Classification of ametropia

To create a working, i.e., practical, classification of ametropia, it is necessary to identify a number of features. One of the variants of such a classification is as follows.

Working classification of ametropia

Sign	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) Aspheric (with astigmatism)
Degree of ametropia	Weak (less than 3.0 Dptr)
	Average (3.25-6.0 Dptr)
	High (more than 6.0 Dptr)
Equality or inequality of refractive values of both eyes	And so tropical
	Anisometropic
Time of formation of ametropia	Congenital
	Acquired (at preschool age)
	Acquired at school age
	Late acquired
Features of pathogenesis	Primary
	Secondary (induced)
The nature of the influence on the anatomical functional state of the eye	Complicated
	Uncomplicated
Refractive stability	Stationary
	Progressive

Some points of this classification require clarification.

1. Although the distinction between weak (3.0 D and less), moderate (3.25-6.0 D) and high (6.0 D and more) ametropia has no clear justification, it is advisable to adhere to the specified gradations that have become generally accepted. This will help to avoid misinterpretations when establishing a diagnosis, as well as to obtain comparable data when conducting scientific research. From a practical point of view, it should be taken into account that high-degree ametropias are usually complicated.
2. Depending on the equality or inequality of the refraction values of both eyes, a distinction should be made between isometropic (from the Greek isos - equal, metron - measure, opsis - vision) and anisometropic (from the Greek anisos - unequal) ametropia. The latter is usually distinguished in cases where the difference in refraction values is 1.0 diopters or more. From a clinical point of view, such 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 hand, complicate binocular correction of ametropia using spectacle lenses (see below for more details).
3. A common feature of congenital ametropia is low maximum visual acuity. The main reason for its significant reduction is the disruption of conditions for the 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 that occurs in adults is often professional, i.e. caused by working conditions.
4. Depending on the pathogenesis, primary and secondary (induced) ametropias can be conditionally distinguished. In the first case, the formation of an optical defect is caused by a certain combination of anatomical and optical elements (mainly the length of the anteroposterior axis and corneal refraction), in the second, ametropia is a symptom of some pathological changes in these elements. Induced ametropias 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 corneal refraction (and, as a consequence, clinical refraction) may occur as a result of disturbances in its normal topography of various genesis (dystrophic, traumatic, inflammatory). For example, in keratoconus (a dystrophic disease of the cornea), significant increases in corneal refraction and disturbances in its sphericity are observed (see Fig. 5.8, c). Clinically, these changes are manifested in significant "myopization" and the formation of irregular astigmatism.

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

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

• One of the symptoms of initial cataract may be an increase in clinical refraction, i.e. its shift towards myopia. Similar changes in refraction may be observed in diabetes mellitus. Cases of complete absence of the lens (aphakia) should be discussed separately. Aphakia is most often a consequence of surgical intervention (cataract removal), less often - its complete dislocation (dislocation) into the vitreous body (as a result of injury or degenerative changes in the zonular ligaments). As a rule, the main refractive symptom of aphakia is high hyperopia. With a certain combination of anatomical and 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 quite rare in clinical practice. These are primarily cases of "myopization" after cerclage - one of the operations performed for retinal detachment. After such an operation, the shape of the eyeball may change (resembles an hourglass), accompanied by some elongation of the eye. In some diseases accompanied by retinal edema in the macular zone, a shift in refraction towards hyperopia may be observed. The occurrence of such a shift can be explained with a certain degree of conventionality by a decrease in the length of the anteroposterior axis due to the prominence of the retina to the front.

5. From the point of view of the influence on the anatomical and functional state of the eye, it is appropriate to distinguish complicated and uncomplicated ametropia. The only symptom of uncomplicated ametropia is a decrease in 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 and optical elements. However, in some cases, ametropia can serve as the causes 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 distinguished in which a causal relationship between ametropia and pathological changes in the visual analyzer can be traced.

• Refractive amblyopia (with congenital ametropia, astigmatism, refractive anomalies with an anisometropic component).
• Strabismus and impaired binocular vision.
• Asthenopia (from the Greek astenes - weak, opsis - vision). This term covers various disorders (fatigue, headache) that occur during visual work at close range. Accommodative asthenopia is caused by accommodation overstrain during prolonged work at close range and occurs in patients with hypermetropic refraction and reduced accommodation reserve. So-called muscular asthenopia can occur with inadequate correction of myopia, as a result of which convergence may increase due to the need to examine objects at close range. G Anatomical changes. With progressive high myopia, changes in the retina and optic nerve occur due to significant stretching of the posterior pole of the eye. Such myopia is called complicated.

6. From the point of view of the stability of clinical refraction, one should distinguish between stationary and progressive ametropia.

True progression of ametropia is characteristic of myopic refraction. Progression of myopia occurs due to stretching of the sclera and increase in the length of the anteroposterior axis. To characterize the rate of progression of myopia, the annual gradient of its progression is used:

GG = SE2-SE1/T (Dopters/year),

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

With an annual gradient of less than 1 D, myopia is considered slowly progressing, with a gradient of 1.0 D or more - rapidly progressing (in this case, it is necessary to decide on performing an operation that stabilizes the progression of myopia - scleroplasty). Repeated measurements of the length of the eye axis using ultrasound methods can help in assessing the dynamics of myopia.

Among the progressive secondary (induced) ametropias, keratoconus should be singled out first. Four stages are distinguished during the course of the disease, the progression of keratoconus is accompanied by an increase in corneal refraction and irregular astigmatism against the background of a noticeable decrease in maximum visual acuity.

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