Anisocoria: Causes, Symptoms, Diagnosis & Treatment Options

With anisocoria, one pupil is significantly larger or smaller than the other. This condition affects approximately 20 percent of the population. 

In the majority of cases, anisocoria is not dangerous and does not signal any underlying eye or health issues. However, in some cases, anisocoria may indicate a more serious condition that requires medical intervention. Because of this potential, it’s important to be evaluated by an eye doctor if you notice your pupils are different sizes.

Key Facts About Anisocoria

• Anisocoria affects roughly one-fifth of the population, making it a relatively prevalent condition. The degree of pupil size difference differs from individual to individual. For many people, the difference is so slight that the condition may go unnoticed.
• The most prevalent form of anisocoria is benign. It accounts for approximately 80 percent of instances of anisocoria. 
• With physiological anisocoria, the disparity in pupil size is typically less than 1 millimeter (mm) and remains constant regardless of the lighting conditions.
• In some instances, anisocoria can be transient, meaning that it comes and goes. This could be due to ocular fatigue, chemical or substance exposure, or as a side effect of certain medications or eye drops. 
• Unless accompanied by other symptoms or if it persists over time, transient anisocoria is typically not cause for concern. Approximately 50 percent of people experience a full recovery.

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What Causes Anisocoria?

Anisocoria can have numerous causes, including these:

• Physiological anisocoria: The difference in pupil size is typically less than 1 millimeter with physiological anisocoria, the most common and benign form of anisocoria. In most cases, physiological anisocoria is harmless and does not require treatment.
• Eye trauma or injury: Eye trauma or injury, such as a blow to the eye, can injure the iris or the muscles responsible for controlling the pupil, resulting in anisocoria.
• Inflammation and infection: Inflammation or infection can cause anisocoria, such as uveitis or iritis, which induces inflammation in the eye.
• Neurological problems: Problems with the nerves or muscles that control the irises can cause anisocoria. Uneven pupil diameters can result from conditions such as Horner’s syndrome, Adie’s syndrome, and third nerve palsy.
• Medications and eye drops: Certain medications and eye drops can alter pupil size, either directly or indirectly. Pilocarpine, which constricts the pupil, and atropine, which dilates the pupil, are examples.
• Other health conditions: Anisocoria can sometimes be a sign of more severe underlying medical conditions, such as a brain tumor, brain aneurysm, or stroke.

How Is Anisocoria Diagnosed?

Immediately seek medical attention if you notice a sudden change in pupil size or if anisocoria is accompanied by other symptoms such as eye pain, vision changes, headache, or facial drooping. It may indicate a more serious underlying condition requiring immediate evaluation and treatment. 

During a comprehensive eye examination, an eye care professional, such as an optometrist or ophthalmologist, can diagnose anisocoria. Typically, the diagnostic procedure consists of a series of tests and evaluations to determine the cause of unequal pupil sizes and rule out underlying health problems. 

Your doctor may complete the following diagnostic tests:

• Medical history: The eye care professional will inquire about your medical history, medications, and any symptoms you may be experiencing, such as vision changes, pain, or discomfort.
• Visual inspection: The doctor will visually examine your eyes. They’ll look for any potentially underlying causes of anisocoria, such as injury, swelling, or structural abnormalities.
• Pupillary light reflex test: A test of the pupillary light reflex involves projecting a light into each eye to observe the pupil’s response. The physician will evaluate the direct and consensual pupillary light reflex, searching for any abnormalities in the way the pupils constrict or dilate in response to light.
• Slit-lamp examination: A slit-lamp is a specialized microscope used to examine the cornea, sclera, and lens of the eye’s anterior structures. This examination can assist the physician in identifying any problems with these structures that could be causing anisocoria.
• Pharmacological testing: In some instances, the physician may use pharmacological agents, such as pilocarpine, atropine, or apraclonidine eye drops, to determine the cause of anisocoria. These medications can help differentiate between conditions, such as Horner’s syndrome, Adie’s tonic pupil, and third nerve palsy, that can produce anisocoria.
• Additional testing: If the eye care professional suspects a neurological condition or other medical condition, they may order additional tests such as neuroimaging (CT scan or MRI) or blood tests. They may also refer you to a neurologist or other specialist for further evaluation.

Is Anisocoria Dangerous? 

Anisocoria is usually not a cause for concern, but it may be an indication of an underlying and potentially life-threatening condition, such as an aneurysm or stroke. Seek medical attention immediately if you notice symptoms. 

Who Is at Risk?

A variety of factors can cause anisocoria, and certain conditions, lifestyle factors, or medical histories may place certain individuals at a higher risk. The following are potential risk factors for anisocoria:

• Eye trauma or injury: For example, blunt force trauma can cause anisocoria by damaging the iris or muscles responsible for regulating the pupil.
• Eye surgery: Those who have undergone eye surgery, especially surgery involving the iris, lens, or other anterior segment structures, may be at a higher risk for anisocoria.
• Eye conditions: Eye conditions like uveitis or iritis, that trigger inflammation or infection can result in anisocoria.
• Neurological disorders: People with neurological disorders that affect the nerves or muscles that control the pupils, such as Horner’s syndrome, Adie’s tonic pupil, or third nerve palsy, are at higher risk of developing anisocoria.
• Medications: Certain medications or eye drops can alter the size of the pupil, either directly or indirectly. For example, pilocarpine constricts the pupil, and atropine dilates the pupil.
• Health conditions: Some systemic health conditions, such as diabetes or hypertension, can increase the likelihood of developing ocular problems, such as anisocoria.
• Other substances: Certain chemicals, toxins, and other substances that can affect the nervous system may increase the likelihood of anisocoria.
• Age: As individuals age, their risk of developing certain eye conditions, such as anisocoria, may increase, particularly when associated with presbyopia or other age-related changes in the eye.
• Genetics: Due to genetic factors, individuals with a family history of anisocoria or related ocular conditions may be at a higher risk.

Treatment Options for Anisocoria

Anisocoria treatment options depend on the underlying cause of the condition. In many instances, anisocoria is benign and treatment is unnecessary. However, if the unequal pupil diameters are the result of an underlying condition, it is essential to treat the specific cause. 

Here are some treatment options for various anisocoria causes:

Eye Injury or Trauma

Immediate medical attention is required in the event of an eye injury or trauma. To repair any injury to the eye’s structures, treatment may include painkillers, anti-inflammatory drugs, and surgery.

Inflammation & Infection

Depending on the specific cause, corticosteroid eye drops, nonsteroidal anti-inflammatory medications (NSAIDs), or antibiotics may be used to treat anisocoria caused by inflammation or infection, such as uveitis or iritis.

Neurological Issues

The treatment will target the underlying cause of anisocoria caused by a neurological disorder, such as Horner’s syndrome, Adie’s tonic pupil, or third nerve palsy. This may involve medication, surgery, or referral to a neurologist for additional evaluation and treatment.

Pharmacological Anisocoria

If the condition is caused by a medication, such as an eye drop, talk to your doctor about how to proceed. You may need to discontinue use and switch to an alternative medication.

Eye Alignment Problems

If anisocoria is accompanied by eye alignment issues, such as strabismus or double vision, the doctor may prescribe corrective lenses with prism correction, vision therapy, or surgery to modify the eye muscles.

Underlying Health Issues

A doctor must properly manage the underlying condition caused by systemic health issues, such as diabetes or hypertension. This may involve medications, adjustments in lifestyle, or additional interventions to control the systemic condition and reduce its impact on eye health.

Anisocoria FAQs

Is anisocoria serious?

It can be. Seek medical attention immediately if you notice symptoms. 

While anisocoria is typically innocuous, it is occasionally a sign of neuro-ophthalmic disorders. Horner’s syndrome, which affects about 1 in more than 100,000 individuals, impacts the sympathetic nerves of the face and eyes. This can cause anisocoria, in which the afflicted pupil is smaller than the unaffected pupil. 

Another example is third nerve neuropathy, which can cause one pupil to be larger than the other in addition to other symptoms like drooping eyelids and difficulty moving the eye. In such cases, anisocoria can serve as an important diagnostic indicator.

What kind of nerve damage causes anisocoria?

Nerve damage to parasympathetic and sympathetic nerves that originate in the cervical spine and smooth muscle within the iris can cause anisocoria. Nerve-related conditions linked to anisocoria include third nerve palsy, Horner syndrome, traumatic mydriasis, and Adie pupil. 

Can anisocoria be caused by anxiety?

Anxiety will most likely cause both pupils to dilate, not just one. Anxiety can potentially exacerbate symptoms of anisocoria. 

References

1. Anisocoria. (December 2022). StatPearls.

2. Anisocoria. (2023). University of Kentucky Neuroscience Institute.

3. Anisocoria. (March 2023). American Academy of Ophthalmology.

4. Nationwide Population-Based Incidence and Etiologies of Pediatric and Adult Horner Syndrome. (October 2020). Journal of Neurology.

5. Transient Anisocoria: A Pesky Palpitation. (July–December 2011). Journal of Neurosciences in Rural Practice.

6. The Prevalence of Physiological Anisocoria and its Clinical Significance – A Neurosurgical Perspective. (December 2019). Neurology India.

7. Anisocoria Correlates With Injury Severity and Outcomes After Blunt Traumatic Brain Injury. (December 2021). Journal of Neuroscience Nursing.

8. Adie Syndrome. (January 2023). StatPearls.

9. Influence of Pupil Size, Anisocoria, and Ambient Light on Pilocarpine Miosis. (February 1993). Ophthalmology.

10. Influence of Strategic Cortical Infarctions on Pupillary Function. (October 2018). Frontiers in Neurology.

11. Horner Syndrome: Clinical Perspectives. (April 2015). Eye Brain.