Why is one pupil larger than the other?

The medical condition where one pupil is larger than the other has several causes. Known as anisocoria, it can be attributed to head injuries like concussions, skull fractures and brain tumors. Internal bleeding of the brain and pooling of blood between the brain and skull are also causes.

Conditions like optic nerve inflammation, shaken baby syndrome, seizures, brain aneurysms and brain tumors can also cause anisocoria. Meningitis, glaucoma and low blood sugar can also dilate the pupils differently. The majority of these conditions are considered serious enough to warrant immediate medical attention, making anisocoria a valid symptomatic marker of significant underlying causes.