Fri. Aug 12th, 2022
    Myopia (Near-sightedness) | It affects distance vision and Different Types of Myopia

    Myopia

    Myopia is an eye condition in a person who cannot see objects at a far distance. It is corrected perfectly thanks to thinned glasses, if the myopia is medium or strong. This is due to the too long eye bulges the cornea or the lens, preventing the rays of light from reaching the retina.

    Nearsightedness is often detected during schooling, because a child who sees very badly from a distance cannot read what is written on the board; it usually occurs between the ages of 7 and 12. In adults, symptoms such as blurred vision, headaches, sensitivity to light, problems driving particularly at night, because reading traffic signs is made difficult or even impossible, are indicative of a problem. myopia. In 80% of cases, astigmatism is associated with myopia.


    What is this vision disorder due to?

    The inheritance factor is important, the risk of being prone to myopia is higher if one of the two parents has it. Often several children of the same family are affected. Nearsightedness can be present from childhood and become even more evident in adolescence, as our eyes grow larger during periods of growth.

    However, several factors can favor it such as:

    • A deficiency in vitamins D, E and A
    • Prolonged exposure to screens
    • Learning to read too early mobilizes the center of the eye called the macula
    • More light, for example too much sun without glasses or children sleeping with a night light
    • A lack of natural light, causing dopamine to secrete, this hormone prevents too much growth of the eye

    Currently, the explosion of myopia is linked to our lifestyles and the time spent in front of screens. But a few reflexes and attitudes to adopt can prevent this vision disorder.

    The causes of myopia are therefore both genetic and environmental.


    How to prevent this visual defect?

    Above all, you must be careful not to focus your attention for a long time and therefore your eyes on objects that are too close: avoid reading too close, or standing too close to your tablet, phone or television. It is also recommended to work in a sufficiently bright environment and to rest your eyes regularly. You can also focus on outdoor activities, perfect for stimulating distance vision.

    Read also: Best Visible Colour by the Human Eye

    It is particularly important to pay attention to the blue light present on all our screens, it is very harmful to the eyes. To guard against this, a filter can help. It has been observed that more and more people are developing near-sightedness linked to the daily and sometimes abusive use of screens.


    Myopia Treatment

    Nearsightedness can be corrected with glasses, contact lenses or refractive surgery. It depends on the degree of your myopia. You may need to wear glasses or contact lenses all the time or only when you need very clear distance vision, such as when driving, looking at the blackboard or watching a movie. If you are nearsighted, the first number (“ball”) on your eyeglass or contact lens prescription is The recipe will start with a minus sign (-). The higher the number, the nearsighted you are. Refractive surgery can reduce or even eliminate the need for glasses or contacts. The most common procedure is done with a laser.

    How do I checkthe degree of my disorder?

    You need to go to an ophthalmologist who will measure your eye power in diopters. It is considered that myopia is low if less than 3 diopters, average between 3 and 6 and high above 6 diopters.

    If you have high myopia, you should pay particular attention to the legal threshold of visual acuity recommended for driving. Surgery to correct your high myopia may allow you to correct it (including beyond – 10 diopters).

    The degree of myopia is indicated in “diopter” (is a unit of measurement of the optical power of a lens). We can say, rather roughly, that myopia is low between -1 and -3 diopters (about one third of myopic), average between -3 and -6 diopters (about two thirds of myopic), high beyond – 6 (5% of myopic), very rare above -20 diopters.

    How to correct myopia?

    The ophthalmologist will recommend divergent glasses or soft or rigid contact lenses to correct these overly converging eyes. These are thicker at the edges to reflect the image on the retina.

    However, if you cannot stand glasses and lenses, you can request refractive surgery, that is to say laser surgery, possible once myopia has stabilized. We talk about stabilized myopia when the patient has not had to change his corrections for at least two years.

    If no contraindication, such as the formation of a keratoconus (when the cornea takes a conical shape – more information on kerotoconus here), is observed, then your doctor may suggest an intervention (more information on the progress of the preoperative assessment here).

    Thanks to this laser procedure, patients regain flawless visual acuity and rediscover sharper, more vibrant images. This operation leads to a gain in quality of life for the patients who can benefit from it.

    The decision to go into surgery for myopia must be made by the patient. The ophthalmic surgeon will provide a surgical response to the patient only if the preoperative assessment is favorable and if the patient’s expectations are in line with the expected results after the operation.

    Can myopia be treated?

    Even though it is not a disease, myopia can be treated very effectively with laser treatment or an implant (very high myopia).


    Myopia AND Presbyopia: AFTER 40 YEARS OF AGE, DO MYOPIA ALSO BECOME PRESBYTES?

    Presbyopia is linked to the aging of the lens, regardless of the visual defect. Short-sighted people seeing well up close can, in some cases (depending on the age and level of myopia), do without their glasses to continue to see clearly up close … But they must quickly put them back on to see clearly from afar. Progressive lenses therefore remain the best solution to avoid these acrobatics and to see clearly at all distances after 40-45 years, without changing or removing and then putting back your pair of glasses.


    Different Types of Myopia

    We can distinguish several types of myopia, depending on the mechanism involved, the type of development, or the existence of associated pathologies.
    Remember that the focal power of the eye depends on the refractive elements of the eye: cornea and lens. The focal power (inversely proportional to the focal distance) of these elements therefore depends on the power of the cornea, the distance between the cornea and the lens, and the power of the lens. Below are the classification of myopia according to its mechanism:

    Refractive

    Excessive axial length (axial length is the distance between the top of the cornea and the fovea which is the retinal seat of fine vision) is an obvious cause of myopia. However, some myopic eyes have an axial length identical to that of emmetropic eyes, or even less!
    There are thus significant variations between human eyes for a parameter such as corneal power (between 40 and 47D for healthy corneas), as well as for the depth of the anterior chamber, (and probably the power of the lens).

    Therefore, it is difficult to systematically incriminate an isolated excess of axial length to explain the occurrence of myopia; we speak of refractive myopia when it is more related to a kind of “disharmony” between axial length and focal power of the refractive elements of the eye. The length of the eye is not excessive in itself (close to 24 mm) compared to a reference population composed of non-myopic eyes; but it is excessive in relation to the optical power of the eye. This type of myopia usually does not exceed 3 or 4 diopters; beyond that, the probability of axile myopia is high.

    Secondary

    A myopic refractive state for which a single, specific cause (e.g., drug, corneal disease or systemic clinical syndrome) can be identified that is not a recognized population risk factor for myopia development.

    Corneal myopia

    It is a special type of refractive near-sightedness. The increase in corneal power leads to an increase in the focal power of the eye: the light rays from a distant source are too refracted by the cornea (they converge in front of the retina), while the axial length n is not statistically above average. This mechanism is at least partially implicated in the myopia associated with keratoconus, because this condition causes a central corneal arch, and therefore a marked increase in corneal vergence. The risk factors for keratoconus are genetic and environmental. According to the author of this site, repeated rubbing of the eyes is the direct and necessary cause of keratoconus.

    Here is an example of corneal myopia, for which the measurement of the axial length in optical biometry is normal, while the keratometry is high.

    Axile

    It is myopia, the main mechanism of which is the elongation of the eyeball. The average axial length of the “emmetropic” human eye is close to 23 mm (2.3 cm).

    A study found that the axial length of hyperopic eyes was on average 22.62 ± 0.76 mm), and was therefore logically shorter than that of myopic eyes, which was measured at 25.16 ± 1.23 mm (axile and refractive myopia combined) . These figures show that the axial length values ​​differ on average, but with regard to the standard deviation of the respective distributions, that they overlap significantly between the two groups (myopic vs hyperopic). In this study, myopic eyes had a slightly more powerful cornea than hyperopic eyes, but the difference measured was not significant (Llorente et al. Myopic versus hyperopic eyes: axial length, corneal shape and optical aberrations. Journal of Vision, 2004; 4, 288-298).

    It is difficult to distinguish certain axial myopia from “refractive” myopia, because the variability of the axial length in the general population is important. However, most myopia greater than 4 D are axile, and most eyes with an axial length greater than 25.5 mm are affected by axial myopia (Hendicott and Lam. Myopic crescent, refractive error and axial length in Chinese eyes. Clin Exp Optom, 1991; 74: 168-174).

    Among axile myopia, we can distinguish high progressive myopia, which is characterized by progressive distension of the posterior pole and the appearance of associated pathologies linked to it. These myopias are often qualified as pathological.

    Pathological

    The most easily recognizable near-sightedness is certainly pathological myopia, also called “progressive” myopia, progressive myopia, degenerative myopia. It is characterized by an earlier onset (early childhood, before adolescence) and progresses rapidly. It is characterized by excessive distension of the posterior pole of the eye, with significant lengthening of the eye, and is accompanied by chorioretinal degeneration. This degeneration is linked to the fact that the little extensible chorioretinal tissue undergoes a significant increase in the internal surface of the eye (linked to scleral expansion). This tissue thins, tears and atrophies depending on the degree of myopia. Fortunately, the prevalence of this type of myopia seems to be declining. Myopia can exceed 10 D (up to 30 D in severe forms, sometimes called “malignant myopia”). The axial length reached, or even exceeds 30 mm.

    The optical correction of these myopia uses rigid contact lenses. In glasses, the thickness of the concave lenses (even with a high index) is such that special equipment sometimes has to be designed.

    These myopia are also associated with a risk of loss of best corrected visual acuity. In other words, the retinal lesions observed in patients with severe myopia can lead to functional loss that cannot be corrected by lenses or glasses. The refractive surgery of this type of myopia, when it is possible, is generally based on the placement of an intra ocular implant (phakic implant) or the excision of the lens and its replacement by an implant whose power is calculated to greatly reduce the myopia.

    An interesting phenomenon is often noted during lens surgery on eyes with high myopia; by focusing on the cornea and then moving the microscope a few centimeters away from the eye slightly, it is possible to observe the details of the fundus (retina) of the very nearsighted person. This is because the retina of the strong myopic is conjugated with the plane of the punctum remotum, which is located at a distance (expressed in meters) equal to the inverse of the myopic correction (expressed in diopters). For example, the plane of the punctum remotum is located at approximately 7 cm for a myopic of -15 D (1/15 = 0.07 approximately).

    Rarer forms

    There are forms of myopia much less frequent than refractive and / or axile myopia.

    Transient

    It is associated with diabetes, certain pathological conditions and pregnancy, and is based on probable chemical modifications of the refractive media (acquiring humor), responsible for an increase in their refractive index, more or less associated with accommodative spams.

    Index

    It is a form of refractive myopia: it is linked to nuclear cataracts: the nucleus of the lens bulges out and its index increases. Index myopia can appear in an emmetrope, in a former hyperopia, or worsen a pre-existing myopia (see the page dedicated to the aberrometric study of index myopia)

    Accommodative (or by “accommodative spasm”):

    It is caused by a prolonged contraction of the ciliary muscle, secondary to sustained efforts in near vision (fine craftsmanship, very prolonged school or computer work, etc.). The instillation of “cycloplegic” eye drops (paralyzing the ciliary muscle) corrects this myopia.

    Linked to a subluxation of the lens

    A condition in which the lens shifts from its normal position but is still within the pupillary area. Acquired lens subluxation is often the result of trauma. Lens subluxation can also occur spontaneously.

    This exceptional shape surprises by the “normal” dimensions of the eyeball with respect to the degree of myopia, which is linked to a relaxation of the suspensory ligament of the lens (zonule). The lens spontaneously adopts a more rounded shape, which increases its refractive power and causes excessive convergence of the refracted rays (see the description of a case of myopia by subluxation of the lens).

    Diseases | List of Diseases: dermatological, cardiovascular, respiratory, cancer, eye, genetic, infectious, mental illness, rare


    Information: Cleverly Smart is not a substitute for a doctor. Always consult a doctor to treat your health condition.


    Sources: PinterPandai, Essilor, All About Vision, National Center for Biotechnology Information (NCBI), Medlineplus

    Photo credit: Essilor