1. In most cases, dilated pupils are normal
We have small muscles located in the colored part of our eyes called the iris. There are two types of iris muscles - those that dilate the pupil are those that constrict it - and both are necessary for us to see normally. In fact, our pupils must dilate naturally when we’re in dark conditions to maintain good vision in low-light conditions. You’ll know what we’re talking about if you’ve ever seen a cat in the dark. The opposite occurs when we’re out in the sun - the pupils become much smaller.
However, light isn’t the only cause of dilated pupils. Certain emotions, like fear or even attraction, for example, have the same effect on our eyes. In fact, only looking at a picture of an object that we fear or a person we find attractive can yield this result. So, in most cases, if you notice that your pupils have suddenly gotten bigger, there’s no reason for concern, especially if they return to their normal size after a few minutes.
While dilated pupils are infamously linked to recreational drugs like cocaine or LSD, actual medications can also cause your pupils to widen as either a side effect or part of a treatment. For example, some medications, such as atropine, are used during an eye exam, and they intentionally prevent your pupils from getting smaller.
That said, the same medication, atropine, is also used to treat problems with stomach issues, poisoning, and heart rhythm, so in those cases, the big pupils may come as a surprise to some patients. But atropine is not the only type of medication that can dilate the pupils as a side effect.
In fact, many over-the-counter meds can have the same effect. This includes certain allergy medications, like diphenhydramine (Benadryl), flu medications, such as pseudoephedrine (Sudafed), and anti-nausea medicines, e.g. Dramamine. Prescription medications that treat depression, Parkinson's, and even anti-seizure drugs have a similar effect.
Your eyes will get back to normal when the medication wears off. If you're taking the medication regularly and it is causing your eyes to dilate, consult your doctor to see if there are any alternatives. Be mindful that your eyes are more sensitive to light when they're dilated, so it will probably feel nice to wear sunglasses when you go outside, dim the lights at home in the evening, and close the blinds during the day. Drive carefully, or even better, avoid driving altogether, if you notice that your vision becomes impaired.
3. Eye injury
One of the most common medical causes of dilated pupils is an eye injury. All kinds of eye injuries, be it a blunt object hitting your eye, something sharp getting into the eye, or a chemical or a thermal burn can result in your pupil becoming dilated either temporarily or permanently. This occurs because trauma can damage the muscles or nerves located in the iris.
As a result, the eye loses the ability to control pupil size. In some cases, the dilation is caused by the bruising of the muscles and it will clear up eventually, but when the muscles or nerves are torn or badly damaged, the condition can be permanent. Temporary eye dilation is also common after eye surgery, such as corneal transplants and cataract removal.
4. Benign episodic unilateral mydriasis
Benign episodic unilateral mydriasis (BEUM) is a very interesting condition. As the name suggests, the condition is benign, so it’s not associated with any serious ailments. It’s also unilateral, which means that only one pupil is dilated. Although it isn’t clear what causes BEUM, it often occurs in women suffering from migraines.
When someone experiences BEUM, one of the pupils spontaneously dilates and it can take from a few minutes to days before going back to its normal size. If you ever experience or get diagnosed with BEUM, there’s nothing to worry about, as the condition doesn’t cause any long-term harm to the eyes.
5. Brain disease or injury
The most serious cause of dilated pupils is a brain injury or an underlying brain disease. It occurs when the pressure builds up in the brain so much that it starts damaging the iris muscles, making it impossible for the pupils to constrict. If you've hit or otherwise injured your head and notice that your pupils have become wider than usual, go to the emergency room immediately, as this is a sign that you've sustained severe head trauma.
Likewise, dilated pupils may be a sign of internal damage to the brain, such as that caused by a brain hemorrhage, a brain tumor, or a stroke. One or both of your pupils can widen and stop reacting to light. Shining a bright light like a flashlight in your eyes to have a friend or relative check if that makes your pupils constrict is a good test. Needless to say, all of the above-mentioned brain conditions require immediate medical attention.
To sum up, mydriasis is usually a normal reaction to changes in light or strong emotions, but it can also be a sign of a medical emergency, so it should never be taken lightly. Seek immediate medical help if you shine a bright light in your eyes and the pupils don't react to it, especially if you've sustained an eye or head injury or can observe other symptoms of a stroke.
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