When one pupil (of the eye) is a different size from the other one, there may be a serious underlying cause.

Occasionally, differing pupil size is inherited and there is no underlying disorder. If other family members also have this condition, then pupil size is probably genetic and is nothing to worry about.

At times, for unknown reasons, pupils may differ in size. If there are no other symptoms and if the pupils return to normal, then the temporary condition is nothing to worry about.

Head injuries that cause differing pupil sizes are usually curable with early recognition of danger signs and medical treatment. Complications, however, can be life-threatening or cause permanent disability.

• Bleeding inside the skull caused by head injury
• Brain tumor or abscess
• Infection of membranes around the brain caused by meningitis or encephalitis
• Expanding brain lesion such as an aneurysm
• Excess pressure in one eye caused by glaucoma
• Birth injury, tumor in the chest, or lymph-node pressure that may be accompanied by decreased sweating, a small pupil, or drooping eyelid all on the affected side
• Eyedrops

Treatment depends on the cause of the enlargement of one pupil. For example:

• For pupil size inequality caused by a head injury, get immediate medical help.
• For pupil size inequality caused by a tumor, follow prescribed therapy. Stay active as strength allows. Work and exercise moderately. Rest when tired.
• For pupil size inequality caused by meningitis or encephalitis, follow prescribed therapy and rest in bed in a darkened room.
• For pupil size inequality caused by acute glaucoma, avoid emotional upset, which raises pressure in the eye. Don't smoke. Tobacco constricts blood vessels, reducing the blood supply to the eye. Follow prescribed therapy that may include eye drops to lower pressure inside the eye, diuretics to decrease fluid pressure in the eye, or pain relievers. After treatment, resume normal activities, but avoid fatigue. A low-salt diet is recommended.

• Differing pupil size follows an eye or head injury -- get medical help immediately!
• Differing pupil size is accompanied by headache, nausea or vomiting, blurred vision, or double vision.
• Differing pupil size is accompanied by fever, stiff neck, eyes sensitive to light, or headache that worsens when bending forward -- get medical help immediately!
• There is severe eye pain and loss of vision -- get medical help immediately!
• Differing pupil size is persistent or unexplained.

The medical history will be obtained and a physical examination performed.

Medical history questions documenting differing pupil size in detail may include:

• Is this a new finding?
• When did it start?
• What other symptoms are also present?
  • Is there a headache?
  • Is there nausea?
  • Is there vomiting?
  • Is there blurred vision?
  • Is there double vision?
  • Is there a fever?
  • Is there a stiff neck?
  • Are the eyes light-sensitive (photophobia)?
  • Is there eye pain?
  • Is there loss of vision?

The physical examination will include a neurological examination.

Diagnostic tests that may be performed include:

• Blood studies such as CBC and blood differential
• Cerebrospinal fluid studies (lumbar puncture)
• CT scan of the head
• EEG
• Head MRI scan
• Tonometry (if glaucoma is suspected)
• X-rays of the skull
• X-rays of the neck

Depending on the diagnosis, medications may be prescribed including cortisone (to diminish swelling of the brain tissue), anticonvulsant drugs (to control seizures), pain relievers, antibiotics (for bacterial meningitis), or anticancer drugs.