January 2025
Volume 66, Issue 1
Open Access
Eye Movements, Strabismus, Amblyopia and Neuro-ophthalmology  |   January 2025
Fixation Stability Deficits in Anisometropic Amblyopia
Author Affiliations & Notes
  • Yusong Zhou
    State Key Laboratory of Ophthalmology, Zhongshan Ophthalmic Center, Guangdong Provincial Key Laboratory of Ophthalmology and Visual Science, Sun Yat-Sen University, Guangzhou, China
  • Wentong Yu
    State Key Laboratory of Ophthalmology, Zhongshan Ophthalmic Center, Guangdong Provincial Key Laboratory of Ophthalmology and Visual Science, Sun Yat-Sen University, Guangzhou, China
  • Qingqing Ye
    State Key Laboratory of Ophthalmology, Zhongshan Ophthalmic Center, Guangdong Provincial Key Laboratory of Ophthalmology and Visual Science, Sun Yat-Sen University, Guangzhou, China
  • Zixuan Xu
    State Key Laboratory of Ophthalmology, Zhongshan Ophthalmic Center, Guangdong Provincial Key Laboratory of Ophthalmology and Visual Science, Sun Yat-Sen University, Guangzhou, China
  • Yunsi He
    State Key Laboratory of Ophthalmology, Zhongshan Ophthalmic Center, Guangdong Provincial Key Laboratory of Ophthalmology and Visual Science, Sun Yat-Sen University, Guangzhou, China
  • Ying Yao
    State Key Laboratory of Ophthalmology, Zhongshan Ophthalmic Center, Guangdong Provincial Key Laboratory of Ophthalmology and Visual Science, Sun Yat-Sen University, Guangzhou, China
  • Yangfei Pang
    State Key Laboratory of Ophthalmology, Zhongshan Ophthalmic Center, Guangdong Provincial Key Laboratory of Ophthalmology and Visual Science, Sun Yat-Sen University, Guangzhou, China
  • Yudan Zhong
    State Key Laboratory of Ophthalmology, Zhongshan Ophthalmic Center, Guangdong Provincial Key Laboratory of Ophthalmology and Visual Science, Sun Yat-Sen University, Guangzhou, China
  • Qiuying Li
    State Key Laboratory of Ophthalmology, Zhongshan Ophthalmic Center, Guangdong Provincial Key Laboratory of Ophthalmology and Visual Science, Sun Yat-Sen University, Guangzhou, China
  • Lei Feng
    State Key Laboratory of Ophthalmology, Zhongshan Ophthalmic Center, Guangdong Provincial Key Laboratory of Ophthalmology and Visual Science, Sun Yat-Sen University, Guangzhou, China
  • Yun Wen
    State Key Laboratory of Ophthalmology, Zhongshan Ophthalmic Center, Guangdong Provincial Key Laboratory of Ophthalmology and Visual Science, Sun Yat-Sen University, Guangzhou, China
  • Xuan Qiu
    State Key Laboratory of Ophthalmology, Zhongshan Ophthalmic Center, Guangdong Provincial Key Laboratory of Ophthalmology and Visual Science, Sun Yat-Sen University, Guangzhou, China
  • Zhonghao Wang
    State Key Laboratory of Ophthalmology, Zhongshan Ophthalmic Center, Guangdong Provincial Key Laboratory of Ophthalmology and Visual Science, Sun Yat-Sen University, Guangzhou, China
  • Jinrong Li
    State Key Laboratory of Ophthalmology, Zhongshan Ophthalmic Center, Guangdong Provincial Key Laboratory of Ophthalmology and Visual Science, Sun Yat-Sen University, Guangzhou, China
  • Correspondence: Zhonghao Wang, State Key Laboratory of Ophthalmology, Zhongshan Ophthalmic Center, Guangdong Provincial Key Laboratory of Ophthalmology and Visual Science, Sun Yat-Sen University, 54S Xian Lie Rd., Guangzhou 510060, China; [email protected]
  • Jinrong Li, State Key Laboratory of Ophthalmology, Zhongshan Ophthalmic Center, Guangdong Provincial Key Laboratory of Ophthalmology and Visual Science, Sun Yat-Sen University, 54S Xian Lie Rd., Guangzhou 510060, China; [email protected]
  • Footnotes
     YZ and WY contributed equally to this work and share first authorship.
  • Footnotes
     ZW and JL contributed equally to this work and share corresponding authorship.
Investigative Ophthalmology & Visual Science January 2025, Vol.66, 14. doi:https://doi.org/10.1167/iovs.66.1.14
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      Yusong Zhou, Wentong Yu, Qingqing Ye, Zixuan Xu, Yunsi He, Ying Yao, Yangfei Pang, Yudan Zhong, Qiuying Li, Lei Feng, Yun Wen, Xuan Qiu, Zhonghao Wang, Jinrong Li; Fixation Stability Deficits in Anisometropic Amblyopia. Invest. Ophthalmol. Vis. Sci. 2025;66(1):14. https://doi.org/10.1167/iovs.66.1.14.

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Abstract

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.

Anisometropic amblyopia is a type of unilateral amblyopia where differences in refractive errors between the two eyes result in varying focal points on the retinas, leading to interocular suppression that contributes to the development of amblyopia.1 Consequently, the amblyopic eye suffers from visual function impairments, including reduced visual acuity, decreased contrast sensitivity, and unstable fixation.24 Chung et al. found that fixation stability in the amblyopic eye of patients with anisometropic amblyopia is diminished, whereas the fixation stability of the fellow eye does not significantly differ from that of healthy controls.5 However, Subramanian et al. reported that fixation instability is present not only in the amblyopic eye but also in the fellow eye.6 Similarly, Kelly et al. demonstrated that even when the fellow eye has relatively normal visual acuity, it can still exhibit deficits in fixation stability.7 
Current research indicates that fixation instability in anisometropic amblyopia is primarily associated with abnormal fixation saccades, ocular drifts, and fusion maldevelopment nystagmus (FMN) within the components of eye movement.811 Regarding visual functions, some studies have suggested that fixation stability in the amblyopic eye may be related to stereopsis. Birch et al. found that among children with amblyopia, those with poorer stereopsis tend to have more unstable fixation in the amblyopic eye compared with those with better stereopsis.8 Kang et al. proposed that fixation stability in the amblyopic eye could be a predictor of stereopsis function.12 
However, the relationship between fixation stability deficits in anisometropic amblyopia and other visual functions, as well as fundus structure, remains to be further explored. The MP-3 microperimeter is a novel device for assessing retinal function, capable of measuring fixation stability, and marking the preferred retinal locus. It also provides color fundus images, linking retinal anatomic structures to their functional attributes.13,14 To further investigate the factors associated with fixation stability in both the amblyopic and fellow eyes of patients with anisometropic amblyopia, this study recruited 164 patients with anisometropic amblyopia at Zhongshan Ophthalmic Center between June 2023 and July 2024. We measured and recorded various visual functions, including best-corrected visual acuity (BCVA), contrast sensitivity function (CSF), and fixation stability, in both the amblyopic and fellow eyes, and conducted corresponding analyses. 
Methods
Participants
A total of 164 participants were recruited for this cross-sectional study at Zhongshan Ophthalmic Center from June 2023 to July 2024. The inclusion criteria were as follows: (1) male or female patients older than 5 years with anisometropic amblyopia; (2) had undergone refractive correction for at least 3 months; and (3) had received occlusion therapy for at least 6 months or had never received. Amblyopia was defined as BCVA > 0.1 logMAR in the affected eye and 2 or more lines of visual acuity difference between the 2 eyes. Anisometropic amblyopia was defined according to the Preferred Practice Pattern from the American Academy of Ophthalmology as an interocular difference (IOD) of ≥ 1.00 diopters (D) in spherical equivalent and/or ≥ 1.50 D interocular difference in astigmatism between any meridians at the time of the initial diagnosis.15 The exclusion criteria included: (1) manifest strabismus; (2) nystagmus; (3) a history of ocular or refractive surgery or other ocular organic diseases, diabetes, or any systemic disease that might affect the eyes; and (4) brain or other conditions affecting vestibular-related balance. Participants who received occlusion therapy for more than 6 months were defined as having a history of occlusion therapy, whereas those who never received occlusion therapy were defined as having no history of occlusion therapy. All collected study data weres recorded in the Uniting Functions in Ophthalmology and Optometry (UFOs) database.16,17 Written informed consent was obtained from each participant. This study adhered to the tenets of the Declaration of Helsinki, and the study protocol was reviewed and approved by the Zhongshan Ophthalmic Center Ethics Committee. Our study results were presented in accordance with the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) reporting guidelines.18 
Measurements of Visual Function
Refraction and Visual Acuity
The refractive error was determined using cycloplegic refraction. BCVA was assessed with a Tumbling-E Early Treatment Diabetic Retinopathy Study (ETDRS) chart (WEHEN Vision, Guangzhou, Guangdong, China) from 4 m at a luminance of 200 cd/m2 and expressed in logMAR units. The charts consist of 5 optotypes per line for a total of 12 lines, with optotype size increasing from −0.3 logMAR to 1.0 logMAR in steps of 0.1 logMAR. Each falsely identified optotype will add 0.02 logMAR in BCVA.19 All participants were first measured on the fellow eye, followed by the amblyopic eye. 
Stereopsis
All tests were conducted with participants having fully corrected refractive errors. Near stereoacuity was measured with the Random Dot Stereo Acuity Test (Vision Assessment Corporation, Elk Grove Village, IL, USA) at 40 cm using sections B and C with disparities ranging from 12.5 to 400 arc-seconds, which was made up of contour-based circle and symbol targets with monocular cues. For results where near stereoacuity exceeded 160 arc-seconds, we recorded them as nil, because the patterns could be recognized under monocular conditions.20 The distance stereoacuity was measured using the Randot Stereoacuity Tests (Stereo Optical Co., Inc., Chicago, IL, USA) at 4 m, including disparities ranging from 60 to 400 arc-seconds with no monocular cues. Each measurement was repeated twice. Nil stereopsis was recorded as 10,000 arcsec. A logarithmic transformation with a base of 10 was applied to the stereopsis measurement results. 
Contrast Sensitivity Function
The CSF was measured using the qCSF method (Manifold Contrast Vision Meter, Adapative Sensory Technology, Inc., San Diego, CA, USA), a Bayesian adaptive active learning procedure for quantitative CSF assessment.2124 The qCSF algorithm selects the optimal test stimulus from a total of 2432 possible stimuli in each trial; these stimuli consisted of 3 spatial frequency bandpass-filtered digits with different contrasts and were presented on a gamma-corrected 46-inch LCD monitor (Model: NEC LCD P463, luminance: 50 cd/m2, 60 hertz [Hz]) with a resolution of 1920 × 1080 pixels. Subjects needed to identify the numbers shown on the screen from 4 m in a dark room in each trial after their refractive error had been best corrected. The qCSF algorithm controls the contrast and spatial frequency of the stimuli in each trial by resizing the digits in response to the result of the feedback of each subject. A new trial started 500 ms after the subject’s response. Each test consisted of 35 trials and took approximately 3 to 5 minutes. The test generated the monocular contrast sensitivities at 19 spatial frequencies (from 1.27 cpd to 32.88 cpd) in each visit. The trapezoid method was used to calculate the area under the log CSF (AULCSF), and it was a broad measure of spatial vision as a summary metric based on the curve for the entire frequency range.25,26 All participants were first measured on the fellow eye, followed by the amblyopic eye. 
Microperimetry Examination
The fixation stability of each eye was measured using a microperimeter (MP-3, Nidek, Hiroshima, Japan) under dark room conditions. Before the assessment, the examiner would adjust the chin rest to stabilize the participant’s head. The microperimeter then projected an approximately 2 degrees by 7 degrees red cross into the tested eye as a fixation target against a dark background according to the feedback of participants.27 Participants were instructed to continuously fixate on the center of the cross while a fundus photograph was being taken. Then, the microperimeter recorded eye movements at a sampling rate of 30 Hz. During the measurement process, the MP-3 would use auto-tracking and auto-alignment technologies to enhance measurement accuracy, patient comfort, and operational efficiency.28 These features can reduce inter-examiner variability, ensuring well-aligned and reliable results. The assessment concluded after approximately 1 minute of effective recording time. All participants were first measured on the fellow eye, followed by the amblyopic eye, with a 5-minute break between the 2 measurements for a rest. 
The MP-3 generated bivariate contour ellipse area (BCEA) values at three different probabilities (68.2%, 95.4%, and 99.6%) and the preferred retinal locus, with only the 68.2% @ BCEA used in this study to assess fixation stability.29 A logarithmic transformation with a base of 10 was applied to the BCEA measurement results. Fixation stability was also assessed by measuring the percentage of fixation points within 1 degree and 2 degrees from the preferred retinal locus, referred to as P1 and P2, according to Fujii et al.’s criteria.30 Fixation is classified as stable when more than 75% of fixation points fall within P1. If less than 75% of fixation points are within P1 but more than 75% are within P2, fixation is considered relatively stable. Fixation is classified as unstable if less than 75% of fixation points fall within P2. The eccentricity was defined as the offset between the preferred retinal locus and the fovea on the retinal images captured by the MP-3. Additionally, the mean eccentricity (0.14 degrees) from the MP-3 results of 10 normal volunteers (Supplementary Table S2) was used to correct the participants’ measurements to avoid potential measurement errors associated with the MP-3. Eccentric fixation is defined as an eccentricity greater than 1 degree (Fig. 1). To minimize the potential impact of learning effects on the results, all participants underwent three practice tests before the formal assessment. 
Figure 1.
 
Examination results of a 12-year-old male participant (right eye). The MP-3 established a polar coordinate system with a radius of 22 degrees centered on the participant’s location of preferred retinal locus in the right eye. The heatmap centered at the origin reflects the density of fixation points. The position of the red circle indicates the location of the fovea.
Figure 1.
 
Examination results of a 12-year-old male participant (right eye). The MP-3 established a polar coordinate system with a radius of 22 degrees centered on the participant’s location of preferred retinal locus in the right eye. The heatmap centered at the origin reflects the density of fixation points. The position of the red circle indicates the location of the fovea.
Data Analysis
Statistical analysis was conducted using SPSS Statistics version 22.0 (IBM, Chicago, IL, USA). Continuous variable data were reported as mean ± standard deviation. Categorical data were described as frequencies and percentages. The IODs were calculated uniformly as the parameters of the fellow eyes minus those of the amblyopic eyes. The normality of the data was assessed with the Shapiro‒Wilk test. The independent samples t-test or independent nonparametric test was used to compare the data difference between the fellow eyes and amblyopic eyes. To determine the factors associated with fixation stability in amblyopic and fellow eyes, potential influencing variables were included based on relevant literature and clinical practice experience. Collinearity was analyzed using the Pearson correlation, and a correlation coefficient (|r|) greater than 0.7 was considered indicative of collinearity between variables. When collinearity was present among the independent variables, a ridge regression model was used for fitting. Otherwise, multivariable linear regression was applied for fitting. A two-sided P value of less than 0.05 was considered statistically significant. 
Results
Participants
A total of 164 participants were recruited for this study at Zhongshan Ophthalmic Center from June 2023 to July 2024. Three participants (two 6-year-old and one 7-year-old children) were excluded due to their inability to complete the required examinations. Ultimately, 161 patients with anisometropic amblyopia (87 men and 74 women, average age = 20.26 ± 8.79 years, ranging from 5 to 51 years old) were included in this study. The demographic and visual function characteristics of the participants are detailed in Table 1. In addition, we have provided a table describing the baseline characteristics of participants with and without patching history separately (Supplementary Table S1). The interocular difference in spherical equivalent refraction is presented as absolute values. All participants had undergone refractive correction for more than 3 months, using either spectacles or rigid gas permeable lenses. Among them, 44 (27.3%) had received occlusion therapy for at least 3 months, whereas the remaining 117 (72.7%) had never received any amblyopia treatment other than refractive correction. 
Table 1.
 
Demographic and Baseline Clinical Characteristics of Participants
Table 1.
 
Demographic and Baseline Clinical Characteristics of Participants
Visual Acuity and Contrast Sensitivity Function
In this study, all participants underwent measurement of BCVA after full refractive correction. The average spherical equivalent refraction (SER) was 1.94 ± 5.54 D for amblyopic eyes and −1.19 ± 2.44 D for fellow eyes, with a significant difference (P < 0.05). The absolute interocular difference in SER (SER-IODabs) was 5.12 ± 3.01 D. The average BCVA was 0.68 ± 0.34 logMAR for amblyopic eyes and −0.04 ± 0.06 logMAR for fellow eyes, showing a significant difference (P < 0.05), with an average BCVA-IOD of −0.71 ± 0.35 logMAR (Fig. 2). The average AULCSF was 0.70 ± 0.40 for amblyopic eyes and 1.53 ± 0.30 for fellow eyes, which was also significantly different (P < 0.05), with an average AULCSF-IOD of 0.83 ± 0.43 (Fig. 3). 
Figure 2.
 
(A) Spherical equivalent of refraction in amblyopic eyes and fellow eyes. (B) Best corrected visual acuity in amblyopic eyes and fellow eyes. Triangles represent individual values. Error bars represent one standard error of the mean. * Indicates statistical significance at 0.05.
Figure 2.
 
(A) Spherical equivalent of refraction in amblyopic eyes and fellow eyes. (B) Best corrected visual acuity in amblyopic eyes and fellow eyes. Triangles represent individual values. Error bars represent one standard error of the mean. * Indicates statistical significance at 0.05.
Figure 3.
 
(A) AULCSF in amblyopic eyes and fellow eyes. (B) Contrast sensitivity function in amblyopic eyes. (C) Contrast sensitivity function in fellow eyes. Triangles represent individual values. Error bars represent one standard error of the mean. * Indicates statistical significance at 0.05.
Figure 3.
 
(A) AULCSF in amblyopic eyes and fellow eyes. (B) Contrast sensitivity function in amblyopic eyes. (C) Contrast sensitivity function in fellow eyes. Triangles represent individual values. Error bars represent one standard error of the mean. * Indicates statistical significance at 0.05.
Stereopsis
In this study, 145 (90.1%) participants had no measurable near stereoacuity, and 159 (98.8%) participants had no measurable distance stereoacuity. The average near stereoacuity for the participants was 3.80 ± 0.61 log arcsec, and the average distance stereoacuity was 3.98 ± 0.16 log arcsec (see Table 1). 
Microperimetry
The MP-3 was used to assess the fixation stability and preferred retinal locus eccentricity of the participants. According to Fujii et al.’s criteria, 77 (47.8%) amblyopic eyes had unstable fixation, and 69 (42.9%) amblyopic eyes had relatively unstable fixation. In contrast, 32 (19.9%) fellow eyes had unstable fixation, and 57 (35.4%) fellow eyes had relatively unstable fixation. The average log BCEA for amblyopic eyes was 0.96 ± 0.43 log deg², whereas for fellow eyes, it was 0.60 ± 0.45 log deg², showing a significant difference (P < 0.05). The average preferred retinal locus eccentricity was 1.90 ± 1.91 degrees for amblyopic eyes and 0.73 ± 0.63 degrees for fellow eyes, also showing a significant difference (P < 0.05; Fig. 4). In the amblyopic eyes, 104 eyes (64.6%) had an eccentricity greater than 1 degree, whereas in the fellow eyes, 45 eyes (28.0%) had an eccentricity greater than 1 degree. 
Figure 4.
 
(A) Log BCEA in amblyopic eyes and fellow eyes. (B) Fixation eccentricity in amblyopic eyes and fellow eyes. Triangles represent individual values. Error bars represent one standard error of the mean. * Indicates statistical significance at 0.05.
Figure 4.
 
(A) Log BCEA in amblyopic eyes and fellow eyes. (B) Fixation eccentricity in amblyopic eyes and fellow eyes. Triangles represent individual values. Error bars represent one standard error of the mean. * Indicates statistical significance at 0.05.
Association of Fixation Stability and Visual Functions
Based on previous clinical experience and literature reports, we included the following variables in the analysis: gender, age, BCVA, AULCSF, SER-IODabs, AULCSF-IOD, eccentricity, patching history, and the log BCEA of the contralateral eye.31 Because the majority of the participants in this study had no measurable stereoacuity, these variables were not included in the model fitting. According to the results of Pearson correlation analysis (Supplementary Fig. S1), ridge regression was used to explore the factors influencing fixation stability (log BCEA) in amblyopic and the multivariable linear regression analyses was used to fellow eyes. As shown in Tables 2 and 3, the log BCEA of the amblyopic eye was significantly associated with age (coefficient = −0.003, 95% confidence interval [CI] = −0.004 to −0.002, P = 0.032), BCVA of amblyopic eyes (coefficient = 0.115, 95% CI = 0.086 to 0.144, P < 0.001), AULCSF of the amblyopic eyes (coefficient = −0.050, 95% CI = −0.075 to −0.025, P = 0.044), AULCSF-IOD (coefficient = 0.080, 95% CI = 0.055 to 0.105, P = 0.002), eccentricity of the amblyopic eyes (coefficient = 0.029, 95% CI = 0.022 to 0.036, P < 0.001), and the log BCEA of the fellow eyes (coefficient = 0.225, 95% CI = 0.196 to 0.254, P < 0.001). For the fellow eyes, the log BCEA was significantly associated with eccentricity of the fellow eyes (coefficient = 0.298, 95% CI = 0.254 to 0.342, P < 0.001), and the log BCEA of the amblyopic eyes (coefficient = 0.527, 95% CI = 0.458 to 0.596, P < 0.001; Fig. 5). 
Table 2.
 
Ridge Regression Analyses to Determine Association of Fixation Stability (log BCEA) of Amblyopic Eyes With Clinical Features and Visual Functions
Table 2.
 
Ridge Regression Analyses to Determine Association of Fixation Stability (log BCEA) of Amblyopic Eyes With Clinical Features and Visual Functions
Table 3.
 
Multivariable Linear Regression Analyses to Determine Association of Fixation Stability (Log BCEA) of Fellow Eyes With Clinical Features and Visual Functions
Table 3.
 
Multivariable Linear Regression Analyses to Determine Association of Fixation Stability (Log BCEA) of Fellow Eyes With Clinical Features and Visual Functions
Figure 5.
 
(A) The log BCEA of amblyopic eyes as a function of the age. (B) The log BCEA of amblyopic eyes as a function of the BCVA of amblyopic eyes. (C) The log BCEA of amblyopic eyes as a function of the AULCSF. (D) The log BCEA of amblyopic eyes as a function of the AULCSF-IOD. (E) The log BCEA of amblyopic eyes as a function of the fixation eccentricity of amblyopic eyes. (F) The log BCEA of amblyopic eyes as a function of the log BCEA of fellow eyes. (G) The log BCEA of fellow eyes as a function of the fixation eccentricity of fellow eyes. (H) The log BCEA of fellow eyes as a function of the log BCEA of amblyopic eyes. Triangles represent individual values.
Figure 5.
 
(A) The log BCEA of amblyopic eyes as a function of the age. (B) The log BCEA of amblyopic eyes as a function of the BCVA of amblyopic eyes. (C) The log BCEA of amblyopic eyes as a function of the AULCSF. (D) The log BCEA of amblyopic eyes as a function of the AULCSF-IOD. (E) The log BCEA of amblyopic eyes as a function of the fixation eccentricity of amblyopic eyes. (F) The log BCEA of amblyopic eyes as a function of the log BCEA of fellow eyes. (G) The log BCEA of fellow eyes as a function of the fixation eccentricity of fellow eyes. (H) The log BCEA of fellow eyes as a function of the log BCEA of amblyopic eyes. Triangles represent individual values.
Discussion
This study measured the clinical characteristics and various visual functions of 161 patients with anisometropic amblyopia. The results showed that the BCVA and AULCSF of the amblyopic eyes were significantly worse than those of the fellow eyes. According to MP-3 measurements, 47.8% of the amblyopic eyes and 19.9% of the fellow eyes exhibited unstable fixation based on Fujii et al.’s criteria. Additionally, 64.6% of the amblyopic eyes and 28.0% of the fellow eyes had eccentric fixation with an eccentricity greater than 1 degree. Gender, age, BCVA, AULCSF, SER-IODabs, AULCSF-IOD, eccentricity, patching history, and the log BCEA of the contralateral eyes were selected as independent variables for fitting a regression model. The model fitting results indicated that, for the amblyopic eyes, age and AULCSF of amblyopic eyes were negatively correlated with the log BCEA of amblyopic eyes, whereas AULCSF-IOD, eccentricity of the amblyopic eyes, and the log BCEA of the fellow eyes had a positive correlation. For the fellow eyes, the eccentricity of the fellow eyes and the log BCEA of the amblyopic eyes had a positive correlation. Gender, SER-IODabs, and patching history did not have significant correlation with the log BCEA of either the amblyopic or fellow eyes. 
The qCSF method was used to measure the contrast sensitivity function in both the amblyopic and fellow eyes. Compared with traditional methods, qCSF offers faster and more accurate measurements. CSF provides a more comprehensive assessment of visual function.21 Unlike BCVA, which primarily reflects the eye's ability to resolve high-frequency details, CSF also captures sensitivity to low-frequency components. Our results, consistent with previous studies,3234 demonstrate that the contrast sensitivity function and BCVA in the amblyopic eyes of patients with anisometropic amblyopia are significantly worse than those in the fellow eyes. 
In 1961, Lang observed 18 cases of anisometropic amblyopia without strabismus and found that all amblyopic eyes exhibited eccentric fixation.35 Helveston’s research suggested that uncorrected anisometropia might be the cause of eccentric fixation.36 However, subsequent studies have shown that not all cases of anisometropia lead to eccentric fixation.37 This finding aligns with our results: the MP-3 measurements revealed that approximately 64.6% of the amblyopic eyes exhibited eccentric fixation. Surprisingly, 28.0% of fellow eyes also displayed this defect, with both amblyopic and fellow eyes showing impairment in fixation stability. This indicates the importance of paying attention to the functional deficits in the fellow eye, even in cases of unilateral amblyopia. 
Many studies have suggested that the severity of amblyopia, as assessed by BCVA, is a function of the degree of anisometropia.3841 However, our regression model indicates that there appears to be no association between fixation stability (log BCEA) and the degree of anisometropia (SER-IODabs). This lack of correlation may be due to fixation stability impairment being potentially associated with higher-level neural function deficits. Yin et al. used resting state functional magnetic resonance imaging (rs-fMRI) to collect the amplitude of low-frequency fluctuation in patients with anisometropic amblyopia and reported changes in spontaneous brain activity. Their study revealed functional impairments in neural pathways associated with anisometropic amblyopia, particularly in the left lingual gyrus, right middle occipital gyrus, right anterior central gyrus, left posterior cingulate gyrus, left superior parietal lobule, left precuneus, and left cuneiform lobe.42 Mendola et al. investigated children and adults with anisometropic amblyopia and reported a reduction in gray matter volume in the visual cortex hemisphere associated with amblyopia. They found that this reduction was less pronounced in adults compared with children,43 which may explain why our regression model shows that age has a negative correlation with the log BCEA of amblyopic eyes, suggesting that fixation stability seems to improve with age. Another possible reason is the lack of attention in children. In the study by Jones et al., fixation stability in children aged 9 to 12 years old, measured using macular integrity assessment microperimetry, was significantly worse than in adults.44 This difference could not be attributed to retinal immaturity, and the lack of attention in children may be the underlying cause of this result.45 Black et al.’s study found that children with amblyopia have slower visual processing times on the Useful Field of View and slower completion times on the Trail Making Tests, indicating deficits in visual attention.46 Similar results have been reported in other studies.47,48 Verghese et al. hypothesized that these deficits in visual attention are a consequence of fixation instability, but the relationship still requires further investigation.49 However, in all the current methods for assessing fixation stability, there is no reliable way to evaluate the participants’ attention during the examination process. Chung et al.’s study reported a correlation between fixation stability and visual acuity in amblyopia, indicating that better visual acuity is associated with more stable fixation, which is consistent with the results of amblyopic eyes in our study.5 Our results similarly suggest that AULCSF is related to fixation stability in amblyopic eyes. This is understandable, as AULCSF is a more comprehensive measure that includes the high-frequency resolution represented by BCVA. According to our regression model, an increase in AULCSF-IOD is positively associated with increased fixation instability in amblyopic eyes. Pardhan’s research has shown that AULCSF-IOD is correlated with interocular suppression.50 Raveendran et al.’s study has indicated that artificially reducing interocular suppression through contrast rebalancing significantly improves fixation stability in the amblyopic eyes of patients with strabismic amblyopia.51 These findings may help explain the impact of AULCSF-IOD in the regression model for amblyopic eyes. 
Fixation stability in both amblyopic and fellow eyes is correlated with the eccentricity of the preferred retinal locus. Fixation instability increases as the preferred retinal locus moves further away from the foveal.52 In the regression model, a history of occlusion therapy did not show a significant impact, suggesting that unilateral occlusion therapy may be not directly improve fixation stability in either the amblyopic or fellow eye. Carpineto et al.’s study demonstrated that fixation stability in unilateral amblyopia improves with prolonged treatment duration.53 Similarly, research by Wang et al. found that fixation stability in patients with severe amblyopia significantly improved after occlusion therapy.54 Notably, these patients also exhibited a significant improvement in BCVA following occlusion, leading us to reasonably hypothesize that the improvement in fixation stability is attributable to the improvement in BCVA, rather than being a direct result of occlusion therapy itself. The log BCEA of the fellow eye was positively correlated only with eccentricity and the log BCEA of the amblyopic eyes, which may indicate an interaction between the amblyopic and fellow eyes and the visual function impairment of the fellow eye also warrants attention. Fusion maldevelopment nystagmus is a characteristic oculomotor deficit that suggests a disruption in binocularity during the first few months of life.55 Although we made every effort to exclude patients with amblyopia with nystagmus, subtle FMN may have gone undetected, which could also account for the positive correlation between fixation stability in the amblyopic eyes and the fellow eyes. Currently, amblyopia treatment is evolving from a monocular approach to a binocular approach.56 Binocular digital therapy is an emerging paradigm that aims to balance visual function between the eyes and reduce interocular suppression,57 and it has been recommended in the 2023 Amblyopia Preferred Practice Pattern.15 However, little attention has been paid to the correction of abnormal fixation patterns or eccentric fixation in amblyopia treatment. Fixation stability assessments for amblyopia are not yet widely implemented in clinical practice. Its deficits may lead to slower reading speeds and difficulties under both binocular and monocular conditions.58,59 Some studies suggest that abnormal eye movements in amblyopia may be related to attentional deficits.6064 This implies that fixation stability deficits could have potential impacts on daily visual tasks in patients with amblyopia, indicating a need for clinical assessments to expand beyond traditional measures. Moreover, perceptual training has gradually become an effective clinical intervention for amblyopia,65,66 primarily aiming to improve visual function through visual stimuli and tasks. However, the stimulus presentation during training might be influenced by the patient's fixation instability, and the effect of this influence on treatment outcomes remains unclear, asking further investigation. 
This study has some limitations. First, as a cross-sectional study, it identified factors associated with fixation stability in amblyopic and fellow eyes using ridge regression model and multivariable linear regression model, but it cannot establish causal relationships between these factors. Additionally, it does not allow us to observe changes in fixation stability over time in patients with anisometropic amblyopia as they age. Further longitudinal studies are necessary to address this limitation. Second, our study may be subject to bias, as most of the recruited participants had no measurable stereopsis, which hindered our ability to investigate the relationship between binocular visual function and fixation stability. Additionally, patients with nystagmus were excluded during recruitment.67 Those may limit the generalizability of our findings. Future research involving larger, multicenter studies with more diverse samples will be essential. 
In conclusion, this study provides comprehensive information on various visual functions in patients with anisometropic amblyopia, including fixation stability, fixation characteristics, and contrast sensitivity function. Using the ridge regression model and the multivariable linear regression, we explored the factors influencing fixation stability in both amblyopic and fellow eyes. We found that both eccentric fixation and fixation stability deficits were observed in both amblyopic and fellow eyes, with fixation stability in both eyes being correlated with the eccentricity of the preferred retinal locus. These findings offer valuable insights for future amblyopia research and underscore the importance of addressing fixation stability and fixation characteristics in the clinical management of amblyopia, regardless of whether it involves the amblyopic or fellow eye. 
Acknowledgments
Supported by National Natural Science Foundation of China (82371088), National Key Research & Development Project (2020YFC2003905), and Fundamental Research Funds for the Central Universities, Sun Yat-Sen University (24pnpy245). 
Disclosure: Y. Zhou, None; W. Yu, None; Q. Ye, None; Z. Xu, None; Y. He, None; Y. Yao, None; Y. Pang, None; Y. Zhong, None; Q. Li, None; L. Feng, None; Y. Wen, None; X. Qiu, None; Z. Wang, None; J. Li, None 
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Figure 1.
 
Examination results of a 12-year-old male participant (right eye). The MP-3 established a polar coordinate system with a radius of 22 degrees centered on the participant’s location of preferred retinal locus in the right eye. The heatmap centered at the origin reflects the density of fixation points. The position of the red circle indicates the location of the fovea.
Figure 1.
 
Examination results of a 12-year-old male participant (right eye). The MP-3 established a polar coordinate system with a radius of 22 degrees centered on the participant’s location of preferred retinal locus in the right eye. The heatmap centered at the origin reflects the density of fixation points. The position of the red circle indicates the location of the fovea.
Figure 2.
 
(A) Spherical equivalent of refraction in amblyopic eyes and fellow eyes. (B) Best corrected visual acuity in amblyopic eyes and fellow eyes. Triangles represent individual values. Error bars represent one standard error of the mean. * Indicates statistical significance at 0.05.
Figure 2.
 
(A) Spherical equivalent of refraction in amblyopic eyes and fellow eyes. (B) Best corrected visual acuity in amblyopic eyes and fellow eyes. Triangles represent individual values. Error bars represent one standard error of the mean. * Indicates statistical significance at 0.05.
Figure 3.
 
(A) AULCSF in amblyopic eyes and fellow eyes. (B) Contrast sensitivity function in amblyopic eyes. (C) Contrast sensitivity function in fellow eyes. Triangles represent individual values. Error bars represent one standard error of the mean. * Indicates statistical significance at 0.05.
Figure 3.
 
(A) AULCSF in amblyopic eyes and fellow eyes. (B) Contrast sensitivity function in amblyopic eyes. (C) Contrast sensitivity function in fellow eyes. Triangles represent individual values. Error bars represent one standard error of the mean. * Indicates statistical significance at 0.05.
Figure 4.
 
(A) Log BCEA in amblyopic eyes and fellow eyes. (B) Fixation eccentricity in amblyopic eyes and fellow eyes. Triangles represent individual values. Error bars represent one standard error of the mean. * Indicates statistical significance at 0.05.
Figure 4.
 
(A) Log BCEA in amblyopic eyes and fellow eyes. (B) Fixation eccentricity in amblyopic eyes and fellow eyes. Triangles represent individual values. Error bars represent one standard error of the mean. * Indicates statistical significance at 0.05.
Figure 5.
 
(A) The log BCEA of amblyopic eyes as a function of the age. (B) The log BCEA of amblyopic eyes as a function of the BCVA of amblyopic eyes. (C) The log BCEA of amblyopic eyes as a function of the AULCSF. (D) The log BCEA of amblyopic eyes as a function of the AULCSF-IOD. (E) The log BCEA of amblyopic eyes as a function of the fixation eccentricity of amblyopic eyes. (F) The log BCEA of amblyopic eyes as a function of the log BCEA of fellow eyes. (G) The log BCEA of fellow eyes as a function of the fixation eccentricity of fellow eyes. (H) The log BCEA of fellow eyes as a function of the log BCEA of amblyopic eyes. Triangles represent individual values.
Figure 5.
 
(A) The log BCEA of amblyopic eyes as a function of the age. (B) The log BCEA of amblyopic eyes as a function of the BCVA of amblyopic eyes. (C) The log BCEA of amblyopic eyes as a function of the AULCSF. (D) The log BCEA of amblyopic eyes as a function of the AULCSF-IOD. (E) The log BCEA of amblyopic eyes as a function of the fixation eccentricity of amblyopic eyes. (F) The log BCEA of amblyopic eyes as a function of the log BCEA of fellow eyes. (G) The log BCEA of fellow eyes as a function of the fixation eccentricity of fellow eyes. (H) The log BCEA of fellow eyes as a function of the log BCEA of amblyopic eyes. Triangles represent individual values.
Table 1.
 
Demographic and Baseline Clinical Characteristics of Participants
Table 1.
 
Demographic and Baseline Clinical Characteristics of Participants
Table 2.
 
Ridge Regression Analyses to Determine Association of Fixation Stability (log BCEA) of Amblyopic Eyes With Clinical Features and Visual Functions
Table 2.
 
Ridge Regression Analyses to Determine Association of Fixation Stability (log BCEA) of Amblyopic Eyes With Clinical Features and Visual Functions
Table 3.
 
Multivariable Linear Regression Analyses to Determine Association of Fixation Stability (Log BCEA) of Fellow Eyes With Clinical Features and Visual Functions
Table 3.
 
Multivariable Linear Regression Analyses to Determine Association of Fixation Stability (Log BCEA) of Fellow Eyes With Clinical Features and Visual Functions
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