Strabismus (heterotropia) means disruption in the position of one or both eyes during which looking forward results in the deviation. With the symmetric position of the eyes, the image of objects falls on the central sections of the retina of both eyes. The cortical sections of the visual analyzer merge the two images into a single binocular picture. In case of strabismus, this merging does not occur. In order to prevent ghosting vision the central nervous system blocks the image coming from the squinting eye. The lengthy existence of this state leads to amblyopia (a functional and reversible reduction of vision during which one of the eyes is not involved partly or fully in the vision process).
Causes for development of strabismus
Strabismus may be caused by numerous factors of an inherent or acquired nature:
- the existence of ametropia (hyperopia, myopia, astigmatism) of a moderate or high degree;
- paralysis and paresis;
- anomalies in the development and attachment of eye muscles;
- diseases of the central nervous system;
- infectious diseases (measles, scarlet fever, diphtheria, flu, etc.);
- somatic diseases;
- psychic traumas (fright);
- sharp reduction of vision acuity of one eye.
Symptoms of strabismus
The normal eyesight of a person is binocular which means vision with two eyes with the visual analyzer (in the brain cortex) combining the image produced by each eye into a single picture. Binocular vision provides stereoscopic vision, i.e. the potential to see the surrounding world in three dimensions, to estimate the distance between objects, to perceive the depth and the corporal nature of the surrounding world. In case of strabismus, the visual analyzer fails to ensure this combination and the central nervous system excludes images from the squinting eye in order to avoid ghosting images.
Type of strabismus in terms of its nature
Specialists differentiate two forms of strabismus: conjugated and paralytic.
In case of conjugated strabismus, it sometimes the left or at times the right eye that squints. The deviation from direct position is approximately same.
Practical medicine shows that strabismus develops in patients with ametropia and anisometropia with dominating hyperopia. Incidentally, hyperopia dominates in cases of conjugated strabismus while short sightedness dominates diverging strabismus. The main cause of conjugated strabismus, more often lies in ametropia and the more it is manifested the greater its role in the development of the pathology.
Specialists also consider the following factors leading to conjugated strabismus:
- the state of the visual system when the visual acuity of one eye is much lower than that of the other;
- disorders of the visual system leading to dramatically reduced vision or blindness;
- uncorrected ametropia (myopia, hypermetropia, astigmatism);
- transparency deterioration of the refracting medium of the eye;
- disorders of the retina and the optic nerve;
- diseases and damage of the central nervous system;
- congenital difference in the anatomic structures of the eyes
Conjugated strabismus is accompanied with the following main symptoms:
- with vision fixed to a stationary object, one of the eyes is turned sideways (towards the nose, the temple, upward, downward);
- there may be alternating deviation of one or the other eye;
- the angle of (primary) deviation–of the (often or permanently) squinting eye, when involved in vision, almost always is equal to the deviation angle (secondary) of the other eye;
- the mobility of the eye (field of vision) is retained in full in all directions;
- there is no image diplopia;
- there is no binocular (3D, stereoscopic) vision ;
- there may be poor vision in the squinting eye;
- frequently, various types of ametropia are detected (far sight, short sight, astigmatism) and of various degrees (asiometropia).
Paralytic strabismus displays squinting of one eye. The main symptom of this disorder is the limitation or absence of eye movement in the direction of the affected muscle resulting in disruption of binocular vision and image doubling. The causes for this type of strabismus may lie in the affection of corresponding nerves or disrupted morphology and functions of the muscles themselves. These disorders may be of an innate nature or the result of infectious diseases, traumas, tumors, and vascular diseases.
The symptoms of paralytic strabismus are:
- limited or full immobility of the eye in the direction of the affected muscle (or muscles);
- the primary angle of deviation is less than the secondary angle;
- absence of binocular vision, possible image ghosting;
- forced turning of the head in the direction of the affected muscle;
This type of strabismus may occur at any age and be caused by traumas, toxicosis, poisoning, etc.
The following types of strabismus are also identified:
- converging squint (often combined with far sight) when the eye is directed at the bridge of the nose;
- diverging squint (often combined with short sight) when the eye is directed at the temple;
- vertical squint (in cases when the eye is turned up or down).
In case of converging strabismus, the visual axis of one eye is diverted towards the nose. This type of squint usually develops at an early age and is often intermittent. More frequently this type of squint is accompanied by a high degree of hyperopia.
In case of diverging strabismus, the visual axis is directed towards the temple. This type of squint often occurs in the case of inherent or early myopia. The causes of diverting strabismus may be traumas, cerebral diseases, fright, infectious diseases.
There may also be various other factor combinations. Strabismus may be permanent or intermittent
Atypical cases of strabismus are very rare and are caused by anatomic anomalies of development (the Douan, Brown, DVD syndromes, etc.)
Strabismus may be classified on the basis of several criteria.
In terms of time of its occurrence:
In terms of deviation stability:
In terms of origin:
In terms of type of deviation:
- converging (the eye is pointed at the bridge of the nose)
- diverging (the eye is pointed at the temple)
- vertical (the eye is turned upward or downward)
Diagnostics of strabismus
A comprehensive testing of the visual system is required in order to confirm or refute the diagnosis of strabismus. The Excimer Eye Clinic performs diagnostics with help of a complex of up-to-date computerized equipment which provides a complete analysis of the patient’s vision. One of the diagnostic criteria for strabismus is binocular vision testing.
Treatment for strabismus
In case of strabismus, it is the eye which is employed in vision that retains normal sight. The eye that is diverted sideways sees worse and worse with time and its visual functions are suppressed. This means that treatment has to be started as early as possible.
Treatment for strabismus may comprise:
- optical correction (glasses, soft contact lens);
- stimulation of vision acuity of both eyes (treatment of amblyopia) with the help of instrumental procedures;
- orthoptic and diploptic treatment (development of binocular vision);
- consolidation of monocular and binocular functions;
- surgical treatment.
Surgery is usually resorted as a cosmetic treatment since it can rarely by itself restore binocular vision (when two images from the eyes are brought together by the brain into a single picture).
The type of operation is selected by the surgeon directly when the patient is on the operating table since this type of surgery requires taking into account the specific features of the eye muscles of the patient. Sometimes both eyes are operated, in some cases only one eye is subject to surgery. This surgery is directed at strengthening or weakening of one of the muscles which activate the eyeball.
Surgery for strabismus correction is performed on an out-patient basis with local drop anaesthesia. The same day the patient returns home. Final rehabilitation takes about one week but doctors recommend the patients to take a course of instrumental treatment for the optimum restoration of the visual functions.