Age-related changes in accommodation

In preschool and school-age patients, against the background of hypermetropic refraction and "weakness" of the accommodation apparatus, the so-called spasm of accommodation may be observed. In this case, there is no complete relaxation of accommodation during distance vision and there is an increase in clinical refraction, i.e. myopia occurs, which is called false. Differential diagnostics with true myopia is based on the implementation of drug cycloplegia.

Accommodation disorders in elderly people are most often caused by age-related changes in the lens: its size, mass, color, shape and, most importantly, consistency, which are mainly associated with the characteristics of its growth and biochemical shifts (see the corresponding section).

The gradual decrease in the elasticity of the lens is the cause of the age-related physiological weakening of the volume of absolute accommodation, established by F. C. Donders in 1866. According to his data (Fig. 5.8), with emmetropia, the nearest point of clear vision gradually moves away from the eye with age, which leads to a decrease in the volume of accommodation. At the age of 65-70 years, the nearest and farthest points of clear vision coincide. This means that the accommodative ability of the eye is completely lost.

The weakening of accommodation in old age is explained not only by the compaction of the lens, but also by other reasons: degenerative changes in the zonule and a decrease in the contractility of the ciliary muscle. It has been established that with age, changes do occur in the ciliary muscle that can lead to a decrease in its strength. Distinct signs of involutional dystrophy of the ciliary muscle appear already at the age of 35-40 years. The essence of the dystrophic changes in this muscle, which slowly increase, consists in the cessation of the formation of muscle fibers, their replacement with connective tissue and fatty degeneration. Gradually, the nature of the muscle structure is disrupted.

Despite these significant changes in the ciliary muscle, its contractile ability is largely preserved, although weakened, due to adaptive-compensatory mechanisms. The relative insufficiency of the ciliary muscle is also aggravated by the fact that, due to the decrease in the elasticity of the lens, the muscle has to work harder to ensure the same degree of curvature. The possibility of secondary atrophic changes in the ciliary muscle due to its insufficient activity in old age is also possible.

Thus, the weakening of the contractile ability of the ciliary muscle plays a certain role in the age-related decrease in the volume of accommodation. However, the main reasons for this are undoubtedly the compaction of the lens substance and the decrease in its elasticity.

The development of presbyopia is based on the process of decreasing the volume of accommodation, which occurs throughout life. Presbyopia manifests itself only in old age, when the distance of the nearest point of clear vision from the eye is already significant and this point approaches the average working distance (approximately 33 cm).

The term "presbyopia" (from the Greek presbys - old man, opsis - vision) - "senile vision" - does not reflect the essence of the process and is a broader, collective concept. The term "senile farsightedness" cannot be considered successful either, since presbyopia differs significantly from hypermetropia in its mechanism of origin and clinical manifestations.

In people with emmetropia, presbyopia usually begins to manifest itself at the age of 40-45 years. During this period, the closest point of clear vision moves away from the eyes by approximately 23-31 cm, i.e. approaches the average working distance (33 cm). For clear recognition of objects at this distance, an accommodation strain of approximately 3.0 D is required. Meanwhile, at the age of 45, the average value of the accommodation volume is only 3.2 D (see Fig. 5.9). Consequently, it is necessary to use up almost the entire accommodation volume remaining at this age, which causes its excessive strain and rapid fatigue.

In hyperopia, presbyopia occurs earlier, in myopia - later. This is due to the fact that in people with hyperopia, the closest point of clear vision is further from the eyes and its removal beyond the average working distance occurs faster with age than in people with emmetropia. In people with myopia, on the contrary, the area of accommodation is closer to the eye, it is necessary to strain accommodation during work at a close distance only with myopia less than 3.0 D, therefore, symptoms of presbyopia with a greater or lesser delay can occur only with a weak myopia. With uncorrected myopia of 3.0 D or more, presbyopia does not appear.

The main symptom of uncorrected presbyopia is difficulty in examining small objects at close range. Recognition of the latter is somewhat facilitated if they are moved some distance away from the eyes. However, with a significant distance of visual work objects, their angular dimensions decrease and recognition again deteriorates. The resulting fatigue of the ciliary muscle, caused by its excessive tension, can lead to visual fatigue.

Anything that causes even a short-term removal of the nearest point of clear vision from the eyes and worsens the distinguishability of objects of visual work contributes to an earlier manifestation of presbyopia and a greater expression of its symptoms. In this regard, all other things being equal, presbyopia occurs earlier in people whose everyday or professional activities are associated with examining small objects. The lower the contrast of objects with the background, the stronger this factor acts. Difficulties in visual work at close range in people with presbyopia increase with reduced illumination due to some removal of the nearest point of clear vision from the eyes. For the same reason, the manifestations of presbyopia increase with visual fatigue.

It is also noted that with the onset of cataract, the manifestations of presbyopia may appear somewhat later or weaken if presbyopia already exists. On the one hand, this is explained by some increase in the volume of accommodation due to hydration of the lens substance, which prevents a decrease in its elasticity, on the other hand, by some shift in clinical refraction towards myopia and the approach of the further point of clear vision to the eye. Thus, an improvement in vision with presbyopia can serve as an early sign of the onset of cataract. The principles of presbyopia correction will be described below.

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