So, there are three kinds of clinical refraction: emmetropia, hypermetropia and myopia. The refraction depends on two factors: the force of the eye refractive apparatus and length of the eye axis. The majority of children are born with hypermetropia and short eye axis. In the process of organism growth the eye grows also, the degree of hypermetropia decreases up to 20-25 years, when the refraction is formed the hypermetropia remains in 50-55% of the people, the emmetropia comes to 30% and in the others (20-25%) the nearsightedness and astigmatism develops.
Emmetropia (E) — the proportional refraction. Such an eye connects the parallel rays on the retina. It is established on the indefinitely remote point. The people with emmetropia can see well both at a distance and closely; with the age, after 40 years old when accommodation starts to weaken they begin to use glasses only for near vision.
At hyperopia (H) the main focus lies behind the eye, and the circle of light dispersion is formed on the retina. In order that the focal point has got on retina, the rays must gather in the front of the eye and come into the eye already convergent. Such rays are not present in the nature, therefore hypermetropic eye does not see well either far, or near, all the time it must accommodate.
That part of hypermetropia, which is determined by lenses, is named as obvious hypermetropia, and that, which is hidden by accommodation tension is latent hypermetropia. The full hypermetropia is the sum of its obvious and latent parts. Degrees of hypermetropia: weak — up to 2 D, mean — up to 5 D and high — more then 5 D.
In connection with that hypermetropic eye always accommodates, and the fatigue of the eye comes while working without glasses. There is feeling of pressure, nausea, dull ache around the eye and in the forehead. It is the accommodative asthenopia. In small children the development of convergent strabismus, spasm of accommodation, refractive amblyopia is possible. Frequently chronic blepharoconjunctivitis is watched.
The weak degrees of hypermetropia, at which the vision is normal do not require the correction. When the hypermetropia causes asthenopic phenomena, it requires the correction by biconvex lenses the same as in hypermetropia of mean and high degrees.
The hypermetropia is only mismatch between dioptric force of the eye and length of eye axis because of this any changes on fundus of the eye is not watched. Sometimes in cases of its high degree the boundaries of optic papilla seem unclear. Such picture is like in neuritis, but as the visual functions are not broken, the treatment is not required.
Myopia — is a refraction, at which the parallel rays converge before the retina. The further point of clear vision lies in front of eye. The visual acuity at a distance is reduced considerably, closely — it is good. There are the circles of light dispersion on the retina. To reduce them the people with myopia must screw up their eyes. In such a way the visual acuity increases. There are three degrees of myopia: weak — up to 3.0 D, mean — 3.0-6.0 D and high — more than 6.0 D. In etiology of myopia the following factors have a large value:
1. Genetic predisposition (Steiger's theory).
2. Unfavorable conditions of environment (professional and school myopia).
3. The primary weakness of accommodation, that conducts to compensatory stretch (Avetisov's theory).
4. Disorder between accommodation and convergencia that conducts to spasm of accommodation (false myopia) which then turns into true myopia.
5. Changes in the posterior part of the eye — dystrophy processes in a sclera that promotes its stretch and increase of length of eye axis.
In the pathogenesis of nearsightedness the lengthening of the eye axis plays a great role. The malignant myopia is accompanied by pathologic processes on the fundus of the eye. There are subjective complains (flying flies), which can be explained by opacities of vitreous, decrease of visual acuity into the distance, the muscle asthenopia, spasm of accommodation, development of divergent strabismus. At ophthalmoscopy near temporal edge of optic papilla it is marked a white sickle — myopic conus, at high degree of myopia — posterior staphyloma (false). At malignant myopia it may be limited sclera prominence — true staphyloma.
If myopia degree increases by 0.5 D within a year, it is considered as slowly progressive myopia, if more than ID — it is fast progressive myopia. At high degree of nearsightedness, as a rule, the pathologic changes on fundus of the eye develop. It is a so-called complicated nearsightedness. The most severe myopic changes, which lead to loss of central vision are the changes in the field of a macula lutea, i.e. central chorioretinites. These are atrophic processes owing to strong stretch of posterior part of eyeglobe. In the course of time the white foci of irregular configuration in the field of macula lutea occur. They can occupy a large part of fundus of the eye. Quite often in the macula lutea field the pigmented focus (Fuks spot) develops after a haemorrhage or without it. There can be too heavy complications: detachment of retina, retinal or vitreous haemorrhage. At high myopia the complicated nuclear cataracts and rough opacities of the vitreous frequently develop.