Strabismus - what's going on?

Sensory adaptation to strabismus

The visual sensory system in children is able to adapt to pathological conditions (confusion and diplopia) through two mechanisms: suppression, abnormal retinal correspondence. Their occurrence is associated with the plasticity of the developing visual system in children up to 6-8 years old. Adults with strabismus are only rarely able to ignore the second image and not experience diplopia.

Suppression occurs when the visual cortex actively suppresses the image coming from one eye with both eyes open. Stimuli for suppression include diplopia, confusion, and defocused images due to astigmatism or anisometroia. Clinically, suppression is subdivided as follows:

• central or peripheral. In central suppression, the image from the fovea of the deviated eye is suppressed to avoid confusion. Diplopia, on the other hand, is eliminated by peripheral suppression, which suppresses the image from the peripheral retina of the deviated eye;
• monocular or alternating. Suppression is monocular if the image from the dominant eye dominates over the image from the deviated (or ametronic) eye, the image of the latter is constantly suppressed. This type of suppression leads to the development of amblyopia. If suppression is alternating (i.e. the image from one and the other eye is alternately suppressed), then amblyopia does not develop;
• obligatory or optional. Optional suppression occurs only when the eyes are in the wrong position. Obligatory suppression is constant, regardless of the position of the eyes.

Anomalous retinal correspondence is a condition in which noncorresponding retinal elements require a common subjective visual direction: the fovea of the fixating eye is paired with the nonfoveal element of the deviating eye. Anomalous retinal correspondence is a positive sensory adaptation to strabismus (as opposed to suppression) that maintains some binocular vision with limited fusion in the presence of heterotropia. Anomalous retinal correspondence is most common in small-angle esotropia and is rare in accommodative strabismus due to angle inconsistency or at large angles due to separation of the retinal images. Anomalous retinal correspondence is also rare in exotropia due to the frequent intermittent deviation. When strabismus occurs, the following occurs:

• the fovea of the squinting eye is suppressed to eliminate confusion;
• diplopia occurs because non-corresponding retinal elements receive the same image;
• to avoid diplopia, the phenomenon of peripheral suppression of the squinting eye or abnormal retinal correspondence occurs;
• the occurrence of suppression leads to disbinocular amblyopia.

The disadvantage of the impaired abnormal retinal correspondence is that after surgical correction of strabismus, the patient does not acquire normal retinal correspondence, so the angle of strabismus can be restored when trying to restore binocular vision.

Motor adaptation to strabismus

It is expressed in a change in the position of the head and occurs in those adults who do not have the suppression phenomenon, or in children with potentially good binocular vision. In strabismus, the forced position of the head allows maintaining binocular vision and eliminates diplopia. The head is turned towards the zone of action of the affected muscle, thus the gaze is diverted to the opposite side, as far as possible from the zone of the affected muscle (turning the head to the side in which turning the eyeballs is impossible).

Horizontal deviation is characterized by a turn of the face. For example, if one of the horizontal muscles that turns the eyeballs to the left is paralyzed, turning the face to the left will compensate for the lack of movement in this direction.

Vertical deviation is characterized by the chin rising or falling. When one of the levators is weak, the chin rises, thus causing a relative lowering of the eyeballs.

Torsion deviation is characterized by a tilt of the head toward the right or left shoulder. For example, with paralysis of the intortor (the superior oblique muscle of the left eye), the left eye will be in a state of extorsion. Tilting the head toward the right shoulder effectively compensates for the deviation of the left eye.

As a rule, the head tilt accompanies the vertical deviation. The tilt towards the eye with hypotropia is determined not by the vertical deviation, but by the accompanying (but less pronounced) torsional deviation.