3.2.1.7. The dominant eye, strabismus, amblyopia and refractive surgery
3.2.1.7. The dominant eye, strabismus, amblyopia and refractive surgery
1. Centro Hospitalar do Porto, Portugal
2. Instituto de Ciências Biomédicas Abel Salazar (ICBAS), Universidade do Porto, Portugal
The concept of ocular dominance corresponds to the tendency to prefer visual input from one eye over the other eye1. Shneor et al2, in their study, suggest that the dominant eye has perceptual processing priority, even at times when there is no competition between the two eyes, results that are also suggested in monocular stimulation imaging studies, which found greater bilateral activation after stimulation of the dominant eye3,4. This dominance is accompanied by different perceptual effects: individuals are more precise when they use their dominant eye5-7; the images appear clearer8 and larger1,9 when viewed by the dominant eye; and the images stabilized in the retina disappear more slowly when viewed by the dominant eye8.
Diagnostic tests to determine ocular dominance can be divided into two groups. The first group forces subjects to use monocular visualization, for example by looking at an object through a small aperture. Faced with these challenges, people tend to favor one eye over the other. The second group consists of tests that measure the balance of sensory input between the eyes. Most of these use binocular rivalry (stereo-disparity) to assess the magnitude of the ocular domain10. Generally, the first group of tests is referred as ocular dominance tests and the second as ocular prevalence tests or sensory dominance tests. The equivalence between these two groups of tests is still under study: in fact, some authors suggest that there is a dichotomy between the two types of ocular dominance10-12, while others show a correlation between the prevalence and ocular dominance tests13,14.
Classically, ocular dominance was considered a fixed phenomenon since most adults show a consistent preference for one eye1. However, in the last few years some studies have shown the possibility of altering the ocular dominance with alterations of the horizontal position of the eyes15 or the relative size of the images in the retina16. A recent study, which evaluated the effect of cataract surgery on ocular dominance, supports the theory that ocular dominance may be an attribute with plasticity, since, in 21.2% of the patients studied, there was a change in dominance after cataract surgery17. This study also suggests that this plasticity decreases with age. In the experience of the Authors, the possibility of altering the ocular dominance in the smallest children is verified, with a rapid reduction in the plasticity that allows it with age.
In the area of refractive surgery, ocular dominance has implications, especially when the monovision strategy is used. Monovision is a form of correction of presbyopia in which one eye, usually the dominant one, is corrected for distant vision and the other, the non-dominant one, for near vision18,19. This strategy is based on the hypothesis that the non-dominant eye will be more easily suppressed by the relatively unfocused image for far, supported, for example, by the studies of Schor and Erikson20 and Collins et al21.
The success of monovision requires that patients can see with quality at all distances. Thus, the extent of clear binocular vision should be continuous and equal to the sum of the extent of sharp monocular vision, without interference from the blurred vision of one eye. However, as noted earlier, the input of the dominant eye produces a greater response to a given stimulus than the input of the non-dominant eye. Recent studies have shown that there is a strong correlation between the magnitude of ocular dominance and patient satisfaction after induction of monovision with the implantation of intraocular lenses13,22. In this sense, some authors argue that for monovision success, image suppression should be flexible between the eyes and may be advantageous in patients with less marked ocular dominance or alternating dominance22-24.
In summary, one of the most important factors for the success of monovision is the effect of ocular dominance11,18. Nonetheless, other factors such as stereopsis or binocular addition also play their role in monovision. A better understanding of the interconnection of these factors and the development of more precise techniques for their evaluation are necessary so that in the future a better preoperative selection of the candidates for this treatment may be possible, leading to better results and greater patient satisfaction.
It is fundamental to establish differentiated strategies in three specific circumstances arising from the study of dominance and binocularity:
The monovision strategy is supported by studies such as those by Schor and Erikson20 and Collins et al21. It is based on the hypothesis that the non-dominant eye will be more easily suppressed by relatively unfocused images in the distance. It is recalled that there may be greater advantage in patients with less marked ocular dominance or alternating dominance22-24.
Thus, with regard to monovision strategies, either with monofocal intraocular lenses or in cases of LASER Excimer, emmetropia is usually chosen for the dominant eye. Hypercorrection in the power of intraocular lenses is reserved for the non-dominant eye, choosing postoperative target refraction close to -1.25 diopters, and similarly, in the cases of LASER correction the target is a light residual myopia for the non-dominant eye. With regard to EDOF lenses, there is often a need for reading glasses for shorter distances. Since these lenses are often chosen for users with a predominance of distance and intermediate (computer) vision, several authors use overcorrection in one eye to reduce the insufficiency of the range of glasses at close range. This strategy, the mini-monovision, is conditioned by the prior determination of ocular dominance and the choice of a discrete myopic refractive target (-0.50 diopters) for the non-dominant eye, similar to the classic monovision strategy25-27.
Generally, most surgeons consider it to be a better indication for multifocal lenses – the presently preferred technique for presbyopia correction – when faced with individuals with good visual acuity in both eyes, with no pathology that reduces contrast sensitivity, and where division of light by two or more foci is attenuated by the sum of the improvement of the vision obtained by the two eyes in simultaneous perception. This addition often allows you to overcome the visual acuity of the best eye for distance, but also significantly improve results at close range. Thus, binocular results are clearly superior to those obtained in monocularity28.
Not infrequently, it is verified that only with the surgery of the two eyes one obtains visual acuity, namely for near, sufficient for the intended independence of reading glasses. However, there are no studies of the implementation of multifocal intraocular lenses in cases of changes in binocularity and absence of stereopsis that condition the result of the addition of the vision of the two eyes. Even in cases of good visual acuity in both eyes but in the presence of alternating suppression, such as in people with micro-strabismus, this additive effect will not be present and its absence will potentially limit functional results29,30.
Classically, amblyopia was defined by Von Noorden31 as "a decrease in visual acuity, for which no cause is found in the ocular physical examination, caused by visual deprivation or abnormal binocular interaction." It is the most common cause of decreased monocular visual acuity in children and young adults, affecting 2-5% of the population32.
Amblyopia is characterized by an altered visual function, affecting not only visual acuity but other parameters of visual function, namely Vernier acuity, contrast sensitivity and reading speed, which are also decreased. The most frequent cause of amblyopia is strabismus, usually infantile endotropia.
Strabismus refers to an ocular misalignment caused by abnormalities in binocular vision or by anomalies in the muscular control of ocular motility. The second cause is anisometropia, followed by the association between strabismus and anisometropia, and the less common cause is sensory deprivation33.
The abnormal development produced by sensory deprivation, anisometropia or strabismus results in changes in the primary visual cortex. The developing visual system uses activity patterns to refine neural connections and is extremely sensitive to uneven binocular competition and competitive inhibition. Studies in monkeys have shown that monocular sensory deprivation causes an atrophy of the ocular dominance columns of the eye in the striated cortex. In anisometropia, the blurred image of one eye leads to a lower sensitivity of the corresponding cortical neurons and, subsequently, to a poorer signaling by them. This results in a binocular neuronal imbalance, reducing binocular activity, with alteration of the functioning of the parvocellular system. In strabismus, two factors contribute to the development of amblyopia. The first is interocular suppression due to non-matching of images. The other factor is the blurred image itself of the deviated eye. Both factors cause asynchrony or inhibition of the signals in the 4C layer of the striated cortex34,35.
If, in the past, amblyopia was assumed to be a permanent visual comorbidity, recent advances in understanding the complexity of neuroprocessing in this pathology have led to new visual rehabilitation techniques that can modestly restore vision and even stereopsis in adults36-39.
In this chapter, the authors seek to review the application of different modalities of refractive surgery in the treatment of amblyopia and strabismus.
In the treatment of amblyopia and strabismus, optic correction associated or not to pharmacological occlusion or penalization are the most commonly used forms. However, a subset of patients does not respond to these forms of treatment for a number of reasons, including aniseikonia and lack of adherence to treatment, especially in children with more complex ocular pathologies or associated systemic pathology. In this sense, several studies have shown the potential of keratorefractive surgery as an alternative for the treatment of anisometropic amblyopia in children40-44. The authors, in a study published in 199845, also demonstrated that LASIK showed efficacy in reducing or eliminating ambliogenic anisometropies, allowing, when associated with a conventional treatment, a significant recovery of visual acuity.
Despite the more reserved potential of these treatments in adults, several studies have shown that visual acuity may improve after refractive correction, occlusion or visual loss of the non-amblyopic eye. Therefore, keratorefractive surgery may also play a role in the treatment of anisometropic amblyopia and/or accommodative endotropia. Agca et al46 showed that 25% of the amblyopic eyes submitted to refractive correction with LASIK improved two or more lines in the best corrected visual acuity (BCVA). Roszkowska et al47 have shown that LASER Excimer is a safe and effective option in the treatment of ametropic and anisometropic amblyopia in adults.
Several studies have demonstrated that keratorefractive surgery reduces the angle of deviation in fully or partially accommodative endotropies, both in adolescents and young adults48-54. Shi et al55, in their study in adult amblyopic patients with accommodative endotropia who underwent keratorefractive surgery, obtained satisfactory results both in ocular alignment and visual acuity, and in binocular function, showing worse results in cases of high hyperopia.
The increasing experience in this field, together with the potential advantages of this therapeutic modality (better compliance, less optical aberration, less aniseikonia), may allow in the future that keratorefractive surgery be affirmed as a therapeutic alternative in the treatment of anisometropia in childhood, and may have an eventual role in the treatment of amblyopia, potentiating the treatments with occlusion and atropine. It is noted, however, that many studies in this area lack long follow-up.
Phakic IOLs play an important role in the correction of high refractive errors. Some studies have shown improved amblyopia in children and adolescents treated with phatic IOLs for myopic anisometropia56-60. This improvement in BCVA was observed even after the age of visual plasticity. For example, Alió et al61 studied the response to treatment with phakic IOL implantation of adults with anisometropic amblyopia with myopic astigmatism. They showed improvement of BCVA, with 91.5% of patients improving at least one line in BCVA and no case of loss of visual acuity. In the study by Gonzalez-Lopez et al62, who compared LASIK and phakic IOLs in adults with amblyopia due to moderate myopia, both techniques showed efficacy and safety in correction of myopia, with phakic IOLs showing better visual results than LASIK.
Although there are only few studies on the use of phakic IOLs in the treatment of anisometropic amblyopia, the promising results reported and the advantages of these lenses in relation to keratorefractive surgery (quality of vision and less induction of optic aberrations), make them a possible alternative in the treatment of anisometropic amblyopia. Its serious potential complications however, in particular endophthalmitis, cataracts and endothelial decompensation, require nevertheless extra caution in its use.
Children with congenital cataracts who require cataract surgery are children who are prone to develop amblyopia. The main reasons for the development of amblyopia in these children is not only the deprivation of preoperative visual stimuli but also the refractive power change during the first two decades of life and the lack of multifocality due to the loss of accommodation after surgery.
The implantation of accommodative or multifocal IOLs could be a theoretical solution to alleviate this problem, since they would allow a rapid rehabilitation of the vision for far, intermediate and near, less need of spectacles and, as such, better self-esteem of the child. Gray and Lyall63 reported the first case of multifocal IOL implantation in a 6-year-old child, showing improvement in their quality of life. Jacobi et al64 evaluated 36 eyes of children (ages 2 to 14 years) who underwent multifocal IOL implantation after cataract surgery and concluded that this treatment would be a viable alternative, especially in patients with unilateral cataract, since it would improve binocular vision and stereopsis. Cristobal et al65 showed their experience with the implantation of multifocal IOLs in five children (ages 4 to 6 years) with unilateral cataract, and reported improvement of visual acuity in all cases, with 4 cases also showing fusion and better stereopsis. However, there are still some issues under study and the publications are still scarce66.
Despite these apparently promising results, further studies are needed. Refractive changes inherent in eyeball growth and amblyopia pose hardly surmountable problems and long-term publications are scarce.
There are no published studies using EDOF lenses such as Symfony® (Abbott Medical Optics) in pediatric patients.
Currently, most authors and the FDA do not recommend the use of multifocal IOLs in patients with amblyopia or strabismus. In patients with amblyopia, even a slight decrease in contrast sensitivity may, after implantation of these lenses, produce a disproportionate reduction in visual function29,30.
In patients with strabismus or phoria, multifocal IOLs are not the best option. Patients with alternate monofixation fail to achieve the simultaneous sum of multifocal binocular vision. Patients with small angle endotropia or phoria, in addition to the absence of benefit of binocular multifocality, may also present an increased risk of decompensation of the underlying pathology29,30. In addition, recent studies suggest that the kappa angle alone contributes to the existence of photopic phenomena after the implantation of multifocal IOLs, so this parameter should be evaluated in the preoperative period67,68.
The literature is scarce regarding the use of multifocal lenses in adults with amblyopia. There are no studies in patients with strabismus. The two publications in patients with amblyopia showed good results for distance and near vision and a slight improvement in binocularity in some patients. Petermeier et al69 studied the implantation of the Restor® lens (Alcon Laboratories, Inc.) in patients with mild to moderate anisometropic amblyopia. De Witt et al70 studied the implantation of the MPlus® lens (Oculentis) in patients with anisometropic amblyopia.
In summary, refractive surgery, with its full range of therapeutic modalities, may in the future play a more relevant role in the treatment of amblyopia and some forms of strabismus. On the other hand, the evolution of the knowledge of the pathophysiological phenomena inherent to these pathologies, the evolution of the surgical techniques and the better preoperative planning, may allow us to maximize the visual potential of amblyopic eyes, always focusing on improving the quality of vision and quality of life of patients.