Non-surgical Strabismus Treatment

Abstract

Treatment of strabismus should begin with non-surgical treatments such as visual acuity enhancement through optical correction, amblyopia treatment, prismatic correction, and orthoptic exercises. The goals of the treatment are to align the eyes in cases of accommodative strabismus, improve visual acuity, improve asthenopic problems, and preserve or restore function of binocular vision, when possible. The best way is to eliminate the causes that led to the development of strabismus.

1. Introduction

In the treatment of strabismus, several goals are distinguished: one of the most important is the preservation, development, or restoration of binocular vision especially in children, cosmetic effect, correction of ocular torticollis, increase in eye mobility, elimination of double vision and increase in the field of vision in patients with esotropia. Non-surgical treatments for strabismus include optical correction of refractive errors, treatment of amblyopia, and orthoptic exercises.

2. Correction of refractive error and amblyopia treatment

Strabismus in children leads to inability to the development of binocular vision, which normally provides an assessment of the relative position of objects in space. The basis of binocular vision is normal anatomy, the correct interaction of work between the sensory and motor parts of both eyes together with normal functioning of the visual part of the brain [1].

Motor balance is necessary to work in synergy with sensory fusion and to provide the ability to physically move the eyes in the direction of an object being viewed [2]. Motor imbalance can be caused by anomalies of the extraocular muscles and anomalies of accommodation. These abnormalities make it difficult to hold the eye in a straight position [3].

The sensory part implements the function of binocular vision, the ability of the visual part of the brain to merge two images (from two eyes) into one image [4]. A qualitative characteristic of binocular vision are three stages of binocularity with final result of stereoscopic vision of an object, which allows one to determine the place of an object in space, with depth and volume [5, 6].

To form normal binocular vision, the eyes must see well, with normal motility, properly positioned, looking in the same direction, and properly focused on the same object, the normal functional ability of the retina, conductive pathways, and higher visual centers is required, on the retina of the same size images in both eyes [5, 7].

Strabismus can occur as an imbalance of sensory and motor parts. Impairments in the sensory region prevent the formation of a clear image on the retina.

Causes of sensory imbalance include refractive errors, differences in the anatomical and optical structure of both eyes (anisometropia), a sharp decrease in visual acuity, or blindness in one eye as a result of trauma or surgical procedures, as well as improper correction of refractive errors [3, 5].

When visual acuity is poor or one eye is blind, deviation can occur due to a lack of stimulus for fusion [8]. The cause of strabismus can be diseases affecting the eye or nervous system (for example, tumors), and injuries in any structures and processes that are involved in the process of visual perception and oculomotor control [3]. A small percentage of strabismus is the result of genetic syndromes, acquired stroke, or disease affecting the extraocular muscles or their innervation [3, 8].

In children, the occurrence of strabismus is accompanied by a complex functional restructuring of the binocular visual system to adapt it to work with an asymmetrical position of the eyes. Children with strabismus use the image of only one eye; the image of the second eye is blocked by the brain, which is invisible to the person himself [7, 9]. Adults usually cannot adapt new situation and struggle with double vision.

Strabismus is divided into concomitant (esotropia, exotropia, vertical) (hypotropia or hypertropia), non-concomitant and paralytic forms.

Treatment should begin immediately as soon as strabismus is detected [10].

The method of complex treatment of strabismus consists of the optical correction of ametropia, amblyopia control, surgeries on the oculomotor muscles, and preoperative and postoperative orthoptic exercises [6, 11, 12, 13]. By forcing the brain to use the deviated eye, the visual cortex responsible for that eye receives additional visual stimulation, which allows it to restore or develop a normal level of vision [6].

2.1 Optical correction of refractive error

Treatment of strabismus begins with optical correction of refractive errors: hypermetropia, myopia, and astigmatism to increase visual acuity. For treatment, glasses are prescribed for constant wear and if necessary, occlusion to treat amblyopia [3, 14, 15, 16, 17].

Any significant degree of ametropia must be corrected, and in the presence of strabismus, correction of any degree of ametropia is recommended [7, 18]. Before prescription of glasses, children with strabismus or suspected strabismus should undergo cycloplegia to determine the true refraction without the influence of accommodation [13].

Because of long cycloplegic effect that can be amblyopic atropin is not the first choice for diagnostic cycloplegia anymore. The first choice today in pediatric ophthalmology opraxis are cyclopšentolate and tropicamid. For children under one year of age or children with neurological problems, a 1% solution of tropicamide is a method of choice. Tropicamide is instilled 3x times every 15 minutes before the examination. For children older than 1 year, 1% solution of cyclopentolate is usually used. Cyclopentolate is instilled twice with an interval of 10 minutes directly on the day of the examination. Refraction is examined approximately 30–40 minutes after instillation. However, tropicamide does not provide complete cycloplegia, and therefore its use is limited. For children less than 1 year of age, a 0.5% solution of cyclopentolate or a 0.5% solution of tropicamide can be used [7, 18].

When measuring refraction in very young children usually under the age of 3 years, autorefractometers cannot be used and retinoscopy in cycloplegia has to be performed. Rules for refraction prescription depend on the age of the child and type of strabismus. Although for children under 6 years of age, full refractive correction is not necessary in the cases of strabismus usually is recommended [18].

With esotropia, a complete amount of cycloplegic hypermetropia must be fully corrected with glasses even if the amount of hypermetropia is only physiological hypermetropia. Adjusting to full cycloplegic correction in glasses may require some time for relaxation and accommodation. Full correction of hyperopia should be prescribed to eliminate the influence of accommodative convergence. In particular, accommodative esotropia, which is most common in children, is well corrected by constantly wearing properly selected glasses, which leads to correction of eye position, improved vision, and the development of binocularity [19, 20, 21]. Although, according to some reports wearing complete cycloplegic hypermetropic correction in children can prevent the development of emmetropia of the eyeball [22], insufficient correction can lead to decompensation of esotropia [21].

For variable exotropia, correction of hyperopia also sometimes has a beneficial effect on exodeviation control and improves binocular status in some patients while complete correction enables the patient to have better accommodation and control of the deviation [6].

Astigmatism of even 1.0 D can reduce visual acuity and cause visual discomfort.

When correcting myopia, it is necessary to strive to ensure clear visualization of distant objects for school children. In younger children, sometimes, myopia does not have to be correct in full amount because their vision is developing at near. In the cases of exotropia, full amount of cycloplegic refraction has to be corrected.

Spectacle correction is necessary after the surgical treatment.

Bifocal glasses are indicated for children with high AC/A coefficient (accommodative ratio of convergence and accommodation) and accommodative esotropia with convergence excess. The upper part of bifocals helps align the eyes at distance and lower part with addition corrects residual excess of esotropia at near. It is obligatory to prescribe a full addition of +3,0 in the beginning and then gradually wean off. The dividing strip in bifocal glasses has to pass in through the middle of the pupil in children. The recommended strategy is to continuously and gradually reduce bifocal correction until the patients restore normal accommodation. In some cases, surgery is needed. The AC/A ratio decreases with age [2].

If patients have amblyopia, treatment begins with amblyopia treatment.

Amblyopia is a decrease in best-corrected visual acuity in one or less often two eyes. It is a developmental disorder of the visual system in some cases accompanied by structural changes in the eye [16]. It develops at an early age, as a result of suppression of the image from the deviated eye, or eye with uncorrected refractive error, or some kind of anatomical disorder (cataract, ptosis, hemangioma), the part of cortex responsible for interpreting the images coming from eye does not receive the stimulation necessary for normal development. As a result of untreated amblyopia, the affected eye develops lower visual acuity due to the abnormal development of the cortical visual pathway or abnormal binocular interactions [23, 24, 25, 26].

During occlusion, especially long-term, it is necessary to monitor the visual acuity of the dominant eye, since if it is turned off for a long time, visual acuity may decrease. In such cases, they switch to alternating occlusion.

In some cases when compliance is not good, instead of occlusion, the technique of blurring the image of the eye with better visual acuity can be used. In such cases, atropine penalization and filters are prescribed. These methods are used less frequently but are also useful [14, 16, 26, 27]. Penalization is effective if the vision of the better-seeing eye under conditions of penalization is lower than the visual acuity of the amblyopic eye.

To stimulate the retina of an amblyopic eye, light-color stimulation, the use of reflected laser beams, frequency-contrast stimulation, and electromagnetic stimulation are used. However, the gold standard treatment for amblyopia is correction of the refractive error and occlusion of the better eye [28, 29]. In children, the time of occlusion depends on degree of amblyopia and age of the child (usually, day from 2 to 6 hours). Occlusions are sometimes reasonable in adults [30].

Inverse occlusion therapy is sometimes useful for elimination of inhibition of central fixation to change excentric fixation pattern of squinting eye which can improve control of some forms of strabismus.

Newer binocular treatments for amblyopia are being developed based on neural tasks and games that complement occlusion. Virtual reality is a relatively new intervention that can be used to treat eye and vision problems [21, 31]. Home video therapy can provide patch-equivalent treatment results [32, 33, 34, 35].

This therapy involves visual tasks that are designed so that they can only be completed using both eyes. These range from simple games using red-green glasses to exciting 3D games and movies [27, 28, 32, 34, 36].

The exercises are interesting for both children and adults and require active participation on their part. Computer games have a complex effect on various types of visual sensitivity: light, brightness, frequency-contrast, various semantic content, and form, which significantly increase the effectiveness of treatment [7].

Virtual reality depends on software and application design features [31].

Currently, asynchronous stimulation of two eyes is being developed, when visual stimuli are presented to the better-seeing eye with some temporary short-term delay, which leads to a change in eye dominance [24].

Amblyopia treatment should be monitored by an ophthalmologist or orthoptist and the schedule adjusted according to the improvement of the visual acuity. Refraction also changes with age, and examinations are recommended every six months [18]. All children are born hypermetropes because of their small eyes. During the childhood, eyes are developing and growing hypermetropia always decreases, and in adolescence, children with hypermetropia of less than 2.5 D become emmetropes.

Although children tolerate anisometropia much better than adult patients if there is a significant anisometropia and aniseikonia (refractive asymmetry between the eyes) of more than 5 D, it is reasonable to use contact lenses for correction [37, 38, 39]. Excimer laser surgery should not be performed until the age of at least 18 years.

If the patient has refractive error, especially in combination with hyperopia and convergent strabismus, as a preoperative preparation, it is mandatory to prescribe glasses for constant wear to correct full cycloplegic refractive error, to identify non-accommodative deviation angle. In the process of presurgical preparation, the accommodative nature of strabismus is revealed. In the cases of accommodative refractive strabismus, after the correction of the full refractive error angle of deviation decreases and the need for surgical correction of strabismus disappears, residual angle after the correction has to be treated surgicaly. Therefore, the accommodative part of squinting angle has to be treated with refractive and not strabismus surgery. If patients do not want to wear glasses in adult age, refractive surgery is always an option for accommodative part.

If children and young people refuse to wear glasses, we recommend that they switch to contact lens correction. But usually, if there is good compliance with the patient and his parents, an explanation of the reasons, and the possible result, patients agree to wear glasses until they reach an age when refractive surgery is possible.

2.2 Orthoptic exercises

Treatment is aimed at restoring (forming) binocular functions. The principle of separating the eyes using mechanical, color and other types of visual field separation and training the patient to combine two images into one image with a gradual transition from artificial to natural conditions is used [7, 40].

The exercises are aimed at improving eye mobility, eliminating abnormal retinal correspondence, and restoring normal relationships between accommodation and convergence [41]. Orthoptic treatment does not affect the angle of the strabismus.

Orthoptic exercises may help a group of patients with good fusion potential by teaching patients how to use their fusion abilities more effectively. To obtain the effect of binocular exercises, visual acuity of the worse-seeing eye is desirable not lower than 0.3–0.4 (20/50) [7].

The exercises help a group of patients with exotropia and convergence insufficiency by objectively improving binocular function after fusion exercises [20, 40, 42]. The effectiveness of home-based pencil push-up therapy has worked well in the treatment of convergence insufficiency.

Wearing complete cycloplegic optical correction in combination with orthoptic exercises can help treat accommodative strabismus [43].

The effectiveness of orthoptic exercises in treating other forms of strabismus is debated. A comprehensive review of the evidence supporting orthoptic exercise/vision therapy conducted in the UK found that there was little evidence-based research to support these practices [43, 44, 45].

In post-Soviet countries, synaptophore is used for therapeutic orthoptic exercises, teaching haploscopic vision. The purpose of such exercises is to treat functional scotoma and restore joint functioning of the eyes. However, this vision differs from vision in natural conditions. Under natural conditions, retinal images of the same object merge, and on the synaptophore, images of two objects presented separately for each eye merge. Therefore, it is more expedient to carry out treatment to restore binocular functions without mechanical separation of the visual fields after achieving a symmetrical position of the eyes.

When using games in virtual reality, the conditions are closer to natural, and the games themselves are much more interesting, allowing them to maintain attention and concentration, especially in children. These methods are used with two eyes open, most often by separating the eyes using color filters.

There is also a method of phase haploscopy using liquid crystal glasses. In a certain frequency-phase mode, an electrical pulse passes through the plates of such glasses, changing their transparency: one glass becomes transparent; the other at this moment becomes opaque. Since this change in transparency occurs with high frequency, the patient does not feel it.

In glasses with an autonomous power system, the binocular phase is also turned on, when both eyes look through transparent glass, bringing the patient closer to natural conditions of perception. This treatment method is also used to treat amblyopia, however, due to the high cost of such glasses, the treatment method is not widespread [7].

2.3 Correction with prisms

The use of prisms is an important treatment modality for the management of patients with binocular dysfunction (asthenopia, diplopia). The use of prisms sometimes helps eliminate diplopia in case of small paresis of the eye muscles, reduce or eliminate the compensatory position of the head (torticollis), and help restore binocular vision during the treatment of concomitant strabismus [4]. Prismatic correction is used to eliminate residual small angles after surgical treatment [46], treatment of asthenopia caused by heterophoria [47]. Sometimes good effects can be obtained with prismatic glasses in cases of microesophoria or microtropia [48]. The use of prisms is contraindicated in the absence of symptoms of binocular function and if there is a risk of aggravation of the existing condition in the process of adaptation to fusion.

Prism correction is used to eliminate double vision in two main directions of vision: straight ahead and vertical.

The prism shifts the image towards the base and thus shifts the gaze in the direction of the top of the prism. This effect is used in cases of limited mobility, which is used in the treatment of paretic strabismus [4]. Thus, prisms do not change the position of the eyes; they change the path of rays from the object in question in the eye, affecting sensorimotor relations.

Fresnel prisms are thin flexible plates made of flexible transparent plastic and are attached to ordinary diopter glasses on the back surface of the spectacle glass. Fresnel prisms do not weigh down the glasses. However, there are still some limitations with diminishing visual acuity: the optimally tolerated correction is up to ≤20 D. Long-term wearing leads to successful treatment in 30% of cases [49]. Prisms with a small diopter value can be prescribed to the dominant eye.

One prism diopter corresponds to approximately 0.5° (34″) deviation. Prisms are drawn according to the full (360°) TABO scale, and the direction of the base of the prism is always indicated. If there are horizontal and vertical components of deviation, you can separately indicate the strength of the horizontal and vertical prism for each eye, or you can indicate the resulting prism force, which can be calculated on special websites or using tables.

When restoring impaired functions, prismatic correction should gradually decrease with their subsequent abolition. Some authors report that staged prismatic treatment, with a reduction in the power of the prisms used can lead to good results as a treatment in patients with acute esotropia of 25 D or less [50].

Permanent prismatic correction is reserved for elderly patients and in the absence of functional dynamics.

Contraindications for wearing prisms are subjective intolerance to prisms, an increase in the angle of deviation, or increased diplopia under the prism.

3. Conclusions

Non-surgical treatment methods are effective as a treatment for some types of strabismus such as accommodative strabismus, paretic strabismus, and small-angle strabismus. Non-surgical treatment methods are also used as a preparatory stage before surgical treatment.

Treatment of strabismus is a long process. Correctly chosen tactics for the complex treatment of strabismus start with correctly selected glasses that correct refractive errors, treatment of amblyopia in combination with orthoptic exercises, and, if necessary, wearing prismatic glasses can in some cases achieve satisfactory eye alignment and enable development of binocular vision. Good visual acuity together with the presence of binocular vision helps to keep the eyes in a straight position [27, 51].