FAR-SIGHTEDNESS

From Big Medical Encyclopedia

FAR-SIGHTEDNESS (synonym hypermetropia) — one of types of a clinical refraction of an eye which is characterized by the fact that focus of parallel beams after their refraction in an eye is lying behind a retina (fig. 1). Thereof D., as well as short-sightedness (see), treats ametropias, or anomalies of a refraction, i.e. «disproportionate» refractions.

Fig. 1. The scheme of beams in a far-sighted (gipermetropichesky) eye: focus of parallel beams is located behind a retina. On the right — a path of rays in a normal (emmetropichesky) eye.

Can be D.'s cause or relative weakness of the refracting device of an eye (so-called refraction D.), or a relative korotkost of a perednezadny axis of an eye (so-called axial D.).

Refraction D. almost always is acquired and develops as a result of various patol. processes (flattening of a cornea, lack of a crystalline lens, etc.).

Axial D., as a rule, inborn. Almost all children are born far-sighted, but further in process of development of an organism D.'s degree usually gradually decreases; in many cases of D. passes in emmetropia (see) and even in short-sightedness.

The most cases of D. belongs to so-called combinational when the relative korotkost of a perednezadny axis of an eye and relative insufficiency is observed refractions of an eye (see). At the same time both an axis of an eye, and the refracting sira of its optical apparatus without leaving! out of limits of the sizes determined at an emmetropichesky refraction; however these two decisive factors are combined in such a way that there is D. Obychno in these cases D. of small degrees which is not followed any patol, changes in an eye is noted, and visual acuity remains normal. At absolutely short perednezadny axis of an eye (and and all sizes of an eye are reduced) D. of high degrees is noted. By the time of final formation of an organism apprx. 50% of people are far-sighted, and other 50% have on emmetrop and miop.

In a far-sighted eye owing to its device the beams which are already possessing before their entry into an eye this or that degree of a convergence could agree on a retina; however in the nature there are no such beams. Thus, a far-sighted eye on the device cannot well see afar (from where there are parallel beams), close (from where there are dispersing beams). Nevertheless most of far-sighted people well sees afar and often also quite well close. It speaks accommodation of an eye (see), the beams getting into an eye and to reduce them in focus on the retina that is a necessary condition of clear sight allowing to refract stronger. Therefore, the persons possessing D. for the clearest sight are always forced to strain accommodation even for sight afar and the more so for sight close. Therefore the usual, constant tension of accommodation is created a nek-swarm, a cut part D. can hide (especially young people with a sufficient force have accommodations) part D. of Tu, edges without effort is defined by the corresponding glasses, name explicit D. (hypermetropia manifesta); the part hidden by the usual tension of accommodation is named the hidden D. (hypermetropia la tenta). Explicit and hidden D. make together true, or full, D.'s (hypermetropia totalis) degree. Due to the gradual natural age weakening of accommodation the hidden D.'s degree decreases, and degree of explicit increases, and by 40 — 45 years usually all D. becomes explicit. Therefore at gipermetrop at this age true D.'s degree can be defined by glasses whereas at young faces with D. for this purpose it is necessary to resort to preliminary atropinization (an instillation in an eye of 1% of solution of atropine) for temporary switching off of accommodation precisely.

Can be established at a research by glasses (in the course of simultaneous definition of visual acuity). At the same time D.'s degree is defined by the strongest of collective (convex) lenses, about a cut gipermetrop best of all sees afar; this method carries the name subjective since at it it is necessary to reckon with indications of sagas of oho investigated. There are also objective methods of definition of D. (see. Refractometry of an eye , Skiaskopiya ), by means of which it is possible to establish quite precisely D.'s presence and to find out its degree without testimonies of the examinee that is especially important during the definition of degree of D. at absolutely small children and during the conducting examination. Almost most often it is necessary to meet rather low degrees of D. (to 3 dptr), however also much higher degrees of D. are observed (5 — 10 dptr above). Visual acuity (see) at D. weak and partly average degrees in most cases happens normal; at D. of high degrees it is, as a rule, lowered, despite correction by glasses.

Since gipermetropa are forced to resort constantly to tension of accommodation, at them (in the absence of correction by glasses) the phenomena of exhaustion of eyes, so-called akkomodativny easily develop asthenopia (see), expressed in emergence of a headache, dull ache in a forehead and about eyes, pressure sense in eyes and especially that letters during the reading merge, become not clear; having rummaged in visual work usually temporarily eliminates these feelings, but during the resuming of occupations they arise again. These phenomena are caused by overfatigue of a ciliary (akkomodatsionny) muscle. At persistent headaches at school students it is necessary to investigate a refraction of eyes and in case of detection D, to appoint the corresponding points.

At more or less considerable D. quite often develops (especially at the inborn or acquired weakness of the neuroreflex device) the concomitant meeting strabismus owing to disturbance at gipermetrop of normal ratios between accommodation and convergence of eyes (see). By outward gipermetropichesky eyes at small degrees of D. differ in nothing from eyes with an emmetropichesky refraction. At higher degrees of D. some features are usually noted: an anterior chamber of eyes more small, than usually, the pupil is narrowish, the sizes of an eye are reduced. At an oftalmoskopichesky research at the bottom of eyes, especially at high degrees of D., the peculiar picture which received the name a false optic neuritis (pseudoneuritis optica) is sometimes noted: the optic disk is hyperemic, its borders several stushevana, vessels of a retina are expanded. Gipermetropichesky eyes are considered as more predisposed to a disease glaucoma (see), than eyes with other types of refractions that connect with rather small anterior chamber, considerable development of a ciliary muscle and reduction of space between ciliary shoots and a crystalline lens.

Fig. 2. The scheme of a path of rays in a far-sighted eye with correction by biconvex glass (convex): parallel beams agree on a retina.

Apply convex glasses (convex) strengthening a refraction of a gipermetropichesky eye to norm (fig. 2) to D.'s correction. At weak degrees of D., especially at young age at high visual acuity and in the absence of the phenomena of an akkomodativny asthenopia, usually there is no need to use glasses. However this requirement arises at development of the phenomena of exhaustion of eyes (especially during the work at a short distance — reading, the letter, etc.), and also, as a rule, at more expressed degrees of. With age in process of the hidden D.'s transition in explicit points should be strengthened. Developing presbyopy (see) in turn demands doubler of a corrective spectacles. Points certainly are necessary in cases when sight without them is incomplete and when visual acuity increases during the use of points. The special attention should - be paid to D.'s correction at children at bent to to squint (see), and also at already developed concomitant strabismus.


Bibliography: Averbakh M. I. Ophthalmologic sketches, M., 1949; Merkulov I. I. Introduction to clinical ophthalmology, Kharkiv, 1964; The Multivolume guide to eye diseases, under the editorship of V. N. Arkhangelsky, t. 1, book 1, page 239, M., 1962.

M. L. Krasnov.

Яндекс.Метрика