Presbyopia is a gene in near vision that appears a bit after the age of 40, and is caused by the gradual reduction of the ability of the eye to focus in close vision (mechanisms of the human eye for near vision are explained on the page dedicated to the)accommodation the lens). Installation of presbyopia at midlife is inevitable, and translates into a growing difficulty to read small print in near vision, making it necessary to use an optical aid (glasses, magnifiersMultifocal lenses, etc). Presbyopia is changing over 15 years ago; the first glasses are prescribed to 45 years (see the page dedicated to the) prevalence of presbyopia, and the correction increases and stabilizes around 55/60 years old then.
Mechanism of presbyopia
Presbyopia translated the gradual reduction of the power of accommodation of the eye (amplitude of accommodation), which is directly related to the reduction of the elasticity of the lens and its capsule. The lens is a lens that is natural, located in the anterior segment of the eye, which allows to make the update to have a clear vision of close objects. It is the equivalent of a 'autofocus' for a camera. In a subject Emmetropic (net without correction in the distance seeing subject) become presbyopic, near vision as one used for playback is increasingly blurred, while distance vision remains normal.
The following image details the optical mechanisms that allow to understand the mechanism of presbyopia.
Functional consequence of presbyopia
Presbyopia results in the appearance of a growing gene closely reading or the identification of fine details within a pattern observed at close range (i.e. less than 1 meter). The status of distance vision, the absence or the presence of optical default (myopia or hyperopia) determine the type and extent of symptoms of presbyopia.
-At a Emmetropic (Good vision from afar), presbyopia results in the appearance of a near-vision discomfort, especially for small print, and when ambient light is low. Over the years, the vision of the near continues to deteriorate, and requires an optical correction towards 43 years, which will be renewed and increasingly strong as the accommodation decreases.
-In a subject short-sighted Having become longsighted, the removal of the correction from afar can allow a closer vision to be sharper. The comfortable reading distance without glasses depends on the degree of myopia (the higher the myopia, the closer the distance is). But the removal of the glasses is accompanied by a blurred vision from afar (remember that myopia corresponds to a net vision effortlessly close, but blurred by far without correction). To see net from afar, the myopic shoes his glasses and to see closely, he removes them (the adaptation of progressive lenses allows to keep the same pair of glasses for these different distances). To learn more about the relationship between myopia and presbyopia: see: myopia and presbyopia
-In a subject farsighted become presbyopic, installation of presbyopia is particularly "bad living." At the hypermetropia moderate (less than + 3 D of correction for distance vision), the effect of the reduction in the accommodative power of presbyopia appears earlier: it makes more difficult the efforts that the farsighted had used to do to offset the effects of hyperopia, and improve distance vision. That is why that the far-sighted became farsighted subject feels so an overall reduction in the quality of his vision: not only the near vision decreases, but the vision from afar, which was good or even excellent, degrades too quickly to quarantine. The optical prescription for near vision is then accompanied by a prescription for distance vision (progressive glasses, or the addition is expressed in) diopters). For more information: see the page on the peculiarities of the association hyperopia and presbyopia
Signs of presbyopia
Presbyopia is probably the sign related to aging whose age of installation is the most consistent: it truly is around 43 years that the effects of the loss of the amplitude of accommodation are beginning to be felt, especially for the emmetropes.
Symptoms begin with an appearance of a difficulty to see net of the small print, in particular when the ambient light is low. It may be a new difficulty to read a label on bottle of wine at the restaurant: intrinsic difficulties like that labels have an often colorful background, and employed calligraphy and sometimes in addition to the generally Dim light, normal.
In most of the case, the beginner presbyter feels the need to do a 'effort' to readwhereas previously the focus of almost was "unconscious" and easily.
Instinctively,. the presbyter back his reading plan (for example, 35 to 50 cm), extending the arms, less soliciting his accommodation. Finally, after some time, this type of strategy fails and it becomes necessary to wear a correction for near vision at least for the reading of texts printed. Indeed, reading on tablet or smartphone can be accomplished without optical aid initially; Unlike the label of a bottle of digital screens contrast is high, and especially it is possible to increase the size of the characters to read them more easily.
Short-sighted becoming presbyopic often tend to remove their glasses from afar to see better close (the nearsighted eye blur by far because it is as "set" to see net, at a distance which is even lower than the myopia is strong). This strategy works poorly among wearers, because it is not practical to remove and replace the lenses over the day punctually. Presbyopic short-sighted have generally less difficult to read when they wear their glasses by far in correction by lenses; This is related to the fact that the objects seen through myopic glasses are certainly smaller, but especially appear as being located further away (so soliciting under the development closely and the convergence of the eyes).
As mentioned above, the hypermetropia experience symptoms of presbyopia of acutely, because the accommodation is often used to hone their distance vision (which can occaseionner a recurrent visual fatigue). The hyperopic eye is an eye too short; the incident rays from "Afar" converge at the back of the retina; the image that is formed in terms of the retina is blurred because insufficiently focused. The accommodation, which is the increase of the power (by bulge) of the lens, is used in the farsighted to improve distance vision. This works well in children and young adults because the amplitude of accommodation is important and allows to correct some diopters of farsightedness without too much effort. But at the age of presbyopia. the vision from afar seems to deteriorate in parallel with near vision: this reflects the exhaustion of the accommodative mechanism of vision by far (see compensation hyperopia and presbyopia). Presbyopia is sometimes responsible for the return of the prescription of corrective lenses for vision far beyond those for vision closely ('progressive' lenses), including at low hypermetropes who wore glasses of 'rest' intermittently during childhood, for the school tasks including.
Can we delay presbyopia?
Presbyopia is not related to the reduction of the strength of the ciliary muscle (which is put in play with the accommodative effort to see up close), but to the loss of elasticity of the crystalline lens (understand the) mechanism of accommodation Allows to apprehend the one of the presbyopia). The ciliary muscle contracts, the zonule (the ligament of the Crystalline lens) relaxes, but the Crystalline lens itself does not bomb anymore enough (a Crystalline lens young bomb when releasing the tension of the Zonule) . If presbyopia was related to a reduction in the intensity of ciliary muscular contraction, treatments could be imagined to "restrengthen" the eye to maintain accommodation. Unfortunately, during the installation of the presbyopia, the ciliary muscle contracts well, but the Crystalline lens, which tends to increase in volume and lose its flexibility during existence, no longer bomb enough. Currently, no effective therapy is known to make its elasticity lost at Crystalline lens. Therapeutic tests were carried out using the femtosecond laser to make incisions directly in the Crystalline lens, in the hope of restoring any flexibility to it. The results of these techniques do not appear to be sufficient, and the risk of induced cataracts is real. The answer to the question of a possible prevention of presbyopia is therefore negative, and it is also remarkable to see how consistently the presbyopia settles in time in humans (between 42 and 44 years most often).
Principles of the correction of presbyopia
We can't stop the presbyopia, but fortunately, it is quite possible to correct.
Presbyopia can be corrected logically by the port (or adding to the correction by far) convex lens designed to compensate for the lack of focal power (originally from the defocus) to form a clear picture of the objects located at a distance of about 35 cm.
The value of this addition (expressed in) diopters (: ex: + 1 D) is a function of the degree of presbyopia, him even in relation to age. The first glasses prescribed early presbyopia have usually close power of ' addition + 0.75 D. This addition must be increased over time, and reaches + 2.75 D or + 3.00 D in the late fifties.
The importance of presbyopia is quantified by the number of diopters for near vision (between + 0.75 D and + 3 D by no + 0.25 D). In a subject Emmetropic and far-sighted, lenses + 3 D reflects the fact that presbyopia is "total": the vergence provided by glass of + 3D equipped eye vision is sharp at 1/3 = 0, 33 meters, approximately 33 cm. A myopic-3D sees net effortlessly to 33 cm, and it is for this that in by wearing its correction by far, this type of medium nearsighted (between-2 and-3.50 D about) can be read closely by removing his glasses. Yet it is far-sighted, because when he's wearing his correction by far (glasses or lenses), it is no more able to see net nearly a no myopic presbyter.
Optical correction adds close initially ametropic topic (nearsighted or farsighted, and/or astigmatism) so the lens correction which is necessary to correct distance vision. It is performed only on the lower part of the corrector glass lunette (half-moon, progressive lens or bifocals) because the drive tends to look down to the bottom to read his book.