Dilated pupils are pupils that are larger than normal. Dilation is not a natural state for your eyes, and it usually happens because of eye drops given prior to an eye examination. Pupils will also dilate, or grow large, under poorly lit conditions.
But pupils can also dilate because of a trauma or injury, particularly a brain injury, as a reaction to medication and because of exposure to some plants. If your eyes stay dilated for an extended time, doctors will investigate whether you have mydriasis or whether you have another underlying condition.
Pupils dilated for long periods can be harmful to your long-term vision.
What Are Dilated Pupils?
You see it on every television medical show. One of the first things the doctor does is flash a little penlight into a person’s pupil, usually wagging it back and forth. It generally turns out well for the patient, but it underscores the importance of the pupil and the information it contains about your overall health.
While this will mostly explore eye health, dilated pupils in particular, it is a good chance to learn about one of the more predictive parts of the body. First, some basics. The pupil is an aperture that allows light to enter the eye. Its size varies depending on the environment. The pupil widens in a poorly lit areas to let in more light while the opposite happens in brightly lit areas.
These changes allow you to see well in different lighting environments. However, if your pupils remain dilated even in bright conditions, you might be having mydriasis. If the pupil doesn’t dilate it can cause blurry vision and discomfort, possibly needing medical attention.
Mydriasis usually affects both eyes, and the pupils are typically the same size. But some individuals have unequal pupils, regardless of the lighting conditions. When the diameter of the pupil differs by more than 1 mm, the disorder is referred to as anisocoria.
Although your anisocoria might be nothing to worry about, especially if you were born with it, unequal pupils could be a sign of a serious underlying brain disorder.
What Causes Dilated Pupils?
The causes of larger-than-normal pupils are many, ranging from nothing to worry about to very serious. Your eye doctor evaluates the cause of your mydriasis and recommends the appropriate treatment. The potential triggers of pupil dilation are:
- Traumatic eye injury
- Brain injury
- Prescribed medications and other drugs
- Exposure to some plants
Traumatic Eye Injury
An injury to the eye can damage the muscular structures in the iris responsible for controlling pupil size. When you get a blow or chemical splash to the eye, you are likely to injure the iris leading to vision changes or permanent blindness if not treated immediately.
Dilated pupils can occur in those with brain injuries because the elevated intracranial pressure causes compression of eye nerve fibers, including those in the iris. The result is usually a blown pupil (unilateral fixed mydriasis), usually in just one eye. A tumor or a stroke can also cause a blown pupil. In the most severe cases, non-reactive pupils can indicate irreversible brain stem damage, which is not a good outcome.
Prescribed Medications and Other Drugs
Medicines that can affect your pupil size include mydriatics and anticholinergics. Mydriatics are an essential part of a dilated eye exam. Before your doctor can look into your eye, they apply eye drops that cause the iris muscles to widen your pupils and let in more light. Although the effect of these medicines usually wears off in 4-8 hours, sometimes the dilation persists for a day.
While mydriatics are topical medicines, anticholinergics are primarily systemic drugs prescribed for different conditions. Anticholinergics act on the iris and lens, causing dilated pupils and other eye effects, such as blurred vision.
Some of the commonly prescribed drugs that cause pupil dilation include:
- Botox and botulinum toxin-containing drugs
- Antihistamines for treating allergies
- Motion sickness drugs like scopolamine
- Anti-seizure medications
- Tricyclic antidepressants prescribed for depressions, insomnia, or anxiety
- Decongestants including Sudafed
- Anti-parkinsonism medications
Besides the prescribed medicines, commonly abused drugs such as methamphetamines, LSD, cocaine, and ecstasy also cause mydriasis.
Exposure to Some Plants
Certain plants contain substances that trigger mydriasis. For example, Angel’s trumpet, Jimson weed, and belladonna plants contain compounds that lead to pupil dilation. Exposure to such plants could give you mydriasis.
Rare conditions that can cause unusual pupil dilation include Adie’s Syndrome, benign episodic unilateral mydriasis (BEUM), and microvascular cranial nerve palsy. BEUM is often associated with migraines.
Adie’s syndrome is a neurological disorder that can affect either one or both pupils. Individuals with microvascular cranial nerve palsy have impaired vision as the disease affects the blood supply to the nerves supplying the eyes.
When to See a Doctor
Whether in one or both eyes, mydriasis can signify stroke, head injury, brain tumor, or eye injury, which are all emergency conditions.
Your doctor will take a detailed medical history, perform a comprehensive eye exam, and order brain imaging scans to determine the cause of your mydriasis.
The treatment of mydriasis depends on its cause. For instance, if your pupils remain dilated after an eye exam, you do not need any treatment because the effect will wear off in a day or so. If the cause is a prescription medicine, just ask your doctor to change the medication.
Dilated pupils due to palsy of the third cranial nerve can be corrected by surgery if there is no improvement in six months. Brain or eye injury-related mydriasis might also require surgery to treat.
Your doctor might also recommend special contact lenses or light-sensitive (polarized) sunglasses in treating dilated pupils.
Why Do Doctors Dilate Pupils in an Eye Exam?
During an eye exam, the doctor or technician uses mydriatic eye drops to dilate your pupil. This allows more light to enter your eyes, thereby making the evaluation of the posterior chamber easier. Problems in the retina such as diabetic retinopathy and age-related macular degeneration are typically identified during dilated eye exams.
While the exam might cause slight discomfort because of the associated temporary mydriasis, the diagnoses that can be made because of greater access to full eye are well worth it and ca facilitate early treatment.
What does it mean if your pupils are dilated?
Dilated pupils might be a sign of underlying brain tumor, head injury, stroke, cranial nerve palsy, migraine or eye injury. However, in most cases, it is temporary with no cause for concern.
Which drugs cause dilated pupils?
Dilated pupils can be caused by both prescribed medicines and drugs of abuse. Anticholinergics such as atropine, antihistamines, tricyclic antidepressants such as amitriptyline, botox, anti-seizure medications and scopolamine for treating motion sickness are the most common drugs causing mydriasis.
People who abuse drugs such as cocaine, methamphetamines, LSD and ecstasy may experience dilated pupils.
Concerned About Dilated Pupils? Causes and Treatment. (July 2021). American Academy of Ophthalmology.
Anisocoria. (August 2021). StatPearls.
Traumatic Brain Injury. (March 2020). American Association of Neurological Surgeons.
Don’t Believe Your Eyes” Ipratropium Induced Mydriasis: A Case Report and Review of the Literature. (June 2016). General Medicine: Open Access.
Pupil cerclage technique for treatment of traumatic mydriasis. (April 2019). European Journal of Ophthalmology.
Get a Dilated Eye Exam. (May 2021). National Eye Institute.
Last Updated February 26, 2022
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