Thursday, January 21, 2010

Physiological defect: Presbyopia.(3) Solutions

Enlace a entrada en español.


  • In the case of myopic people with fewer than -3.00 diopter, compensate presbiopia for a long time simply by removing their eyeglasses in order to read. I remind you that the myopic people focus the image in front of the retina, and that the presbyopic people focus the image behind it. Here I explain it better:

    A young myopic that looks far away through her well corrected eyeglasses, focuses the image on the retina; when she has to look something at close with her glasses, the image goes behind the retina, but her accommodation helps her to focus it on the retina again.
    Thus, in the case of presbiopic myopic without eyeglasses, when she looks far away, she sees blurry because the image is focused in front of the retina; but when she looks something at close, the image goes back, bringing it closer to the retina, and consequently, seeing it more clear.

  • The most common method to compensate the presbyopia is through some OPHTALMIC LENSES in eyeglasses; but depending on the use of the eyeglasses, that is, for what activities or distances you will use them, there are different options. Therefore, whenever you receive a visual examination, you must give the optician some information about your needs in near vision (reading, computer, work distance, etc.), so that, she can recommend you the best option in your case.

    Basically the compensation of presbyopia requires the use of convergence or positive lenses (magnifying glasses) for near vision tasks. From here, we play with different variations.

    Progressive lenses: These are the lenses that are most used, although their adaptation is not easy.

    These lenses allow to have clear vision at all distances (from far away to up close) simply by changing the inclination of the eyes or the head; this is something everyone wishes, specifically if the focus distance has to be constantly changed. But these lenses have got two main inconvenients:

    1. they have a vertical “corridor” of central vision that is more narrow than any other lens (due to optic aberrations in both side of the lens); therefore, when we look out of the corridor, the vision is blurred;
    2. and we have to learn to use these lenses, since we have to learn by which areas of the lens we must look at, depending on the distance where we want to see. Once achieved, the effort will be worthwhile.

    Bifocals lenses: These are less used lenses (before the progressive lenses appeared in the market, these were the best option), but for those people with certain binocular problems, whose daily activities require clear distance-near vision, and when they do not accept the progressive lenses, the bifocal ones may be their only option.

    The top of the lens has a graduation which is required to see clear distance objects; and the bottom part has a “segment” that has the addition required to see clear close up objects AT A CERTAIN DISTANCE.

    Advantage: The adaptation is much easier than the progressive ones, because the bifocal lenses do not have so many aberrations, nor so many areas with different graduation for different near distances; simply by performing two eye movements, we can focus far away or close.

    Disadventages: Precisely the last advantage is a disadvantage as well: the bifocal lenses allow clear vision in only two specific distances (they lose the intermediate distance) and they are less aesthetic, “they betray the age”.

    Trifocal lenses: They are lenses basically obsolete.

    Reading lenses: These are the best option for those that see very well far away, so they need nothing for distance. They are lenses with only one graduation, so, they are prefect in order to perform prolonged near tasks.

    In this option there is not any specific size of eyeglasses. But if you need to alternate your vision in different distances, mainly far away-close up, it is more recommended to use narrow glasses, known as “half moon reading glasses” which let you look over them when you look far away, without performing strange head positions or movements.
    These small glasses have one inconvenient: as the bifocals lenses, they also “betray the age”, although nowadays there are many models of eyeglasses which do not have the typical shape of “half moon”, and that can be perfectly fine.

    Reading glasses work well with contact lenses.

    Ready-made reading glasses: They are standard reading glasses, that is, they are the same for everyone. THIS IS NOT AN OPTION. These glasses are only good for pulling through at a specific given time, nothing else. But I will write about these glasses in other post.

  • PROGRESSIVE CONTACT LENSES: The adaptation is even more complicated than with progressive lenses in eyeglasses, but the success rate is increasing. The complication resides in that the brain is responsible for learning what RING of the contacts has to look for, depending on whether it wants to focus far o close. It is not easy for everybody, and not everyone achieves a successful behavior.

    With this kind of contacts, a type of adaptation is usually performed called MONOVISION: one eye has better vision for looking far away and the other eye has better vision for looking at near.

  • Besides these solutions, I have mentioned above, during last years the cataract surgeries have promoted the research of the INTRAOCULAR LENSES with accommodative capacity: the ophthalmologist removes the crystalline lens and puts a new lens inside of the eye, with the suitable graduation.
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