Anisocoria: what it is, types and risk factors. What is the cause?


What is anisocoria?

Anisocoria is a condition in which one of the pupils (the black in the center of the eye) is larger than the other, even when there is the same stimulus in both. In ICD-10, its number is Q13.2.

The pupils are like the diaphragm of the camera, that part that opens and closes to let the light in. It is through them that light enters, allowing the brain to interpret and form images. When there is a lot of light in the environment, the pupils contract to avoid too much light. If it’s dark, they dilate, allowing more light to enter your eyes.

This is one of the ways in which the eyes get used to changes in light. That is why our eyes burn when we leave a dark room for sunlight and when we enter a closed room after leaving the sun, everything seems very dark and it takes a few minutes for us to see right again.

It can affect animals other than humans, such as dogs, cats and anyone with pupils.

When there is anisocoria, which can be caused by different conditions or be natural, one of the pupils is more dilated than the other. Singer David Bowie (1947 – 2016) had different sized pupils and was a famous case of anisocoria.


Anisocoria has two types, but they are not necessarily different. The only difference is the cause.

Physiological anisocoria

In the case of physiological anisocoria,  the pupils are naturally of different sizes. They contract in a normal way in response to light and have been that way since the person was born.

Eye exams show no abnormalities in vision and there is no cause for concern. Usually the difference is at most 1 mm from one pupil to another.

Pathological anisocoria

With pathological anisocoria ,  more care should be taken. Various conditions can cause this type of anisocoria and it is usually a sign that something is wrong in the person’s body.

Anisocoria, until there is evidence of its cause, should be treated as an emergency as it can mean serious problems in the brain.


The most common cause of anisocoria is the physiological cause. Being born with small variations in the size of the pupils represents 20% of the cases of the condition. However, several diseases and situations can cause anisocoria. The most common are:

Eye injury

Eye damage can cause anisocoria. Singer David Bowie was known to have a larger pupil than the other due to an injury. As a teenager, he got into a fight and got punched in the eye by someone wearing a ring. Even after the surgical procedure, it was not possible to recover the eye completely and he was left with anisocoria.

Horner syndrome

Horner’s syndrome is rare and is caused by injuries to the facial and ocular nerves that make up the sympathetic nervous system (SNS) . The SNS, in short, controls the body’s reactions to certain stimuli such as stress (causing the heart rate to increase) or the dilation of the pupils (when there is light or lack of it).

Usually only one side of the face is affected and in addition to the anisocoria the eyelid of the eye is drooped and the affected side of the face loses the ability to sweat. It can be caused by a stroke or tumor, in addition to direct injuries.

Congenital iris defect

A congenital defect is a lifelong defect such as a cleft lip or a broken one, as is Down’s syndrome.

Chemicals and medicines

Certain chemicals, when in contact with the eyes, can cause anisocoria. The use of certain drugs can also. Among the substances that cause anisocoria are:

  • Cocaine;
  • Scopolamine adhesive;
  • Pilocarpina;
  • Flea collars or sprays;
  • Organophosphates;
  • Ipratropium spray (if in contact with the eyes);
  • Mydriatics (pupil dilators);
  • Clonidine;
  • Apraclonidina.


This condition causes the pupil’s muscles to become paralyzed, making it dilate regardless of the light.

Pupil of Argyll Robertson

Argyll Robertson’s pupil is a condition that is related to syphilis neurological complications . The condition causes the pupil to lose its photomotor reflex, which causes it to dilate in the dark and contract in the light. The pupils still contract when the eyes focus on very close objects.

Adie’s tonic pupil

This condition has symptoms similar to Argyll Robertson’s pupil. The eye loses the photomotor reflex, maintaining the accommodation reflex (adapts to the approach of objects). It is usually caused by damage to the post-ganglionic fibers, part of the sympathetic nervous system. In most cases the culprit is a bacterium or virus. The condition affects more women than men and in 80% of cases it only appears in one eye.


Tumors in different parts of the brain can cause anisocoria if they affect or are close to the parts of the brain that control the sympathetic nervous system.


Like tumors, strokes can cause the most varied neurological problems, including anisocoria.

Third cranial nerve palsy

Aneurysms or tumors can cause this type of paralysis.

Risk factors

There are few risk factors for anisocoria. The diseases that cause it are, in general, genetic. However, some acquired conditions can still cause anisocoria. Are they:


Trauma to the head or eyes can damage the nerve that controls pupil contraction and dilation, so it is possible that they cause anisocoria. It was through trauma that David Bowie acquired the condition.


Tumors that affect the sympathetic nervous system, specifically the third cranial nerve, can cause anisocoria.


Like tumors, aneurysms can cause paralysis of the third cranial nerve, resulting in anisocoria.


Strokes can affect all brain functions, including pupillary reactions to the environment.


Anisocoria is a symptom in itself. However, when accompanied by other symptoms, it can indicate serious illnesses.

Pain, redness, blurred vision or sensitivity to light

When uneven pupils accompany these symptoms, the cause of anisocoria may be eye problems.

Drooping eyelid, diplopia (double vision) or difficulty with balance

If these symptoms accompany the anisocoria, there may be a medical emergency. In these cases, there is often Horner Syndrome or paralysis of the third cranial nerve.

The latter can be caused by a brain tumor or a brain aneurysm , both of which are extremely dangerous.

Recent blurred vision

In these cases, especially if the blurred vision is more pronounced when trying to focus on nearby objects, the anisocoria may be the result of the use of medication or drugs.

No extra symptoms

When there is no extra symptom, the anisocoria is likely to be physiological or congenital, that is, natural for the person.

How is the diagnosis made?

The diagnosis of anisocoria is simple and can be performed by the ophthalmologist and even by the patient himself, who with a mirror, can see that the size of the pupils are different.

In less pronounced cases, with small differences in size, the patient may not notice the condition.

The doctor, however, should keep an eye out for extra symptoms so that he can diagnose the cause. Some can be extremely serious.

The exams are performed through eye observation, which can be done in the dark and in the light so that the ophthalmologist can see differences more clearly.

The slit lamp, a device that allows magnified viewing of the eye, can be used.

Anisocoria in cure?

Anisocoria, in itself, is a symptom and has no cure. However, the cause can be cured, which leads to the recovery of the original size of the pupil.

What is the treatment of anisocoria?

Anisocoria has no direct treatment. What it is about is your cause. Surgeries can be used to remove clots or tumors that are possibly interfering with the ocular nerve or to solve problems caused by trauma, for example.

Despite this, treatment is not necessary. In most cases, anisocoria is natural and does not interfere with the patient’s life.

Living together

Living with anisocoria can be easy. In physiological and less severe cases, the patient can live his whole life without realizing that he has the condition. When it is severe, problems with vision and sensitivity to light can affect the patient. Visiting the ophthalmologist can help with these problems.


In 20% of cases, anisocoria is physiological and difficult to perceive. There are no consequences for the eyes and nothing even needs to be done to correct the pupils. However, when the cause is different, treatments may be necessary.

Surgeries can resolve the condition that causes the anisocoria, as can medications in cases where it is applied. The causes for the appearance of the symptom are varied, so only the doctor can indicate which is the appropriate path to follow.


Anisocoria, when severe, can cause problems with sensitivity to light and difficulty in focusing on distant or close objects, depending on whether the affected eye keeps the pupil dilated or contracted.

How to prevent?

It is not possible to prevent anisocoria directly. Its causes are often unpleasant surprises in life or genetic conditions. However, it is possible to reduce the possibilities of some of them.

Do not smoke

Cigarettes are often associated with lung and throat cancer , but make no mistake. It can cause harm to the entire body, and can create brain cancer . In addition, tobacco increases the chances of aneurysms and strokes, all of which are capable of causing anisocoria and other, much more serious symptoms.

Beware of trauma

Probably one of the most difficult to avoid is trauma to the eyes or head, but you can also prevent the incidence of these events. Wearing a helmet where it applies, such as when riding a bicycle, skateboard or in construction, avoids accidents and protects the head.

Use your seat belt to avoid impact in the event of a car accident.

Anisocoria is a symptom that can arise due to serious conditions, trauma or simply being a genetic trait. Share this text with your friends to let them know a little more about uneven pupils!