Amblyopia

One of the most frequently occurring sensory disturbances in unilateral strabismus is amblyopia, i.e. functional reduction of vision of the eye due to its inactivity, disuse.

Normally, fixation is foveal. Non-central fixation can be parafoveal, macular, paramacular, peridiscal (peripheral), with the image falling on an eccentric area of the retina.

[ 1 ], [ 2 ], [ 3 ]

Causes of amblyopia

According to the mechanism of occurrence, amblyopia can be dysbinocular, i.e. arising as a result of a violation of binocular vision, which is observed with strabismus, when the participation of the deviated eye in the visual act is significantly reduced, or refractive, which is a consequence of the untimely prescription and inconsistent wearing of glasses with ametropia, creating a blurry image on the fundus.

In the presence of uncorrected anisometropia, anisometropic amblyopia occurs. Refractive amblyopia can be overcome quite successfully by means of rational and constant optical correction (glasses, contact lenses).

Clouding of the ocular media (congenital cataract, leukoma) can cause obscuration amblyopia, which is difficult to treat and requires timely surgical intervention to eliminate (for example, extraction of congenital cataract, corneal transplant).

[ 4 ], [ 5 ], [ 6 ], [ 7 ]

Symptoms of amblyopia

Amblyopia can be unilateral or bilateral.

With amblyopia, color and contrast sensitivity also decreases.

When strabismus occurs, double vision inevitably occurs, since the image in the squinting eye falls on the disparate area of the retina, but due to the adaptation mechanisms, the visual-nervous system adjusts to the asymmetric position of the eyes and functional suppression, inhibition, or "neutralization" [according to the terminology of L. I. Sergievsky (1951)] of the image in the squinting eye occurs. Clinically, this is expressed in the occurrence of functional scotoma. Unlike true scotomas observed in organic lesions of the organ of vision, functional scotoma in strabismus exists only if both eyes are open, and disappears with monocular fixation (when the other eye is covered). Functional scotoma is a form of sensory adaptation that eliminates double vision, which is observed in most patients with concomitant strabismus.

In case of monolateral strabismus, the presence of a constant scotoma in the squinting eye leads to a persistent decrease in vision. In case of alternating strabismus, the scotoma appears alternately in the right or left eye depending on which eye is squinting at the moment, so amblyopia does not develop.

One of the forms of sensory adaptation in concomitant strabismus is the so-called abnormal retinal correspondence, or asymmetric binocular vision. Diplopia disappears due to the appearance of the so-called false macula. A new functional connection appears between the fovea of the fixating eye and the area of the retina of the squinting eye, which receives the image due to deviation (deviation of the eye). This form of adaptation is observed extremely rarely (in 5-7% of patients) and only at small angles of strabismus (microdeviations), when the area of the retina of the deviated eye differs little organically and functionally from the fovea. At large angles of strabismus, when the image falls on the insensitive peripheral area of the retina, the possibility of its interaction with the highly functional fovea of the fixating eye is excluded.

Amblyopia degrees

According to the degree of visual acuity reduction, according to the classification of E. S. Avetisov, amblyopia of a low degree is distinguished - with visual acuity of the squinting eye of 0.8-0.4, average - 0.3-0.2, high - 0.1-0.05, very high - 0.04 and lower. High degree amblyopia is usually accompanied by a violation of visual fixation of the squinting eye.

[ 8 ], [ 9 ], [ 10 ], [ 11 ], [ 12 ], [ 13 ], [ 14 ]

Classification of amblyopia

Amblyopia is a unilateral or bilateral (more often) decrease in maximum corrected visual acuity due to deprivation of form vision and/or pathological binocular connections in the absence of organic pathology of the eye and visual pathway.

1. Disbinocular (strabismic) amblyopia develops with pathological binocular connections with prolonged monocular suppression of the deviated eye. Decreased vision is typical even with forced fixation.
2. Anisometropic amblyopia occurs when the difference in refraction is >1 spherical diopter. Abnormal binocular connections occur when different sized focused and unfocused visual images overlap (aniseikonia). An element of deprivation of formed vision also occurs, since a constant projection of a blurred image occurs. It is often combined with microstrabismus and can be combined with disbinocular amblyopia.
3. Obscuration amblyopia occurs with visual deprivation and can be unilateral or bilateral. The cause may be opacity of the optical media (cataract) or grade III ptosis.
4. Isoametropic amblyopia occurs when there is deprivation of formed vision. Bilateral amblyopia is usually caused by symmetrical refractive errors, most often with hyperopia.
5. Meridional amblyopia occurs when there is visual deprivation in one meridian and can be unilateral or bilateral. The cause is uncorrected astigmatism.

[ 15 ], [ 16 ], [ 17 ], [ 18 ], [ 19 ], [ 20 ], [ 21 ]

Diagnosis of amblyopia

Visual acuity. In the absence of organic changes, a difference in corrected visual acuity of two lines or more indicates amblyopia. Visual acuity in amblyopia and the process of studying visual acuity by individual optotypes is higher than by line. This phenomenon of "crowding" can also occur normally, but is more pronounced in amblyopia.

The neutral dense filter allows indirect differentiation of reduced vision in organic pathology from amblyopia. The filter reduces visual acuity in the norm by two lines. It is used in the following cases:

• when determining visual acuity with correction;
• when determining visual acuity with a filter
  installed in front of the eye;
• If visual acuity does not decrease when using the filter, this indicates amblyopia;
• If visual acuity decreases when using a filter, then the presence of organic pathology is assumed.

Visual acuity determined by sinusoidal gratings (i.e. the ability to distinguish gratings of different spatial frequencies) is often higher than visual acuity determined by Snellen optotypes.

[ 22 ], [ 23 ], [ 24 ], [ 25 ], [ 26 ], [ 27 ]

Amblyopia: Treatment with Occlusion, Pleoptics and Penalization

The sensitive period during which treatment of amblyopia is effective is 7-8 years for dysbinocular amblyopia and 11-12 years for anisometropic amblyopia.

Pleoptics is a section of ophthalmology that develops methods for treating amblyopia, which affects about 70% of children with strabismus. The main goal of treating amblyopia is to achieve visual acuity that allows for binocular vision. This should be considered as visual acuity equal to 0.4 D or higher. Treatment of amblyopia begins after wearing glasses.

The main methods of treating amblyopia include direct occlusion, treatment using a negative sequential image, and local “blinding” stimulation of the central fovea of the retina with light.

Occlusion is the exclusion of one eye from the act of vision. The purpose of permanent exclusion of the leading eye is to achieve equal visual acuity in both eyes and to convert monolateral strabismus into alternating. Such treatment is carried out for at least four months.

Occlusion of the healthy eye to increase the visual load of the amblyopic eye is the most effective treatment method. The mode of wearing the occluder (all day or periodically) depends on the patient's age and the degree of amblyopia. The younger the patient, the faster the improvement occurs, but at the same time the risk of amblyopia in the healthy eye increases. In this regard, during the treatment it is necessary to monitor the visual acuity of both eyes. The higher the visual acuity when occlusion is prescribed, the shorter the time of wearing the occluder. If visual acuity does not improve within 6 months, it is unlikely that the treatment will be effective.

The use of a negative successive image consists of illuminating the retina of the posterior pole of the eye while simultaneously covering the uveal zone with a ball. As a result, a successive visual image arises, which has a central field corresponding to the covering object.

Local "blinding" stimulation of the central fovea of the retina with light consists of stimulation of the central fovea with light from a pulsed lamp or a helium-neon laser introduced into the system of a large non-reflex ophthalmoscope.

Orgoptics - development of binocular vision. As soon as orthophoria is established under the influence of treatment or surgery with visual acuity of the amblyopic eye of 0.4 and higher, exercises for the development of binocular vision should be added to pleoptics. This treatment is carried out on haploscopic devices - synoptophores.

The synoptophore is an improved stereoscope. It consists of two tubes with eyepieces, through which each eye is presented with a drawing separately. If the patient has the ability to merge foveal images of objects, exercises are performed on the synoptophore to develop fusion reserves.

If strabismus is not eliminated after a set of pleopto-orthoptic exercises, then surgical treatment is used. In some cases (usually with large angles of strabismus), surgical intervention may precede pleopto-orthoptic treatment.

Penalization is an alternative method in which the vision of the better seeing eye is blurred by instillation of atropine. The method can be effective in the treatment of mild amblyopia (6/24 and above) when combined with hyperopia. Penalization does not produce an effect as quickly as occlusion and is effective only if the vision of the normal eye under penalization is lower than the visual acuity of the amblyopic eye, at least when fixating a close object.

[ 28 ], [ 29 ], [ 30 ], [ 31 ], [ 32 ]