Unequal pupil size/anisocoria 

Although an uncommon presentation to the eye casualty, causes of unequal pupil size ranges from benign to life-threatening conditions. The history and physical examination are important to determine which patients needs urgent referrals or routine clinical appointment. The main task of the attending doctor is to exclude third nerve palsy which requires immediate referrals as the patient may harbour a cerebral aneurysm.


  • Patients may present with unequal pupils as an incidental finding which realtived or friends comment on. 
  • More seriously are patients who present with sudden onset unequal pupils associated with headache +/- double vision
  • Determine which is the abnormal pupil, as a rule of thumb, if the abnormal pupil is small immediate referral to the casualty is not needed.  If the abnormal pupil is the small one, the anisocoria tends to increase in the dark; on the other hand, if the abnormal pupil is the large one, the anisocoria increases in bright light. A large pupil that does not respond or responds sluggishly to light is the abnormal pupil.
  • Look for associated signs such as ptosis (third nerve palsy) and abnormal eye movement (third nerve palsy).
  • A dilated pupil with ptosis and/or poor eye movement should be assumed to be caused by third nerve palsy until proven otherwise.
  • Also refer unequal pupils associated with head injury to the eye casualty within 24 hours
  • Unequal pupils without other associated signs can be referred to the clinic
Figure 1. 
This 70 year-old man presented with a sudden onset left dilated pupil with ptosis and 
ocular palsy. He also suffered from headache. He was seen at the eye casualty and was
referred to the neurosurgeon. MRI scan with contrast revealed a posterior communicating 
artery aneurysm which was clipped surgically.
Figure 2
This man had unequal pupils with the left pupil being smaller. Note the partial ptosis. 
The patient had a left Horner's syndrome. He was referred to the clinic for further 
evaluation. In Horner's syndrome, it is important to exclude pathological process such as 
tumour in the pulmonary apex. Once this is excluded, most cases are benign.

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