APHAKIA

From Big Medical Encyclopedia

APHAKIA (grech, and - otritsa. + phakos lentil, lens) — lack of a crystalline lens.

In most cases the aphakia is a consequence of extraction of a crystalline lens at to a cataract (see), results from a severe wound of an eye less often, in rare instances can meet inborn And. in combinations with other malformations.

Find And. on lack of characteristic pupillary reflections from front and back surfaces of a crystalline lens (Purkinye's figure), on a deep water of an anterior chamber of an eye, trembling of an iris of the eye (iridodonesis) at the sharp movements of eyes, and also on lack of an easy grayish reflection from depth of a pupil which is often observed, especially at elderly people if the crystalline lens is on site.

As a result And. sharply changes refraction of an eye (see) since the crystalline lens is one of the main refracting optical environments of an eye. If the general refracting force of all optical system of a human eye makes, on Gullstranda, 58,64 dptr, then falls to the share of a crystalline lens over 1/4 all the refracting force. Parallel rays of light as a result of refraction in the eye deprived of a crystalline lens gather in focus not on distance of 22,8 mm that corresponds to the average length of a healthy eye (Gullstrand), and at distance of 31 mm. For correction of such hypermetropia (see Far-sightedness) it is necessary to put before an eye glass by force in + 10,0 or + 11,0 dptr. However the refracting force of such corrective lens is much lower than the optical power of the crystalline lens removed from an eye, makes edges, on Gullstranda, + 19,11 dptr. Difference between the valid refraction of lens and optical power corrective And. glasses is explained by the fact that glass takes not that place what was taken by a crystalline lens in optical system of an eye; besides, in an eye the crystalline lens is surrounded not with air, but intraocular liquid from all directions, the index of refraction a cut (1,336) is close to index of refraction of cortical layers of a crystalline lens (1,386).

As at And. an eye loses not only parts of the refraction force, but also the main akkomodatsionny device (see. Accommodation of an eye ), it loses an opportunity to adapt for clear sight close. For work with small objects and reading at And. appoint special glasses, approximately on 3,0 dptr is stronger than the points appointed for sight afar.

See also Eye (dioptrics), Crystalline lens .


Bibliography: Dymshits L. A. Fundamentals of ophthalmology of children's age, page 372, D., 1970; The Multivolume guide to eye diseases, under the editorship of V. N. Arkhangelsky, t. 5, page 248, M., 1960.

A. Ya. Samoylov.

Яндекс.Метрика