What is miosis?

The Miosis (Due to diseases, clinical conditions, syndromes and other alterations in the health) or pharmacological (eye reactions), are all that decrease of the diameter of the pupils, either by physiological causes (natural reflexes of the sight before the light), Which have been produced by the action of chemical substances, such as drugs and related drugs). The term miosis comes from the Greek Muein , Which means"close your eyes").

Miosis (a word that should not be confused with mitosis , Meiosis neither Myositis ) Is in direct opposition to mydriasis-which is discussed in more detail in a separate article-in which the pupils dilate, and is different from anisocoria, in which one pupil is more dilated than the other.

In fact, in a broader sense the miosis also applies to the excessive constriction of the pupils that can have a diameter smaller than two millimeters, in very extreme cases.

The eye and the photomotor reflex

As is evident in any book of human anatomy, the pupil is what is commonly referred to as that"disk"or"nugget"of black color that is located in the center of the eye, between the lens and the anterior chamber (the one containing The aqueous humor, the transparent 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: to regulate the amount of light passing through the view.

Through this mechanism of regulation is that the retina receives the light that needs to be able to have a correct vision of the observed object. In the human eye - that of other animals will not be treated here for reasons of space - the pupil follows this rule for its operation: the more light in the environment, the smaller its diameter (ie the pupil contracts) , And the less light is received, the greater its diameter (i.e., 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, in which the diaphragm has different levels of aperture that adjust according to the amount of light that is wanted to let pass (in this section, see image below). In fact, the scientific principle of constriction and dilation remains the same, saving differences.

Taking into account the above, the miosis is produced by what in physiology is known as the photomotor reflex. In this reflex, light causes the pupil sphincter to contract or relax, which explains why the pupil"closes"or"opens"in the presence or absence of a light stimulus that is received by the retina .

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

The pupils are then closed equally in both eyes by the action of the cranial nerve III, which receives direct orders from the brain for this to happen. At this moment two photomotor effects occur: the direct, which is in the same eye where the light is captured, and the consensual, which is in the eye opposite to that had that direct photomotor effect.

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

The photomotor reflex is not only suitable for observing how miosis occurs, but has a purpose that goes beyond mere observation.

In medicine, examination of this reflex serves to diagnose certain diseases or health problems that may affect the patient, especially if the patient can not tell how he or she is for any reason (eg, psychic disability, unconscious state, etc.). It is also useful to know if you have lost sight in one of the eyes.

Causes of meiosis

Miosis, as has been said, has three causes: physiological, pathological and pharmacological. While the physiological ones strictly obey the reactions of the sight, the pathological ones usually come from diseases.

Pharmacological drugs, on the other hand, are directly linked to the use of chemical substances, with a greater preponderance of drugs and even of drugs. The paragraphs that follow further delve into these factors that cause constriction of the pupils.

Physiological Causes

Physiological causes are those in which the pupils contract naturally, without alterations in the health of the person or in the functions inherent to the organs that correspond to the sense of sight.

Consequently, the miosis here indicates, without a doubt, that the individual is healthy and that therefore there is no reason to be alarmed by his state of well-being. In the human body, non-pathological miosis may be due to elements such as:

• Age: Is in fact called senile miosis, as aging causes the size of the pupil to diminish over the years. It follows that the elderly, on average, have the smallest pupils and therefore their eyesight receives a much lower amount of light than in the younger people. This also explains why many older adults find it difficult to see at night.
• Excess light: Can occur because the view is exposed to a high amount of light, which can come from very (or too) illuminated environments, such as sunny places in the open and various buildings in cities with lamps everywhere. The retina therefore receives enough light stimulus and the pupils end up contracting frequently to obtain a greater and better visibility of the surroundings.

Pathological causes

[ Preliminary note for the reader: Any health problem that is causing the miosis should be consulted with the physician in order to obtain an accurate diagnosis that can give him an appropriate treatment. Therefore, what is in this section only has an informative role and in no way should be taken as a specialized guide to detect or alleviate any of these illnesses. A visit to an expert is strongly recommended to clarify doubts.]

Pathological causes are those in which the pupils contract in an induced manner, that is, they do not react normally to the stimulus of light but does so due to some syndrome, clinical condition, disease or condition that compromises health or Quality of life of the person. Some of the ills that afflict patients due to abnormal constriction of the pupils are:

• Microcoria: Is the name that has the congenital myosis. This is a genetic disease in which the baby is born with the small pupils and without the sphincter of the pupil, reason why this part of the eye can not dilate. Microcoria is also associated with other vision problems, such as juvenile glaucoma.
• Horner's Syndrome: The miosis occurs because there is damage to certain nerves of the face (sympathetic facial nerves). There may be muscle injury to the eyes. In addition, this syndrome may be congenital, that is, that may occur from the birth of the patient.
• Argyll-Robertson syndrome, also known as reflex iridoplegia.
• Miscellaneous genetic disorders, some hereditary.
• Hemorrhage within the skull (hemorrhage in the trunk-encephalic bridge, also called Pons , pons or Annular protuberance ).
• Uveitis: Inflammation of the uvea (layer of the eye that is responsible for blood supply to the retina) that can have several causes. Here is an obvious constriction of the pupils.
• Headache in clusters accompanied by a drooping eyelid.
• Neural paralysis in one of the sympathetic nerves.

Pharmacological causes

Pharmacological causes are those in which the pupils are contracted due to the action of chemicals that, consumed by the person, pass through the bloodstream and therefore end up altering the normal functioning of the pupillary sphincter.

In this regard, miosis does not necessarily indicate that the person is ill, but the individual did test for a substance that impacted his nervous system .

This is a fundamental knowledge that cops often have on hand when they want to find out, for example, if the driver of a car took illicit substances that caused him to drive at speeding. The ocular test with the flashlight is very useful to discover the offender by just seeing the constriction of the pupils in his eyes.

Among the chemicals that cause constriction of the pupils by miosis are:

• Opioid drugs: Examples are drugs that have analgesic and anesthetic (ie, calming pain) effects, such as fentanyl, heroin, morphine, methadone, among others.
• Cholinergic drugs that stimulate the parasympathetic nervous system, such as acetylcholine.
• Nicotine in any of its presentations and products, whether in cigarettes, chewing gum or tobacco leaves (used for chewing).
• Camptothecin and some other anticancer drugs that are used in chemotherapies.
• Haloperidol, risperdal and other drugs used as antipsychotics and neuroleptics (drugs that mostly treat psychosis, among other psychiatric conditions).
• Imidazoline-based drugs such as clonidine and naphazoline.

References

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