Classification of ametropia

To create a working, that is, having a practical orientation, the classification of amethropy requires the identification of a number of characteristics. One of the variants of this classification is as follows.

Working classification of ametropia

Symptom	Clinical manifestations
Correspondence of physical refraction to the size of the eye	Severe refraction (myopia) Weak refraction (hypermetropia)
Sphericity of the optical system of the eye	Conditionally spherical (without astigmatism) Aspherical (with astigmatism)
The degree of ametropia	Weak (less than 3.0 D)
	The average (3.25-6.0 D)
	High (more than 6.0 D)
Equality or inequality of refraction values of both eyes	And the zomotropic
	Anisometropic
Time of formation of ametropia	Congenital
	Rapopreobretepnaya (at the preschool age)
	Acquired at school age
	Late-acquired
Features of pathogenesis	Primary
	Secondary (induced)
The nature of the effect on the anatomofunkionnoe state of the eye	Complicated
	Uncomplicated
Stability of refraction	Stationary
	Progressive

Some items of this classification need clarification.

1. Although the separation of ametropia is weak (3.0 diopters and less), medium (3.25-6.0 diopters) and high (6.0 diopters and more) degree has no clear justification, it is advisable to adhere to these grades, which have become generally accepted. This will avoid different interpretations when establishing a diagnosis, as well as obtain comparable data in the conduct of scientific research. From a practical point of view, one should take into account the fact that high-grade ametropia, as a rule, is complicated.
2. Depending on the equality or inequality of refraction values of both eyes, it is necessary to distinguish isometropic (from Greek isos - equal, metron - measure, opsis - vision) and anisometropic (from Greek anisos - unequal) ametropia. The latter is usually chosen in cases where the difference in refractive index is 1.0 dptr and more. From a clinical point of view, such a gradation is necessary, because significant differences in refraction, on the one hand, have a significant impact on the development of the visual analyzer in childhood, and on the other, make it difficult to binocular correction of ametropia with spectacle lenses (for more details see below) .
3. A common feature of congenital ametropia is a low maximum visual acuity. The main reason for its significant decrease is the violation of conditions for sensory development of the visual analyzer, which in turn can lead to amblyopia. The prognosis is also unfavorable for myopia acquired at school age, which, as a rule, tends to progress. Myopia, which occurs in adults, is often professional, that is, conditioned by working conditions.
4. Depending on the pathogenesis, it is possible to distinguish conditionally the primary and secondary (induced) ametropia. In the first case, the formation of an optical defect is due to a certain combination of anatomical-optical elements (mainly the length of the anteroposterior axis and refraction of the cornea), in the second - ametropia is a symptom of any pathological changes of these elements. Induced ametropia are formed as a result of various changes in both the main refractive media of the eye (cornea, lens) and the length of the anteroposterior axis.

• Changes in the refraction of the cornea (and as a consequence of clinical refraction) can occur as a result of violations of its normal topography of various genesis (dystrophic, traumatic, inflammatory). For example, with keratoconus (dystrophic corneal disease), a significant increase in the refraction of the cornea and a violation of its sphericity are noted (see Figure 5.8, c). Clinically, these changes are manifested in a significant "myopia" and the formation of an incorrect astigmatism.

As a result of traumatic damage to the cornea, corneal astigmatism is often formed, most often incorrect. As for the influence of such astigmatism on visual functions, localization (in particular, remoteness from the central zone), depth and extent of the cornea scars are of primary importance.

In clinical practice, it is often necessary to observe the so-called postoperative astigmatism, which is a consequence of scar tissue changes in the area of the surgical incision. Such astigmatism often occurs after such operations as cataract extraction and corneal transplantation (keratoplasty).

• One of the symptoms of an initial cataract may be an increase in clinical refraction, that is, a shift towards myopia. Similar changes in refraction can occur in diabetes mellitus. Separately, we should dwell on cases of complete absence of the lens (aphakia). Afakia is most often the result of surgical intervention (removal of cataracts), less often - its full dislocation (dislocation) in the vitreous humor (as a result of trauma or dystrophic changes in the cynic ligament). As a rule, the main refractive symptom of aphakia is hypermetropia of a high degree. With a certain combination of anatomical-optical elements (in particular, the length of the anteroposterior axis of 30 mm), the refraction of the aphakic eye may be close to emmetropic or even myopic.
• Situations in which changes in clinical refraction are associated with a decrease or increase in the length of the anteroposterior axis are rarely encountered in clinical practice. These are, first of all, the cases of "myopyzation" after the cirque - one of the operations performed with detachment of the retina. After such an operation, a change in the shape of the eyeball may occur (resembling an hourglass), accompanied by some lengthening of the eye. In some diseases, accompanied by retinal edema in the macular area, a shift of refraction towards the hyperopia can occur. The appearance of such a shift with a certain degree of conventionality can be explained by a decrease in the length of the anteroposterior axis due to the prominence of the retina anteriorly.

5. From the point of view of the effect on the anatomical and functional state of the eye, it is advisable to single out complicated and uncomplicated ametropia. The only symptom of uncomplicated ametropia is a decrease in the uncorrected visual acuity, while the corrected or maximum visual acuity remains normal. In other words, uncomplicated ametropia is only an optical defect of the eye caused by a certain combination of its anatomical-optical elements. However, in a number of cases, ametropia can serve as the cause of the development of pathological conditions, and then it is appropriate to talk about the complicated nature of ametropia. In clinical practice, the following situations can be identified in which the causal relationship between ametropia and pathological changes in the visual analyzer can be traced.

• Refractive amblyopia (with congenital ametropia, astigmatism, refractive anomalies with anisometropic component).
• Strabismus and a violation of binocular vision.
• Asthenopia (from the Greek astenes - weak, opsis - sight). This term combines various disorders (fatigue, headache), which arise from visual work at close range. The accomodative asthenopia is caused by overstrain of accommodation with long-term work at close range and occurs in patients with hypermetropic refraction and a reduced supply of accommodation. The so-called muscle asthenopia can occur with inadequate correction of myopia, as a result of which it is possible to increase convergence in connection with the need to examine objects at close range. D Anatomic changes. With progressive high degree myopia due to significant stretching of the posterior pole of the eye, changes in the retina and optic nerve occur. Such short-sightedness is called complicated.

6. From the point of view of the stability of clinical refraction, stationary and progressive ametropia should be isolated.

The true progression of ametropia is characteristic of myopic refraction. Progression of myopia occurs due to the extension of the scleral membrane and an increase in the length of the anteroposterior axis. To characterize the rate of progression of myopia, the annual gradient of its progression is used:

ГГ = СЭ2-СЭ1 / Т (diopters / year),

Where GG is the annual gradient of progression; SE2 is the spherical equivalent of the refraction of the eye at the end of the observation; SE1 - the spherical equivalent of the refraction of the eye at the beginning of the observation; T is the time interval between observations (years).

With a yearly gradient of less than 1 D, myopia is considered slowly progressing, with a gradient of 1.0 D and more rapidly progressing (it is necessary to solve the problem of performing an operation that stabilizes the progression of myopia, scleroplasty). In assessing the dynamics of myopia, repeated measurements of the length of the axis of the eye with the help of ultrasonic methods can help.

Among the progressing secondary (induced) ametropia, first of all, it is necessary to isolate the keratoconus. In the course of the disease, four stages are distinguished, the progression of keratoconus is accompanied by increased refraction of the cornea and abnormal astigmatism against a noticeable reduction in maximum visual acuity.

[1], [2]