Anisocoria (Unequal Pupil Sizes): Causes & Treatments

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What is Anisocoria (Unequal Pupil Sizes)?

If you have anisocoria, it means your pupils are not the same size

The pupil is the dark circle that appears in the middle of the eye. Its role is to let light come into the eye so you can see. 

Any person can have pupils of different sizes. However, having uneven pupils can sometimes indicate a serious eye issue or underlying health condition.

For example, people who are at risk of developing anisocoria include those with:

  • A nervous system problem
  • Eye trauma
  • Possible stroke 
  • Viral infection 

If you have anisocoria and experience accompanying symptoms like headaches or eye pain, you should seek immediate care. 

You may require emergency intervention to address the cause of the eye condition. 

According to the American Association for Pediatric Ophthalmology and Strabismus, approximately 30% of the average population will have anisocoria. The degree of anisocoria can differ each day and even switch eyes.2 

Types of Anisocoria

Characterization of anisocoria includes unequal pupil sizes. While the condition is common, the causes may or may not be benign

In some cases, anisocoria can develop due to a possibly life-threatening condition. 

The following list shows different types of anisocoria and their causes. 

Simple Anisocoria

Simple anisocoria (otherwise known as physiologic or essential) is the most frequent cause of uneven pupil sizes. 

This particular type can affect up to 20% of the population. It is benign. The difference in pupil size will be less than or equal to 1 mm, and the condition may be intermittent, persistent, or self-resolving. 

Health professionals do not know exactly why people develop simple anisocoria. 

Some believe that the condition occurs due to a type of inhibition of the Edinger-Westphal nucleus. This nucleus is responsible for the pupillary sphincter. This is a muscle located in the colored part of the eye called the iris.

Mechanical Anisocoria

Mechanical anisocoria will happen because of damage to either the iris or its supporting structures.  

Eye trauma, surgery, or inflammatory conditions like uveitis (inflammation of the middle tissue layer of the eye) are just some examples that could lead to mechanical anisocoria. 

Angle-closure glaucoma or intraocular tumors may also result in mechanical anisocoria. 

Pharmacologic Anisocoria

Pharmacologic anisocoria can appear as either mydriasis (dilation of the pupil) or miosis (constriction of the pupil). 

The following list details agents that can affect the pupillary dilator or sphincter muscles and cause this anisocoria:

  • Anticholinergics, like atropine, homatropine, tropicamide, scopolamine, and cyclopentolate
  • Glycopyrrolate antiperspirants
  • Nasal vasoconstrictors
  • Herbal plants, like Jimson weed, blue nightshade, and Angel’s trumpet
  • Pilocarpine 
  • Sympathomimetics (type of stimulant compounds), such as adrenaline, clonidine, and phenylephrine
  • Prostaglandins
  • Opioids 
  • Organophosphate insecticides 

Pathologic Anisocoria

Pathologic anisocoria occurs due to an underlying disease or condition. If you experience symptoms alongside anisocoria, you should seek medical care. An eye doctor will be able to rule out any life-threatening conditions and perform a diagnosis. 

Examples of conditions that can result in pathologic anisocoria include:

  • Iritis. This type of uveitis (inflammatory eye disease) results from an eye infection, eye injury, or separate inflammatory eye disease. 
  • Horner’s syndrome. The disruption of a nerve pathway that runs from the brain to the face and eye (on one side of the body). It triggers a series of signs and symptoms, including a drooping eyelid.
  • Adie’s tonic pupil. Also referred to as Adie's Syndrome or Holmes-Adie Syndrome, this rare neurological disorder often involves non-progressive or limited damage to the nervous system. The disorder can also impact deep tendon reflexes. 
  • Third nerve palsy. The third cranial nerve is responsible for moving four of the six eye muscles and pupil constriction, eye focusing, and upper eyelid positioning. Damage to the nerve can be due to various causes. It has been associated with brain tumors, diabetes, high blood pressure, and aneurysm. 

Accompanying Symptoms of Anisocoria

Many people do not realize that their pupils vary in size. It may become apparent when they compare old and newer photos of themselves. 

If you develop anisocoria, you may also experience symptoms. Symptoms may be the sign of a more severe health issue. You should describe and report any symptoms or signs present during anisocoria to a healthcare professional. 

Symptoms may include:

  • Drooping eyelid (ptosis)
  • Trouble moving your eye
  • Eye pain
  • Fever
  • Headache
  • Reduced sweating

If you develop unequal pupil sizes of more than 1 mm and do not return to equal size, you may have an eye, brain, blood vessel, or nerve disease or condition. 

How is Anisocoria Diagnosed?

If you think you have anisocoria, you should speak with your ophthalmologist or healthcare professional. 

You’ll undergo an eye examination to receive a diagnosis, which may include information on the cause and follow-up care. If you are experiencing additional symptoms, you might also need a neurological examination.

The examination will also let your specialist compare pupils in size, shape, position, symmetry, and reactivity in both light and dark settings. Depending on observations recorded, your specialist should better understand what is causing the anisocoria. Additionally, your specialist may use a slit lamp during the examination to obtain more details about any possible associated or coexisting eye condition. 

In the neurological examination, your specialist will look for lesions and neurological deficits in sensory, motor, and deep tendon reflex pathways. 

If anisocoria is more extensive in the dark, you may have a lesion in the sympathetic pathway (a type of pathway related to the nervous system). Possible causes could be: 

  • Horner’s syndrome (disrupted nerve pathway that runs from the brain to the face and eye on one side)
  • Argyll Robertson pupil (small pupils that reduce in size on a near object but not when exposed to bright light) occurs in advanced stages of syphilis
  • Iritis (inflammatory eye disease of the iris)
  • Mechanical anisocoria (due to damage to the iris or supporting structures)
  • Pharmacologic anisocoria due to miotics, narcotics, or insecticides

When is Anisocoria Normal? When is Treatment Necessary?

Not all types of anisocoria will require treatment or indicate a serious health condition. 

Some cases of anisocoria may be asymptomatic. For example, chronic anisocoria without accompanying symptoms is likely a benign form of physiologic anisocoria. 

However, a sudden onset of anisocoria with symptoms should raise concern. There may be an underlying neurological condition that requires immediate medical attention.

You should speak with your healthcare professional if you develop anisocoria. 

Anisocoria itself does not generally impact eye health. Visual acuity (sharpness) is minimally affected in most cases. 

How to Correct Different Pupil Sizes 

Correcting different pupil sizes will depend on the type of anisocoria you have. 

For example, mechanical anisocoria due to eye trauma could require surgery to correct the structural defect causing the issue. 

Physiologic anisocoria does not often present with symptoms, nor does it require medical attention. 

If you have anisocoria due to Adie’s tonic pupil (rare neurological disorder), glasses can help vision. Pilocarpine (a medication used to reduce eye pressure) can also be used to constrict the pupil. 

In pharmacologic anisocoria cases, stopping the administration of the causative pharmacologic agent should address the issue. 

In atypical cases like anisocoria due to autoimmune autonomic ganglionopathy (when the immune system mistakenly attacks a person’s own autonomic nervous system), a consultation with a neuro-ophthalmologist or neurologist will be necessary. The consultation can help determine the proper treatment. 

You should seek medical care if you develop anisocoria to rule out any underlying life-threatening conditions. 

Is Anisocoria Preventable? 

Anisocoria may be preventable in some cases. For example, in pharmacologic anisocoria, stopping the administration of the pharmacologic agent causing the issue could provide relief. 

In other cases, anisocoria can occur due to an underlying health condition, like a stroke. 

While you cannot prevent anisocoria due to a stroke, you could minimize the risk of suffering a stroke by eating a well-balanced diet and staying active. 

Resources
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Al Othman, Bayan. “Anisocoria.EyeWiki, 16 July 2021.

Anisocoria and Horner's Syndrome.Anisocoria and Horner's Syndrome - American Association for Pediatric Ophthalmology and Strabismus.

Anisocoria: MedlinePlus Medical Encyclopedia.MedlinePlus, U.S. National Library of Medicine, 21 Apr. 2021.

Boyd, Kierstan. “What Is Anisocoria?American Academy of Ophthalmology, 2 July 2021.

Payne, William N. “Anisocoria.StatPearls [Internet]., U.S. National Library of Medicine, 8 Aug. 2021.

Pupillary Disorders Including Anisocoria.Johns Hopkins Medicine

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