Pupil Size and Your Health

The size of your pupil can tell your healthcare provider 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 its continually changing size. Your pupil naturally enlarges and contracts based on the intensity of the light around you and whether you are looking at near or far objects.

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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. They widen or narrow to let more or less light in. Pupils 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 so you can see better.

Your pupils also constrict slightly to look at close objects and dilate slightly to look far away.  

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 in normal light.

Testing Pupil Size

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

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

Pupil size is controlled by the optic nerve and oculomotor nerves. These nerves receive some of their messages from the autonomic nervous system, which involves the brain, spinal cord, lungs, heart, and subclavian artery.

A disruption of the autonomic nervous system might cause changes in pupillary reaction. That's why the size of 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 there are also other conditions that can cause irregular pupil function.

Conditions include:

  • Brain aneurysm: An aneurysm that pushes on nerve pathways 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, recreational drugs and alcohol can cause the pupils to dilate or constrict abnormally. This is the reason why pupils are often checked when there is a concern about intoxication or overdose.

Some prescription and over-the-counter medications, including antihistamines and some medications used to treat glaucoma, 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, but they can also be a sign of 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 healthcare provider 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.