ACCOMMODATION OF A EYE

From Big Medical Encyclopedia

ACCOMMODATION OF A EYE (accomodatio oculi) — process of change of the refracting force of an eye for adaptation to perception of the objects which are from it at various distances.

Mechanism

Fiziol. mechanism A., according to Helmholtz's theory (N. of Helmholtz, 1853), is as follows: at reduction of fibers of a ciliary muscle of an eye there is a relaxation of a ciliary corbel, by means of to-rogo the crystalline lens concluded in a bag is attached to a ciliary body (see. Eye ), weakening of a tension of fibers of this sheaf weakens in turn tightness of a bag of a crystalline lens, and the crystalline lens having elastic properties gets more convex form therefore the refracting force of all optical system of an eye (fig. 1) changes. At relaxation of a ciliary muscle there is a reverse.

At And. the crystalline lens changes the form unevenly: especially convex is a central, antipupillary area having major importance for optics of an eye; on the periphery it is flattened. According to some authors, in strengthening of a refractivity of a crystalline lens in the course of accommodation plays a role not only a factor of change of curvature of its surface, but also mutual movement of the fibers of lenticular substance having various index of refraction thanks to what the refracting force of its central part increases (so-called vnutrikapsulyarny And.). In the course of accommodation the crystalline lens falls a little from top to bottom.

Fig. 1. Scheme of accommodation of an eye: the solid line designates the provision of a crystalline lens at rest; dotted — at reduction of a ciliary muscle

The innervation of a ciliary muscle is carried out by the parasympathetic nerve fibrils going as a part of a third cranial nerve.

Sympathetic fibers also take part in this process. At the same time excitement of the parasympathetic fibers of a third cranial nerve reducing ring fibers of a ciliary muscle causes tension of accommodation necessary at installation of an eye for examining of a subject at a short distance; excitement of the sympathetic nerve fibrils reducing radiarny fibers causes the opposite phenomenon — weakening of accommodation (installation of an eye for examining of the remote objects).

And. it can be carried out only in the known limits which are defined, on the one hand, as so-called. the closest point of clear sight (see), with another — as further point of clear sight (see). The space between the closest and further points of clear sight, in limits to-rogo an eye can accommodate, call area or length of the accommodation measured in linear sizes; increase in the refracting force of optical system of an eye, a cut is reached at the maximum tension of accommodation, call the volume, force, amplitude or width And. also express in dioptries. The accommodation measured for each eye separately carries the name of absolute; the accommodation measured at simultaneous watching by two eyes i.e. at the known extent of convergence (data of visual axes of both eyes during the fixing of the objects which are close located from an eye), is called relative. Relative And. there is always less absolute. It depends on the existing communication between two fiziol. processes — accommodation and convergence of eyes (see).

This communication, however, is not continuous and at each certain extent of convergence of an eye to a certain extent can change the accommodation. Part relative And., edges it is spent for performance of this or that visual work at a certain extent of convergence, is called a negative part relative And.; same part relative And., edges remains at the same time in a stock, is called a positive part relative And.

A proper correlation between negative and positive parts relative And. has great practical value: visual work at a short distance can be carried out is long without the phenomena of exhaustion of an eye only if a positive part relative And. it is more or it is at least equal to negative; if a positive part is less than negative, there can come the exhaustion of eyes — akkomodativny asthenopia (see).

Disturbances of accommodation of an eye

With age And. gradually changes because the crystalline lens gradually loses the elasticity and ability to change a form (fig. 2). Dependence of volume And. from age it was for the first time established by Donders (F. Bonders, 1818 — 1889), and later it is precisely studied by Dwayne (A. Duane, 1858 — 1926).

Weakening of accommodation affects in gradual moving away from an eye of the next point of clear sight and in reduction of length of accommodation with age. These disturbances carry the name presbyopies (see).

From other disturbances of accommodation it is necessary to point to a spasm and paralysis And.

Understand more or less steady and excessive stress it proceeding as a spasm of accommodation and after eyes ceased to fix a close subject. The spasm arises usually at young people (especially at neurasthenics) as a result of steady stress of accommodation, and also at injuries, effect approximately of very bright light. The spasm of accommodation can make an impression of short-sightedness.

Fig. 2. Dependence of volume of accommodation on age

Paralysis of accommodation is characterized by total loss of ability to distinguish a small print at a short distance: at paresis of accommodation this ability is only weakened. Paralyzes and paresis And. can be the central origin (the nuclear, most often found at defeat of that part kernels of a third cranial nerve, edges is related to And.); these paralyzes of accommodation are usually caused by intoxications or infections (syphilis, encephalitis, flu, diphtheria, diabetes, botulism, etc.). Defeats of a trunk of a third cranial nerve on the basis of a brain (basal paralyzes) caused by fractures of base of the skull, meningitis, tumors can also conduct to a clinical picture of paralysis of accommodation (at the same time pupillary disorders and paralyzes of outside muscles of an eye are observed).

The similar picture is observed also at defeats of a third cranial nerve within an eye-socket (orbital paralyzes).

Flaccid paralyzes of accommodation owing to damage of an akkomodatsionny muscle or nerve terminations in it are observed at bruises (contusions) of an eye. The flaccid paralysis of accommodation can develop at intakes of drugs of atropine or a belladonna, and also can be caused artificially for a short time in clinic when for a research and treatment resort to a mydriasis, digging in in a conjunctival sac solutions of atropine, Scopolaminum and other means expanding a pupil operating at the same time and on And.

The fact of easing is noted And. in the conditions of the lowered barometric pressure and air hunger (anoxemia) observed at height. Spasms and paralyzes And. can be bilateral and unilateral. At paresis And. a peculiar phenomenon of a so-called micropsia when all objects seem reduced in sizes can be observed. It is explained by disturbance of the normal relations between the size of images a retina and rate of strain of accommodation.

The forecast depends on the basic disease which caused disturbance And.

The diagnosis is based on subjective complaints, data of clinical inspection and emergence at an objective research of strengthening of the refracting force of optical system of an eye (see. Refraction of an eye ).

Treatment of a spasm and paralysis And. it is carried out depending on the reason which caused these disturbances. At a spasm And. it is recommended to dig in in eyes drugs of atropinic group for weakening of a tone of a ciliary muscle.

Research A. see below.

See also the Closest point of clear sight , Short-sightedness , Further point of clear sight , Far-sightedness , Refraction of an eye .

Devices for a research of accommodation of an eye

For a research A. apply akkomodometra (optometra). The research is conducted in the darkened room, showing to the studied eye an object, on Krom the attention of the patient (subject to fixing) is fixed. By change of distance between subject to fixing and an eye that is an incentive to accommodation, it is possible to come into a certain fortune of accommodation, a cut define in dioptries.

The simplest are devices for subjective (according to answers of the patient) researches A. (subjective optometra). They consist of the optical system projecting the image of a test object on a retina of the studied eye. The provision of a test object corresponding to accurate vision by his patient characterizes And. These devices are used mainly for definition of the next and further point of clear sight.

The device allowing to conduct an objective research A. by consecutive measurements of the separate fixed conditions of a refraction of an eye, there can be an eye refractometer (see. Refractometry ) in combination with the device for presentation to an eye of the subject to fixing stimulating accommodation. Such device allows to estimate And. at the distances defined, established in advance between subject to fixing and the studied eye by means of the test object (measuring brand) observed by the doctor. Test object is established by the doctor in the provision of the largest clearness that corresponds to accommodation of the studied eye which at this moment observes the subject to fixing placed at the set distance.

However as And. is physiological process, that its research shall be conducted in dynamics. Devices with automatic simultaneous registration as distances between subject to fixing and an eye, and the accommodation of an eye arising in response to this change of distance are used to these purposes. In such devices as a sensitive element various photo-electric receivers of infrared radiation and therefore the patient actually does not participate in assessment are used And. There are devices of two types. Devices of the first type carry out registration And. by measurement of radius of curvature of a front surface of a crystalline lens (see. Crystalline lens ), what only partially characterizes And.

Fig. 3. The elementary optical diagram of an infrared optometr of Campbell — Robsona: 1 — filament; 2 — the infrared light filter; 3 — the condenser; 4 — a two-slit diaphragm; 5 — measuring brand in the form of a slit diaphragm; 6 — a projective lens; 7 — a translucent mirror; 8 — the studied eye; 9 — the lens projecting the secondary image on a photodetector; 10 — couple of photodetectors

Devices of the second type allow to judge about And., considering the changes of optical system of the studied eye in general happening at accommodation. These devices have two principles of measurement And. One are constructed on the principle of a research of the out-of-focus (indistinct) image of the test object which is turning out on a retina of an eye of the patient, others — on the principle of measurement of distance from an eye of the patient to the image of a test object on condition of receiving on a retina of an eye of the patient of a sharp image of the test object which is automatically controlled by the device.

One of the first devices for measurement And., using the principle of the out-of-focus image, the infrared opto-meter of Campbell — Robsona (fig. 3) is. By the same principle are constructed optometr Elyulya, infrared optometr Allen — Carter, infrared akkomodometr Avetisova — Urmakhera — Shapiro — Nabatchikova, akkomodometr Avetisova — Ananina — Kipriyanova, infrared optometr Roth (Vildt's device) which differ on a way of a research of the out-of-focus image.

Fig. 4. Elementary optical diagram of an infrared optometr of Roth: I \dorsal view; II \a look on an arrow A. 1 — a projective lens; 2 — the infrared filter; 3 — a translucent mirror; 4 — the studied eye; 5 and 7 — the lenses projecting the secondary image of brand on a photodetector; 6 — the diaphragm eliminating a lid reflex; 8 — the aperture diaphragm interfaced to the plane of a pupil of an eye; 9 — a wedge; 10 — a plane-parallel plate; 11 — two couples of photodetectors; T — the light source which is carrying out a role of measuring brand; T' — the image of measuring brand on a mesh cover of the studied eye; T

The most successful device can be considered infrared optometr Roth (fig. 4) improved by Vildt. The optical scheme (fig. 5) of the device allows to receive two images of a test object which move in the optical way everyone concerning pair of receivers in the plane of two pairs of photo-electric receivers. Simultaneous passing of receivers each image of a test object does not cause a difference of phases of the signals removed from each pair of receivers. At change of accommodation of distance between these images change, and each image passes the pair of receivers in various time that causes a difference of phases of signals. Value of a difference of phases of signals also is a measure And. The advanced device of Vildt allows to register at the same time accommodation, akkomodatsionny convergence of an eye and diameter of a pupil.

Fig. 5. The elementary optical diagram of the device of Vildt for measurement of accommodation: 1 — the studied eye; 2 — a lens; 3 — a bilens; 4 — couple of photodetectors; 5 — couple of photodetectors (and and — extreme points of the secondary out-of-focus image); 6 — a light source; 7 — the rotating cube providing scanning of the image concerning photodetectors; 8, 9, 10 and 13 — system of mirrors; 11 — a projective lens; 12 — the infrared filter

Example of the optometr using the principle of measurement of distance from an eye of the patient to a test object is the device Warsaw, improved at first by S. Oshima, and then T. N. Cornsweet. The optical system of this device allows to receive on pair of photo-electric receivers the image of a test object in the form of two strips of light which are combined in that case when the image of a test object is focused on a retina and the receiver that is reached by means of automatic movement of a test object and optical elements of system. At this moment the recorder of the device registers And. in dioptries.



Bibliography: Averbakh M. I. Ophthalmologic sketches, page 213, M., 1940; Dashevsky A. I. Refraction and accommodation of an eye, Mnogotomn. the management on glazn. to diseases, under the editorship of V. N. Arkhangelsky, t. 1, book 1, page 252, M., 1962; Kravkov S. V. Eye and its work, page 74, M., 1945; Donders F. Die Anomalien der Refraction und Akkomodation des Auges, Wien, 1888; Helmholtz H. Handbuch der physiologischen Optik. Bd 1, S. 120, Hamburg — Lpz., 1909; Hess C. Die Refraction und Akkomodation des men-schlichen Auges und ihre Anomalien, Handb. eres. Augenheilk., hrsg. v. A. Graefe u. T. Saemisch, Bd 1, Abt. 1, V., 1910; Landolt E. Die Untersuchung der Refrak-tion und der Akkomodation des Auges, ibid., Bd 1, S. 4, V., 1920.

Devices for a research And. — Ananin V. F., Avetisov E. S. and Kipriyanova T. I. Objective registration of accommodation of an eye by method of scanning of the slot-hole image reflected from a retina of an eye, Vestie, oftalm., No. 2, page 61, 1971; Soft T. N. The characteristic of limit of sensibilitys at a research of a refraction of ideal optical system of an eye by objective methods, News of medical instrument making, century 1, page 62, 1972; it, Influence of various factors on results of measurement of a refraction by objective methods, in the same place, page 73; Allen M. J. and. Carter J. N of An infrared optometer to study the accommodative mechanism, Amer. J. Optom., v. 37, p. 403, I960; Campbell F. W. a. Rоbsоn J. G. High-speed infrared optometer, J. Opt. Soc. Amer., v. 49, p. 268, 1959; Campbell F. W. a. Westheimer G. Dynamics of accommodation responses of the human eye, J. Physiol. (Lond.), v. 151, p. 285, 1960, bibliogr.; Cornsweet T. N. and. Crane H. D. Servo-controlled infrared optometer, J. Opt. Soc. Amer., v. 60, p. 548, 1970, bibliogr.; Roth N. Automatic optometer for use with the undrugged human eye, Rev. Sci. Instr., v. 36, p. 1636, 1965; van der Wildt G. J. and. Bouman M. A. An accommodometer, Appl. Optics, v. 10, p. 1950, 1971; Warshawsku J. High-resolution optometer for the continuous measurement of accommodation, J. Opt. Soc. Amer., v. 54, p. 375, 1964.


M. L. Krasnov; T. N. Soft (tekhn.).

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