Refractive errors – Myopia, hyperopia, astigmatism and presbyopia
Ametropia refers to vision problems that can be corrected with glasses, contact lenses, or refractive surgery. There are four main types: myopia ( nearsightedness), hyperopia (farsightedness) , astigmatism , and presbyopia . These conditions respectively describe an eye that is too long, too short, asymmetrical, or has lost its ability to focus. Each results in different types of vision problems: blurred vision, eye strain, and difficulty seeing clearly at distances and/or nearsightedness.
What is ametropia?
Ametropia is an anomaly in the focusing of the image on the retina. This means that the light signals captured by the eye and successively deflected by the cornea and the lens, are projected in front of or behind the retina.
To correct them, a converging or diverging corrective lens must be placed in front of the eye. This can be a spectacle lens, a contact lens, or refractive surgery (by reshaping the cornea, for example).
Ametropia is to be distinguished from the emmetropic eye, for which images naturally focus directly on the retina.
Emmetropic eye – Perfect eye
As a reminder, the functioning of the eye can be likened to a darkroom, like the one used in photographic cameras, with the pupil being the diaphragm and the retina the film.
When an eye is normal, the image of an object is formed exactly on the retina. Neither too far in front, nor too far back.
This corresponds to an emmetropic eye. In all other cases, the eye is said to be ametropic.
The main refractive errors
Myopia – If the image is formed in front of the retina (eye too long). Vision will be blurry at a distance, clear at close range.
Hyperopia – If the image is formed behind the retina (eye too short). Vision will be good at a distance (not in cases of high hyperopia), blurry up close (or eye strain, called asthenopia).
Astigmatism – If, instead of being perfectly spherical, the eye is slightly oval, like a rugby ball rather than a football, the image will be constantly slightly blurry.
Presbyopia – With age, the lack of accommodation (focusing, which is made possible by the lens) impairs near vision.
Concepts of visual acuity and optical correction
Visual acuity and optical correction are often confused.
Visual acuity: This is expressed in tenths and is measured, with or without correction, by the size of the letters read during the examination. Ten tenths, for example, written 10/10, corresponds to a maximum angle of resolution of one minute of arc. It is most often not included on a glasses prescription!
Optical correction: This corresponds to the measurement of your "ametropia" and gives the value of the lenses to wear to correct this visual disorder.
Myopia – Eye too long / strong
Myopia is characterized by an image forming in front of the retina because the eye's focusing power is too great for its length. It generally affects people with large eyes.
Distance vision is blurry, but near vision is possible. Myopia correction aims to restore clear distance vision and can be achieved through glasses, contact lenses, or surgery.
Wearing glasses is recommended primarily for distance vision to restore clarity.
Myopia and overcorrection
Slight overcorrection is often appreciated for its sharpness, but can cause headaches and discomfort in the long run. You are likely overcorrected if you are nearsighted and:
Feel the need to take off your glasses at the end of the day to read.
Do you have headaches in the evening after working?
Your vision is blurry in the distance after work.
Hyperopia – Eye too short / straining
Hyperopia, or farsightedness, is characterized by an image forming behind the retina because the eye's focusing power is too weak for its length. It generally affects people with small eyes.
An effort of accommodation is possible to focus light rays onto the retina. The closer the object being observed, the greater this effort. This is why hyperopia is characterized primarily by difficulty seeing close objects.
Symptoms may be perceived as eye strain or genuine blurriness and the perception of needing to "focus," which corresponds to accommodation.
The correction of hyperopia aims to reduce the accommodative effort responsible for the discomfort and can be done by wearing glasses, contact lenses, or by surgical intervention.
Resting glasses
In the case of low power, the correction is referred to as "resting".
Wearing it is recommended primarily for close vision and prolonged work.
The goal is to reduce accommodation efforts from the beginning of the day, in order to limit the symptoms of visual fatigue (asthenopia) at the end of the day.
Wear your glasses first thing in the morning when you start working.
Full optical correction
In cases of severe hyperopia in children, it is important to correct all hyperopia with glasses to be worn permanently.
The goal is to completely relax the accommodation in order to help him develop his vision properly.
Total optical correction is the treatment for amblyopia.
Astigmatism – Asymmetrical/blurry eye
Astigmatism is characterized by an eye that is slightly oval in shape instead of being perfectly spherical.
Roughly speaking, the eye has a shape closer to that of a rugby ball than a football.
From near and far, the vision is blurry and the letters blend together.
Note that unlike myopia and hyperopia, which are opposites and therefore cannot be present in the same eye, astigmatism can be added to either one (myopia + astigmatism or hyperopia + astigmatism).
Astigmatism is defined by two values: its power (its magnitude) and its axis, noted as follows (-2.00 at 180°) being read as astigmatism of 2 diopters at 180°.
The concept of regular and irregular astigmatism
Regular astigmatism (direct or indirect) is the most common type. In this case, the corneal curvature is uniform. It can be corrected with glasses, contact lenses, or refractive surgery.
Irregular astigmatism is rarer but also more difficult to correct. It occurs in cases of diseases such as keratoconus. In this case, the cornea is irregular, and only rigid contact lenses can effectively correct this condition if it is severe.
Presbyopia – An eye that no longer focuses
Presbyopia, independent of the three previous refractive errors, develops with age. It is linked to the loss of accommodation which most often occurs around age 45.
In the case of myopia, it often occurs later, but earlier in the case of hyperopia. It can be corrected with glasses, contact lenses, or surgery.
Progressive lenses
Glasses should only be chosen for near vision if the problem only occurs at close range. For patients with other vision problems such as myopia or astigmatism, progressive lenses will be recommended.
They allow both near and far vision.
Wearing contact lenses is possible, but difficult. Adaptation is often lengthy, and the result is not guaranteed because these lenses, known as multifocal lenses, can cause discomfort that is not tolerated by all patients. Other correction methods, such as "switching" lenses, are also available.
Surgery is a possibility. You should discuss it with your ophthalmologist.
Correction of retractile dysfunction
Glasses should only be chosen for near vision if the problem only occurs at close range. For patients with other vision problems such as myopia or astigmatism, progressive lenses will be recommended.
They allow both near and far vision.
Wearing contact lenses is possible, but difficult. Adaptation is often lengthy, and the result is not guaranteed because these lenses, known as multifocal lenses, can cause discomfort that is not tolerated by all patients. Other correction methods, such as "switching" lenses, are also available.
Surgery is a possibility. You should discuss it with your ophthalmologist.
Frequently Asked Questions
Do I need to wear my prescription glasses all the time?
Your corrective device must be worn in the cases that constitute the discomfort for which you requested it. Except in rare cases, described in the following chapter, it is not detrimental not to wear a corrective device, apart from the fact that you will not be relieved of the problem it corrects.
Myopia: You are slightly nearsighted, for example, and it doesn't bother you in certain activities, such as at work. In this case, it's not harmful not to wear your glasses in these situations. You could, for example, only wear them when driving or at the movies.
Hyperopia: If you have been prescribed a pair of glasses for eye strain, you should only wear your glasses in circumstances that cause discomfort, in this case, when your eyes are strained by close-up vision.
Astigmatism: same.
Presbyopia: same applies. Just because you've been prescribed progressive lenses doesn't mean you have to wear them "all the time." Simply wear them in situations that cause you discomfort.
In what situations is it essential to wear corrective glasses?
Certain situations require the MANDATORY wearing of glasses, including:
Children under 6/8 years old: In this situation, glasses are often prescribed for amblyopia and/or strabismus to correct them. Wearing them as prescribed by your ophthalmologist is absolutely essential, generally from morning to night without interruption. Talk to your ophthalmologist about it.
Strabismus: Some cases of strabismus require permanent optical correction to stabilize. Talk to your ophthalmologist.
Progressive myopia and childhood: Some studies show that under-correction and/or not wearing appropriate optical correction during childhood is a risk factor for the increase of this myopia.
Other cases: Sun protection, protection during work with a risk of splashing…
How do I read my glasses prescription?
On your prescription, the sphere is the value indicated before the parentheses. If written as a negative value, it corresponds to the power of myopia; if written as a positive value, it corresponds to the degree of hyperopia.
The values in parentheses correspond to astigmatism. The axis is noted in degrees (°), its power preceded by a negative sign (or positive sign depending on the nomenclature used).
The last number, preceded by the word Add, corresponds to the addition, the value of the correction to be made to correct presbyopia.
Examples:
- 3.00 = Myopia of 3 diopters
+ 3.00 = Hyperopia of 3 diopters
(-3.00 to 180°) = Astigmatism of 3 diopters with an axis of 180°.
Add +3.00 = Addition of 3 diopters.
Reading a glasses "formula":
-4.00 (-2.00 at 180) Add +2.5 is read as follows: Myopia of 4 diopters, astigmatism of 2 diopters at 180°, addition of 2.5 diopters.
Should I request protection against blue light?
-
Blue light is the subject of many misconceptions: toxicity, eye strain, myopia, etc. To demystify it, an article is dedicated to it here.