innervation to the pupil. Affected pupil dilated (mydriasis; oculoparasympathetic paresis):
      • Holmes-Adie pupil (vermiform movements of the pupil margin may be visible with a slit-lamp)
        Oculomotor (III) nerve palsy (efferent path from Edinger-Westphal nucleus)
        Mydriatic agents (phenylephrine, tropicamide)
        Anticholinergic agents (e.g., asthma inhaler accidentally puffed into one eye)

Clinical characteristics and pharmacological testing may help to establish the underlying diagnosis in anisocoria.



Kawasaki A. Approach to the patient with abnormal pupils. In: Biller J (ed.). Practical neurology (2nd edition). Philadelphia: Lippincott Williams & Wilkins, 2002: 135-146


Cross References

Argyll Robertson pupil; Holmes-Adie pupil, Holmesadie syndrome; Horner’s syndrome; Miosis; Mydriasis