Miosis – Constriction of the Pupil: Causes, Symptoms and Treatment


Medical definition:

Miosis is a medical term that refers to the constriction of the pupil. It relates to the closure of the pupil by the action of small muscles in the eye.

Miosis reduces the amount of light that enters the eye, which is necessary for daily life as light levels change in the environment.

Various medicinal and recreational drugs can also induce it.

Miosis should not be confused with meiosis or mitosis, both forms of cell division.

Causes of miosis

The local actions that cause miosis are the relaxation and contraction of two muscles in the eye’s iris. The iris is the colored part of the eye and is anchored just below the protective cornea.

The dilator muscle of the iris and the sphincter muscle of the iris work against each other to adjust the size of the pupil, which is simply the hole in the center of the iris that allows the passage of light.


The light will pass through the lens, which focuses the image and continues towards the back of the eye. The retina is a highly specialized tissue that contains cells that can detect light; these cells pass the signal to the optic nerve and brain.

Some of the specialized cells in the eye are called photosensitive ganglion cells and are responsible for detecting environmental levels of light in the eye.

The ganglion signals to the brain and the autonomic nervous system automatically and continuously adjust the iris.

If too much light hits the retina, miosis will occur, and the pupil will contract. For miosis to happen, the dilator muscle of the iris must relax while the sphincter muscle of the iris acquires at the same time.

This causes the pupil in the iris to shrink, which effectively limits the amount of light that enters the eye. If the pupil closes too much, the ganglion cells will detect this, send the information to the brain, and the muscles will dilate the pupil.

This is called mydriasis, and it is the opposite of miosis. The nerves that connect to the iris dilator and sphincter muscles circle different brain parts.

The ultimate causes of miosis can be variable. In most healthy and sober people, miosis is simply a reaction to the amount of light received by the eyes.

Because miosis is caused by a complex pathway that involves many nerve cells, chemicals, and different parts of the brain, miosis can also be induced by a variety of medications, such as opioids, nicotine, antipsychotic drugs, and certain eye drops and a wide variety of other substances.

Changes in available light caused by miosis and myopia due to drug use may contribute to the “altered perception” that drug users claim to experience.

Other times, miosis may be due to various diseases or other degenerations of nervous tissue.

Miosis is not limited to the human eye; it occurs in all animals that have a mechanism to restrict the amount of light that enters the eye. Only two groups of animals have developed eyes like this, vertebrates and cephalopods.

Other animals with eyes, like insects, do not use miosis because they do not regulate the amount of light that enters their eyes in the same way that vertebrates and cephalopods do.

Vertebrates include all organisms with a spinal column, while cephalopods have octopuses and relatives.

Although the structure and mechanisms of the eye are so similar that they undergo identical processes, scientists believe that the eyes emerged through convergent evolution or a similar evolutionary outcome caused by similar environments and environmental pressures.


The constriction of the pupil is a symptom, not a disease. The accompanying symptoms can provide a clue as to what is causing the problem.

If you take opioids, you may also experience:

  • Drowsiness
  • Nausea and vomiting
  • Confusion
  • Delirium
  • Difficult breathing

The symptoms will depend on how much medicine you take and how often you take it. In the longer term, the use of opioids can reduce lung function; signals that can be an addiction to opioids include:

  • Intense cravings for more drugs
  • Needing a higher dose to achieve the desired effect
  • Problems at home, at work, or financial difficulties due to drugs.

Intracerebral hemorrhage can cause severe headaches, nausea, and vomiting. It may be followed by loss of consciousness.

You may also have a drooping eyelid if your miosis is due to Horner’s syndrome. Babies with Horner’s syndrome may have a lighter iris than the other.

Additional symptoms of anterior uveitis include redness, swelling, blurred vision, and sensitivity to light.

Nervous agents can also cause tearing, vomiting, convulsions, etc.

Intoxication by insecticides causes salivation, tearing, excessive urination, defecation, and vomiting.


There is no specific treatment for miosis because it is not a disease. However, it can be a symptom of one. The diagnosis will guide your treatment options.

In the case of an overdose of opioids, emergency personnel may use a medication called naloxone to reverse the life-threatening effects of opioids. If you are addicted, your doctor can help you stop safely.

In some cases, intracerebral hemorrhage may require surgical intervention. The treatment will also include measures to keep your blood pressure under control.

There is no treatment for Horner’s syndrome. It can be improved if the cause can be determined and treated.

Corticosteroids and other topical ointments are typical treatments for anterior uveitis. Additional steps may be necessary if the cause is an underlying disease.

Insecticide poisoning can be treated with a medication called pralidoxime.

When should you seek help?

For unknown reasons, consult your ophthalmologist or a general practitioner if you have miosis for unknown reasons. It’s the only way you’ll get an adequate diagnosis.

An overdose of opioids can be fatal. These symptoms, which may indicate an overdose, require emergency medical attention:

  • Pale or wet face
  • Purple or blue nails
  • Flaccid body
  • Vomiting
  • Heartbeat slowed
  • Slow breathing or difficulty breathing
  • Loss of consciousness.