Friendly strabismus
Last reviewed: 23.04.2024
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[What causes a friendly strabismus?
The causes of friendly strabismus can be congenital and acquired diseases of the central nervous system, ametropia, a sharp decrease in visual acuity or blindness in one eye. The immediate causes of the emergence of friendly strabismus are the lack of precise alignment of the visual axes of eyeballs with the object of fixation and the impossibility of keeping them on this object, since the main regulator (binocular vision) is upset.
The most important factor in the emergence of friendly strabismus is the accommodation-refractive factor. The optimal relationship between accommodation and convergence is in the conditions of emmetropy: each diopter of accommodation corresponds to one metropolitan convergence. At farsighted accommodation is excessively strengthened. As a result of this, hyperopia develops an increased impulse to convergence. On the contrary, with short-sightedness, the need for accommodation is either significantly reduced or absent. This weakens the incentive for convergence. Thus, with uncorrected hypermetropia, there is a tendency for convergent strabismus, with uncorrected myopia - to divergent.
The nature of friendly strabismus is associated with an inborn lack of ability to develop fusions (fusional theory) and an inherent lack of binocular vision (functional theory). A number of researchers assign an important role to heredity, and not a squint but a set of factors contributing to its appearance is inherited.
Symptoms of Friendly Strabismus
Under the primary angle of deviation is understood the angle of deflection of the mowing eye, under the secondary one - the angle of deviation of the healthy eye. To determine the angle of strabismus, Hirschberg's method is convenient. The patient fixes the eye of the opening of the manual ophthalmoscope, and the doctor observes from a distance of 35-40 cm behind the position of light reflexes on the cornea of one and the other eye. The coincidence of the light reflex with the pupil's edge (at an average width of 3.5 mm) corresponds to the angle of strabismus at 50 °, the reflex on the iris near the edge of the pupil is 20 °, at the midpoint of the distance between the pupil's edge and the limb is 30 °, on the limb 45 ° , on the sclera in 3 mm from the limb - 60 °.
According to the clinical classification of friendly strabismus, the following types of strabismus are distinguished: periodic, constant, monolateral (mows one eye), alternating (alternately mowing both eyes), converging (eye deviates from fixation point to nose), divergent (eye deviates to the temple), supraverting strabismus upward), infravagating (strabismus downwards). A friendly strabismus is called accommodative if, under the influence of wearing glasses, deviation is eliminated, and unaccustomed, when the optical correction does not affect the position of the mowing eye. If the angle of deflection when wearing glasses is not completely eliminated, one speaks of a partially accommodative strabismus.
Accommodation strabismus
Accomodation strabismus is formed in 2-4 years with uncorrected hypermetropia above the age norm (+3 diopters).
In these years, the child begins to consider closely located and small objects, which makes more demands for accommodation. Excessive accommodation tension, especially with uncorrected hypermetropia, causes an excessive reflex to convergence. The eyes deviate to the inside, initially impermanent, and then quite quickly the strabismus becomes permanent.
Partially accommodative strabismus has all the features of accommodative strabismus, as well as motor disorders: incomplete abduction, nystagmus with extreme positions of the eyes, vertical deviations.
At the heart of non-accommodation strabismus lie paresis of oculomotor muscles caused by intrauterine and birth trauma or a disease in the postnatal period.
Regardless of the type of strabismus, complications arise that complicate its correction: scotoma of inhibition, dysbinocular amblyopia, abnormal correspondence of the retina.
The scotoma of inhibition is the suppression by the consciousness of the image coming from the mowing eye, which frees the patient from doubling. Once the fixing eye is turned off from the act of binocular vision (covered), the scotoma disappears and the central vision in the mowing eye is restored. Therefore, cattle inhibition is also called a functional scotoma.
With monocular strabismus, the constantly existing scotoma of inhibition can lead to a significant decrease in the sight of the mowing eye, despite the absence of changes on the eye bottom. Such a decrease in the sight of a mowing eye without visible organic causes is called amblyopia due to disuse or dysbinocular amblyopia.
An adaptive reaction of the eye, relieving the patient of diplopia (double vision), is abnormal correspondence of the retina. Its essence lies in the fact that a new functional connection arises between the yellow spot of the mowing eye and the retinal part, onto which the image of the object in the mowing eye falls, adapting the rejected eye to the binocular vision at the angle of strabismus. In this case, binocular vision is inadequate, this fusion of images does not occur (simultaneous vision is noted).