Why Do Pupils Dilate During a Migraine?

Have you ever noticed your pupils changing size during a migraine attack, or experienced that strange sensation of light sensitivity that forces you to seek darkness? You're not alone.

Migraine pupil dilation is a puzzling yet common phenomenon that many of the 1 billion migraine sufferers worldwide experience.

This mysterious pupillary response—where your eyes' black centers expand during an attack—can be both alarming and confusing. Is it normal? Is it dangerous? And most importantly, why exactly does it happen?

While you might expect your pupils to constrict when facing painful light sensitivity, the opposite often occurs during migraine attacks.

This article explores the fascinating connection between migraine and pupil dilation, unraveling the complex autonomic nervous system disruptions involved.

The Autonomic Nervous System and Pupil Control

Pupil size is regulated by a delicate balance between the sympathetic and parasympathetic divisions of the autonomic nervous system.

The sympathetic nervous system is responsible for pupil dilation (mydriasis), while the parasympathetic nervous system controls pupil constriction (miosis).

Under normal circumstances, these two systems work in harmony to adjust pupil size in response to changing light conditions and emotional states.

During a migraine attack, this autonomic balance can be disrupted, leading to changes in pupil size.

Research has shown that migraine is associated with autonomic nervous system dysfunction, which can manifest as alterations in pupillary responses.

This dysfunction may result from the complex neurochemical changes that occur during migraine episodes, including fluctuations in serotonin levels and other neurotransmitters that influence autonomic function, as well as the vasodilation of blood vessels in the brain and surrounding tissues that is characteristic of migraine attacks.

Interestingly, studies have found conflicting evidence regarding whether migraine typically causes pupil dilation or constriction.

Some research indicates that during migraine attacks, particularly those with aura, there may be a shift in the pupillary balance toward the parasympathetic system, which would actually result in pupil constriction rather than dilation. 

This parasympathetic shift has been observed measuring pupillary parameters in migraine patients during headache-free periods compared to healthy controls.

Pupillary Changes During Migraine Attacks

Contrary to what many migraine sufferers might expect, research has revealed that pupil behavior during migraine attacks follows complex patterns that can vary between individuals and even between different types of migraine.

The Paradox of Pupil Constriction During Attacks

While light sensitivity might intuitively suggest pupil dilation would occur, clinical studies have documented the opposite phenomenon in many cases.

This creates a paradoxical situation where patients experience increased light sensitivity despite having pupils that are actually more constricted.

This heightened photophobia despite smaller pupils suggests that migraine photosensitivity stems from central processing abnormalities in the brain rather than simply from too much light entering the eye.

Research has consistently shown that pupil size often decreases during migraine attacks compared to headache-free periods:

• One comprehensive study found that mean pupil diameter significantly decreased during migraine attacks (5.85 ± 0.19 mm) compared to the interictal (between-attack) phase (6.05 ± 0.19 mm).

• This constriction suggests a subtle oculosympathetic hypofunction (reduced sympathetic nervous system activity affecting the eye) during the ictal (attack) period.

• The effect is particularly notable in patients with unilateral migraine, where the pupil on the headache side is often measurably smaller than on the pain-free side.

This pupillary asymmetry between the affected and unaffected sides displays several important characteristics:

• It is most pronounced in darkness

• It decreases progressively with increasing illumination

• It represents a physiological manifestation of a pupillary sympathetic deficit in migraine patients

When Pupils Dilate: The BEUM Phenomenon

Despite the general trend toward constriction, some migraine sufferers experience the opposite effect—episodes of pupil dilation, particularly during the aura phase.

This condition, known as Benign Episodic Unilateral Mydriasis (BEUM), is characterized by:

• Temporary, one-sided pupil enlargement

• Associated symptoms of blurred vision and orbital pain

• Episodes lasting from minutes to several hours

• Spontaneous resolution without medical protocol

The pathophysiology of BEUM remains incompletely understood, but likely involves either:

• Parasympathetic nervous system hypoactivity (reduced constriction signals)

• Sympathetic nervous system hyperactivity (increased dilation signals)

It is important to note that, although intriguing, BEUM is a rare phenomenon reported in isolated cases, and not a core migraine feature.

To better grasp this topic, here is a comparison of pupillary changes in different migraine types

Clinical Detection of Pupillary Changes

Pupillometry, the precise measurement of pupil size and reactivity, has become an important tool for studying these changes.

Modern digital pupillometers can detect subtle differences that might be missed during standard clinical examination, revealing:

• Reduced velocity of pupil constriction in migraine patients

• Smaller amplitude of pupil movement in response to light stimuli

• Faster latency of pupil contraction (time between light stimulus and pupillary response)

• Persistent asymmetry even during headache-free periods

These findings collectively suggest that migraine involves a fundamental disruption of normal autonomic control of pupillary function, with evidence pointing toward a shift in pupillary balance that favors the parasympathetic system in many cases.

Pupillary Changes During the Aura Phase

The migraine aura phase is characterized by transient neurological symptoms that typically precede or accompany the headache phase.

Visual disturbances are the most common aura symptoms, affecting approximately 90% of migraine sufferers who experience aura.

These visual phenomena can include flashing lights, zigzagging lines, bright spots, and blind spots in the visual field, foggy vision, and wavy or "watery" vision. The visual aura typically moves across the field of view and disappears within 30 minutes.

During the aura phase, some patients may experience pupil dilation as part of the complex neurological changes occurring in the brain.

The aura is believed to be caused by a wave of neuronal depression spreading across the cerebral cortex, known as cortical spreading depression (CSD).

This wave of depolarization can affect the visual cortex and other brain regions involved in autonomic control, potentially leading to changes in pupil size.

These visual disturbances can occur in both eyes and may persist even with the eyes closed, indicating their origin in the brain rather than the eyes themselves.

Migraine Medication Effects on Pupil Size

Many medications used to treat migraine can affect pupil size as a side effect.

Here's what you should know about how common migraine treatments influence pupillary response:

Triptans and Pupillary Response

• Triptans (commonly prescribed for acute migraine support) work by activating serotonin receptors, leading to constriction of dilated blood vessels in the brain

• While primarily targeting 5-HT1B and 5-HT1D receptors, triptans may indirectly affect pupil size through their influence on serotonergic pathways

• Research on sumatriptan (a common triptan) found no direct effect on pupil size at doses of 50, 100, and 200 mg, suggesting its serotonin receptor effects don't significantly impact pupillary function

Serotonin Syndrome

Despite limited direct effects, some patients report pupil dilation when taking triptan medications.

This could potentially be related to serotonin syndrome, a rare but serious condition occurring when there is excessive serotonergic activity in the central nervous system.

Warning signs of serotonin syndrome include:

• Dilated pupils

• Agitation and confusion

• Rapid heart rate

• Excessive sweating

• Muscle rigidity

Patients should seek immediate medical attention if experiencing these symptoms after taking a triptan, especially if they're also taking other serotonergic medications such as SSRIs or MAOIs.

Differential Diagnosis of Pupillary Changes

While pupillary changes during migraine attacks are generally benign, it is important to distinguish them from other causes of pupil abnormalities that may require urgent medical attention.

For those concerned about serious conditions, understanding the differences between migraine vs aneurysm is crucial.

Anisocoria, or unequal pupil size, can be a sign of various neurological conditions, including third nerve palsy, Horner's syndrome, and intracranial aneurysms.

Anisocoria affects about 20% of the population and is often a benign condition when both pupils react normally to light but vary in size by more than half a millimeter.

However, new-onset anisocoria, especially when accompanied by other neurological symptoms such as double vision, drooping eyelid, or severe headache, should be evaluated promptly to rule out serious underlying conditions.

Other causes of pupil dilation include certain medications (such as anticholinergics, sympathomimetics, and some recreational drugs), traumatic brain injury, increased intracranial pressure, and seizures.

If pupil dilation is a frequent symptom during migraine attacks, it is important to discuss it with a healthcare provider to rule out these other potential causes.

When to Seek Medical Attention

While pupillary changes during migraine attacks are usually benign and resolve once the migraine subsides, certain situations warrant immediate medical attention.

These include:

1. Sudden, severe headache accompanied by abnormal pupil size or reactivity, especially if different from typical migraine symptoms.

2. Persistent pupillary abnormalities that do not resolve after the migraine attack.

3. Pupillary changes accompanied by other neurological symptoms such as double vision, drooping eyelid, or weakness/numbness on one side of the body.

4. First-time experience of significant pupillary changes during a headache.

5. Pupillary changes accompanied by symptoms of serotonin syndrome after taking migraine medications, such as agitation, confusion, rapid heart rate, and muscle rigidity.

In these situations, prompt medical evaluation is necessary to rule out more serious conditions that may require specific treatment.

Frequently Asked Questions

If you've been reading about migraine-related symptoms, you might still have some questions regarding pupil dilation during these episodes.

Here, we've compiled answers to some of the most frequently asked questions to shed light on this topic.

How can I distinguish between migraine-related pupil dilation and more serious health conditions?

Migraine-related pupillary dilation is typically temporary and resolves when the migraine attack subsides.

However, sudden pupil dilation accompanied by severe head pain could indicate more serious health conditions such as aneurysm, stroke, or injury to the oculomotor nerve.

If you experience pupil dilation with symptoms like intense eye pain, double vision, drooping eyelid, or confusion following a head injury, seek immediate medical attention.

Unlike migraine, which are common conditions that follow predictable patterns for most sufferers, serious neurological emergencies often present with additional symptoms and don't follow your typical migraine pattern.

Can bright light trigger both migraine and pupillary changes simultaneously?

Yes, exposure to bright light can initiate a complex cascade of neurological responses that trigger both migraine and pupillary changes.

Blue light and migraine have a particularly strong connection, with blue wavelengths often exacerbating symptoms.

For many migraine sufferers, bright light serves as both a trigger for attacks and an aggravating factor during episodes.

When exposed to bright light, the cranial nerves involved in visual processing become hyper-excited, potentially triggering the migraine process.

Simultaneously, this stimulation can disrupt the normal light reflex pathway, creating unusual pupillary responses.

Some patients report experiencing a paradoxical pupillary dilation when exposed to bright light during a migraine, rather than the expected constriction, further intensifying their discomfort and necessitating retreat to dim light environments.

How does ocular migraine affect pupil dilation compared to other migraine types?

Ocular migraine (sometimes called retinal migraine) typically involve more pronounced and noticeable pupillary changes than other migraine types.

During an ocular migraine, patients may experience temporary vision loss or disturbances in one eye, often accompanied by significant pupillary dilation on the affected side.

This specific pattern of unilateral pupillary dilation differs from the more subtle or bilateral changes seen in common migraine.

The physiological mechanism relates to the direct involvement of retinal blood vessels and local cranial nerves, rather than the more generalized cortical spreading depression seen in typical migraine with aura.

Patients with ocular migraine often report seeing a dark spot in their vision that expands over minutes, followed by dull pain behind the eye and noticeable pupillary changes.

Can over-the-counter eye drops help manage pupillary dilation during migraine?

Most over-the-counter eye drops are not specifically formulated to address pupillary dilation during migraine.

However, some patients find that artificial tears can provide support from the discomfort associated with light sensitivity when pupillary changes occur.

These drops moisturize the eye surface, potentially reducing irritation from bright environments.

It's important to note that eye drops containing vasoconstrictors (for redness support) should be used cautiously by migraine sufferers, as they can potentially influence blood pressure and cranial blood flow.

Never use prescription eye dilation drops to try to counteract migraine-related pupillary changes, as this could mask important diagnostic signs.

Always consult with your healthcare provider before using any eye medications to manage migraine symptoms.

Is blurry vision with pupillary dilation during migraine a sign of eye injury?

Blurry vision accompanied by pupillary dilation during migraine typically does not indicate eye injury but rather reflects the neurological nature of migraine attacks.

This temporary visual disturbance results from abnormal brain activity affecting visual processing centers and the nerves controlling pupil size.

Unlike eye injuries, which often cause persistent symptoms, migraine-related blurry vision and pupillary changes usually resolve completely after the attack.

However, if you experience sudden blurry vision with pupil dilation that differs significantly from your usual migraine pattern, particularly following trauma or accompanied by eye pain, seek immediate medical evaluation to rule out potential eye injury or other serious conditions.

Why do some people develop dark circles under their eyes during migraine episodes with pupillary dilation?

Dark circles that appear during migraine episodes with pupillary dilation likely result from several physiological changes occurring simultaneously.

This symptom is part of a broader spectrum of facial changes some patients experience, with migraine and puffy eyes often appearing together during attacks.

During migraine, blood vessels dilate and constrict abnormally, potentially causing blood pooling in the thin skin around the eyes.

Additionally, the autonomic nervous system disruption that affects pupillary response can also influence blood flow and fluid distribution in facial tissues.

These temporary dark circles typically resolve after the migraine attack concludes and normal vascular tone returns to the facial vessels.

Can changes in blood pressure cause both migraine and pupillary dilation?

Fluctuations in blood pressure can indeed influence both migraine occurrence and pupillary responses.

Sudden increases in blood pressure can trigger headaches in some individuals, while also affecting the sympathetic nervous system that controls pupillary dilation.

During a rapid blood pressure elevation, pupils may dilate as part of the body's stress response.

Conversely, some migraine medications that help minimize attacks by regulating blood pressure can also normalize pupillary function.

If you notice a correlation between blood pressure changes and your migraine patterns with pupillary symptoms, tracking these measurements may help your healthcare provider optimize treatment.

How does dim light exposure affect pupillary dilation during migraine recovery?

Retreating to dim light environments is a common coping strategy for migraine sufferers, and it can significantly affect pupillary function during the recovery phase.

In dim light, healthy pupils naturally dilate to gather more available light.

For migraine patients whose autonomic nervous system is already disrupted, this environment allows a more normalized pupillary response without the painful stimulation of bright light.

Conclusion

The relationship between migraine and pupillary changes reveals the complex neurological nature of this common yet debilitating condition.

Whether experiencing pupil constriction, dilation, or the rare BEUM phenomenon, these eye changes highlight the profound impact migraine have on the autonomic nervous system.

Understanding these pupillary responses not only helps distinguish migraine symptoms from more serious conditions but also provides insight into the underlying mechanisms of migraine attacks. While pupillary changes can be alarming, they typically resolve as the migraine subsides.

By recognizing these patterns and working with healthcare providers, migraine sufferers can better manage their symptoms and make informed decisions about when to seek medical attention.