What Is Mydriasis (Dilated Pupils)?

Mydriasis occurs when the pupil remains dilated (widened) and doesn't respond to light changes in an environment. It can happen due to an injury, medications, or psychological factors. Usually, the pupil becomes wider to allow more light into your eye in dim environments and gets smaller to prevent light from entering your eye in bright places. The pupil also dilates in response to factors such as touch, sound, and emotion. In people with mydriasis, the pupil will remain dilated even in bright places and in the absence of any stimulus.

In many cases, mydriasis is benign and can be triggered by drops used during an eye exam or a health condition. Fixed mydriasis in one eye can also be a red flag for severe brain injuries. 

How Pupils Change Size

A pupil is the round black circle in the center of the iris. It controls how much light enters the eye before it reaches the retina. The standard pupil size ranges between 2 millimeters (mm) and 8 mm, depending on the lighting.

Two muscles control the size of the pupil. One of them is the circumferential sphincter, which is located at the iris’s margin and innervated by the parasympathetic nervous system. The other one is the iris dilator muscle, which contains fibers that extend through the iris and expand or contract in response to light.

Stimulation of the sympathetic nervous system, which is known for triggering a fight-or-flight response when the body is under stress, can also cause the pupil to dilate. 

Causes of Mydriasis

Eye Injury

Eye injuries can damage nerves responsible for controlling the pupil and iris, triggering traumatic mydriasis. This form of mydriasis is often followed by other symptoms, such as eye pain and discomfort when reading and photophobia (light sensitivity).

Traumatic Brain Injury

Mydriasis can occur in people with traumatic brain injuries. These often increase intracranial pressure, which can put pressure on and affect the eye system. It tends to happen on just one side, and this type of mydriasis is called unilateral fixed mydriasis, or a blown pupil. The presence of a dilated pupil on only one side suggests that a large mass or lesion may be present.

Lasting bilateral mydriasis and the absence of a light reflex in the pupil following a severe traumatic brain injury are considered signs of irreversible brain stem damage and have been strongly associated with poor outcome.

Medications

Mydriasis can also be a side effect of different medications. While systemic medications cause bilateral mydriasis, direct ocular inoculation with topical medications can cause unilateral mydriasis.

Some of the medicines that can cause the problems are:

• Antihistamines
• Botox
• Atropine
• Scopolamine patches
• Amphetamines
• Serotonergic medications
• Muscle relaxants
• Anticholinergics

Exposure to Certain Plants

Some plants have substances such as scopolamine and atropine that can cause mydriasis. The Angel's trumpet plant, for example, contains hyoscyamine, atropine, and scopolamine, which can dilate the pupil. Plants of the belladonna family and Jimson weed can also cause mydriasis. 

Drug Use

Drugs can expand or shrink the eye muscles, causing mydriasis. Substances such as cocaine increase serotonin level, which can lead to pupil dilatation. LSD has a direct effect on serotonin receptors in the brain, which can also cause mydriasis. Other drugs that have a similar side effect are crystal methamphetamine and ecstasy.

Increased Oxytocin Levels

Oxytocin is a hormone. High levels of oxytocin can lead to mild or moderate mydriasis.

Migraine

Migraines can occasionally cause benign episodic unilateral mydriasis due to hyperactivity of the sympathetic nervous system or hypoactivity of the parasympathetic system. It is a temporary condition and is more common in young women. 

Cranial Nerve Neuropathy

Neuropathy is a disorder that impairs the ability to feel and move. When it affects the third cranial nerve, also known as the oculomotor nerve, the pupil becomes fixed and dilated due to paralysis of the iris sphincter.

Pupil Dilation in Eye Exams

Not all causes of mydriasis are problematic. Optometrists and ophthalmologists often use drops to dilate your pupils during an eye exam. This allows them to examine the optic nerve and retina. This dilation typically lasts four to six hours, but sometimes a doctor will use a drop that can last 24 hours or more. Other causes, however, may be serious and require treatment.

Two types of ophthalmological eye drops are used during eye exams. One will dilate the pupil by contracting the muscles in the iris, while the other relaxes the muscles that shrink the pupil, allowing the eye to focus the lens inside the eye.

It often takes up to 30 minutes for a pupil to dilate after receiving the eye drops. Your eye doctor cannot tell you how blurry your vision will be and for how long. That depends on the type of dilating eye drop used and how your eyes react to it.

When your eyes are dilated, they are often extremely sensitive to light. Your vision becomes blurry and you will have trouble focusing on near objects. 

Treatment

Treatment will depend on what causes mydriasis. When prescribed medications cause it, the pupils will go back to normal after the effect diminishes and you stop taking the medication. No treatment is required in this case.

When mydriasis is caused by third cranial nerve palsy, surgery may be recommended after six months if there is no improvement.

When to See a Doctor

If your pupils don’t change when the light changes or one is noticeably larger than the other, you should talk to your eye doctor. In these cases, mydriasis might be a sign of an eye injury or a problem that affects the brain, like a tumor, stroke, or head injury.