Optical Coherence Tomography in Children With Microtropia

Amblyopia and fixation eye movements

Amblyopia is a neurodevelopmental disorder caused by abnormal visual experience in early life that affects 3-5% of the population. Amblyopia results in a host of monocular and binocular visual afferent function deficits including reduced visual acuity, contrast sensitivity, depth perception, interocular suppression, and efferent function abnormalities such as unstable and inaccurate fixation. Conventional treatments such as patching therapy and newer dichoptic treatments are not always successful as 30-40% of patients experience recurrence/regression of amblyopia. There are numerous review articles focused on visual afferent function deficits and treatment modalities and outcomes in amblyopia. Recently, the advent of high spatial and temporal resolution eye trackers has spurred studies on fixation eye movements (FEMs) in healthy controls and neurologic and ophthalmic disorders. In this focused review, we will summarize studies evaluating FEM abnormalities in amblyopia. We will first describe the common devices and techniques used to quantify fixation abnormalities, and then highlight the importance of systematically evaluating the eye movements under different viewing conditions and describe the parameters crucial in assessing FEM abnormalities in amblyopia. We will summarize the evidence suggesting that FEM abnormalities are not limited to the amblyopic eye only but also affects the fellow eye and that FEM abnormalities can serve as biomarkers to predict the impact of amblyopia on visual functions. Beyond diagnosis, we will discuss the treatment and prognostic implications of the evaluation of FEM abnormalities in clinical practice.

The agreement between the Irvine 4 diopter prism test and assessment of ocular fixation in microtropia with identity

Microtropia describes a primary ocular deviation of less than 10 prism diopters associated with harmonious anomalous retinal correspondence and reduced stereopsis. It is routinely accepted that children with microtropia are less likely to achieve equal vision following occlusion therapy than those with bifoveal fixation. The most commonly used methods of diagnosing a microtropia are the 4 diopter prism test (4^∆PT) and assessment of ocular fixation. This study examines the agreement between the two tests. One hundred and twelve typically developing children without a manifest strabismus who were able to undertake a linear visual acuity test and had two or more lines of anisoacuity following refractive adaption to their full cycloplegic correction underwent assessment of the 4^∆PT and ocular fixation along with their routine orthoptic examination. One hundred and twelve children (46 boys and 66 girls) attending the Orthoptic department who fitted the above criteria were included in the analysis. The mean age at examination was 6 years. 80.3% had anisometropia of at least 1.25 diopters. The 4^∆PT indicated a microtropia in 74 cases, whereas assessment of fixation indicated a microtropia in 68 cases. In 88 cases (78.6%), the results of the two tests agreed. Analysis found only moderate agreement between the two tests (k = 0.540 (CI 0.379-0.700)). Logistic regression analysis comparing those children where the two tests agreed with those where they disagreed found no difference in the level of anisoacuity (p = 0.7823), degree of anisometropia (p = 0.9385), the vision in the worst eye (p = 0.5260), the refractive error in the "worst" eye (p = 0.865), or the age at the time of testing (p = 0.4485) between the two groups. When assessing for a microtropia, it was found that not all children who elicit a central suppression response on the 4^∆PT are found to be fixing eccentrically and vice versa. This could potentially have implications for the treatment of their amblyopia. It is important not to rely on just one test at one time to make the diagnosis of microtropia. Rather, if one or other test indicates a microtropia the first time they are attempted, this should be reassessed regularly as treatment progresses and certainly before treatment is stopped and suboptimal visual acuity is accepted.

Using OCT Fixation Shift to Assess Eccentric Fixation in Children With Residual Amblyopia

Purpose

Eccentric fixation in amblyopia is often estimated grossly without precision. Although the usefulness of optical coherence tomography (OCT) fixation shift in the quantification of eccentric fixation in a small cohort of amblyopic children was recently reported, there is a lack of understanding of characteristics of OCT fixation shift. In a retrospective cohort study, we evaluated eccentric fixation with OCT in a large cohort of children with residual amblyopia.

Methods

Children, age 4 to 17 years, with residual amblyopia (amblyopic, n = 56) and without amblyopia (control, n = 75) were enrolled. Amblyopia was associated with anisometropia alone (anisometropia subtype, n = 28) and strabismus without or with anisometropia (strabismic subtype, n = 28). Spectral domain OCT was used to estimate fixation. The OCT fixation shift, defined as the distance between the fovea and the fixation point, was measured and adjusted with calculated axial length and converted into visual degrees. Fixation shift in amblyopic eyes, fellow nonamblyopic eyes, and right eyes of the control group were compared. Fixation shift between the anisometropic and strabismic amblyopia subtypes was also compared. Its correlation with visual acuity was estimated.

Results

The mean fixation shift was significantly different: 0.17° ± 0.29° for control right eyes, 0.94° ± 1.24° for amblyopic eyes, and 0.34° ± 0.57° for fellow eyes (χ² = 23.3; P < 0.001). There was no significant difference between fellow eyes and control eyes (P = 0.11). Fixation shift in amblyopic eyes was significantly correlated with visual acuity (R = 0.44; P < 0.001), and it was significantly smaller in the anisometropic subtype than in the strabismic subtypes (0.34° ± 0.46° vs. 1.54° ± 1.48°, W = 338, P < 0.001).

Conclusions

OCT fixation shift can be used both in detection and quantification of eccentric fixation in children with residual amblyopia, especially in those with strabismus.

Translational Relevance

OCT fixation shift offers a convenient clinical approach in quantitative evaluation of eccentric fixation in children with strabismic amblyopia.

Characterization, passive and active treatment in strabismic amblyopia: a narrative review

Strabismic amblyopia is characterized by a distorted spatial perception. In this condition, the neurofunctional disorder occurring during first years of life provoke several monocular and binocular anomalies such as crowding, deficits in the accommodative response, contrast sensitivity, and ocular motility abilities. The inhibition of the binocular function of the brain by the misaligned amblyopic eye induces a binocular imbalance leading to interocular suppression and the reduction or lack of stereoacuity. Passive treatments such as occlusion, optical and/or pharmacological penalization, and Bangerter foils has been demonstrated to be potentially useful treatments for strabismic amblyopia. Recent researches have proved new pharmacological options to improve and maintain visual acuity after occlusion treatment in strabismic amblyopia. Likewise, the active vision therapy, in the last years, is becoming a very relevant therapeutic option in combination with passive treatments, especially during and after monocular therapy, in the attempt of recovering the imbalanced binocular vision.

Imaging Amblyopia: Insights from Optical Coherence Tomography (OCT)

Amblyopia refers to visual impairment resulting from perturbations in visual experience during visual development, typically secondary to strabismus, uncorrected refractive error, and/or deprivation. Amblyopia has traditionally been considered a cortical disease, but the depth of our understanding of this complex neurodevelopmental condition is limited by our ability to appreciate structural pathophysiology in the visual pathway. Recent advances in Optical Coherence Tomography (OCT) have facilitated numerous studies of the structural changes in the retina and optic nerve, thereby expanding our appreciation for the pathogenesis of this condition. In this review, we summarize findings from studies evaluating retinal, retinal nerve fiber layer, and choroidal thickness changes in patients with amblyopia. Focusing on the largest and most recent studies, we discuss common limitations and confounding variables in these studies. We summarize recent advances in ocular imaging technology and reconcile the findings of early histological reports with those of structural OCT in amblyopia.