Anisometropia in children and adults

Among the existing pathologies of vision, ophthalmologists noted anisometropia. What it is? This is a refractive imbalance - when a person's right and left eyes have unequal refractive power, and this difference can be several diopters. This violation of refraction (ametropia) in the ICD-10 has the code H52.3. [1]

Epidemiology

Some studies have reported an increase in the prevalence of anisometropia with age [2],  [3] while others have shown a non-linear relationship between age and anisometropia [4],  [5]or a lack of relationship between age and the prevalence of anisometropia. [6],  [7]Gender differences in the prevalence of anisometropia in schoolchildren, as a rule, were not found. [8], [9]However, it has been reported that the prevalence of anisometropia and astigmatic anisometropia [10]may be higher in girls than in boys.

The prevalence of anisometropia at various ages is approximately 2% on average (range from 1% to 11%).

This anomaly of refraction is detected in about 6% of children aged 6-18 years.

Atkinson and Breddik [11],  [12]showed that less than 1.5% of children (aged 6 to 9 months) anisometropia was greater than or equal to 1.5 diopters. Anisometropic amblyopia is less common than anisometropia, and usually affects less than 1.5% of the population.

According to experts, in a third of cases, bilateral refractive disorders of the same magnitude prevail (both eyes are myopic or hyperopic).

Causes of the anisometropia

Despite studies of the structural and biomechanical characteristics of the eyes, as well as the characteristics of the  optical system of the eye , the main causes underlying anisometropia are still not well understood. In children, it is most often congenital, in adults - acquired.

There are various  refractive errors : myopia (myopia), farsightedness (hyperopia), astigmatism, and presbyopia (decreased accommodation capacity due to loss of lens elasticity in old age).

The reason for myopia is too much optical power of the eye (back focal length) or too long sagittal (anteroposterior) axis of the eye, for example, due to lengthening of the eyeball. This leads to the displacement of the main optical focus of the eye in front of the retina of its posterior chamber. When anisometropia and myopia are combined, anisometropic myopia is determined  .

With hypermetropic anisometropia, anisometropia and hyperopia coexist , the reasons for which are also associated with the morphometric features of the eye: a shortened anteroposterior axis or insufficient optical power - with a focus shift beyond the retina.

The reason for the development of anisometropia in some adults is unclear, but, as expected, in most cases is a consequence  of the lazy eye syndrome  (amblyopia). [13]

Acquired anisometropia in adults can also be associated with  age-related changes in refraction  or changes in the lens in one eye against the background of farsightedness.

But anisometropia in children and adolescents is etiologically associated not only with impaired  development of refraction , but also with:

• congenital anatomical ophthalmic defects;
• heredity, which initially determines the state of the optical system of the eyes;
• different eye sizes, for example, with unilateral microphthalmia - a congenital decrease in the eyeball.

Moreover, anisometropia in a teenager with myopia continues to increase throughout adulthood. More information in the material -  Anomalies of refraction in children .

Risk factors

Experts associate risk factors for the development of anisometropia in adults with certain diseases, in particular, myopia, a history of eye injury,  [14]cataracts,  [15]retinal degeneration,  [16]lens displacement, vitreous hernia, ptosis, microvascular complications of diabetes and asymmetric diabetic retinopathy,  [17]diffuse toxophthosis autoimmune connective tissue diseases.

In children, congenital toxoplasmosis, [18]premature retinopathy,  [19]capillary hemangioma of the eyelids, oculomotor glioma (developing within the orbit),  [20]unilateral congenital obstruction of the nasopharyngeal duct, congenital myasthenia gravis  [21], etc. Are considered risk factors .

Pathogenesis

The development mechanism, that is, the pathogenesis of anisometropia is not fully understood.

Perhaps the fact is that very few people are born with the same optical power of both eyes, but the brain compensates for this, and the person does not even suspect that his eyes are not the same.

Hence, the development of the ciliary muscles and their functional usefulness with the growth of the eyeball may be different; weakening of the sclera (the main support of the eyeball); retinal distension due to increased intraocular pressure, etc. [22]

The relationship of anisometropic refractive errors with the difference between dominant and non-dominant eyes during the progression of myopia is studied. As it turned out, with the development of myopia, the size of the left eye increases to a lesser extent than the right - when the right eye is "aimed", that is, dominant (oculus dominans).

In children, the prevalence of anisometropia increases between 5 and 15 years, when in some children the eyes become longer and myopia develops. However, anisometropia accompanying hyperopia, suggests the existence of other mechanisms of refractive imbalance.

Symptoms of the anisometropia

Sometimes anisometropia may be present at birth, although it is often asymptomatic up to a certain age.

Key symptoms of anisometropia are manifested:

• eye strain and visual discomfort;
• deterioration of binocular vision;
• diplopia (double vision), which is accompanied by dizziness and headaches;
• hypersensitivity to light;
• decrease in the level of contrast of vision (visible images are blurred);
• the difference in the field of view of the eyes;
• violation of stereopsis (lack of perception of the depth and volume of objects).

Anisometropia and aniseikonia. A symptom of a pronounced difference in the refractive power of the eye is aniseikonia - a violation of the fusion of images, as a result of which one sees a smaller image with one eye and a larger image in the other with the other. In this case, the overall image is blurry. [23]

Forms

The following types of anisometropia are distinguished: [24]

• simple anisometropia, in which one eye is nearsighted or farsighted, and the refraction of the second eye is normal;
• complex anisometropia, when there is bilateral myopia or hyperopia, but its magnitude in one eye is higher than in the other;
• mixed anisometropia - with the myopia of one eye and the farsightedness of the other. 

In addition, three degrees of anisometropia are determined:

• weak, with a difference between the eyes of up to 2.0-3.0 diopters;
• medium, with a difference between the eyes of 3.0-6.0 diopters;
• high (more than 6.0 diopters).

Complications and consequences

During the development of the optical system of the eye, anisometropia leads to  amblyopia . It is believed that almost a third of all cases of uncorrectable amblyopia are caused by anisometropia. This is explained by a violation of binocular vision, when the visual cortex of the brain during its development (during the first 10 years of life) does not use both eyes together, suppressing the central vision of one of them. [25],  [26], [27]

At the same time, the risk of amblyopia is approximately two times higher with hyperopia.

In addition, the consequences and complications of anisometropia include strabismus or  strabismus in children who suffer from at least 18% of patients with this type of ametropia, as well as accommodation esotropia (convergent strabismus) and exotropia (divergent strabismus).

Diagnostics of the anisometropia

Early detection and treatment of anisometropia are important for the development of optimal visual function.

Anisometropia can initially be detected by checking the binocular red reflex of each eye using the Bruckner test.

Read more about how the diagnosis of refractive errors is carried out, read in a separate publication -  Eye examination .

Be sure to carry out instrumental diagnostics, see -  Methods for the study of refraction

The goal of differential diagnosis is to identify congenital anomalies of the eyeball, lens, vitreous body, retina, one way or another affecting the refractive power of the eyes.

Treatment of the anisometropia

Currently, the initial treatment for young patients who have anisometropia and amblyopia begins with optical correction and then adds additional treatment if necessary (for example, occlusion). [28] If the human visual system demonstrates the process of isoemetropization, it is advisable to leave these patients without treatment in order to allow anisometropia to disappear and, therefore, to improve the quality of the retina image in the amblyopic eye. 

The most effective correction methods are presented in the materials:

• Correction of refractive errors in children
• Vision correction with spectacle lenses [29]

By the way, with a high degree of anisometropia, the glasses do not give the desired effect, moreover, they can aggravate the binocular vision impairment, therefore they use contact lenses, in detail in the article -  Contact vision correction . [30]

Surgical treatment of anisometropia and its methods are given in the publications:

• Surgical Ametropia Correction
• Excimer laser correction of refractive errors

Prevention

There are no special methods for the prevention of anisometropia.

Forecast

Mild anisometropia may disappear during the development of eye refraction. An average degree (≥ 3.0 diopters) can persist for a long time, and amblyopia often appears in preschool children.

With age - after 60 years - the risk of anisometropia increase only increases.