Anisocoria

Contents

  1. Definition and Clinical Features
  2. Pathophysiology and Causes
  3. Clinical Examination and Testing
  4. References

Definition and Clinical Features

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 the degree of ambient illumination; or it may be pathological, presenting with many possible underlying causes.

Patient presenting with unequal pupil sizes (anisocoria)

Anisocoria is the presence of unequal pupil sizes. It is crucial to determine which pupil is abnormal by testing pupillary responses in both bright and dim light.

Pathophysiology and Causes

Pathological anisocoria can be broadly divided into structural and neurological etiologies:

  • Structural Causes: Ocular infection, trauma, inflammation (e.g., iritis), or prior ocular surgery.
  • Neurological Causes:
    • Sympathetic Defect (Anisocoria greater in dim light or darkness): Darkness normally stimulates the dilatation of a normal pupil. If anisocoria worsens in the dark, it suggests the affected pupil is abnormally constricted (miosis; oculosympathetic paresis). Causes include:
    • Parasympathetic Defect (Anisocoria greater in bright light / less in dim light): Bright light normally stimulates pupillary constriction. If anisocoria worsens in bright light, it suggests a defect in parasympathetic innervation, leaving the affected pupil abnormally dilated (mydriasis; oculoparasympathetic paresis). Causes include:
      • Holmes-Adie pupil (vermiform movements of the pupil margin may be visible with a slit-lamp)
      • Oculomotor (III) nerve palsy (indicating an efferent path lesion from the Edinger-Westphal nucleus)
      • Mydriatic agents (e.g., phenylephrine, tropicamide)
      • Anticholinergic agents (e.g., an asthma inhaler accidentally puffed into one eye)

Clinical Examination and Testing

Evaluating anisocoria requires a careful assessment of pupillary size and reactivity under varying lighting conditions. A combination of distinct clinical characteristics and targeted pharmacological testing (such as the instillation of cocaine, apraclonidine, or pilocarpine eye drops) can significantly help to establish the underlying diagnosis in anisocoria.

 

References

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, Holmes-Adie syndrome; Horner’s syndrome; Miosis; Mydriasis