Wednesday, February 25, 2009

Refractive disorders: Myopia (2). Kinds and Factors

Enlace a entrada en español

Different kinds of myopia, which one is yours?

CONGENITAL MYOPIA – It is the myopia of the newborn, mainly those who are born with low weight or are premature babies (“20% of preterm infantas suffer from myopia; as the greater the degree of prematurity is, the level of myopia is higher, reaching 8 to 10 diopters, according to a study by the International Center of Optometry –IOC-”). This myopia persists in the childhood, and remains when the school stage begins (6 years) (2%). It is not usually a low myopia degree and as time goes by, it will increase.


Myopia caused in the youth – This myopia appears between 6 and 12 years old. This percentage increases from 2% at 6 years old to 20% at 20 years old :-O

Usually, it is a low myopia degree (-0.50 diopters or more), mainly if it appears from the age of 12. Normally these myopia will decrease, turning into emmetropia or even hyperopia in adult age.

Myopia of young adult person – This appears between 20 and 40 years old, as well in low degree, but the prevalence increases up to 30%. As the previous one, this myopia will decrease too, turning into emmetropia or even hyperopia at the last stage of the life.

These two last myopias are low and they are usually due to environmental factors (*), as for instance, excessive near-point visual activities or bad conditions of visual hygiene at school/home/work/… These myopias may increase but they will not do it in high degree.

Myopia at the beginning of the maturity – This appears from the age of 40-50 years old and the prevalence increases gradually at the last years of the life. It is the case of those people that when reaching this age, say they do not have any problem at reading a book unlike their friends of the same age. The reason is that this myopia is usually associated with the changes of the density of the ocular structures, typical of the age. When the density changes, the “refractive index” of these structures changes too, and therefore, as well the power of the whole ocular optic system. This kind of myopia is directly associated with the development and the progression of Cataracts. At a first stage myopia may be compensated through eyeglasses, but if the cataract keeps on going the natural course, the last step is surgery.

NOCTURNAL MYOPIA or NIGHT MYOPIA – It becomes apparent just in low light conditions when we want to look at a distant object. It may affect any people (myopic or not), mainly young people, and even if daytime vision is normal. Its value is usually -0.50 diopter, but may reach even -1.00 diopter in extreme cases, and in myopic people may increase the refraction at the same proportion.

This myopia is due to a disruption in the “accommodation process” (briefly, accommodation is the ability of our eyes in order to focus on different distances – I will write more about it in another later post -). When we look at distant object, accommodation does not work, so, it must be totally relaxed; but for that to happen, we need to see all details of the object so a clear image is achieved in the retina. If the light decreases, it is more complicated to see those details; so accommodation, in an attempt to clear the image, starts to work and focuses on an intermediate distance that it is not really where the object is.
Besides this, when the light decreases, the pupil is dilated (increases its size) in order to allow more light to get into the eye. But when the pupil diameter increases, the “depth of focus(**) decreases and spherical aberrations of the visual system increase.
Because of all the previous, this nocturnal myopia is more noticed when we drive at night, since we need to see well, and we feel some insecurity behind the wheel because of this reduced vision that this myopia causes.

A little myopia of just -0.50 diopter is equivalent to losing around 15% of visual acuity, which is enough to make nocturnal driving difficult.

In the case of a emmetropic person (without refraction on daytime conditions) or a myopic person, both may solve the problem by wearing eyeglasses that correct this myopia in those conditions; in the case of the low hyperopic person, sometimes, she may remove her eyeglasses in order to drive at night and feel better; but in many occasions, the performance of some visual exercises focused to use that accommodation correctly, avoids the use of those “extra-glasses”.

Here you have an interesting article, that I encourage you to read (though it is written in Spanish) “The drivers visual acuity is reduced 70% at night”

PSEUDOMYOPIA – During many years, the progression of myopia has been researched, but nowadays there are few specific findings; as the factors are not very clear, neither what happens in the eye when the myopia increases.

Some people that say when a myopic person performs near work for a long time and a prolonged accommodation is made, a strain is carried out in the vitreous camera (9), that causes an increase of the axial length of the eye; this is why the image is not focused on the retina, but in front of it and this causes an increase of myopia.

In the other hand, others say this progression is due to a “ciliary spasm” because of a prolonged accommodation: the tonicity of ciliary muscle increases until that in a certain moment, this muscle can not get totally relaxed in order to allow seeing at distance. It is like when you take a very heavy object, and keep it strongly with your arms during some specific time, although that involves a great effort. When you let the object go, you are not capable of stretching your arms, they are rigid in that position; it is what you call “the muscle is spasmodic”. That is why, when myopia appears, or initially increases, the patient usually complains that distance vision is blurred after working at near a prolonged time, but after some minutes the vision is clear. This is called “pseudo-myopia” or “false myopia”; a real myopia does not allow that after any short time, distance vision improves. When this pseudo-myopia begins to become evident, the degree is only -0.50 to -1 diopter, this is the reason why if we can not avoid it, the best thing would be for you to receive visual therapy in order to relax that muscle, and teach your eyes to work at near without the effort that favors the increase of myopia. Thereby, we avoid that myopia to be irreversible and lastly, that an increase of the eye axial length is made, leaving myopia already structured.

MAGNA or PATHOLOGICAL MYOPIA – They are very high myopias (more than -10.00 diopters). These people should encourage periodic eye checks, since they may suffer complications as cataracts, glaucoma, retinal detachment (sudden and abundant floaters) or vitreous detachment, or macula pathologies.

(*) Factors that cause myopia

If you have always asked what you could do in order for you and your children to avoid having myopia, or even to avoid that the myopia that you already have, increases, I am so sorry tell you it is very difficult to achieve it :-(
This is because the factors that cause the myopia and its progression are nowadays a very controversial subject and have carried out much research, without finding out exactly which is the main factor: whether the hereditary character, the environment factors or both of them.

We are more worried about the progression of myopia than of any another refractive error, because, as I wrote before, although only a 2% of the children that start school suffer from it, this percentage increases very much during school stage, and also its progression is very fast during the later years. In the case that the myopia appears in adult stage, the progression is slower, but unlike other ametropias, once the myopia begins, the progression is sure. This might mean that exists an environment influence in this progression.

(**) DEPTH OF FOCUS: Until now, I had explained that in order to see an object clear, its image had to be just in the retina (fovea), but that is not totally correct. A small MARGIN is accepted, in front and behind of the retina, where the image of the object that we see, can be located, without losing clarity; this is called “depth of focus”.

Refractive disorders: Myopia (1). Vision and Symptoms
Refractive disorders: Myopia (3). Solutions
Why does a myopic person’s vision improve when she squeezes her eye lids to a narrow opening?
Refractive visual Disorders. Some clarifications.
Some numbers...