Accommodation: natural to see close mechanism
Accommodation allows the human eye to see net at different distances: its mechanism involves a contraction of the ciliary muscle, which induces a passive bulge of the lens due to its suspensory ligament (called zonule) is released. It is this bulge (increase in curvature) which allows the eye to reach the required close vision vergence. Presbyopia occurs when the accommodation isn't enough to allow the eye to the development of some sufficiently. Knowing the mechanisms of accommodation allows to better understand the reasons for the installation of presbyopia, and develop relevant strategies of surgical correction.
Why is the accommodation necessary?
A static optical system can be adjusted to form a sharp image from a source located at a certain distance. Form a sharp image is to converge the retinal plane captured light rays initially issued by each point source of the observed object. This is the case of the eye says 'Emmetropic' in vision from afar, which captures parallel rays (or almost) and refracts them (through the cornea and the lens) so that they converge in the plan of the retina. If the observed source gets closer, the rays emitted from the source will reach the eye by forming a divergence angle. If the optical power of the eye remains unchanged, the rays will be refracted as well behind the retina, and will form in the plan of the retina a spot of defocus (the image is then perceived as fuzzy).
Issued by a closely observed source (ex: reading page, cell phone screen), the incident rays present a discrepancy when they reach the eye, but they must always be focused the same focal distance (in terms of the retina). This need to focus requires a larger vergence of the couple 'horny + crystalline' - to compensate for the divergence of the incident rays.
The human cornea is a fabric which can be considered as immovable, and which does not deform for near vision. However, the lens is a lens with flexibility, and elasticity properties: this flexibility allows it (at least before the introduction of presbyopia) deform to increase its vergence, to allow the eye to see net a close target. The lens takes a more rounded form, in order to increase its vergence (more a lens is curved, more she brings together the incident light rays) (1). Accommodation thus provides the necessary to the couple 'horny + lens' optical power increase to form retinal image of close objects: this increase in the vergence is due to a distortion of the lens. When an Emmetropic eye is at rest, it is focused "at Infinity", and sees net (to more than 5 meters) distant objects.
The Optical power of the lens in these conditions is estimated at 19 diopter. "To the point" on an object closer, 10 cm from the cornea, this optical power should increase 10 diopters. When an eye is fine, he can clearly see the object they observe closely, but if he keeps the same accommodation to observe a remote source, it will form an image blurred, similar to one seen by a myopic; It is therefore necessary for the eye of "désaccommoder" quickly when he see again from afar.
Accommodation meets a need similar to that of autofocus cameras: this system allows the photographer to choose the plan where the image will be sharp. However, most of the photographic goals using an automated system (or a manual focus ring) which allows to modulate their optical power by a game of translation some lenses, and not a property of deformation. In the case of accommodation of the eye human, even if the increase in the vergence of the lens is mainly handled by its deformation, a slight shift in terms of the lens to the front can be observed and helps increase the vergence of the couple 'cornea + crystalline' (2). All of the mechanisms put into play during accommodation (and the desaccommodation, that is to say the return to the State not accommodated) is not known in full, but the main steps allowing the lens to bomber (the vergence increase) and then resume its not rosy form are properly identified. The mechanism of accommodation involves three main players: the ciliary muscle, the cristallinienne, and of course the lens zonule. In the animal Kingdom, there are many species that have eyes able to accommodate; the type of accommodation put at stake in human eyes (deformation of crystalline) is usually present in most mammals. However, other species, particularly aquatic or flying, use other mechanisms: movement of the lens, or deformation of the cornea.
The actors of the accommodation
The anatomical elements involved in the human eye accommodation are located in the posterior segment of the eye behind the iris.
The lens is a natural lens of the eye. It consists of different layers of concentric, and surrounded by a transparent capsule, whose thickness is variable, and whose elasticity gives a spherical shape at rest (not subject to the forces of tension of the zonule). If the lens was not subject to any force, he would naturally adopt a form more curved (particularly at the level of its front), close to the one whom he married during accommodation. The lens 'to the rest of accommodation' is actually a crystalline 'in tension', which the front is particularly more flattened than it would be in the absence of this tension. Presbyopia is a result of the loss of the flexibility of the lens, which deforms more enough when the zonule relaxes, and causes problems in near vision from the quarantine.
This fibrillar fabric fits on one side on the lens (around), and the other on the ciliary body, 360 °. Part of the zonular fibers fits on the part of previous Equatorial pre (previous fibers), the other on the pre equtoriale posterior (posterior fibers) part. Finally, there is a contingent of fibres which fit very close to the Equator (Equatorial fibers). The zonule has elastic properties, and its fibers are in tension in the idle state (absence of the ciliary muscle contraction).
The ciliary muscle
It is a special, ring, eye muscle that has contractile properties: it is innervated by so-called fibre parasympatiques, as a result of which he suffered a contraction that projected on the inside and in front. The diameter of the ciliary muscle and the trial ring is reduced: thereby the previous zonular fibers undergo then a loosening. This relaxation of the zonular fibers passes to the anterior capsule, which incorporates a more circular shape and then causes curvature of the lens. This bulge is especially marked at the level of the side front of the lens (2). The lens moves slightly forward.
Theories of accommodation
Various theories of accommodation have occurred over time. The astronomer Kepler who was the first to mention the role of the lens in the process of accommodation (near vision) in 1611: he was joined by Descartes a few decades later, the latter indicating the intervention of a modification of its curvature. The astronomer Scheiner highlights the beneficial role of pupillary contraction (accommodative myosis) for vision closely (increase in depth of field). Today, the Helmholtz theory (1909) is one that allows to better describe the phenomena at the convenience store (3). Other theories exist (ex: theory of Schachar, Tscherning theory), but they are not confirmed by experimental data, and are often used to justify certain techniques for correcting presbyopia as advocated by Schachar sclerale expansion (which incidentally is not very effective).
Hermann Ludwig von Helmholtz (1821/1894) is a prolific German scientist, who became interested in many areas of Physics (acoustics, sound physical), chemistry, and a treatise on physiological optics published in 1867. Helmholtz is the inventor of the ophthalmoscope, small portable instrument that allows to observe the retina through the pupil, and is still used today. Helmholtz stipulates that accommodation is well linked to the deformation of the front of the lens, itself caused by a relaxation of the zonule. It establishes that the lens is, to a State of rest (distance vision), put in power by the zonular fibers. The release of these fibers, due to contraction of the ciliary muscle, allows the lens to a more rounded 'natural' form.
This theory is now invalidated by numerous experimental results and the absence of results observed after correction of presbyopia techniques claiming to (ex: sclerale expansion surgery). This theory assigns to the Equatorial fibers of the lens a different role. Schachar, and unlike the Helmholtz theory, in the absence of accommodation, the pre-and post-zonular fibers would be released, while the Equatorial zonular fibers would be under tension (4). The contraction of the ciliary muscle will cause a layout disinsertion anterior and posterior tension, with capsular flattening peripheral and secondary central bulge (by a mechanism of conservation of curvature). During the ciliary contraction, only Equatorial zonular fibres emit truly. This theory implies that the mechanism of presbyopia is related to the increase in the volume of the lens, that occurs way inevitable by addition of cortical fibers by the previous germ cells. This growth is that the equator of the lens approximates the ciliary muscle, which break the zonule and reduced its start-up voltage (required according to Schachar to the accommodation, unlike Helmholtz theory). Logically, Schachar has proposed a novel technique for correction of presbyopia called sclerale expansion. This is to insert a strip circular in the sclera (wall eye), next to the ciliary muscle, to increase the diameter, and put in power the zonule. In reality, plenty of experimental evidence contradicts the validity of this theory, that today only a few practitioners wishing to justify a technical support any action on the sclera for correcting presbyopia: sclerales strips, cilio-disinsertion tension ring... To date, all the techniques of correction of presbyopia based on the theory of Schachar overall ineffective and are abandoned; some like to ask a tension ring in the anterior segment to "tighten the zonule" was more relatively invasive. The results of the experimental and clinical studies validate today Helmholtz theory: accommodation is linked to a loosening of the zonule of the lens, and the persistence of a sufficient elasticity of the lens capsule. With age, the loss of elasticity of the capsule decreases and reduced the importance of accommodation. Presbyopia occurs when the power of accommodation of the eye no longer allows to close properly, without optical aid (glasses, lenses, magnifiers).
Effect of accommodation on the lens and the eye
The relaxation of the zonule due to contraction of the ciliary body causes a distortion of the lens, where the front became more arched more aspherical (hyperboloide), and moves slightly forward. The thickness of the cristallinien kernel increases slightly (2.5). It is the front of the lens of the eye which deforms the most. For an effort to accommodation of 6 D for a 19-year-old patient, a modification of the anterior radius of 12.8 mm was measured at 7.7 mm, that of the posterior also undergoing a slight reduction (7.1 to 6.5 mm). During the accommodation and the ciliary contraction, the choroid (serving as support to the retina vascular tissue) is pulled forward, which could direct more directly the retinal photoreceptors to the entrance of the eye pupil. Ciliary contraction, slightly reducing the equatorial diameter of the eye, could also cause a slight elongation of it (any increase in the axial length of the eye for near vision!) Finally, the ciliary contraction causes enlargement of the trabecular meshwork, which is a natural drainage of aqueous humor, which tends to lower intra ocular pressure.
Stimulus of accommodation
If the way in which the eye does its change of optical power is currently well understood, it is not the same for the mechanisms that control the eye ' to the point '. Chromatic aberrations are certainly a trigger of the accommodative stimulus, because experiments have shown that it was easier to accommodate on black and white targets (polychromatic, so prone to induce polychromatic aberrations) monochromatic (6). Indeed, "retinal blur" observed closely target the defocus-induced depends on the wavelength, and provides information to the visual system to trigger an accommodative response. This response is relayed by a part of the frontal cortex, a secondary nerve center called nucleus of Edinger-Westphal, which is itself connected to the ciliary ganglion, from which emerge parasympathetic fibers that innervate the ciliary muscle and control its contraction, as well as the iris sphincter (narrows the pupil: myosis). Jointly, there is a simulation of the internal rights engine eye muscles, which align the eyes. In the accommodative effort occurs so a triple action: contraction of the ciliary muscle (release disinsertion and bulge of the lens), contraction of the iris sphincter (myosis), and convergence of the eyes. It is interesting to note that in the absence of any stimulus (ex: field of view neutral or 'empty'), the accommodation is not zero as one might think, but rather of about 1 to 1.50 D. This can also occur at night or in the dark: this may contribute to what is called the phenomenon of "night myopia".
Amplitude of accommodation
It is related to the distance between the furthest point seen sharp when the eye is at rest, and the closest point seen sharp when the eye accommodates maximum. This distance separates the punctum remotum and the punctum proximum of the eye. It is calculated in diopters, as the vergence required for seing clearly each of these points. For example, for a myopic including the punctum point at1 meter and the punctum proximum at 33 cm, the amplitude of accommodation is 1/0.3-1/1 = 2 diopters. The amplitude of accommodation is maximal at birth, but decreases with age, and becomes null around the age of 55. This gradual reduction is related to a progressive loss of elasticity of the crystalline lens capsule. A regression formula was proposed to calculate the amplitude of accommodation from the age of a subject (7): Amplitude (dioptres) = exp (1.93 + 0.0401 x age - 0.00119 (age) 2) this formula predicts that the amplitude of accommodation is close to 10 D at the age of 20 years, but is roughly 3.5 D at 45 years. There are other simpler formulas, based on the assumption of a linear decay of accommodation with age (8). They reflect the dispersion of the amplitude of accommodation with age: the likely amplitude is given by the equation: 18-0.3 x age. At the age of 45, there may be more than 4.5 diopters of accommodation; However, to work comfortably in near vision, should not exceed half this maximum accommodative effort (or approximately 2.25 diopters). Three diopters being required to read comfortably to 33 cm, one can deduce that a prescription of 0.75 D for a reading addition in a patient who is 45 years year old (young presbope) so that he could read comfortably. This gradual reduction of accommodation is not related to a lesser contraction of the ciliary muscle, but a gradual reduction of the elasticity of the capsule of the lens, which retains a circular shape and no longer allows the lens to steepen enough.
Reflex syncinetique of accommodation
The neuropathways of accommodation are coupled with those of the eye movement (eye movements) and motility of the pupil (pupillary diameter control). The eye syncinesie is about the convergence of the eyes, which occurs at the same time as the accommodation and myosis (pupillary contraction). The association of an excess of accommodation and convergence during efforts accommdatifs is a cause of strabismus said "accommodative" in children. The use of so-called eyedrops "cycloplegics" cause a paralysis of the ciliary muscle: atropine and the cyclopentolate to measure refraction in children (and adults) after blocking of accommodation. These agents block receptors known as "muscarinic", and also cause a dilation of the pupil, which can last a few hours. They are used also in the framework of the bilant preoperative of refractive surgery to measure refraction while being sure that the accommodation is blocked. Their action can sometimes last almost 24 hours (including dilation induced the pupil, which makes the eyes more sensitive to light and "blurs" a little vision).
Accommodation in the animal eyes
Like human eyes, animal eyes do to adjust the vision at different distances: but there are a large variety of game mechanisms to this end (9). Remember that he is to vary the optical system vergence eye, refracting the couple 'horny + crystalline'. Game strategies depend on certain evolutionary traits, and Visual needs that vary depending on whether one is predatory or sedentary, living in the water or in the air... Fish cornea is flat, because the low gradient index between water and the corneal tissue does not offer the possibility of producing a significant vergence. Their lens is generally very convex, which reduces the interest of a mechanism based on the deformation (it would be difficult to further increase the convexity of the lens to accommodate). Sea lions, seals have a very spherical lens, and their accommodation, which is based on a mechanism of cristallinienne deformation (as in man), is not very efficient. In the majority of the fish (teleosts, representing more than 99% of fish species), the accommodation is based on a shift of the lens, which is done to the retina, which reduced the vergence of the crystalline horny couple; the accommodation is used to see from a distance the eye "at rest" with a clear vision of almost (these fish are so naturally "short-sighted", but can accommodate to point "by far". However, the elastomobranches (sharks, rays) have an accommodation based on a displacement of the lens to the cornea, which increases the vergence of the couple "horny and crystalline. These animals are predators that it is more important to see net effortlessly remotely to identify their prey more easily.
Corneal accommodation is a strategy used in the lamprey, which is a particular and relatively primitive fish species. This feature does not seem to have taken in more recent species of fish, outside of the sandeel, which has a striated muscle (the retractor corneal), able to produce very large accommodatives changes (thanks to the presence of a lenticule of corneal intra). The outside of the water (in mid air) accommodation by displacement of the lens is not able to produce very good quality images. In addition, the cornea is terrestrial species is more arched, because the index gradient between the air and the corneal tissue can be used to make the cornea provides the lead role for the ocular vergence; The lens doesn't have to be as curved as for aquatic species, and may adopt a more elliptical shape. If the lens is made of a 'flexible' material, accommodation by distortion of the lens is a powerful mechanism for the development in near vision. Theoretically, the deformation of the cornea would be able to increase the ocular vergence necessary to the development in vision closely. In practice, this mechanism of corneal accommodation exists in birds, where she completed an also cristallinienne accommodation. Two muscles striated, called respectively Brücke muscle and muscle of Crampton contract to enclose the lens and cause the bulge of it. More peripheral insertion of the Crampton muscle also causes some increase in the curve of the cornea. This mechanism is can be used to "enhance" and refine the accuracy of the development closely. While mammals use a smooth muscle for the accommodation by cristallinienne deformation, reptiles have striated muscles to achieve a more efficient and less prone to fatigue of fact accommodation. The lens of reptiles is very flexible, and can very quickly deform under the action of these striated muscles. At the turtles, deformation of the lens is such that it protrudes through the pupil, which creates a sort of «accommodative lenticone» Finally, the snakes have a unique mechanism of accommodation, by translation of the lens to the front (as in sharks). However, this translation is not the result of a muscle pull, but a compression of the vitreous cavity, which propels the lens forward, through the pupil where it protrudes also, at the turtle.
In humans, the fact that the reduction of the amplitude of accommodation, which results in presbyopia, either related to the loss of elasticity of the crystalline lens explains that there is no "training" method to not become presbyopic (or reduce the importance of presbyopia). The contraction of the ciliary muscle helps release the zonule, but the gradual stiffening of the capsule and the lens reduce the induced bulge. He is so used to "beef up" the cilaire muscle, since the reduction of the accommodation is the result of a passive phenomenon.
(1) Koretz and al. Accommodation and presbyopia in the human eyes. Changes in the anterior segment and crystalline lens with focus. Invest invest Vis Sci, 1997; 38:569 - 78
(2) Brown and al. The change in shape and internal form of the lens if the eye we accommodation. Exp Eye Res, 1973; 15:441 - 459
(3) Helmholtz von H. Handbuch der physiological Optik, vol1, 3rd ed.
(4) Schachat RA. Cause and treatment of presbyopia with a method for increasing the amplitude of accommodation. Ann invest, 1992; 24:445 - 447
(5) Garner and al. Changes in ocular dimensions and refraction with accommodation. Ophthalmic B.j. Opt, 1996; 17 (1): 12 6) Fincham EF. The accommodation reflex and its stimulus. BR J invest, 1951; 35:381 - 393
(7) Ungerer J. The optometric management of presbyopic airlines pilots. Unpublished MSc Optom thesis, University of Melbourne
(8) Hofstetter HW. A useful formula age-range. Pensylvania Optom, 1947; 7:5-8
(9) Schwab IR. Evlution's witness. How eyes evolved. Oxford University Press, 2012