What is mydriasis and why does it occur?

What is mydriasis and why does it occur: When we are in places with little or low light, the eye reacts involuntarily , producing a distension of the radial muscles of the iris. This is known as mydriasis , which is also called pupillary dilation or dilation of the pupil .

With this response, the body activates a form of night vision, graduating or adjusting the amount of light that enters. Although in human beings the visual acuity does not intensify as much as it happens in some animals, such as the cat and the owl What is mydriasis and why does it occur?

This is not the only reason why such a phenomenon occurs. What is mydriasis and why does it occur? Mydriasis can also be due to a variety of causes , including mood (fear), injury, illness, and drug reactions, among others.

What is mydriasis?

The pupil is that black circle in the center of the eye ; more exactly, the iris. Its function is to regulate how much light enters the retina. Depending on this, the pupil can expand or contract. It is estimated that the size varies between 2 and 8 millimeters.

Being more exposed to light, contraction or miosis occurs . And on the contrary, when it expands, the opposite phenomenon takes place, which is called What is mydriasis and why does it occur? mydriasis . This enables some degree of night vision, although in humans it is rather limited.

Its etymology is not entirely clear. In Latin, the term “mydriasis” was already used to refer to pupillary dilation. It is thought, on the other hand, that the word could be related to anvil , in ancient Greek.

What is known is that two groups of muscles are involved  in this process , which are the ones that control the size of the pupil. These are the circumferential sphincter and the dilator muscle of the iris.

The most common reason for pupillary dilation is due to being in an environment with low or low light. The same thing usually happens when you wake up in the morning, because after spending time with your eyes closed or in the dark, it cWhat is mydriasis and why does it occur? an take us a while to adapt to daylight again.

Emotion, enthusiasm, excitement and even sexual What is mydriasis and why does it occur? desire are evidenced in pupil dilation. This is a good sign to know if someone likes you.

Causes of mydriasis

In the aforementioned situations (low light, fear, emotion, excitement), pupillary dilation is present for a relatively short time. But there are several reasons why mydriasis would be prolonged , becoming a possible sign that something else is happening.

Vision test

Sometimes the pupil must be dilated in order to examine what is inside the eye , including the condition of the retina and optic nerve. This is important for diagnosing, preventing, or even treating certain eye conditions.

In this case, drugs such as atropine are used . The effect can last for a few hours; in some people, up to a whole day. But then it goes away naturally.

Wounds and injuries to the eye

Some eye injuries could damage the muscles or nerves related toWhat is mydriasis and why does it occur? the control of the iris and the pupil, causing what is known as traumatic mydriasis , which is accompanied by other symptoms, such as pain, photophobia , discomfort when reading or focusing for a long time.

It usually occurs in only one eye, which is the one that has been injured. Meanwhile, the other dilates and contracts normally What is mydriasis and why does it occur?.

Head or brain trauma

Mydriasis also occurs in people who have had traumatic brain injuries  that exert pressure, affecting the functioning of the ocular system. It may be unilateral or bilateral.

According to research , a long-standing bilateral mydriasis, with absence of light reflection in the pupil after severe traumatic brain injury, is considered a sign of irreversible damage.


What is mydriasis and why does it occur? Various medications can alter the functioning of the pupils, triggering mydriasis. These include the following:

  • Atropine.
  • Anticholinergic derivatives of the above, such as ipratropium.
  • Antidepressants with serotonergic effects .
  • Scopolamine.
  • Amphetamines.
  • Muscle relaxants.
  • Antihistamines.

Consumption of drugs

What is mydriasis and why does it occur? According to studies on the subject, drugs such as cocaine, methamphetamine, ecstasy and LSD have an effect on serotonin receptors . Therefore, they also affect the functioning of the eye muscles.

Other causes

What is mydriasis and why does it occur? High levels of oxytocin, as well as exposure to some plant species that contain scopolamine and atropine (belladonna or Jimson’s wort), can trigger mild to moderate mydriasis.

Diseases associated with mydriasis

What is mydriasis and why does it occur? A mydriasis in only one eye, which becomes permanent or occurs frequently and is accompanied by other symptoms, such as headache, blurred or double vision, loss of vision or sensitivity, is a possible sign of some pathologies. The most relevant are the following:

  • Migraine: According to research , migraines can cause temporary unilateral mydriasis, due to hyperactivity of the sympathetic system or hypoactivity of the parasympathetic system.
  • Neuropathy: when the third cranial nerve (common oculomotor nerve) is affected, the pupil can remain fixed in a dilated state, due to paralysis of the iris sphincter.
  • Trigeminal injury: This nerve is linked to the control of certain parts of the face. When affected, for example by trauma, in addition to paralysis, pain and other symptoms, there is mydriasis.
  • Claude-Bernard-Horner syndrome : In this syndrome, one pupil appears constricted or there is a difference in the size of the two pupils (anisocoria).
  • Ocular pathologies: if the condition is not neurological, then the mydriasis could be caused by an injury to the eye, by glaucoma or by ischemia of the iris.
  • A brain tumor or edema: as well as a cerebrovascular accident.

Different types of mydriasis

What is mydriasis and why does it occur? Most cases of pupillary dilation are not serious; although it may be associated with some diseases. In general, there are different types of mydriasis:

  • Physiological: it is the normal pupillary response to little or low light.
  • Bilateral: As its name indicates, it occurs in both eyes. It can be a pupillary reaction to drug or alcohol use, food poisoning, or diseases such as meningitis.
  • Unilateral benign: it is characterized by an asymmetry in which a single pupil is dilated and the other is not. If its appearance is occasional, it may be associated with migraines or a problem with the parasympathetic system .
  • Fixed unilateral: it also occurs in only one eye. If it becomes fixed or permanent, it is a sign of possible brain injuries.
  • Areactive: refers to the fact that the pupils dilate, but do not contract when receiving intense light. It occurs when there is brain damage, cardiorespiratory arrest or coma.

Treatment of mydriasis

In most cases, pupillaryWhat is mydriasis and why does it occur? dilation goes away on its own after a short time. If necessary, the treatment of mydriasis will depend on the cause and will be established after the corresponding evaluation by the specialist.

Surgical intervention is not usually required, except when there is third cranial nerve palsy. And this is only recommended if there is no evidence of improvement within 6 months.

The same applies to angle-closure glaucoma. In this situation, the intervention consists of a laser iridotomy, which is considered a quick, simple surgical procedure with less risk than iridectomy.

When in doubt, consult if mydriasis persists

What is mydriasis and why does it occur? Mydriasis is not a very common condition and is rarely dangerous. People prone to migraines have a higher risk of pupillary dilation, although the episodes are infrequent (two or three times in a month) and last a few hours.

What is mydriasis and why does it occur? Now, if in addition to dilation other signs are present, such as blurred vision, eye pain or headache, you should go to the doctor to rule out any eye injury or pathology that affects the brain. The specialist will carry out the corresponding tests.

Aperture can be measured with an  . In this way, how the pupil responds to light is evaluated . If any treatment is required, the doctor will indicate What is mydriasis and why does it occur?What is mydriasis and why does it occur? it.

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