From the Guidelines
When a patient has one dilated pupil and one normal pupil, this condition is called anisocoria and may indicate a serious neurological problem requiring immediate medical attention. This asymmetry can be caused by several conditions, with the most concerning being pressure on the third cranial nerve from a brain aneurysm, stroke, or increased intracranial pressure 1. Other causes include Horner's syndrome (constricted pupil with drooping eyelid), certain medications (like eye drops containing atropine or scopolamine), direct eye trauma, inflammation, or infection. Some people have physiologic anisocoria, a benign condition where pupils are naturally different sizes.
Key Considerations
- If anisocoria appears suddenly, especially with symptoms like headache, vision changes, eye pain, drooping eyelid, or neurological symptoms, it should be treated as a medical emergency 1.
- Immediate evaluation is necessary to determine if the dilated pupil or the constricted pupil is abnormal, as this distinction helps identify the underlying cause.
- A thorough neurological examination, imaging studies, and possibly a lumbar puncture may be needed for diagnosis.
Diagnostic Approach
- A comprehensive eye examination with particular attention to sensorimotor exam, evidence of ptosis and anisocoria, and pupillary responses in bright and dim illumination should be performed 1.
- Imaging studies such as MRI or CTA may be indicated to rule out compressive lesions, such as aneurysms or tumors 1.
- Vascular imaging either with CTA or MRA is indicated if vascular compression from an adjacent aneurysm is suspected 1.
From the Research
Causes of Unilateral Dilated Pupil
- A unilateral dilated pupil can be caused by various medical conditions, including third nerve palsy, intracranial hemorrhage, and head injury 2, 3, 4.
- The dilated pupil can be a false-localizing sign, meaning that it may not be directly related to the underlying cause of the condition 3.
- In some cases, a unilateral dilated pupil can be a sign of a life-threatening condition, such as a ruptured aneurysm or a subarachnoid hemorrhage 3, 5.
Diagnostic Approach
- The diagnostic approach to a unilateral dilated pupil requires a stepwise evaluation, including a complete history and physical examination, pharmacologic testing, and neuroimaging 2, 6.
- The clinical approach should aim to differentiate between direct third nerve injury and expanding mass lesion 4.
- Advanced imaging modalities, such as CT, CTA, DSA, FLAIR, 3D-TOF-MRA, and CE-MRA, can be used to evaluate the underlying cause of the condition 5.