Accomodation

The eye has the mechanism which is capable to raise a refrac­tion, to adapt it for seeing at different distances. This dynamic proc­ess of intensification of eye refraction and its adaptation to differ­ent distances is named accommodation.

The mechanism of accommodation is following. The lens in the eye is attached with the help of ciliary zonule, the fibers of which go from ciliary body to equator of lens. When the fibers of ciliary muscle are shortened there is a relaxation of ciliary zonule, lens becomes more convex, that increases its refractive force. At relax­ation of accommodative muscle there is a return process. It is the classic theory of the accommodation mechanism created by Helm-holtz and acknowledged by the majority of the ophthalmologists. The point which is seen by the eye at maximum tension of accom­modation is named the nearest point of clear vision (Punctum prox-imum). That quantity of diopters, on which the eye can increase its refraction, expresses the force, volume and amplitude of accom­modation. The volume of accommodation is calculated by the for­mula:

A = P + R,

where A — accommodation, P — force of refraction at the instal­lation of the eye on the nearest point of clear vision, R — refrac­tion of the eye in a state of accommodation rest.

The distance between the nearest and the most remote point of clear vision is named length of accommodation. This linear con­cept is expressed in measures of length. The accommodations of one eye is absolute. At examining a subject by both eyes the con- vergence joins to the act of accommodation. That accommodation, which both eyes are capable of at definite convergence, is named a relative accommodation. It always less then absolute and con­sists of positive and negative parts. The interrelation of these parts has large significance. The less a positive part of accommodation, the faster fatique arises.

Disorders of Accommodation

The anomalies of refraction are frequently accompanied by dif­ferent disorders of accommodation. Hypermetropia and astigma­tism, as it was already indicated above, may cause the accommo­dative asthenopia, pareses of ciliar muscle, reduction of accommo­dative volume, appearance of obvious hypermetropia and spasm of accommodation. It is characterized by strengthening of refrac­tion, appearance of false myopia, which disappear at the altitude of cycloplegia (after atropinization).

The spasm of accommodation can arise also in patients with weak degree of myopia. The paresis and paralysis of accommoda­tion are watched at the lesion of parasympathic part of the oculo­motor nerve owing to traumas or poisonings.

With age the volume of accommodation decreases, the so-called presbyopy develops. The presbyopy arises at the age after 40 years. It is characterized by removal of the nearest point of clear vision from eye, worsening of vision at a short distance. At the age of 65 the accommodation equals 0 and the nearest point of clear vi­sion is removed to infinity.

For correction of presbyopy concave glasses for short distance are prescribed: at 40 years of age — +1.0 D, at 50 — +2.0 D, af­ter 60 and older — +3.0 D. If there are anomalies of refraction, the degree of refraction is added (at hypermetropia) or deducted (at myopia) to force of glasses for appropriate age.

In the conclusion it is necessary especially to underline once more that it is necessary the early diagnosis of refractive anoma­lies and carrying out of the special exercises for strengthening of the tonus of ciliar muscle, the general treatment and observation of children with ametropia in consulting centers.