Pupil Response to Light

Mydriasis – Causes, Symptoms and Treatment

Understanding Mydriasis

The pupil of the eye open (dilate) and close with exposure to light and darkness. (See Figure 1) Mydriasis is a condition when the pupil fails to respond to light and become smaller as it normally should. The pupil opens with darkness to allow more light in and shrinks in bright light to shade the inner eye. Muscles are normally responsible for opening and closing the colored part of the eye, known as the iris and when this action is disrupted, the pupils take longer to shrink. It is actually believed to be a nervous system malfunction that does not send the right nerve signals to the muscles.

Figure 1: Normal Pupil Response to Light/Dark

Photo Courtesy of: Daviddarling.info

Many different factors can be attributed to Mydriasis such as; brain injury, death, drug use, chemical exposure, injuries and infections. When mydriasis only affects one side, doctors will be concerned that it is a sign of head injury.

It is usually found when a medical practitioner shines a light into a person’s eyes and the pupils remain dilated instead of shrinking in response to the light. The condition can be graded in millimeters, between 1mm and 9mm. (See Figure 2)

Pupil Reactions

Figure 2: Pupil Reactions

Photo Courtesy of: Primary-Surgery.org

One major complication of mydriasis is blindness due to overexposure to sunlight. Pupil constriction protects the eyes from the burning rays of the sun. This is why optical doctors give patients sunglasses after dilating the eyes with medications.

What Causes Mydriasis?

Mydriasis happens when the nerves that supply the muscles to the iris (the colored ring in the eye that surrounds the pupil) and causes them to be hyperactive. This is most often caused by stimulant use such as; amphetamines, caffeine and other stimulating substances. The hormone released during childbirth and breastfeeding, Oxytocin can also stimulate pupils to dilate. Optical doctor’s can instill certain medications directly into the eye to dilate the pupil in order to examine deeply into the structure of the eyes.

Sexual stimulation can also cause the pupils to remain in a fixed and dilated position temporarily. It can also happen during epileptic seizures, which are an overabundance of nerve stimulation in the brain.

Head and neck trauma that damages the cranial nerves that supply the eyes can also cause mydriasis. Mydriasis could be a sign of brain swelling and is caused by pressure to the ocular nerves. It also occurs after brain death when the muscles and nerves controlling the pupil cease to function.

Other less common causes include; botulism, aneurysm, viral and fungal infections of the eye or the brain, conditions that cause nerve damage and brain tumors. It is rarely found, but the genetic predisposition to mydriasis is a possibility.

What are the Symptoms of Mydriasis?

People who are conscious with mydriasis will complain of sensitivity to light. Since the pupil does not constrict to block out excess light, light exposure can actually be painful. Blurred vision can occur since the pupil needs to shrink in the light in order to clarify the vision.

Most people with mydriasis do not complain of any symptoms at all. Some complain of headaches but it is still unclear if this is a symptom of mydriasis or another co-existing condition. On the other hand, mydriasis could be a symptom itself of a type of migraine headache.

The most prominent symptom of mydriasis is pupil dilation without responding to light shined into the eye. Normally, light will cause the pupil to constrict. The pupils also become “non-reactive” to the application or removal of light and stay in a fixed position. (See Figure 3)

Pupil Response to Light

Figure 3: Pupil Response to Light

A cardinal sign with mydriasis is if one or both eyelids are “droopy” otherwise known as ptosis. If this symptom is experienced with mydriasis there may be a severe medical emergency and the patient needs to be evaluated by an emergency physician immediately.

What is the Treatment for Mydriasis?

Mydriasis is first treated by finding and treating the underlying condition that caused it. Cases of mydriasis that are caused by head trauma or brain injury are left alone until the swelling in the brain is relieved. Mydriasis is actually a good tool for doctor’s to check for response to treatment.

When Mydriasis is caused by chemical or drug exposure, the substance that caused the condition is stopped and the eyes should return to normal after the effects wear off. When Mydriasis is induced by use of medications to dilate the pupil for examination, the doctor will recommend that the patient stay out of direct sunlight or wear sunglasses to protect the inner parts of the eyes.

There are medications that can treat Mydriasis when there is no exact underlying condition that needs to be treated first. These include:

  • Phenylephrine drops
  • Pilocarpine
  • Steroids
  • Atropine
  • Scopolamine
  • Atropine/Scopolamine combination


In some cases, surgery may be required if the Mydriasis is unresponsive to mediations used for the condition. In any cases of treatment, protective eyewear needs to be used during the entire time of treatment to protect the inside of the eyes from sun damage.


Mydriasis is most often a temporary condition and those who have it do not suffer any complications if eye protection is worn. This condition usually goes away on its own once the substance that is causing it is removed from the body. When induced by an eye doctor for examination, the procedure is completely safe and the patient only feels mild discomfort but usually no pain.

The only pain that is felt with Mydriasis is during exposure to bright light. Headaches that accompany Mydriasis could be a sign of brain injury, trauma or tumor and needs to be checked by a physician. Also, when Mydriasis is only on one side and one pupil is larger than the other could signal an acute brain injury and need to be evaluated immediately.

If you or someone you know suffers from dilated pupils, always take care to protect the eyes from direct sunlight and see a physician as soon as possible.

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