When one pupil is bigger than the other, the condition is called anisocoria. This means the pupils are unequal in size. It is quite common and can be a normal variation in up to 20% of healthy people, called physiologic anisocoria, and usually does not cause symptoms or vision problems. However, it can sometimes indicate underlying issues related to the eye or nervous system, including eye injuries, certain medications, infections, nerve damage, or brain disorders. If the size difference arises suddenly or is accompanied by other symptoms like eye pain, vision loss, drooping eyelid, or headache, it requires urgent medical evaluation.
Causes of Unequal Pupils
- Physiologic Anisocoria: Natural difference in pupil size without illness, common and harmless.
- Eye-related causes: Injuries, inflammation, certain eye medications, or glaucoma.
- Nervous system-related causes: Disorders affecting the cranial nerves or autonomic pathways, such as Horner syndrome (which includes drooping eyelid and decreased sweating), stroke, brain injury, or brain tumors.
- Other causes: Exposure to certain chemicals, infections, or tumors pressing on nerves.
When to Seek Medical Attention
- Sudden onset of unequal pupils
- Accompanying symptoms like severe eye pain, vision changes, double vision, headache, or drooping eyelid
- History of head injury or neurological symptoms
In many cases, especially if the difference has been present a long time without other symptoms, anisocoria is harmless. But any new or worsening signs should prompt a medical exam for proper diagnosis and treatment.
