The Pupils they are the black sphere that is in the central part of the eye color zone, the iris and its function is to dilate or contract to control the amount of light that enters the eye.

Occasionally, the pupils may present problems that prevent them from contracting or dilating, when they remain dilated, this condition is known as mydriasis o dilated pupils.


The pupils, like the diaphragm of a camera, open or expand automatically when the person is in an area with low light. However, there are also certain conditions in which the eye reacts to stimuli that dilate the pupil and that they have nothing to do with the absence of lighting, being able to be physiological, pharmacological or pathological.

En Advanced Ophthalmology Area We explain what mydriasis is, causes and treatment.

What is mydriasis?

Mydriasis is the permanence of dilated pupils when they should be contracted, even in an illuminated environment, quite the opposite of myosis or punctate pupils.

This occurs when the iris muscles are unable to close the pupil.

To dilate the pupil, the radial iris muscles and it is controlled through the sympathetic system, while to contract the pupil the circular iris muscles o pupil sphincter, through the parasympathetic system.

The pupil is the space that remains in the center of the iris and is responsible for controlling the amount of light that enters the eyeball and reaches the retina, essential to obtain a good vision (for example, to avoid glare). The pupils, normally, dilate in low light conditions, so that more amount of light to the eye y the images can form correctly on the retina. Under normal conditions, pupillary dynamics, size and speed of dilation of both eyes, is similar and symmetrical.

Types of mydriasis

Below we explain the most frequent types of mydriasis found in ophthalmology:

Physiological mydriasis

Pupil dilation (mydriasis) in an environment with a shortage of light is a normal process in healthy eyes. Physiological mydriasis is the process by which both pupils dilate symmetrically in both eyes in low light environments.

Episodic benign unilateral mydriasis

It is identified by a clear asymmetry between both pupils or anisocoria, being one dilated while the other remains in its normal size. This is a rare type of mydriasis that occurs especially in patients with migraines, parasympathetic deficits or sympathetic hyperactivity in the iris. It is very frequent in cases of major stress as in students at the time of final exams, or work stress.

Arreactive symmetric mydriasis

The pupils remain dilated in an environment with high luminosity or even when we stimulate with a light directly on the eye (photomotor examination). One of the main causes of symmetric arreactive mydriasis are brain damage caused, in many cases, by cardiorespiratory arrest or by severe trauma where we can make a first assessment of the brain state of the injured.

Asymmetric Arreactive Mydriasis

In this case we have a pupil that normally reacts to light stimuli while in the other eye there is a dilated, mydriatic pupil that does not respond to light. This is mainly due to traumatic causes (a blow to the head that injured the pupillary sphincter muscle), brain tumors that affect only one hemisphere or after an inflammatory eye disease (uveitis) or acute intraocular hypertension (angle-closure glaucoma) that left the pupil attached to crystalline in mydriasis (posterior synechiae).

Bilateral mydriasis

It is usually caused by a process of intoxication caused by the intake of methanol, drugs, opiates or toxins that cause cholinergic activity. Some other causes that can be associated with bilateral mydriasis are hypothermia and lesions in the middle area of ​​the cerebral protuberance.

Causes of the most common dilated pupils

Now that we know what mydriasis is and how it occurs, it is important to know its main causes:

Ambient light

The most normal cause of dilated pupils is the absence of light in certain places and situations. The pupil dilates as an automatic reaction to see correctly.

Emotional state

Few people notice that the pupils dilate during an intense emotional state. Nervous crises, fear and enthusiasm are some emotions that can cause mydriasis suddenly.

Certain drugs

The use of anticholinergic drugs blocks the transmission of some chemicals to the eyes and dilates the pupils. Similarly, the ophthalmologist uses drugs (cycloplegic eye drops, atropine, among others), to dilate the pupils and to explore the fundus or before certain surgeries such as cataracts or the retinal detachment.

Drugs or alcohol

Consumption of alcoholic beverages and narcotic substances can cause dilated pupils for several hours. The drugs that produce mydriasis are cocaine, ecstasy, THC, methamphetamines and hallucinogens.

The drugs affect serotonin receptors and can increase or decrease the production of this transmitter, causing dilated pupils as a result.

mydriasis definition


When mydriasis has no apparent cause, a neurological or brain condition that is permanently stimulating pupil dilation may be suspected.

Some non-ophthalmic diseases can significantly affect the layers of the eye and their proper functioning.

Among the diseases that can cause mydriasis as a side effect, we find:

  • Cerebral haemorrhage.
  • Stroke
  • Tumor in the eye or in the brain
  • Trigeminal nerve injury.
  • Trauma or collateral damage due to glaucoma.


If the dilated pupils present as an abnormal reaction that remains despite being in a very bright place it is important to go immediately to the ophthalmologist.

Before a case of permanent mydriasis, the most important thing is to detect the cause that is generating this abnormal and involuntary reaction, since it can be dangerous conditions such as a stroke.

The treatment of mydriasis will depend on the cause that is affecting the ocular structure.

While the person has dilated pupils, it is important to wear sunglasses, not expose to sunlight and do not drive.

From a purely ophthalmological point of view, in cases of severe mydriasis, we can perform specific treatments to close the pupil. You can perform a surgery to selectively close the pupillary diameter, adjusting the amount of light entering the eye, an intervention we call Pupiloplasty.

En Advanced Ophthalmology Area We have the best human and technological team to attend to visual health. Do not hesitate to contact us, we are happy to assist you!

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Dr. Carlos Verges Roger, PhD. Ophthalmologist and Medical Director
Ophthalmologist surgeon specializing in ocular surface, corneal transplant, refractive surgery, cataract surgery and dry eye and works as Medical Director in Advanced Ophthalmological Area. His training highlights his stay at Harvard University and his Doctoral Thesis on the mechanisms of lacrimal secretion, which will be basic to understand the pathology of Dry Eye Syndrome and which represent the National Research Award (1983). He continues his training by completing a Corneal Fellow in the Department of Ophthalmology at the Harvard Medical School, Massachusetts Eye and Ear Infirmary (1985) and in Ocular Pathology at the Moorfield Eye Hospital in London and the Hôpital de Dieu de Paris (1986). Throughout his career he has received prestigious awards such as the INQUIFARMA National Award. In 1991 he won the oppositions of Professors Full Professor and went on to serve as Head of Service of the Department of Ophthalmology at Hospital del Mar.
Dr. Carlos Verges Roger, PhD. Ophthalmologist and Medical Director

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