Accommodation ( Latin accommodare "to adapt, create") is a dynamic adjustment of the refractive power of the eye . It means that an object that is located at any distance between the individually different optical near and far point is sharply imaged on the retinal plane and thus an essential requirement for clear vision is met. The near point indicates the shortest and the far point the farthest distance to the eye in which this is possible. The processes when changing from far to near focus are referred to as near accommodation , those when changing from near to far setting are referred to as far accommodation . In the narrower sense, however, “accommodation” is often only understood as close fitting. Not all of their mechanisms and control processes have yet been fully clarified. The ability to accommodate near-by is gradually lost with increasing age ( presbyopia ). People with normal vision then need reading glasses . Due to the increased depth of field with a narrowed pupil, the decreasing visual acuity in the close-up range is less noticeable in daylight than in poor light conditions.
The accommodation represents a higher reflex circle that can be influenced at will. In order to vary the refractive power, mammals , birds and reptiles change the shape of the elastic lens during accommodation . In the ciliary body of the eye characterized opto-geometric changes, and therefore adjustments to the overall refractive power of the eye set (in humans, depending on the age up to 15 diopters ). In fish and amphibians, the distance between the rigid lens and the retina is changed by muscles for accommodation.
There are various, sometimes contradicting, theories about the mechanism of changing the refractive power. The so-called von Helmholtz theory, which is largely accepted today and essentially confirmed experimentally, is based on the article " Ueber die Accommodation des Auges " published by Hermann Helmholtz in 1855 . She assumes a lens deformation during accommodation. Accommodation is controlled by the ring-shaped ciliary muscle to which the lens of the eye is suspended by the zonular fibers . The elastic lens of the eye is drawn into a flatter ellipse shape by the zonular fibers on the lens capsule when looking at a distance. The elastic fibers pull the relaxed ciliary muscle back, causing the lens to enlarge in diameter. With near accommodation, the ciliary muscle is tensed, the diameter of the circular muscle and the lens decrease. As a result, the zonular fibers are stretched and the radiating body is concentrically narrowed. The lens is deformed by the elastic forces of the lens capsule into its more spherical rest shape, which causes an increase in the refractive power. The change in the radius of curvature of the lens is also called external accommodation . In addition, mechanisms are known which lead to a rearrangement of the microstructures and a change in shape of the lens fibers in the lens interior, and which are also effective for accommodation. These processes are called internal accommodation .
The Nahakkommodation is an active process, which is caused by contraction of the annular muscle fibers of the ciliary muscle. In ignorance of an antagonistic muscle force, it was long assumed that remote accommodation, on the other hand, was a purely passive process, triggered by the slackening of these muscle contractions and innervation impulses. More recent research results expand Helmholtz's theory to the effect that there are mechanisms that are antagonistic to near accommodation , which actively support remote adjustment of the eye through the contraction of meridional muscle fibers of the ciliary muscle, although the proportion of passive processes clearly predominates. The removal of the accommodative setting is called disaccommodation .
A controversially discussed theory in contrast to Helmholtz was formulated by the American scientist Ronald A. Schachar .
Evolution of accommodation
Accommodation in invertebrates
The relative simplicity of the invertebrate eyes does not require an elaborate accommodation mechanism. Only in a few cases does a muscularly controlled apparatus exist that resembles that of vertebrates. In most cases, however, the accommodation is static in nature and is based on the use of different optical systems in one or more eyes, one for vision in the distance and the other for near vision.
A muscularly controlled adaptation is found in a developed form mainly in molluscs , whereby the position of the eye lens is changed secondarily by the compression of the eyeball. A similar functionality can also be found in snakes . The cephalopods , however, show this in the highest form: The contraction of the ciliary muscle leads to compression of the eyeball. This leads to an increase in the intraocular pressure , so that the vitreous humor pushes the lens forward passively, which results in an increase in the refractive power of around 10 to 14 diopters.
In the eyes of certain polychaetes (for example Alciopa ), the stimulation (excitation) of secretory cells leads to an increase in volume of the distal vitreous body, which is located directly behind the lens. It is assumed that this pushes the lens forward and thus enables close focus. Furthermore, there is a muscle in this eye, which in its function resembles the contraction mechanism in cephalopods.
However, such active systems are the exception. Accommodation is more often achieved through certain optical arrangements. The simplest example of this is provided by the ocellus insects: In grasshoppers , for example, there is a double curvature on the proximal surface of the corneal lens, which can be used in the manner of bifocal glasses to focus two images at different distances at the same time.
The optical structure of the compound eye does not allow an accommodative setting, which is not necessary because of the mosaic image . It appears, however, that in certain compound eyes segments with optically different refractive properties are arranged in such a way that short ommatidia with strong lenses lie in one region and long ommatidia with weak lenses in other regions. This can be particularly pronounced in the compound eyes of certain Ephemeroptera , Diptera , Hemiptera and some pelagic schizopods , in which a part is designed for near vision and another part for distance vision.
Finally, there can also be two different eyes, one of which is optically adjusted for near and the other for distant. An example of this can be found in the side and center eyes of spiders. The same concept also applies to the dorsal and belly facet eyes of the tumbler beetle , the former being used for seeing in the air and the latter for seeing in water.
Accommodation in vertebrates
While emmetropia is usually a prerequisite for good visual acuity , the ability to adjust the optical system for near and far vision is almost as important a trait, especially for activities such as snatching prey. In contrast, with amphibious forms of life, the adaptation to the different optical conditions between water and air is of greater importance. For the safety of tree dwellers, on the other hand, a particularly fast and effective adjustment is essential. Finally, for higher primates and humans, it is of crucial importance to be able to see objects that are manipulated with the hands clearly and thus better examine them.
Only a few species of vertebrates have no accommodation whatsoever. For the majority of them, vision is of little biological importance. Most representatives of the primitive group of fish, such as cartilaginous organoids , dipnoi and coelacanth, lack such a mechanism . Furthermore, there is no accommodation in primitive mammals, which are primarily nocturnal. In monotremes and pouch mammals as well as in many placental animals (except the squirrel ) is also not formed. Even in ungulates such as horses, sheep and pigs, there is hardly any evidence of accommodative activity. With the exception of the weak adaptability in squirrels and carnivores , a usable range of accommodation in mammals can only be determined in otters and primates, especially humans.
Within the vertebrate tribe, accommodation is achieved through a number of different, basic mechanisms. Obviously, in every development phase of the various species, all conceivable variants that seemed suitable for the distance-dependent adaptation of the dioptric system were "tried out". These different approaches can be divided into two types:
- Static systems in which the optical variability is achieved through structural features;
- Dynamic systems based on an active change of the dioptric apparatus through muscle strength.
Impulse accommodative go from the visual cortex and reach the nuclei pretectales the pretectal area in the midbrain . From here fibers run to the accessory nucleus nervi oculomotorii ( Edinger-Westphal nucleus ) in the midbrain , with some of the fibers crossing via the epithalamic commissure to the opposite side - this leads to a bilateral reaction, even if one eye is blind. In addition, this core area , which is responsible for the parasympathetic innervation of the inner eye muscles, is also reached by fibers from the adjacent lateral mesencephalic reticular formation , which in turn receives afferents from visuomotor areas of the endbrain.
The axons of the parasympathetic neurons of the accessory nucleus N. III leave the brain in the oculomotor nerve and run as preganglionic fibers to the parasympathetic ciliary ganglion . Here, their impulses are switched to the postganglionic fibers of the Nervi ciliares breves , which also innervate the Musculus ciliaris ( ciliary muscle ). Its fibers are arranged in two different directions, which are represented by the Müller muscle and the Brück muscle . The Müllerian muscle is parasympathetically innervated and brings about near accommodation, while the sympathetically supplied Brückian muscle makes at least a small active contribution to the remote adjustment of the eye ( double innervation ).
If the ciliary muscle is no longer actively innervated in any way, one speaks of the resting position of accommodation (also visual equilibrium ), whereby the relaxation tone is somewhere between the far and near point. The forces and elasticity elements of the zonula, lens and basic tone of the ciliary muscle still at work here lead to myopia, the extent of which is between 0.5 and 4.0 D. Such a position of rest occurs when the visual field is stimulus-free or has no stimulus, for example in pilots at high altitudes ( space myopia ) or when seeing at night ( night myopia ).
The duration of accommodation when switching from far to near is around 0.5–1.5 seconds, and that from near to far vision around 0.8–1.3 seconds. It can vary in the case of fatigue, loss of elasticity of the lens or increased ciliary muscle tone ( tonic A., pupillotonia ).
Physiology and pathophysiology in humans
When looking at an object nearby, the eyes converge and constrict the pupils ( miosis ). Together with near accommodation, these two mechanisms belong to a superordinate neurophysiological control loop and are collectively referred to as the near focus trias . So far, however, it is not clear what the primary mechanism is. The accommodation provided is directly related to the necessary convergence movement. This ratio is expressed in what is known as the AC / A quotient . If this relationship is disturbed, it can lead to a squint .
The maximum possible change in refractive power is referred to as the accommodation width or also the accommodation amplitude . In the case of small children it is on average about 14 dpt. In relation to the total refractive power of the eye of around 58 D, this corresponds to a variation of around 25%. In old age, the range of accommodation falls to values below 0.5 D or 1%. This increases the smallest distance, the so-called accommodation near point, at which objects can still be seen clearly without close-up correction, from approx. 7 cm to more than 150 cm. The range of fluctuation in the population is around ± 2.0 D in childhood, although this value decreases to around ± 1 D with increasing age. The reason for the reduction in the range of accommodation is a progressive age-related loss of elasticity in the lens capsule or lens thickening due to lifelong growth of the lens shell (Helmholtz theory). The area between the minimum and maximum accommodation effort is called the accommodation area.
The age dependence of the average range of accommodation is classically described by a curve according to Duane. It is shown in the diagram above (in the original with minimum and maximum curves). The corresponding age-dependent minimum object distance (“minimum visual distance” = position of the accommodation near point) for people with normal vision and ametropia with optimal distance correction is shown in the diagram below.
In principle, it is always essential to completely correct any existing ametropia before taking measurements . There are various apparatus methods for measuring the width of accommodation. The devices with which this is carried out are called an accommodation meter or optometer . Otherwise it is also possible to determine the accommodation near point by means of simple, small fixation objects . These objects are brought so close to the eye that their contours are no longer clearly visible to the test subject. Based on the accommodation near point, the accommodation width can then be calculated using the formula:
- Accommodation width in dpt = 1 / near point in m (corresponds to: 100 / near point in cm).
Another way of calculating the minimum object width results from the following formula.
- b: = image distance with relaxed eyes without glasses (in meters)
- f: = image distance of the viewed object (in meters)
- g: = near point. The reciprocal value is the accommodation effort A (in dpt ): 1 / g = A
idealized the lens equation applies : 1 / g + 1 / b = 1 / f
Using the full range of accommodation, i.e. the maximum accommodation effort (A max ), results in the minimum object distance ("minimum visual distance") without glasses (f min )
- (1) A max + 1 / b = 1 / f min
or (with 1 / A = g)
- (2) f min = 1 / (1 / g + 1 / b)
This can be used to estimate the strength of reading glasses for presbyopia .
In emmetropia ( normal vision ), by definition, b is infinite and the minimum object distance is the reciprocal of the accommodation width. Correspondingly, f min can be calculated for a spectacle wearer by substituting the negative spectacle value for the distance (in diopters) for 1 / b - provided that the eye is relaxed and can see clearly when wearing the glasses and looking into the distance.
Another possibility is to keep minus lenses in front of you when you are looking into the distance , with maximum visual acuity requirements and to increase them until the optotypes can no longer be clearly recognized. The value of the minus lens, at which the optotypes could just be seen clearly, represents the range of accommodation.
Use accommodation width
While the width of accommodation indicates the maximum value by which a change in refractive power is possible, the width of accommodation for use only accounts for about two thirds of this maximum. It is not determined monocularly for the right and left eye, but rather with binocular fixation of a nearby object and a slight lowering of the gaze. Only the near point is taken into account, which can be viewed effortlessly and without great effort after a remote setting.
External success in accommodation
In the case of an existing axial refractive error (A), a correction lens (spectacle lens) and an error lens (eye) together form a so-called Dutch telescope . This results in an enlargement (V) of the image in the case of corrected hyperopia , and a reduction (V) in the case of myopia . With the distance (e) of the spectacle lens from the eye and an increase in the ametropia, the change in image size increases. This telescope effect influences the success of external accommodation . There is the following relationship between the external success of accommodation (AE) and accommodation expenditure (A 0 ):
- A 0 = V 2 * AE.
The enlargement / reduction results from the formula: V = 1 + e * A . In practice this means that a hyperoper corrected with glasses has to make more accommodation expenditure than an emmetroper, a myoper corrected with glasses less. Example:
- Hyperopia or axial refraction A = 5.0 dpt
- Distance between glasses and eyes e = 20 mm = 0.02 m
- V = 1 + 0.02 * 5 = 1.1
If accommodation is desired for a distance of 25 cm (external accommodation success AE = 4.0 D ), the actual accommodation effort is:
- A 0 = 1.1 2 * 4 = 1.21 * 4 = 4.84 dpt .
With presbyopia , or presbyopia is called the progressive age-related loss of Nahanpassungsfähigkeit the eye .
In the case of accommodation paralysis , also known as cycloplegia , there is a loss of function of the ciliary muscle. This can have pathological causes (for example if the parasympathetic nerve fibers of the oculomotor nerve are damaged) or for diagnostic purposes (for example refractometry ) it can be actively brought about by appropriate pharmacological agents ( cycloplegics ). The function restriction triggered in this way should be as complete as possible for the period of the investigation. For emmetropes and hyperopes , sharp vision in the vicinity is no longer possible for a certain period of time in this state.
Under Hypoakkommodation means a significant impairment of amplitude of accommodation that is not neurologically related and often a manifest or latent Convergence excess with increased AC / A ratio triggers. The success of the accommodation does not correspond to the applied innervation impulse, whereby the near point is clearly shifted into the distance and not age-appropriate. In addition, there are asthenopic complaints, reading difficulties and fluctuating visual acuity in the vicinity, which is why those affected tend to look for an unnaturally large reading distance. Hypoaccommodation is extremely rare and usually occurs in childhood. The therapy of choice is the prescription of bifocal glasses. However , it is not advisable to perform a squint operation .
In the case of hyperopic eyes, a corresponding amount of accommodation is required to see clearly in the distance. The accommodation near point therefore moves away from the eye by the proportion of accommodation performance that is required to compensate for the hyperopia. In the case of significantly undercorrected hyperopia or overcorrected myopia , an accommodation spasm can develop after a long period of time . This condition manifests itself in headaches and blurred vision. In such cases, optimally adapted glasses help those with a good overview who can see clearly in the distance and near without glasses, but in the long run this is accompanied by corresponding complaints. In some cases, antispasmodic drug treatment may also be an option.
An accommodation spasm can also lead to temporary nearsightedness of the eyes, a so-called pseudomyopia . It is not to be confused with the physiological, refractive-related myopia.
The physical optics also knows the concept of negative accommodation. In the case of the eye, this would mean distant accommodation that goes beyond complete relaxation of the ciliary muscle. However, it is doubtful that such a physiological mechanism exists.
As pseudoaccommodation is the capability to be able to sufficiently sharp recognize both objects in the distance and near without active power change of the eye.
- Herbert Kaufmann, Heimo Steffen: Strabismus . With the collaboration of W. de Decker u. a. Enke, Stuttgart 1986, ISBN 978-3-13-129724-2 .
- Th. Axenfeld, H. Pau: Textbook and Atlas of Ophthalmology . With the collaboration of R. Sachsenweger u. a. Gustav Fischer Verlag, Stuttgart 1980, ISBN 3-437-00255-4 .
- Karl Mütze: The Accommodation of the Human Eye . Akademie-Verlag, Berlin 1956. (with extensive bibliography)
- bio.vobs.at ( Memento from March 11, 2009 in the Internet Archive )
- Change in the wavefront of the eye under accommodation (PDF)
- H. Helmholtz: About the Accommodation of the Eye. In: Albrecht von Graefes Archiv für Ophthalmologie , 2, 1855, pp. 1-74, doi: 10.1007 / BF02720789 .
- Rudolf Sachsenweger: Neuroophthalmology. 3. Edition. Thieme Verlag, Stuttgart 1983, ISBN 3-13-531003-5 , p. 309 ff.
- Ronald A. Schachar: Presbyopia: Cause and Treatment.
- Stewart Duke-Elder: The Eye in Evolution. In: System of Ophthalmology. Vol. 1, Henry-Kimpton, London 1958, pp. 590f.
- P. Gamlin, A. Reiner: The Edinger-Westphal nucleus: sources of input influencing accommodation, pupilloconstriction, and choroidal blood flow. In: J Comp Neurology . tape 306 , no. 3 , April 1991, pp. 425-438 , PMID 1713924 .
- B. Lachenmayr, D. Friedburg, E. Hartmann, A. Buser: Eye - Glasses - Refraction: Schober course: understand - learn - apply. 2005, Fig. 1.29.
- figure , compare Alexander Duane: Studies in monocular and binocolar accommodation with their clinical applications . In: Transactions of the American Ophthalmological Society . Volume 20, 1922, pp. 132-157. PMID 16692582 . PMC 1318318 (free full text).
- M. Koch, A. Langmann: Hypoaccommodation in childhood and prepresbyopic age . In: Spectrum of Ophthalmology , Vol. 19, February 2005, doi: 10.1007 / BF03163192
- Herbert Kaufmann: Strabismus . 3rd, fundamentally revised and expanded edition. with the collaboration of W. de Decker u. a. Georg Thieme Verlag, Stuttgart / New York 2003, ISBN 3-13-129723-9 , p. 171.