Mydriasis – Dilated Pupil: Causes, Symptoms and Treatment



The pupil of the eye opens (dilates) and closes with exposure to light and darkness. The pupil opens with darkness to allow more light to enter and shrinks with a bright light to shade the inner eye.

Mydriasis is an increase in the diameter or dilatation of the pupil of the eye. The muscles are normally responsible for opening and closing the colored part of the eye, known as the iris, and when this action is interrupted, the pupils take longer to shrink.

Actually it is believed that it is a malfunction of the nervous system that does not send the correct nerve signals to the muscles.

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

It is usually found when a doctor lights a light in a person’s eyes and the pupils remain dilated instead of shrinking in response to light. The condition can be classified in millimeters, between 1 mm and 9 mm.

A major complication of mydriasis is blindness due to overexposure to sunlight. The constriction of the pupil protects the eyes from the burning rays of the sun.

This is the reason why optical doctors give patients sunglasses after dilating their eyes with medication.


Mydriasis occurs when the nerves supplying the muscles to the iris (the ring of color in the eye that surrounds the pupil) make them hyperactive.

This is most often caused by the use of stimulants such as; amphetamines, caffeine and other stimulating substances. The hormone released during childbirth and lactation.

Oxytocin can also stimulate the pupil to dilate. The optician can inculcate certain medications directly into the eye to dilate the pupil in order to deeply examine the structure of the eyes.

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

Injuries to the head and neck that damage the cranial nerves that supply the eyes can also cause mydriasis. Mydriasis could be a sign of brain inflammation and is caused by pressure on the eye nerves.

It also occurs after brain death when the muscles and nerves that control the pupil stop working.

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


People with mydriasis will complain about sensitivity to light. Because the pupil does not contract to block excess light, exposure to light can actually be painful. Blurred vision may occur because the pupil needs to shrink in the light to clear the vision.

Most people with mydriasis do not complain of any symptoms. Some complain of headaches, but it is not yet clear if this is a symptom of mydriasis or another coexisting condition. On the other hand, mydriasis could be a symptom of a type of migraine.

The most prominent symptom of mydriasis is dilation of the pupil without responding to the light shining in the eye. Normally, light will make the pupil narrow.

The pupils also become “non-reactive” to the application or removal of the light and remain in a fixed position.

A sign of mydriasis is if one or both eyelids are “drooping”, also known as ptosis. If this symptom is experienced with mydriasis there can be a serious medical emergency and the patient should be evaluated by an emergency doctor immediately.

Treatment of Mydriasis

The treatment is first to find and treat the underlying condition that caused it. Cases that are caused by head injury or brain injury are left alone until the swelling in the brain is relieved.

Mydriasis is actually a good tool for the doctor to verify the response to treatment.

When mydriasis is caused by exposure to chemicals or drugs, the substance that caused the condition stops and the eyes must return to normal after the effects disappear.

When mydriasis is induced by the 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:

  • Drops of phenylephrine.
  • Pilocarpina.
  • Steroids
  • Atropine.
  • Scopolamine.
  • Atropine / scopolamine combination.

In some cases, surgery may be required if mydriasis does not respond to the mediations used for the condition. In any case of treatment, goggles should be worn throughout the treatment to protect the inside of the eyes from damage caused by the sun.


It is usually a temporary condition and those who suffer from it do not suffer any complications if they wear eye protection.

This condition usually goes away on its own once the substance that causes it is removed from the body. When an ophthalmologist induces it for examination, the procedure is completely safe and the patient only feels a slight discomfort but generally does not feel pain.

The only pain felt with mydriasis is during exposure to bright light. The headaches that accompany mydriasis could be a sign of brain injury, trauma or tumor and should be checked by a doctor.

In addition, when mydriasis is only on one side and one pupil is larger than the other, it could indicate acute brain injury and should be evaluated immediately.

If you or someone you know suffers from dilated pupils, always be careful to protect your eyes from direct sunlight and consult a doctor as soon as possible.