The most common cause of anisocoria is physiological anisocoria, which is benign and occurs in up to 20% of the population
. In physiological anisocoria, the difference in pupil size is usually less than 1 mm and remains consistent under different lighting conditions
Pathological causes of anisocoria
- Horner's syndrome: Characterized by ptosis, miosis, and anhidrosis, resulting from disruption of the sympathetic pathways
- Third cranial nerve palsy: Presents with ptosis, mydriasis, and ophthalmoplegia, often due to aneurysm, ischemia, or trauma
- Adie's tonic pupil: Typically unilateral, with a dilated pupil that reacts poorly to light but better to accommodation, often due to postganglionic parasympathetic denervation
- Pharmacologic agents: Exposure to mydriatic or miotic agents can result in anisocoria
Anisocoria in neurological emergencies
- In the context of neurological emergencies, new-onset anisocoria occurs in over 60% of patients
- The occurrence of new-onset anisocoria accompanied by objective evidence of abnormal pupil reactivity is significantly associated with maximum midline shift, while new-onset anisocoria accompanied by objective evidence of normal pupil reactivity is inversely associated with death
In conclusion, while physiological anisocoria is the most common cause of anisocoria, several pathological conditions can also lead to this condition. In the context of neurological emergencies, anisocoria can be a significant clinical sign associated with patient outcomes.