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What Are The Causes Of Anisocoria?

What is Anisocoria?

The sizes of the pupil in both eyes are always equal. However, there are instances when the size of the pupil of one eye becomes different from the other. This is medically referred to as Anisocoria. Anisocoria can be permanent or temporary and depends on the underlying cause.

In addition to unequal pupil size, an individual with Anisocoria also tends to have vision disturbances in the form of blurred or double vision.(1) In some cases there may also be temporary loss of vision as well. In some cases, people with Anisocoria complain of frequent bouts of headaches as well.(1, 2)

Some people also tend to have low grade fever along with nausea with or without vomiting.(1) Neck stiffness is yet another feature that presents itself along with anisocoria.(1) This article highlights some of the potential causes of Anisocoria.

What are the Causes of Anisocoria?

What are the Causes of Anisocoria?

There have been many cases of Anisocoria where no cause is identified. This is referred to as simple anisocoria. Normally these are very mild and the difference between the pupil sizes is barely visible. This is a benign form of Anisocoria and rarely has any other symptoms.

There have been various studies done to look how prevalent Anisocoria is. In one such study, photographs were taken of people with normal pupil size under low lighting conditions. This was done during in the daytime for one week every day. This showed that around 20% of people showed different pupil size suggestive of anisocoria at one point or the other, albeit it was very mild.

The study concluded that anisocoria was in no way dependent on the age, sex, or color of the eye of an individual. This has been corroborated by other studies as well. However, if there is a significant change in the size of the pupil then it may point towards a more serious cause.

Anisocoria Caused by Injury(3): An injury or trauma to the eye is one of the most common causes of anisocoria. This can happen due to a motor vehicle crash where shreds of glass penetrate the eye causing injury to the pupils. Certain medications given for eye infections like pilocarpine tend to cause anisocoria.

Medical Conditions Causing Anisocoria: There are also certain medical conditions, which can cause anisocoria. Inflammation of the iris is a potential cause for Anisocoria. Adie’s pupil is yet another medical condition that causes Anisocoria.(3) It is a benign medical condition and poses no threat to the vision of the affected patient. The pupil that is affected also becomes nonreactive to light. Complication arising out of cataract surgery is one of the many causes for Adie’s pupil resulting in Anisocoria.(4)

Anisocoria Caused by Neurological Conditions or Damage to Brain/Spinal Cord: There are also quite a few neurological conditions which tend to cause Anisocoria. This is usually seen whenever there is damage or injury to the spinal cord or the brain. Horner Syndrome is the most common nerve damage that causes Anisocoria.(1) An individual with nerve damage also tends to have strabismus along with Anisocoria. Damage to the brain causing anisocoria is normally seen in cases of stroke or hemorrhage due to injury to the brain such as being hit on the head after a crash or after being hit by a hard object. Tumors are also one of the reasons why damage to the nerves or brain occur causing Anisocoria.(1, 4)


In summary, Anisocoria is a benign medical condition in which the size of the pupil in one eye is different from that of the other. Along with unequal pupil size, an individual with Anisocoria can also have blurred or double vision along with bouts of headaches. It is quite normal for any individual to have minimal degree of anisocoria especially in dim lights.

An injury or trauma to the eye is one of the most common causes of anisocoria. Medications used to treat eye infections also have the potential to cause anisocoria. The most serious, however, of all the causes of Anisocoria include injury or damage to the brain or nervous system. This can happen after a traumatic brain injury, tumor or hemorrhage.

Thus if an individual has significant anisocoria then it may point towards a serious medical illness involving the brain or the nervous system. This may need urgent and aggressive treatment to prevent any complications.


Team PainAssist
Team PainAssist
Written, Edited or Reviewed By: Team PainAssist, Pain Assist Inc. This article does not provide medical advice. See disclaimer
Last Modified On:May 24, 2022

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