Contrast sensitivity, optotype acuity and fixation eye movement abnormalities in amblyopia under binocular viewing

Fixation Stability Deficits in Anisometropic Amblyopia

Purpose

The purpose of this study was to investigate the relationship between fixation stability deficits in anisometropic amblyopia and various visual functions, as well as the underlying retinal structure.

Methods

All 164 patients with anisometropic amblyopia were recruited in this cross-sectional study. The contrast sensitivity function (CSF) was measured using the qCSF method, whereas the MP-3 microperimeter was used to assess fixation stability and locate the preferred retinal locus. Bivariate contour ellipse area (BCEA) of both the amblyopic and the fellow eyes was used as the dependent variable. Based on previous research and clinical practice, the following variables were selected as independent variables for regression modeling to explore potential influencing factors: gender, age, area under the log CSF (AULCSF), absolute interocular difference (IOD) in spherical equivalent refraction (SER), AULCSF-IOD, the eccentricity of the preferred retinal locus, patching history, and the log BCEA of the contralateral eye.

Results

A total of 161 participants (87 men and 74 women, average age = 20.26 ± 8.79 years, ranging from 5 to 51 years old) completed all examinations. Three participants were excluded due to their inability to complete the required examinations. There were significant differences between the amblyopic and the fellow eyes in terms of SER, best-corrected visual acuity, AULCSF, log BCEA, and the eccentricity of the preferred retinal locus (all P < 0.001). Both the amblyopic and the fellow eyes exhibited fixation stability deficits and eccentric fixation. The regression model showed that fixation stability in the amblyopic eye (log BCEA) was significantly associated with age, AULCSF of the amblyopic eye, AULCSF-IOD, eccentricity of the amblyopic eye, and log BCEA of the fellow eye (all P < 0.05). Fixation stability in the fellow eye (log BCEA) was significantly associated with eccentricity of the fellow eye, and log BCEA of the amblyopic eye (all P < 0.05).

Conclusions

Eccentric fixation and fixation stability deficits were observed in both the amblyopic and the fellow eyes, with fixation stability in both eyes being correlated with the eccentricity of the preferred retinal locus. These findings suggest that in the clinical management of amblyopia, attention should be given to the fixation stability and fixation characteristics of both the amblyopic and fellow eyes.

Evaluating Eye Tracking During Dichoptic Video Viewing With Varied Fellow Eye Contrasts in Amblyopia

Purpose

This study uses eye tracking to investigate how varying fellow eye (FE) contrast during dichoptic video viewing influences eye movement patterns, and their associations with interocular suppression, visual acuity, and stereoacuity deficit in amblyopia.

Methods

Eye movements of 27 amblyopic and 8 healthy control participants were recorded during dichoptic viewing of stationary dots and videos with FE contrasts (100%, 50%, 25%, and 10%). Analysis included durations the amblyopic and FE spent in different stimulus regions, fixation switches, and eye deviation, and correlating these with suppression, visual acuity, and stereoacuity.

Results

Participants with pronounced suppression, visual acuity, and stereoacuity deficits demonstrated reduced amblyopic eye fixation in the amblyopic eye (AE) region at 100% FE contrast. Lowering FE contrast increased amblyopic eye duration in stimuli presented within the AE region, notably in anisometropic and treated strabismic participants, and strabismic participants exhibiting fixation switches during viewing of dichoptic stationary dots. Even at lower FE contrasts, participants with greater stereoacuity and visual acuity deficits continued to exhibit diminished AE fixation in the AE region. Increased eye deviation was seen in strabismic participants with lowering of FE contrasts.

Conclusions

Dichoptic contrast modulation holds promise for reducing suppression with responses varying by amblyopia type and visual function deficits. Larger strabismic angles may hinder binocular benefits of dichoptic treatments. Fixation switches may serve as an indicator of favorable outcomes. Eye tracking is crucial for understanding these dynamics, providing essential insights into visual attention dynamics of the FE and AE, and may serve as a valuable tool in optimization of amblyopia treatments.

Inter-Ocular Fixation Instability of Amblyopia: Relationship to Visual Acuity, Strabismus, Nystagmus, Stereopsis, Vergence, and Age

PURPOSE

Amblyopia damages visual sensory and ocular motor functions. One manifestation of the damage is abnormal fixational eye movements. Tiny fixation movements are normal; however, when these exceed a normal range, the behavior is labeled "fixation instability" (FI). Here we compare FI between normal and amblyopic subjects, and evaluate the relationship between FI and severity of amblyopia, strabismus angle, nystagmus, stereopsis, vergence, and subject age.

METHODS

Fixation eye movements were recorded using infrared video-oculography from 47 controls (15.3 ± 12.2 years of age) and 104 amblyopic subjects (13.3 ± 11.2 years of age) during binocular and monocular viewing. FI and vergence instability were quantified as the bivariate contour ellipse area (BCEA). We also calculated the ratio of FI between the 2 eyes: right eye/left eye for controls, amblyopic eye/fellow eye for amblyopes. Multiple regression analysis evaluated how FI related to a range of visuo-motor measures.

RESULTS

During binocular viewing, the FI of fellow and amblyopic eye, vergence instability, and inter-ocular FI ratios were least in anisometropic and most in mixed amblyopia (P < .05). Each correlated positively with the strabismus angle (P < .01). During monocular viewing, subjects with deeper amblyopia (P < .01) and larger strabismus angles (P < .05) had higher inter-ocular FI ratios. In all, 27% of anisometropic and >65% of strabismic/mixed amblyopes had nystagmus. Younger age and nystagmus increased FI and vergence instability (P < .05) but did not affect the inter-ocular FI ratios (P > .05).

CONCLUSIONS

Quantitative recording of perturbed eye movements in children reveal a major functional deficit linked to amblyopia. Imprecise fixation, measured as inter-ocular FI ratios, may be used as a robust marker for amblyopia and strabismus severity. NOTE: Publication of this article is sponsored by the American Ophthalmological Society.

Leveraging neural plasticity for the treatment of amblyopia

Amblyopia is a form of visual cortical impairment that arises from abnormal visual experience early in life. Most often, amblyopia is a unilateral visual impairment that can develop as a result of strabismus, anisometropia, or a combination of these conditions that result in discordant binocular experience. Characterized by reduced visual acuity and impaired binocular function, amblyopia places a substantial burden on the developing child. Although frontline treatment with glasses and patching can improve visual acuity, residual amblyopia remains for most children. Newer binocular-based therapies can elicit rapid recovery of visual acuity and may also improve stereoacuity in some children. Nevertheless, for both treatment modalities full recovery is elusive, recurrence of amblyopia is common, and improvements are negligible when treatment is administered at older ages. Insights derived from animal models about the factors that govern neural plasticity have been leveraged to develop innovative treatments for amblyopia. These novel therapies exhibit efficacy to promote recovery, and some are effective even at ages when conventional treatments fail to yield benefit. Approaches for enhancing visual system plasticity and promoting recovery from amblyopia include altering the balance between excitatory and inhibitory mechanisms, reversing the accumulation of proteins that inhibit plasticity, and harnessing the principles of metaplasticity. Although these therapies have exhibited promising results in animal models, their safety and ability to remediate amblyopia need to be evaluated in humans.

Multifaceted Interactions of Stereoacuity, Inter-Ocular Suppression, and Fixation Eye Movement Abnormalities in Amblyopia and Strabismus

Purpose

Amblyopic and strabismus subjects experience inter-ocular suppression, impaired stereoacuity, and increased fixation instability. The purpose of the study was to investigate factors affecting suppression and stereoacuity and examine their relationship to fixation eye movement (FEM) abnormalities.

Methods

We recruited 14 controls and 46 amblyopic subjects (anisometropic = 18, strabismic = 14, and mixed = 14) and 11 subjects with strabismus without amblyopia. We utilized the dichoptic motion coherence test to quantify suppression, and stereoacuity was assessed using the Titmus Fly test. We recorded FEMs using high-resolution video-oculography and classified subjects that did not have nystagmus (n = 27) versus those with nystagmus (n = 32; fusion maldevelopment nystagmus [FMN], n = 10) and nystagmus that did not meet the criteria of FMN (n = 20). We also recorded FEMs under dichoptic viewing (DcV) at varied fellow eye (FE) contrasts and computed the amplitude and velocity of the fast and slow FEMs and vergence instability.

Results

Inter-ocular suppression and stereoacuity deficits were closely correlated with an amblyopic eye (AE), visual acuity, and strabismus angle. Subjects with nystagmus displayed more pronounced stereoacuity deficits than those without nystagmus. Strabismic subjects with and without amblyopia, who demonstrated a fixation switch at 100% FE contrast, had lower inter-ocular suppression than subjects lacking a fixation switch under DcV. Amplitude of fast FEMs and velocity of slow FEMs, and vergence instability were increased as the FE contrast was lowered in both amblyopic and strabismic subjects.

Conclusions

The current study highlights the intricate relationships between AE visual acuity, eye deviation, and FEM abnormalities on suppression and stereoacuity deficits and underscores the need to evaluate FEM abnormalities while assessing dichoptic treatment outcomes.