Pupil Size

Pupil size
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Question: Pupil Size

Why is pupil size important?

Answer: Pupil size can tell a doctor a lot about your health. Pupil size is an important key to unlocking possible medical problems inside your body.

What is the pupil?

The pupil is the round black circle in the center of the iris, 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.

 The pupil is similar to a camera aperture that you control when you want more or less light into your camera. The pupil 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 tends to range between 2.0 and 5.0 mm depending on the lighting. Pupil size is typically larger in younger people.

What can pupil size reveal?

When your doctor examines your pupils, he or she will first look for anisocoria. Anisocoria is a condition in which your pupil sizes are unequal. Twenty percent of the general population has normal anisocoria and does not signal anything abnormal.

In some cases, however, unequal pupil sizes can be a symptom of disease.

Your doctor is also looking at the size and shape of the pupil in both bright light and dim light. The speed and quality of pupillary response to stimuli will also be noted. Your doctor may also test your pupillary reaction to near stimuli such as small print.

Any differences between your pupils is also noted.

The pupil is controlled by a very long nerve pathway in the body. The nerve that controls the pupil starts in the brain, then travels down the spinal cord, up over the top of the lung, under the subclavian artery, up the neck and through extensions of the brain, and finally travels close to the optic nerve and then to the pupil. Any interruption along this pathway could possibly affect this nerve and cause changes in pupillary reaction.

What conditions can affect pupil size?

Pupil size abnormalities can sometimes signal disease. The following diseases can affect pupil size:

  • Glaucoma: A mid-dilated pupil can be a sign of glaucoma.
  • Aneurysm: An aneurysm that pushes on certain blood vessels 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.
  • Recreational drug use: Certain drugs can cause the pupils to dilate or constrict abnormally.
  • Medications: Glaucoma and antihistamines can sometimes cause dilated pupils.
  • Head trauma: Head injury or concussion can cause unequal pupils.
  • Cluster headaches: Cluster headaches can cause a constricted pupil.
  • Stroke: Sometimes a stroke can cause changes in the size of the pupil.
  • Syphilis: Syphilis can cause an Argyll-Robertson pupil. Argyll-Robertson pupils are small, unequal, misshapen pupils that constrict with near focusing but do not react normally to light.

How is pupil size measured?

Eye doctors 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.

How does pupil size affect Lasik eye surgery?

People with very large pupils are generally bad candidates for Lasik and other refractive procedures. Having naturally large pupils or pupils that dilate heavily in dim light may increase the occurrence of glare and halos following Lasik. Measuring pupil size is an important step in deciding if Lasik is right for you.


Slmovits, Thomas L and Ronald Burde. Neuro-ophthalmology. Year Book Europe Ltd, 1994.

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