Pupil dilation is a normal, healthy response to a variety of possible stimuli. For example, a pupil should dilate and become larger in the dark, but constrict when there’s ample light. However, a number of issues can cause a pupil to remain dilated, even in well-lit areas. Physicians refer to this condition as mydriasis, though some medical experts use mydriasis as a general term for any instance of dilated pupils, even natural ones. Some fields use the term “blown pupil” to refer to mydriatic pupils that result from specific conditions.

Benign Episodic Unilateral Mydriasis

Benign episodic unilateral mydriasis is an unusual condition that can cause one pupil to dilate. Individuals with this condition may experience additional symptoms while their pupil is dilated. Headaches are common, as are blurry vision, and light sensitivity. People who frequently experience migraines may also develop benign episode unilateral mydriasis. The condition is often non-serious, though a medical examination may be necessary to ensure there isn’t an underlying condition or injury.

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Many medications and drugs contain substances that can cause the pupils to dilate. These substances, anticholinergics, block certain chemicals from reaching the eyes, muscles, and glands. Before some exams, a doctor or optometrist may use mydriatics to intentionally dilate the patient’s pupils. This allows the physician to examine the retina and the other structures deep within the eye. Mydriatics can last as long as 24 hours. Other medications that can cause the eyes to dilate include antihistamines, decongestants, and antidepressants.

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In some cases, eye injuries lead to mydriasis. Usually, this happens as a result of a serious, penetrating injury that damages the iris. Injuries of this type may also cause the pupil to develop an irregular shape. Blunt force trauma may damage the nerves that control the pupils or the iris, preventing the eyes from dilating and constricting properly in response to light levels. These injuries sometimes occur during medical procedures such as cataract surgery or corneal transplant.

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Brain Conditions

Many diseases, disorders, injuries, and other conditions that affect the brain can directly impact the eye and lead to mydriasis. Conditions that result in increased intracranial pressure, such as strokes or trauma, often affect the eyes, causing mydriasis or a “blown pupil.” Cranial nerve neuropathy is a condition that results from gradual damage of the nerves that travel to the eyes. Typically, individuals with cranial nerve neuropathy also develop other eye-related symptoms such as impaired vision.

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Adie’s Pupil

Some people are born with a rare neurological disorder, Adie’s pupil or tonic pupil. This condition causes the pupil of one eye to be larger than other. Additionally, the affected pupil is often slow to react to light and may not constrict at all. Adie’s pupil may also cause poor or absent tendon reflexes. Physicians do not know the cause of Adie’s pupil, though many associate the condition with trauma, poor blood circulation, surgery, or infections.

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The human body produces a hormone called oxytocin. Among other things, oxytocin plays a role in sexual reproduction, childbirth, and social bonding. Typically, the body releases oxytocin during intimate social or physical interactions. When humans see someone they find attractive, or when they experience a positive social interaction, their pupils dilate. Oxytocin is the hormone responsible for dilated pupils. If something causes the body’s levels of oxytocin to dramatically increase, it could lead to mydriasis.

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Though it seems bizarre, some plants can lead to mydriasis in those who touch or consume them. Notably, those in the nightshade family, such as belladonna, jimsonweed, and angel’s trumpet, can cause pupil dilation. This is due to organic compounds called tropanes. If a person has cuts or injuries on their hands and touches a nightshade plant, it is possible for tropanes to enter their system. This could have various effects, with mydriasis being one of the less serious. In cases of overexposure or over-consumption, symptoms include convulsions, comas, and death.

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Rarely, a person is born with a condition where their iris is completely absent. This condition, congenital aniridia, gives the appearance of a fully dilated pupil because there is no iris. In addition, the iris is the portion of the eye responsible for regulating the amount of light that enters the eye. Without an iris, individuals with aniridia develop intense light sensitivity. Aniridia usually affects both eyes, and the affected eyes have significant issues including glaucoma and cataracts.

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The underlying issue causing mydriasis determines treatment and whether or not the individual requires it. Some instances of mydriasis are asymptomatic and cause no issues, thus requiring no treatment. However, if the pupil dilation also features light sensitivity or other issues, a doctor may recommend opaque contact lenses or specialized light-sensitive sunglasses. In the case of medications, a physician may simply order their patient to stop taking them or find an alternative. Rarer conditions require specialized and specific treatment plans.

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Seeking Medical Attention

It can be difficult to notice when your own pupils dilate. Because of this, it can also be difficult to associate other symptoms with pupil dilation. For example, headaches or dizziness alone are often not signs of an underlying issue. However, if the symptoms occur during abnormal pupil dilation, there could be an underlying issue. People with pupil dilation accompanied by frequent or severe symptoms may require medical attention.

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