What is traumatic iridoplegia

What is miosis?

The Miosis it is only this decrease in the diameter of the pupils, either due to physiological causes (natural reflexes of seeing in front of the light), pathological (due to diseases, clinical conditions, syndromes and other changes in health) or pharmacological (reactions of sight) which are caused by exposure to chemical substances such as drugs and related drugs). The term miosis comes from the Greek muinwhich means "close your eyes".

Miosis (a word that does not include Mitosis, meiosis child Myositis) is in direct contrast to mydriasis, which is discussed in more detail in another article in which the pupils are dilated, and differs from anisocoria, in which one pupil is more dilated than the other.

In a broader sense, miosis is also applied to the excessive narrowing of the pupils, which in extreme cases can reach a diameter of less than two millimeters.

The eye and photomotor reflection

As recorded in every book of human anatomy, the pupil is what is commonly referred to as a "disc" or "nugget" of black color, located in the center of the eye between the lens and the anterior chamber (which contains one) of the aqueous humor , the clear liquid that separates the cornea from the iris, see the illustration at the beginning of this section). The function of this circular hole is modest but extremely important: regulate the amount of light that passes through the eye.

This regulating mechanism gives the retina the light it needs so that you have a correct view of the object being observed. In the human eye - the eye of other animals is not dealt with here for reasons of space - the pupil follows this rule: the more light there is in the environment, the smaller its diameter (i.e. the pupil contracts) and the less light is received, the smaller it is larger is its diameter (ie the pupil dilates).

In other words, the size of this "hole" depends on how intense the lighting is. Something very similar to the function of the pupil happens by analogy with the camera where the aperture has different f-stops that adjust according to the amount of light you want to pass through (in this section, see image below). In fact, the scientific principle of narrowing and dilation remains the same and saves the differences.

With the foregoing in mind, miosis is produced by what is known in physiology as the photomotor reflex. In this reflex, the light causes the pupil's sphincter to contract or relax, which explains why the pupil "closes" or "opens" in the presence or absence of a light stimulus received by the retina. .

Then the rest of the work is done by the optic nerve, from which the visual information is transmitted to the brain, which is its ultimate destination.

Then the pupils in both eyes are evenly closed by the action of cranial nerve III, which receives direct commands from the brain. At that moment, two photomotor effects are created: the direct one, which is in the same eye in which the light is captured, and the consensual one, which is in the eye opposite the one who had that direct photomotor effect.

When the optic nerve is healthy, both pupils contract, but when it is injured this will not happen because there is no light stimulus that can be transmitted.

Not only is the photomotor reflex ideal for watching miosis occur, but it has a purpose beyond mere observation.

In medicine, the examination of this reflex is used to diagnose certain illnesses or health problems that may affect the patient, especially if the patient cannot tell how he or she is acting for some important reason (e.g. mental disability, unconsciousness, etc.) .). It is also useful to know if you have lost sight in any of the eyes.

Causes of Miosis

As I said, miosis has three causes: physiological, pathological and pharmacological. While the physiological ones strictly follow the reactions of sight, the pathological ones generally come from diseases.

The pharmacological ones, on the other hand, are directly linked to the use of chemical substances, with drugs and even drugs playing a larger role. The following sections further discuss these factors that cause pupil constriction.

Physiological causes

For physiological reasons, those in which the students contract naturally are classified without changes in the health of the person or the functions of the organs related to the sense of sight.

Consequently, the miosis here clearly indicates that the individual is healthy and therefore there is no reason to be concerned about his welfare state. In the human body, miosis, which is not pathological, can be due to the following elements:

  • Age: In fact, it is called senile miosis because aging reduces the size of the pupil over the years, and it follows that older people, on average, have smaller pupils and therefore their eyesight is much lower than that of younger people. This also explains why many older adults are difficult to see at night.
  • Excess light: This can happen because the view is exposed to a high amount of light, which can come from very (or too) bright environments, such as sunny places in the open field and various buildings in cities that have lights all over the place. The retina therefore receives a lot of light stimulation and the pupils contract frequently to get better and better visibility of the surroundings.

Pathological causes

[Preliminary note for the reader: Any health problem that is causing the miosis should be discussed with the doctor for an accurate diagnosis that will allow for appropriate treatment. Therefore, what is contained in this section is for informational purposes only and should in no way be construed as specialist guide to discovering or alleviating any of these conditions. Visiting an expert is strongly recommended to clarify any doubts in this regard.]

Due to pathological causes, those in which the pupils contract in a provoked manner are classified, that is, they do not respond normally to the pulse of light, but because of a syndrome, clinical condition, disease or disease that affects the health or quality of life of the People. Some of the diseases that affect patients due to abnormal constriction of the pupils are:

  • Microcoria: is the name that has congenital miosis. This is a hereditary disease in which the baby is born with small pupils and without the pupil's sphincter, which is why this part of the eye cannot stretch. Microcoria is also linked to other vision problems such as juvenile glaucoma.
  • Horner syndrome: Miosis occurs because there is an injury to certain facial nerves (sympathetic nerves on the face). There may be muscle injuries in the eyes. In addition, this syndrome can be congenital, that is, it can appear from the patient's birth.
  • Argyll-Robertson Syndrome, also known as reflex iridoplegia.
  • Various hereditary diseases, some of them hereditary.
  • Bleeding inside the skull (also called bleeding in the stem-brain bridge) Pons, Varolium bridge O Ring projection).
  • Uveitis: Inflammation of the uvea (the layer of the eye that is responsible for the bloody irrigation of the retina) which can have various causes. There is a clear narrowing of the students here.
  • Cluster headache accompanied by drooping eyelid.
  • Nerve paralysis in one of the sympathetic nerves.

Pharmacological causes

Pharmacological causes are classified as those in which the pupils contract due to the action of chemicals that, consumed by the person, pass through the bloodstream and thus ultimately change the normal function of the pupillary sphincter.

In this aspect, miosis does not necessarily mean that the person is sick, but that the person has tested a substance that has affected their nervous system.

The above is basic knowledge that the police usually have on hand when trying to find out if, for example, the driver of a car has used illegal substances that have caused them to speed excessively. The flashlight eyepiece test is very useful in discovering the culprit just because he sees the constriction of the pupils in his eyes.

The chemicals that cause miosis to narrow the pupils include:

  • Opioid drugs: Examples of this are drugs that have analgesic and anesthetic effects (that is, that calm the pain), such as fentanyl, heroin, morphine, methadone.
  • Cholinergic drugs that stimulate the parasympathetic nervous system, such as acetylcholine.
  • Nicotine in any of its presentations and products, be it in cigarettes, chewing gum, or tobacco leaves (which are used to chew).
  • Camptothecin and some other cancer drugs used in chemotherapy.
  • Haloperidol, Risperdal, and drugs other than antipsychotics and neuroleptics (drugs that primarily treat psychosis, alongside other psychiatric disorders).
  • Imidazoline-based drugs, such as clonidine and naphazoline.


  1. Alemañy Martorell, Jaime and Villar Valdés, Rosendo (1983). Ophthalmology (4th edition, 2003). Havana, Cuba Editorial Medical Sciences.
  2. Ball, Jane W., Stewart, Rosalin W. et al (1987). Mosby's Guide to Physical Exam (7th edition, 2011). Missouri, United States. Mosby.
  3. Congenital miosis [Article online] (2016). San Francisco, United States. American Academy of Ophthalmology. Retrieved January 27, 2017, from: aao.org.
  4. Galán Terraza, Alicia (2011, October 14). The photomotor reflex or why the pupil is closed with light [Article online]. Barcelona, ​​Spain. DYTO Eye Clinic. Retrieved January 27, 2017 from: clinicadyto.com.
  5. Kozarsky, Alan (2016, December 21). What is eye miosis? [Article online] Atlanta, United States. WebMD, LLC. Retrieved January 27, 2017, from webmd.com.
  6. Llano Sosa, David S. and Ramírez Velázquez, Reyvis (2009). Ophthalmic terminology. Bilingual glossary Spanish-English / English-Spanish. Havana, Cuba Editorial Medical Sciences.
  7. Millodot, Michel (1986). Dictionary of Optometry and Visual Science (7th edition, 2014). Amsterdam, Netherlands. Elsevier Health Sciences.
  8. Traumatic mydriasis and miosis [Article online] (2016). San Francisco, United States. American Academy of Ophthalmology. Retrieved January 27, 2017, from: aao.org.