How Doctors Test Pupil Reflexes

Your pupils control the amount of light that enters your eyes. Testing the pupils is an important part of a comprehensive eye exam. Because you do not have voluntary control of your pupils, pupil testing may uncover possible problems with your autonomic nervous system as well as other problems in the rest of your body.

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.


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.

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.

Eye doctors use three procedures to test pupil reflexes.


Light Response Pupil Test

close up of the eye
Peter A. Kemmer/Getty Images

The light response pupil test assesses the reflex that controls the size of the pupil in response to light. Your doctor will first dim the lights, then ask you to look at an object in the distance. A light will be shone into your eyes from each side. Your doctor will watch your pupils closely to determine whether or not your pupils constrict in response to the light, making note of the size and shape of your pupils.


Swinging Flashlight Pupil Test

The swinging flashlight pupil test is used to compare your pupils' response to light. The lights in the room will be dimmed, and you will again be asked to look at a distant object. Your doctor will "swing" the light rhythmically from one eye to the other, noting the response of each pupil. Your pupils should constrict or stay the same size when the light is shone on them. Dilating pupils may alert your doctor to a possible optic nerve problem.


Near Response Pupil Test

The near response pupil test measures the pupil's response to a near target. This test will be performed in a room with normal lighting. Your doctor will ask you to look at a distant object, then move a small object or card in front of your eyes. As you fixate your eyes on the near object, your doctor will watch your pupils closely to make sure they constrict quickly as your fixation changes from far to near.

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  • Levin, Leonard A. and Anthony C. Arnold. Neuro-ophthalmology: The Practical Guide. Medical Publishers, Inc, 2005.