Your pupils let light into your eyes to help you see clearly. They are controlled by muscles in your iris, the colored part of your eye. Pupils naturally widen or dilate in dim environments to allow more light into the eye.
When pupils dilate without any change in light, this is called mydriasis. Mydriasis can be caused by many different things, including medicine, injury and disease.
If you notice a sudden change to one or both pupils, or if you observe that your pupils frequently stay dilated, you should contact your primary care physician or ophthalmologist to make sure your eyes and vision are safe.
Here are some common and rare causes of mydriasis.
Medicine and Drugs
There are two classes of medicine that can affect the muscles managing your pupils: mydriatics and anticholinergics.
Mydriatics, or dilating eye drops, act directly on the iris muscles that change pupil shape. These are used by ophthalmologists as part of an eye exam or before eye surgery. Mydriatics may cause temporary sensitivity to bright light, but they will rarely have other side effects. Mydriatics gradually wear off after 4 to 8 hours, but can last as long as 24 hours.
Anticholinergics are found in many different prescription drugs. This type of medicine affects both the muscles in your iris and in your lens. It can sometimes cause other side effects, including blurry vision and dizziness. Commonly prescribed anticholinergics include:
Recreational drugs such as cocaine, ecstasy, LSD, methamphetamines and psychedelic mushrooms can also cause mydriasis.
Trauma to the eye from an injury can damage the structures in the iris that control the pupil. Punches, flying balls, or chemical splashes are examples of eye injuries. Eye injuries can result in vision changes or blindness, so they require immediate medical attention.
Migraine, Brain Injury and Disease
- An injury inside the head can lead to a buildup of pressure that damages the nerves in your iris. This can cause mydriasis in one or both pupils. Head injury, stroke and tumor can all cause changes in pupil size.
- Adie’s Syndrome, a rare neurological condition, can also cause one or both pupils to dilate abnormally.
- Microvascular cranial nerve palsy (MCNP), or blocked blood flow to certain nerves going to the eye, can affect pupil size and impair vision.
- Benign episodic unilateral mydriasis (BEUM) is another rare condition that can cause intermittent pupil asymmetry. BEUM is associated with migraines.
A doctor will need to evaluate and diagnose these less common conditions.
The treatment of dilated pupils depends on the cause. Because pupil changes can also affect your vision, you should get examined by a doctor if you experience them. Your doctor may recommend medication changes, light-sensitive sunglasses or special contact lenses to resolve or treat your mydriasis.