PRESBYOPY

PRESBYOPY (presbyopia; grech, presbys old + ops of eyes; a synonym senile far-sightedness) — the weakening of the refracting force of optical system of an eye generally due to change of accommodation during the examining of a subject at a short distance occurring aged after 40 years. According to F. Donders, volume accommodations of an eye (see) depends on age. An eye can see well at a short distance without being tired only if it spends at the same time no more than 1/2 and 2/3 its volumes of accommodation. The most part of visual work is performed at distance apprx. 33 cm that corresponds for a proportional (emmetropichesky) eye to the refracting force in 3,0 dptr. Such tension of accommodation can is long to take place at the capacity of accommodation not less than 6,0 dptr that is generally characteristic aged up to 40 years at an emmetropia.

The item develops as a result of consolidation of a kernel of a crystalline lens. The crystalline lens loses the elastic properties and therefore in response to reduction of a tsiliarny muscle does not change the curvature as at young age any more. At the Item. the closest point of clear sight (see) it is removed from eyes and there is a deterioration in sight close.

Apply collective glasses to P.'s correction (see. Points ). They replace the weakened accommodation and give the chance, despite more or less sharp reduction of volume of accommodation, to work to an eye close at usual distance. Since the volume of accommodation changes with age, it is necessary to change respectively and the refracting force of corrective glass. At selection of glasses at P. at persons with an emmetropichesky refraction (see. Refraction of an eye ) or for each eye define the provision of the next point of clear sight, and, using Donders's formula for scoping of accommodation, calculate the refracting force of necessary glass, or use (without preliminary measurements) empirical data which specify the refracting force of ocular glass depending on age (a working distance of 33 cm). So, at the age of 45 — 50 years the refracting force of necessary ocular glass averages 1 — 1,5 dptr, at the age of 50 — 55 years — 1,5 — 2 dptr, at the age of 55 — 60 years — 2 — 2,5 dptr, at the age of 60 — 65 years — 2,5 — 3 dptr.

In addition to age, at selection of glasses persons with P. need to consider a refraction and visual acuity always. At a hypermetropia to the indication of the refracting force of the glass necessary on age for P.'s correction, add the refracting strength of corrective glass for gave. At a miopichesky refraction value of the refracting force of presbyopic glass is subtracted from value of the refracting force of glass for a distance. Influence of a refraction on P. affects also in what at P.'s hypermetropia comes in earlier, and at a myopia (see. Short-sightedness ) at later age, than at emmetropias (see). At P.'s correction the occupation since the working distance at various professions is unequal for eyes matters. So, e.g., during the sewing the subject is held further from eyes, than the book or a notebook during the reading and the letter. Engravers and watchmakers, on the contrary, hold objects in more short distance from eyes. For work in more short distance prescribe glasses with the bigger refracting force, and at distance, remote from an eye — weaker, than it corresponds to age. At decrease visual acuities (see) owing to any diseases considerable approach of an object of work to eyes and consequently, and purpose of points with glasses of the bigger refracting force, than it corresponds to age is necessary. When decrease in visual acuity reaches high degrees, presbyopic correction should be combined with special magnifying systems and to appoint telescopic spectacles (see). At a combination of a presbyopy to an ametropia it is necessary to appoint a corrective spectacles both for a distance, and for a bliza. Quite often in this case it is possible to use points with bifocal glasses.