Isochoric Pupils: Definition and Conditions Affecting Pupil Size

The iris contains muscles that respond to external stimuli to control the amount of light reaching the retina.

Isochoric means that the pupils are the same size, technically 1mm in diameter.

Most people have a very slight size variation, and it is normal.

The opposite is the isochoric pupils, which means that they are more than 1 mm of different sizes, that is, diameter.

The pupil is the circle that is usually perfectly round and black in the center of the iris, which is the colored part of the eye.

The function of the pupils is to regulate the light that enters the retina, allowing it to focus and start the visual process.

The pupil is a hole through which light passes into the retina, the light-sensitive layer at the back of the eye.


The pupils are similar to the aperture of a camera. These are controlled unconsciously when you want to let more or less light enter the retina.

This control is essentially done in the brain.

It can expand to get bigger (dilate) or contract to get smaller (constriction).

In very bright light, the pupil automatically contracts to control the amount of light that enters it.

The pupil dilates in the very dark or dim light to allow more light to enter the eye to improve vision.

The average size of the pupil tends to range between 2.0 and 5.0 millimeters, depending on the lighting.

The younger you are, the larger your pupils tend to be.

Pupil function

The muscles within the iris control the size of the pupil: one muscle contracts the opening of the pupil (makes it smaller), and another strength of the iris dilates the pupil (makes it more significant).

This process controls the amount of light that enters the eye through the pupil.

When the pupil is dilated in dim light conditions, it allows more light to enter the retina and can improve vision at night.

When the pupil contracts, it limits the amount of light that enters the eye, preventing very bright light from causing glare, causing discomfort, and can damage the retina.

Pupil size

Pupil size varies from person to person and changes with age, children and young adults tend to have large pupils, and older people generally have smaller pupils.

Generally, in adults, the average diameter of the pupils varies from 2 to 4 mm in the presence of solid light and 4 to 8 mm in dim light.

When the eyes focus on a very close object, the pupils narrow as an involuntary reaction called the accommodative pupillary response.

The size of the pupil can also express some health conditions.

This is an essential key to recognizing possible medical conditions that would not otherwise be known.

The eye has many parts, and the pupil is among the most important.

Controlling the amount of light that enters the eye and continually changing its size naturally depending on the intensity of the light surrounding it is ultimately the essential function of the pupils.

Pupil test

In a pupil exam, the ophthalmologist will examine the pupils and perform a test to observe the activity of the pupils.

This test is usually done in a dimly lit room with an object located at a distance, where the doctor will briefly point a beam of light with a small flashlight to observe the pupils’ response in both eyes.

This test is called the Marcus Gunn test or the “oscillating flashlight test.”

Typically, students react directly and indirectly to light stimulation.

When one pupil receives light, this reaction is known as a direct response, and the reaction of the other pupil is known as a consensual response.

The doctor adjusts the lights in the room with a little more light, and they make the patient concentrate on a portable object while bringing that object to the nose.

This test is called the accommodative response of the pupils.

Conditions affecting the pupils

When the doctor examines the pupil, he will first look for an anisocoria. This is a condition in which the pupils are uneven in size.

Twenty percent of the population generally has normal anisocoria and does not indicate anything abnormal.

In some cases, however, uneven pupil size can be a disease symptom.

The doctor will also look at the size and shape of the pupil in both dim and bright light.

The speed and quality of the pupillary response to stimuli will also be noted.

The doctor can also assess the pupillary reaction to nearby stimuli, such as fine print and any differences between the two pupils.

The pupil is controlled by a very long nerve pathway in the body.

The nerve begins in the brain, travels through the spinal cord, over the upper part of the lung, under the subclavian artery, in the neck, and through extensions of the brain.

Finally, it travels close to the optic nerve and then towards the pupil. Any interruption along it could affect the nerve and cause changes in pupillary reaction.

That is why the pupils can indicate health problems unrelated to your eyes.

Conditions that can affect pupil size

Pupil size abnormalities can sometimes be a sign of disease.

These conditions include:

  • Glaucoma – A half-dilated pupil can be a sign of glaucoma.
  • Aneurysm: An aneurysm that pushes on certain blood vessels in the brain can cause a dilation of the pupil and other symptoms.
  • Lung cancer: Lung cancer that affects the upper part of the lung can affect 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: One of the possible indications of multiple sclerosis is an abnormal response of the pupils, known as an afferent pupillary defect.
  • Head trauma: A head injury or concussion can cause uneven 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, uneven, misshapen pupils that contract with close focus but do not usually react to light.

Also, certain recreational drugs and even alcohol can cause the pupils to dilate or constrict abnormally.

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