Pupil Size and Your Health

The size of your pupil can tell your doctor quite a bit about your health. It is an important key to unlocking possible medical conditions you might not otherwise know about.

There are many parts of the eye, and the pupil is among the most important. It controls the amount of light that enters your eye and it's continually changing size. Your pupil naturally enlarges and contracts based on the intensity of the light around you.


What Is the Pupil?

The pupil is the round, black circle in the center of the iris, which is the colored part of your eye. The pupil is actually a hole through which light passes to the retina, the light-sensitive layer in the back part of the eye.

Your pupils are similar to a camera aperture. You control them subconsciously when you want to let more or less light into your camera (essentially, your brain). It can expand to be become larger (dilate) or contract to become smaller (constrict). 

Your iris contains muscles that respond to outside stimuli to control the amount of light that reaches your retina. In bright light, the pupil constricts to reduce the amount of light entering the eye. In dark or dim light, the pupil dilates to allow more light into the eye to improve vision. 

Normal pupil size ranges between 2.0 and 8.0 millimeters, depending on the lighting. The younger you are, the larger your pupils tend to be.

Testing Pupil Size

When your doctor examines your pupil, they will first look for anisocoria—a condition in which the pupils are unequal in size. Twenty percent of the general population has normal anisocoria and it does not signal anything abnormal. In some cases, however, unequal pupil sizes can be a symptom of a disease.

Your doctor will also look at the size and shape of the pupil in both bright and dim light. The speed and quality of pupillary response to stimuli will be noted as well. Your doctor may also test your pupillary reaction to near stimuli such as small print. Any differences between your two pupils are also noted.

The pupil is controlled by a very long nerve pathway in the body. The nerve starts in the brain, travels down the spinal cord, over the top of the lung, under the subclavian artery, up the neck, and through extensions of the brain.

It then travels close to the optic nerve, other nerves that control the eye muscles, and blood vessels that feed the eye, and finally to the pupil. It's a very strange and rather out-of-the-way path to take.

Any interruption along it could affect the nerve and cause changes in pupillary reaction. That's why your pupils can indicate health problems completely unrelated to your eyes.

Associated Conditions

Pupil size abnormalities can sometimes signal disease. This is not an exhaustive list as researchers continue to discover other conditions that can be indicated by irregular pupil function. Conditions include:

  • Glaucoma: Pupil size may be affected in glaucoma.
  • Aneurysm: An aneurysm that pushes on certain nerves in the brain can cause a dilated pupil as well as other symptoms.
  • Lung cancer: Lung cancer that affects the top part of the lung can impact the pupillary nerve fibers.
  • Brain tumor: If a tumor or mass is close to the origin of the pupillary nerve fibers, it can cause problems within the pupil.
  • Multiple sclerosis: Multiple sclerosis can cause optic nerve damage, which leads to an abnormal response of the pupils known as afferent pupillary defect (APD).
  • Head trauma: Head injury or concussion can cause unequal pupils.
  • Cluster headaches: Cluster headaches can cause a constricted pupil.
  • Stroke: A stroke can sometimes cause changes in the size of the pupil.
  • Syphilis: Syphilis can cause an Argyll-Robertson pupil. These are small, unequal, misshapen pupils that constrict with near focusing but do not react normally to light.

In addition, certain recreational drugs and even alcohol can cause the pupils to dilate or constrict abnormally. It is the reason why law enforcement officers will check someone's pupils when they suspect intoxication.

Some prescription and over-the-counter medications, including antihistamines, can sometimes dilate your pupils as well.

How Pupil Size Affects LASIK Surgery

It is possible that the size of your pupils can prevent you from having LASIK eye surgery to correct your vision. People with very large pupils are generally not good candidates for LASIK and other refractive procedures.

Eye doctors may use an infrared pupillometer to measure the size of the pupils. The device consists of a large camera tuned for infrared detection and two infrared side lamps for pupil illumination.

Having naturally large pupils or pupils that dilate heavily in dim light may increase the occurrence of glare and halos following LASIK. This would counteract the clear vision you are hoping for when electing for the surgery. For this reason, measuring pupil size is an important step in deciding if LASIK is right for you.

A Word From Verywell

Though you may not think about them much, your pupils are a very active part of your body. Not only do they help you see better in various situations, they can indicate concerns for your health.

For this reason, it's important to get regular eye exams. If you notice that your pupils are an unusual size or reacting differently than normal, be sure to see your doctor and have it checked out.

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Article Sources
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Additional Reading
  • Doran M, Karmel M, Stuart A. 4 neuro conditions not to be missed. American Academy of Ophthalmology. 2012.

  • Liu GT, Volpe N, Galetta SL. Neuro-Opthalmology: Diagnosis and Management. 2nd ed. Amsterdam, Netherlands: Elsevier Health Sciences; 2010.