Bilateral changes in foveal structure in individuals with amblyopia

Associations between unilateral amblyopia in childhood and cardiometabolic disorders in adult life: a cross-sectional and longitudinal analysis of the UK Biobank

Background

Amblyopia is a common neurodevelopmental condition and leading cause of childhood visual impairment. Given the known association between neurodevelopmental impairment and cardiometabolic dysfunction in later life, we investigated whether children with amblyopia have increased risk of cardiometabolic disorders in adult life.

Methods

This was a cross-sectional and longitudinal analysis of 126,399 United Kingdom Biobank cohort participants who underwent ocular examination. A subset of 67,321 of these received retinal imaging. Data analysis was conducted between November 1st 2021 and October 15th 2022. Our primary objective was to investigate the association between amblyopia and a number of components of metabolic syndrome and individual cardiometabolic diseases. Childhood amblyopia, dichotomised as resolved or persisting by adulthood, cardiometabolic disease and mortality were defined using ophthalmic assessment, self-reported, hospital admissions and death records. Morphological features of the optic nerve and retinal vasculature and sublayers were extracted from retinal photography and optical coherence tomography. Associations between amblyopia and cardiometabolic disorders as well as retinal markers were investigated in multivariable-adjusted regression models.

Findings

Individuals with persisting amblyopia (n = 2647) were more likely to be obese (adjusted odds ratio (95% confidence interval): 1.16 (1.05; 1.28)), hypertensive (1.25 (1.13; 1.38)) and diabetic (1.29 (1.04; 1.59)) than individuals without amblyopia (controls, (n = 18,481)). Amblyopia was also associated with an increased risk of myocardial infarction (adjusted hazard ratio: 1.38 (1.11; 1.72)) and death (1.36 (1.15; 1.60)). On retinal imaging, amblyopic eyes had significantly increased venular caliber (0.29 units (0.21; 0.36)), increased tortuosity (0.11 units (0.03; 0.19)), but lower fractal dimension (-0.23 units (-0.30; -0.16)) and thinner ganglion cell-inner plexiform layer (mGC-IPL, -2.85 microns (-3.47; -2.22)). Unaffected fellow eyes of individuals with amblyopia also had significantly lower retinal fractal dimension (-0.08 units (-0.15; -0.01)) and thinner mGC-IPL (-1.14 microns (-1.74; -0.54)). Amblyopic eyes with a persisting visual deficit had smaller optic nerve disc height (-0.17 units (-0.25; -0.08)) and width (-0.13 units (-0.21; -0.04)) compared to control eyes.

Interpretation

Although further research is needed to understand the basis of the observed associations, healthcare professionals should be cognisant of greater cardiometabolic dysfunction in adults who had childhood amblyopia. Differences in retinal features in both the amblyopic eye and the unaffected non-amblyopic suggest generalised versus local processes.

Funding

Medical Research Council (MR/T000953/1) and the National Institute for Health and Care Research.

Retinal microvascular changes in unilateral functional amblyopia detected by oct-angiography and follow-up during treatment

OBJECTIVE

To evaluate the macular microvascular changes using optical coherence tomographic angiography (OCT-A) in children with unilateral amblyopia and their reversibility during treatment.

METHODS

Patients with unilateral strabismic or anisometropic amblyopia or residual amblyopia from early congenital cataract surgery, examined between October 2019 and March 2021, were included. Vessel density and perfusion density in the superficial capillary plexus and area, perimeter and circularity of the foveal avascular zone (FAZ) were analysed using OCT-A in amblyopic eyes, contralateral eyes and control group healthy eyes. Correlation analyses between the microvascular parameters and the visual acuity were performed. In a pilot study on a few patients from the amblyopic cohort, longitudinal follow-up during treatment was also performed.

RESULTS

A total of 128 eyes of 64 patients were included: 32 amblyopic eyes compared with 32 contralateral eyes and 64 control eyes. Vessel density and perfusion density in the superficial capillary plexus were significantly lower in amblyopic eyes compared to control eyes in 6 × 6 mm (p < 0.02) and 3 × 3 mm (p < 0.01) scans. Correlation analyses showed a linear decrease in vessel density and perfusion density with decreasing visual acuity. The microvascular changes observed were reversible with the occlusion treatment of amblyopia (p < 0.001).

CONCLUSIONS

The study found a decrease in vessel density and perfusion density in the macula of children with unilateral functional amblyopia. These microvascular changes were correlated with visual acuity and appeared to be reversible with treatment of amblyopia. On the whole, OCT-A appears to be a relevant complementary examination when it comes to diagnosing and monitoring functional amblyopia.

Anisometropic amblyopia: A review of functional and structural changes and treatment

Amblyopia is the decreased best-corrected visual acuity (BCVA) in one or both eyes caused by the abnormal processing of visual input during development. One common cause of amblyopia is anisometropia, which has attracted widespread attention. Many structural changes occur in the primary and extrastriate visual areas of the cerebral cortex, as well as in the eyes, in patients with anisometropic amblyopia. Understanding these mechanisms has provided a favorable theoretical basis for treating anisometropic amblyopia. This article reviews the functional and anatomical changes and progress toward the treatment of anisometropic amblyopia.

Long-term effects of early/late-onset visual deprivation on macular and retinal nerve fibers layer structure: A pilot study

BACKGROUND/AIMS

Tomographic analysis of macular and peripapillary retinal nerve fibers layer (RNFL) thickness in patients with history of congenital (CC) and developmental cataract (DC).

METHODS

Analysis of macular and RNFL thickness using a spectral-domain optical coherence tomography was performed. Retinal layers thickness was measured using the internal segmentation software. Measurements of affected (unilateral and bilateral), contralateral eyes and control eyes were compared.

RESULTS

Patients with history of CC or DC (n = 13 and 11 respectively) and 35 healthy control subjects were enrolled. Thicker inner and outer nuclear layers (INL, ONL) and thicker ONL were found when CC and DC group when compared to controls respectively. Bilateral CC showed the most relevant differences. Slight thickening of CC inner retinal layers were found when compared to DC. Increased superonasal RNFL thickness was found in CC group when compared to DC and controls. Thickening of RNFL of contralateral unaffected eyes of unilateral CC were found when compared to controls.

CONCLUSION

Significant macular and RNFL thickness changes between CC, DC patients and controls that partially involve also contralateral unaffected eyes of unilateral congenital cataract were found. CC and DC groups show significant differences only in inner retinal layers thickness. Our data suggest that early visual deprivation may influence retinal arrangements occurring during development involving predominantly the outer nuclear layer and para/perifoveal inner retinal layers, and confirm that early treatment of CC allow to achieve better long-term visual outcome. Moreover functional and structural data support the hypothesis that unilateral amblyopia is not exclusively an unilateral issue.

Macular pigment optical density of hyperopic anisometropic amblyopic patients measured by fundus reflectometry

Purpose

Hyperopic anisometropia is a major cause of amblyopia and may be associated with macular pigment optical density (MPOD) reduction. To explore whether the MPOD changes in hyperopic anisometropic amblyopia, we measured the MPOD using fundus reflectometry in eyes with hyperopic anisometropic amblyopia and normal vision.

Methods

This was a cross-sectional study conducted from January 2017 to June 2017. Forty subjects (25 males and 15 females) between the ages of 6 and 10 years were recruited. The subjects' eyes were divided into two groups: amblyopic eyes (best-corrected visual acuity (BCVA) not more than 20/25 or BCVA of two eyes differing by two or more lines) and fellow eyes. All enrolled subjects underwent a comprehensive ophthalmic examination, including extraocular motility assessment, cover-uncover testing, and refractive error (noncycloplegic), BCVA, axial length (AL), macular foveal thickness (MFT) and MPOD (Visucam^® 200, Carl Zeiss Meditec AG, Germany).

Results

The MPOD of amblyopic and fellow eyes was 0.12 ± 0.03 log units and 0.13 ± 0.04 log units, respectively, with a significant difference (P = 0.026). The MFT of amblyopic and fellow eyes was 241.28 ± 13.95 and 237.13 ± 16.02 μm, respectively, revealing that the MFT was significantly higher in amblyopic eyes than in fellow eyes (P = 0.028). Conversely, there was no correlation between the MPOD and MFT in the two groups.

Conclusions

This study is the first to report that the MPOD is decreased in hyperopic anisometropic amblyopia. In this study, no correlation between the MPOD and MFT was found. In the future, factors that induce a decrease in the MPOD in eyes with hyperopic anisometropic amblyopia should be explored in a large-sample study with follow-up observation.

Analysis of Retinal Microvasculature Features in Amblyopic Eyes: A Meta-Analysis

BACKGROUND

Amblyopia is a common cause of visual disorder with several changes in retinal structure and vessel. To date, alterations in retinal microvasculature features in amblyopia have yielded inconsistent results.

OBJECTIVES

This meta-analysis aimed to evaluate retinal vessel density (VD) in amblyopic patients using optical coherence tomography angiography (OCTA).

METHODS

PubMed, Web of Science, Embase, and Cochrane Library databases were systematically searched for published articles comparing retinal microvasculature characteristics in patients with amblyopia and controls. Continuous variable outcomes were assessed using the mean difference (MD) with a 95% confidence interval. Review Manager Version 5.30 was used for the analysis.

RESULTS

Thirteen qualified articles were pooled in this meta-analysis. Compared with controls, the foveal whole en face VD of superficial capillary plexus (SCP) and deep capillary plexus (DCP) of patients as measured by 3 × 3-mm scans were significantly lower in amblyopia eyes (MD: -1.37, p = 0.0003; MD: 1.70, p < 0.00001, respectively). Similarly, in the 6 × 6-mm scans, foveal whole en face VD of the SCP and DCP were remarkably lower in amblyopia eyes than in controls (MD: -2.24, p = 0.03; MD: -5.08, p = 0.04, respectively). The parafoveal VD of SCP in 3 × 3-mm scans (MD: -1.96, p < 0.00001) was also lower in amblyopic patients than in controls. Similarly, in 6 × 6-mm scans, amblyopia eyes showed a significant decrease and a trending decrease in the parafoveal VD of the SCP (MD: -3.85, p = 0.007) and DCP (MD: -3.03, p = 0.10), respectively. For whole radial peripapillary capillary (RPC), VD was significantly reduced in amblyopic patients compared to controls (MD = -0.83, p < 0.00001). In addition, the deep foveal avascular zone (FAZ) was larger in amblyopic eyes than in the controls (MD = 0.55, p = 0.007).

CONCLUSIONS

Our data suggest that whole foveal and parafoveal VD and RPC whole VD were reduced in patients with amblyopia. Moreover, our results reveal that the FAZ is larger in amblyopic patients. Consequently, OCTA may have the potential for diagnosing and monitoring patients with amblyopia.

Microvascular assessment of macula, choroid, and optic disk in children with unilateral amblyopia using OCT angiography

PURPOSE

To investigate the microvascular changes of macula, choroid, and optic disk in children with unilateral amblyopia.

METHODS

This prospective cross-sectional study involved 39 unilateral amblyopic children and 39 age- and sex-matched heathy participants who served as control. Vessel densities of the superficial and deep capillary plexuses (SCP and DCP), foveal avascular zone (FAZ) area, macular thickness, optic disk vessel density, retinal nerve fiber layer (RNFL) thickness, choriocapillaris vessel density, and subfoveal choroidal thickness were evaluated by OCT angiography (OCTA). Meanwhile, the correlations of microvascular perfusion and structural changes of macula, choroid, and optic disk were analyzed.

RESULTS

The vessel density of SCP and DCP in the whole macula in the amblyopic group was significantly lower than that in the control group after adjusting for age, axial length, and spherical equivalents (all P < 0.05). FAZ area, macular thickness, RNFL thickness, and the optic disk vessel density were not statistically different between the amblyopic group and the control group (all P > 0.05). Subfoveal choroidal thickness of amblyopic eyes was significantly higher than that of control eyes(P = 0.032). Choriocapillaris flow void (FV) in the amblyopic group was greater than that in the control group (P = 0.013). Significant differences were observed between the fellow eyes and the control eyes in choriocapillaris FV and subfoveal choroidal thickness (P = 0.011 and P = 0.042, respectively). Foveal SCP and DCP vessel density in all studied eyes were positively correlated with the whole macular thickness, respectively (r = 0.556 and r = 0.627, respectively, both P < 0.001). Whole SCP and DCP vessel density in the amblyopic eyes were negatively correlated with choriocapillaris FV (r = -0.723, P < 0.001; r = -0.512, P = 0.001, respectively).

CONCLUSION

Children with amblyopic eyes have attenuated macular and choriocapillaris perfusion. There is a need for future studies that will investigate the pathophysiology of amblyopia in children by OCTA.

Associations between visual function and ultrastructure of the macula and optic disc after childhood cataract surgery

PURPOSE

To investigate the association between visual function and ultrastructural characteristics of the retina and optic disc in children operated for cataract and factors influencing this relationship.

METHODS

We included 56 children aged 7-18 years who had been operated for bilateral or unilateral cataract. Three-dimensional swept source OCT scans of the macula and the optic disc were obtained using the Triton OCT (Topcon, Corporation, Tokyo, Japan). Best corrected distance visual acuity was measured using the HOTV 3 Meter visual chart. Contrast vision was evaluated using the Freiburg Visual Acuity Contrast Test (FrACT). Healthy fellow eyes of the unilateral group were used as control eyes.

RESULTS

We did not find any associations between visual acuity or contrast sensitivity and structural retinal parameters. Shorter axial length, the presence of glaucoma and pseudophakia (opposed to aphakia) were all significant predictors of greater mean macular thickness in a multiple regression analysis, (p = 0.039). Two children had foveal hypoplasia, and one child had a right eye with a thin, disorganized retina.

CONCLUSIONS

Children with pseudophakia have a thicker macula but this does not seem to be related to visual function. Obtaining high quality OCT scans of the macula and optic disc in severely visually impaired children with glaucoma and/or nystagmus is challenging. We found structural macular changes which contributed to poor vision but most eyes with very reduced vision were difficult to image, and the prevalence of structural changes may be underestimated.

The long-term observation in Chinese children with monocular myelinated retinal nerve fibers, myopia and amblyopia

BACKGROUND

The syndrome of monocular myelinated retinal nerve fibers (MRNF), myopia and amblyopia threatens visual development of infants and young children. The efficacy of part-time occlusion therapy remains controversial, and the long-term prognosis of the syndrome remains unclear.

METHODS

Ten children (4.40±2.22 years, 4 boys and 6 girls) with monocular MRNF, myopia and amblyopia were recruited. Both the affected eyes (treatment group) and the fellow eyes (control group) underwent routine ophthalmic examinations, including AL, cycloplegic refraction, best-spectacles-corrected distance visual acuity (BSCDVA), cover test, simultaneous perception, corneal curvature, anterior segment and fundus examinations. Refractive error was corrected by either spectacles or rigid gas permeable (RGP) contact lenses. Part-time occlusion therapy was employed for amblyopia treatment. The retinal characteristics were examined only on the final visit with a Fourier-domain optical coherence tomography (FD-OCT). The speed of myopia progression and axial elongation were calculated. Statistical analysis was performed using SPSS 20 software. Cut-off P values were 0.05.

RESULTS

On the final visit, six children had normal simultaneous perception but three had strabismus. The mean AL and spherical equivalent (SE) values of the affected eyes were 28.05±1.59 mm and -12.60±4.20 D, respectively, while those of the fellow eyes were 23.67±1.13 mm and -0.66±1.99 D, respectively. AL and SE were significantly different over time and between groups (P<0.0001), but no time*group interaction effect (P>0.05) was observed. The differences in the corneal curvature, mean speed of myopia progression, axial elongation, foveal thickness and parafoveal thickness between the two groups were not significant (P>0.05). However, the perifoveal thickness in the affected eyes was significantly higher than that of the control eyes (P=0.047). In the treatment group, the occlusion dosage positively correlated with the final BSCDVA (R=0.764, P=0.016) but did not correlate with the speed of myopia progression or axial elongation (both P values >0.05).

CONCLUSIONS

The affected eyes have longer AL, severer myopia and thicker perifoveal retina when compared with the fellow eyes. Both the affected and the fellow eyes have similar speed of myopia progression as well as that of axial elongation.

Macular Microvasculature and Associated Retinal Layer Thickness in Pediatric Amblyopia: Magnification-Corrected Analyses

Purpose

The purpose of this study was to characterize macular microvasculature and structural retinal layers using magnification-corrected optical coherence tomography angiography (OCTA) images in children with amblyopia.

Methods

This prospective cross-sectional study included 22 children with unilateral amblyopia (4–11 years of age) receiving spectral-domain OCTA. Vessel densities in foveal and parafoveal regions of the superficial capillary plexus (SCP) and deep capillary plexus (DCP) were measured in amblyopic and fellow eyes using a customized image analysis program correcting the scale of retinal image with axial length. Iowa Reference Algorithms (Iowa Institute for Biomedical Imaging) were used to measure mean thickness values of 10 intra-retinal layers rescaled for image size correction.

Results

Foveal and parafoveal vessel densities in amblyopic eyes were lower than that of the fellow eyes in the SCP (fovea: P = 0.006 and parafovea: P = 0.003) and the DCP (P = 0.024 and P = 0.025, respectively). Amblyopic eyes had significantly smaller foveal avascular zone (FAZ) area than fellow eyes (P < 0.001). There were significant differences in retinal layer thickness between paired eyes, particularly in the inner retina in both foveal and parafoveal regions; retinal nerve fiber layer (RNFL) (P = 0.024 and P = 0.095, respectively), ganglion cell layer (P < 0.001 and P = 0.008), inner plexiform layer (IPL; P = 0.12 and P = 0.037), inner nuclear layer (P = 0.005 and P = 0.005), and outer plexiform layer (OPL; P = 0.02 and P = 0.057), except in the foveal IPL, the parafoveal RNFL, and OPL.

Conclusions

Unilateral amblyopic eyes demonstrate reduced macular vessel density and thicker inner retinal layers compared with fellow eyes even after correcting for image magnification. Changes in macular microvasculature and structural layers may offer valuable insights in the development of amblyopia.

Association of Optical Coherence Tomography Angiography Metrics With Detection of Impaired Macular Microvasculature and Decreased Vision in Amblyopic Eyes: The Hong Kong Children Eye Study

Importance

Microvascular abnormalities in amblyopia are becoming evident with high-resolution imaging, such as optical coherence tomography angiography (OCT-A); however, to our knowledge, the clinical significance and use of these findings are unknown.

Objective

To assess changes in quantitative OCT-A metrics in amblyopic eyes and explore their association with visual acuity in children.

Design, Setting, and Participants

This population-based nested case-control study included children aged 6 to 8 years who were consecutively recruited between January 2016 and July 2017 from the population-based Hong Kong Children Eye Study (HKCES) at the Chinese University of Hong Kong Eye Centre. All participants underwent OCT-A with a swept-source OCT and detailed ophthalmic investigations. Macular microvasculature of the superficial capillary plexus was quantified by a customized automated image analysis program. A multivariable linear regression was conducted to evaluate the differences in OCT-A metrics between amblyopic and nonamblyopic eyes after adjustment for all known confounders. Data analysis was conducted from September to November 2018.

Main Outcomes and Measures

Differences in OCT-A metric (foveal avascular zone [FAZ]) area, FAZ circularity, vessel density, vessel diameter index, and fractal dimension between amblyopic and nonamblyopic eyes.

Results

There were 30 participants with amblyopia (mean [SD] age, 7.57 [1.2] years; 16 girls [53.3%]) and 1045 controls (mean [SD] age, 7.65 [1.0] years; 580 girls [55.5%]) in this cohort. Compared with control eyes, amblyopic eyes had decreased FAZ circularity (-0.058; 95% CI, -0.096 to -0.021, P = .002), decreased fractal dimension (-0.014; 95% CI, -0.024 to -0.003; P = .01), and increased vessel diameter index (0.002; 95% CI, 0.002 to 0.003; P < .001). A difference was not identified between FAZ area and vessel density. LogMAR visual acuity was associated with FAZ circularity (sβ, -0.133; P < .001) and vessel diameter index (sβ, 0.097; P = .001) but not with vessel density nor FAZ area.

Conclusions and Relevance

The results of this population-based study in children supports the presence of macular microvascular abnormalities in amblyopic eyes. Such changes as measured by OCT-A metrics are associated with visual acuity, inferring retinal involvement in the development of amblyopia and suggesting a potential role of quantitative OCT-A metrics in the diagnosis and recognition of amblyopia.

Amblyopia

Amblyopia is a neurodevelopmental abnormality that results in physiological alterations in the visual pathways and impaired vision in one eye, less commonly in both. It reflects a broad range of neural, perceptual, oculomotor, and clinical abnormalities that can occur when normal visual development is disrupted early in life. Aside from refractive error, amblyopia is the most common cause of vision loss in infants and young children. It causes a constellation of perceptual deficits in the vision of the amblyopic eye, including a loss of visual acuity, position acuity, and contrast sensitivity, particularly at high spatial frequencies, as well as increased internal noise and prolonged manual and saccadic reaction times. There are also perceptual deficits in the strong eye, such as certain types of motion perception, reflecting altered neural responses and functional connectivity in visual cortex (Ho et al., 2005). Treatment in young children consists of correction of any refractive error and patching of the strong eye. Compliance with patching is challenging and a substantial proportion of amblyopic children fail to achieve normal acuity or stereopsis even after extended periods of treatment. There are a number of promising experimental treatments that may improve compliance and outcomes, such as the playing of action video games with the strong eye patched. Although there may be a sensitive period for optimal effects of treatment, there is evidence that amblyopic adults may still show some benefit of treatment. However, there is as yet no consensus on the treatment of adults with amblyopia.

VISUAL FUNCTION AND OPTICAL COHERENCE TOMOGRAPHY ANGIOGRAPHY FEATURES IN CHILDREN BORN PRETERM

PURPOSE

Preterm children have an increased risk of impaired vision from retinopathy, strabismus, and high refractive error. The aim of this study was to investigate the relationship between foveal parameters generated by optical coherence tomography angiography and visual function in preterm children.

METHODS

Eighty eyes (32 eyes of former preterm infants and 48 age-matched full-term control eyes) were analyzed. Subjects underwent complete eye examinations including best-corrected visual acuity and retinal imaging with the Optovue XR Avanti optical coherence tomography angiography device. Foveal morphologic parameters including foveal depth, central foveal thickness, inner retinal area, and outer retinal area were measured on a central horizontal B-scan. Foveal vasculature parameters including foveal avascular zone, superficial capillary plexus-vessel density, and deep capillary plexus-vessel density were measured on optical coherence tomography angiography.

RESULTS

The best-corrected visual acuity was significantly affected in preterm children compared with controls (P < 0.0001). The central foveal thickness (P < 0.0001), inner retinal area (P = 0.01), and outer retinal area (P = 0.03) were significantly increased in preterm compared with control eyes. Foveal depth (P < 0.001) and foveal avascular zone (P < 0.001) were significantly decreased in preterm compared with control eyes. The superficial capillary plexus-vessel density (P = 0.01) and deep capillary plexus-vessel density (P = 0.003) at the fovea (1 mm) were significantly increased in preterm compared with control eyes. The best-corrected visual acuity was negatively correlated with foveal depth (r = -0.42, P = 0.001) and foveal avascular zone (r = -0.53, P < 0.001), and positively correlated with central foveal thickness (r = 0.32, P = 0.01) and inner retinal area (r = 0.32, P = 0.01), indicating that worse visual acuity was associated with a smaller foveal avascular zone, shallower foveal depth, increased central foveal thickness, and larger inner retinal area.

CONCLUSION

Foveal morphology and vasculature changes in preterm children were associated with impaired visual function. Further longitudinal studies are required to evaluate these changes over time.

Rethinking amblyopia 2020

Recent work has transformed our ideas about the neural mechanisms, behavioral consequences and effective therapies for amblyopia. Since the 1700's, the clinical treatment for amblyopia has consisted of patching or penalizing the strong eye, to force the "lazy" amblyopic eye, to work. This treatment has generally been limited to infants and young children during a sensitive period of development. Over the last 20 years we have learned much about the nature and neural mechanisms underlying the loss of spatial and binocular vision in amblyopia, and that a degree of neural plasticity persists well beyond the sensitive period. Importantly, the last decade has seen a resurgence of research into new approaches to the treatment of amblyopia both in children and adults, which emphasize that monocular therapies may not be the most effective for the fundamentally binocular disorder that is amblyopia. These approaches include perceptual learning, video game play and binocular methods aimed at reducing inhibition of the amblyopic eye by the strong fellow eye, and enhancing binocular fusion and stereopsis. This review focuses on the what we've learned over the past 20 years or so, and will highlight both the successes of these new treatment approaches in labs around the world, and their failures in clinical trials. Reconciling these results raises important new questions that may help to focus future directions.

Thicknesses of Macular Inner Retinal Layers in Children with Anisometropic Amblyopia

Objective

To investigate the thicknesses of macular inner retinal layers in children with anisometropic amblyopia using spectral domain optical coherence tomography (SD-OCT).

Methods

Thirty-seven children with anisometropic amblyopia and fifty-seven children with normal vision were recruited in the study. Both eyes of children with anisometropic amblyopia and the right eyes of normal controls underwent scanning with the Spectralis OCT. The segmentation of retinal layers was performed automatically to measure individual inner retinal layers in the five sectors of the macular. An independent sample t-test was applied to compare the mean layer thicknesses of anisometropic eyes and fellow eyes with those of control eyes.

Results

There was no significant difference in the total macular thickness between amblyopic and control eyes. However, in the peripheral macular area, three of the four quadrants of both the ganglion cell layer (GCL) and the inner plexiform layer (IPL) thicknesses were significantly reduced in amblyopic eyes compared to control eyes. Moreover, two of the four quadrants of the GCL thickness and three of the four quadrants of the IPL thickness in the peripheral macular area were significantly reduced in fellow eyes than in control eyes.

Conclusion

The SD-OCT data revealed differences in the thicknesses of some macular inner retinal layers in both eyes of children with anisometropic amblyopia compared with those with emmetropia, indicating that structural changes might exist in the retina of children with amblyopia.

In vivo assessment of foveal geometry and cone photoreceptor density and spacing in children

The fovea undergoes significant developmental changes from birth into adolescence. However, there is limited data examining cone photoreceptor density, foveal pit shape, and foveal avascular zone (FAZ) size in children. The purpose of this study was to determine whether overall foveal structure differs as a function of age and refractive status in children. Forty-eight healthy children (ages 5.8 to 15.8 years) underwent optical coherence tomography imaging to quantify foveal point thickness and foveal pit diameter, depth, and slope. Adaptive optics scanning laser ophthalmoscope (AOSLO) images of foveal capillaries and cone photoreceptors were acquired in a subset of children to quantify FAZ metrics and cone densities at 0.2, 0.3, and 0.5 mm eccentricities. Results show that foveal pit and FAZ metrics were not related to age, axial length, or refractive status. However, linear cone density was lower in myopic versus non-myopic children at eccentricities of 0.2 mm (mean ± SD = 50,022 ± 5,878 cones/mm² vs 58,989 ± 4,822 cones/mm², P < 0.001) and 0.3 mm (43,944 ± 5,547 cones/mm² vs 48,622 ± 3,538 cones/mm², P < 0.001). These results suggest FAZ and foveal pit metrics do not systematically differ with age in children, while myopic eyes have decreased linear cone density near the foveal center. Significance Statement: The development of the fovea begins prior to birth and continues through the early teenage years until it reaches adult-like properties. Although the majority of changes during childhood are related to the maturation and migration of cone photoreceptors, in vivo data describing cone packing in children is limited. We assessed overall foveal structure in children as young as 5.8 years old by quantifying cone density and spacing, foveal avascular zone size, and foveal pit morphometry to investigate potential structural differences as a function of age and refractive status. While foveal avascular zone and foveal pit metrics did not significantly differ with age, results indicate that myopic children have lower linear cone densities close to the foveal center compared to non-myopic children.

Lamina cribrosa thickness in children with hyperopic anisometropic amblyopia

AIM

To determine lamina cribrosa thickness (LCT) in the optic nerve head region of the eyes in children with hyperopic anisometropic amblyopia and to compare this thickness with that of fellow eyes, hyperopic non-amblyopia, and age-matched controls.

METHODS

Thirty-two patients (12.0±1.8y, mean± standard deviation) with hyperopic anisometropic amblyopia, 31 subjects with age- and refractive error-matched hyperopic non-amblyopia (10.7±2.2y), and 32 age-matched controls (11.2±2.0y) were included in this prospective, cross-sectional study. LCT was measured using an enhanced depth-imaging program of a spectral domain optical coherence tomographic instrument in all participants, and the correlation between LCT and axial length was calculated.

RESULTS

The mean LCT was 180.9±29.4 µm in amblyopic eyes, 247.7±19.0 µm in fellow eyes, 251.6±27.3 µm in hyperopic non-amblyopic eyes, and 240.2±15.8 µm in control eyes. Lamina cribrosa in amblyopic eyes was significantly thinner than fellow, hyperopic non-amblyopic, and control eyes (P<0.05). There was no significant correlation in LCT and axial length between amblyopic (P=0.16) and control (P=0.31) group.

CONCLUSION

Lamina cribrosa of eyes with hyperopic anisometropic amblyopia is significantly thinner than that of fellow eyes, hyperopic non-amblyopia, and age-matched controls. The LCT profile in amblyopic eyes is different from that observed in fellow, hyperopic non-amblyopic, and control eyes.

Macular Thickness, Foveal Volume, and Choroidal Thickness in Amblyopic Eyes and Their Relationships to the Treatment Outcome

Purpose

To assess the correlations between the retinal/choroidal structure and the treatment outcomes of amblyopic children.

Methods

This study enrolled eyes with amblyopia resulting from strabismus, anisometropia, or ametropia. All patients underwent detailed eye examinations, including spectral domain optical coherence tomography (SD-OCT) scan. All of the subjects received amblyopic treatment and were divided into 2 groups after 6 months of follow-up: the recovered amblyopic group with a best-corrected visual acuity (BCVA) ≥0.8 and the persistent amblyopic group with a BCVA <0.8 on the Landolt C chart.

Results

Forty-four amblyopic children were included, of which 26 were in the recovered amblyopic group after 6 months of follow-up. The patients with strabismic anisometropic amblyopia and severe amblyopia (initial VA ≤ 0.3) were significantly predisposed to developing persistent amblyopia (P=0.049 and P < 0.001, respectively). After correcting with Littmann's formula, the thickness and volume of the parafoveal and perifoveal retinal regions in the persistent amblyopia group did not show significant differences with the recovered amblyopia group.

Conclusions

The initial severity of amblyopia and the type of amblyopia were the risk factors related to the poor outcome of amblyopic treatment. The foveal thickness, foveal volume, and choroidal thickness were not associated with the treatment outcome.

OCT angiography findings in children with amblyopia

PURPOSE

To compare the microstructure and vascularity of amblyopic eyes in children with their contralateral eye and with eyes from control children using optical coherence tomography angiography (OCT-A).

METHODS

We conducted a prospective, cross-sectional evaluation of macular and optic disk vascular density and flow area using OCT-A (Avanti RTVue XR, Optovue Inc, Fremont, CA). Parameters were calculated using automated software.

RESULTS

A total of 52 children were included: 26 subjects with amblyopia and 26 nonamblyopic controls. The amblyopic eye of subjects showed a statistically significant decrease in macular vascular density (P = 0.0171) of the superficial capillary plexus (SCP), in the optic disk flow area (P = 0.0195) and in the average retinal nerve fiber layer thickness (P = 0.0194) as well as a marginally statistically significant decrease in the macular flow area of the SCP (P = 0.0305) and in the optic density (P = 0.0279). Compared with randomly selected eyes of controls, amblyopic eyes showed a statistically significant decrease in the macular flow area of the SCP (P = 0.005) and of the deep capillary plexus (DCP; P = 0.002), in the macula vascular density of the SCP (P = 0.022), in the optic disk flow area (P = 0.004), and a marginally statistical significant increase in the area of foveal avascular zone of the DCP (P = 0.038).

CONCLUSIONS

In our study cohort amblyopic eyes manifested significant differences in macular and optic disk vascularization. The clinical significance of these findings warrants further research.

Comparison of the Thickness and Volume of the Macula and Fovea in Patients with Anisometropic Amblyopia Prior to and after Occlusion Therapy

Purpose

To compare the thickness of superior, temporal, inferior, and nasal macula and foveal thickness and volume in patients with anisometropic amblyopia prior to and after successful occlusion therapy using optical coherence tomography (OCT) measurement.

Methods

Data were collected prospectively on 30 patients with unilateral anisometropic amblyopia from December 2006 to August 2007. All patients had anisometropia of 2.0 diopters or more. OCT scans were obtained for all patients at diagnosis. Occlusion therapy was then prescribed and OCT scans were obtained again at the time of successful occlusion therapy (defined as interocular difference of <0.1 log units). The Stratus OCT-3 was used to measure fovea thickness and volume and the thickness of superior, temporal, inferior, and nasal macula (within a diameter of 3 mm).

Results

Of 30 patients, 22 (mean age of 5.8 years) had successful resolution of amblyopia. The mean duration of occlusion was 11.24 months and mean best-corrected visual acuity at diagnosis was 0.35 ± 0.12 logarithm of the minimum angle of resolution. The mean thicknesses of the superior, temporal, inferior, and nasal macula prior to and after occlusion were not significantly different (p > 0.05). However, mean foveal volume prior to occlusion therapy (0.15 ± 0.02 mm³) decreased after occlusion (0.14 ± 0.01 mm³) with statistical significance (Wilcoxon signed rank test, p = 0.025).

Conclusions

There was a meaningful decrease in foveal volume in patients with anisometropic amblyopia after successful occlusion therapy. Whether this decrease relates to visual improvement of the amblyopic eye remains to be determined.

A sloped piecemeal Gaussian model for characterising foveal pit shape

PURPOSE

High-quality optical coherence tomography (OCT) macular scans make it possible to distinguish a range of normal and diseased states by characterising foveal pit shape. Existing mathematical models lack the flexibility to capture all known pit variations and thus characterise the pit with limited accuracy. This study aimed to develop a new model that provides a more robust characterisation of individual foveal pit variations.

METHODS

A Sloped Piecemeal Gaussian (SPG) model, consisting of a linear combination of a tilted line and a piecemeal Gaussian function (two halves of a Gaussian connected by a separate straight line), was developed to fit retinal thickness data with the flexibility to characterise different degrees of pit asymmetry and pit bottom flatness. It fitted the raw pit data between the two rims of the fovea to improve accuracy. The model was tested on 3488 macular scans from both eyes of 581 young adults (376 myopes and 206 non-myopes, mean (S.D.) age 21.9 (1.4) years). Estimates for retinal thickness, wall height and slope, pit depth and width were derived from the best-fitting model curve. Ten variations of Gaussian and Difference of Gaussian models were fitted to the same scans and compared with the SPG model for goodness of fit (by Root mean square error, RMSE), model complexity (by the Bayesian Information Criteria) and model fidelity.

RESULTS

The SPG model produced excellent goodness of fit (mean RMSE = 4.25 and 3.89 μm; 95% CI: 4.20, 4.30 and 3.86, 3.93 for fitting horizontal and vertical profiles respectively). The SPG model showed pit asymmetry, with average nasal walls 17.6 (11.6) μm higher and 0.96 (0.61)^° steeper than temporal walls and average superior walls 7.0 (12.2) μm higher and 0.41 (0.65)^° steeper than the inferior walls. The SPG model also revealed a continuum of human foveal shapes, from round bottoms to extended flat bottoms (up to 563 μm). 49.1% of foveal profiles were best fitted with a flat bottom >30 μm wide. Compared with the other tested models, the SPG was the preferred model overall based on the Bayesian Information Criteria.

CONCLUSIONS

The SPG is a new parsimonious mathematical model that improves upon other models by accounting for wall asymmetry and flat pit bottoms, providing an excellent fit and more faithful characterisation of typical foveal pit shapes and their known variations. This new model may be helpful in distinguishing normal foveal shape variations by refractive status as well by other characteristics such as sex, ethnicity and age.

Comparative electrophysiological responses in anisometropic and strabismic amblyopic children

Purpose

To compare anisometropic hypermetropic amblyopic and strabismic amblyopic responses to pattern electroretinogram (PERG) and pattern visual evocated potential (PVEP).

Materials and methods

Fifty-six patients – 18 hypermetropic anisometropic amblyopic children (mean age 9.70±2.5 years), 19 strabismic amblyopic children (mean age 10.30±2.6 years) and 19 normal emetropic subjects (mean age 10.10±2.2 years) – were enrolled in this study. After routine ophthalmic examination, PERG and PVEP were recorded in response to checks reversed at the rate of two reversals/second stimulating macular area.

Results

The difference between hypermetropic anisometropic amblyopia and strabismus amblyopia with respect to P100/P50/N95 wave latencies (P=0.055/0.855/0.132) and P100/P50/N95 amplitudes (P=0.980/0.095/0.045) was not statistically significant. However, there was a significant statistical difference between strabismic amblyopia group and controls for P100/P50/N95 latencies (P=0.000/0.006/0.004).

Conclusion

Our findings indicated that despite clinical differences between anisometropic amblyopic and strabismic amblyopic patients, no differences were found in the responses of PVEP and PERG. The abnormal components of the PVEP and PERG in amblyopic subjects could reflect a retinal dysfunction in the visual pathway.

Thickness of retinal layers in the foveas of children with anisometropic amblyopia

Purpose

To use highly precise spectral-domain optical coherence tomography (SD-OCT) to determine whether there were structural abnormalities in the layers of different regions of the fovea in children with anisometropic amblyopia.

Methods

Eighteen children (mean age 7.8 years old; range 5–11 years) with unilateral anisometropic amblyopia and 18 age-matched control subjects participated. Foveal thickness was measured with an enhanced depth imaging system, SD-OCT and segmented into layers using custom developed software. The thickness of each layer of the fovea was compared among amblyopic eyes, fellow eyes and control eyes with optical magnification correction for axial length and statistical correction for age and sex.

Results

The total thickness and each intra-ocular layer of the central fovea were the same for each group. However, the amblyopic eyes were significantly thicker than the normal control eyes in 2 of 4 quadrants of the peripheral retina. Exploring intra-retinal layers in these two quadrants, the nasal nerve fiber layer (NFL) and inferior inner nuclear layer (INL)were significantly thicker in amblyopic eyes than in control eyes (p = 0.01 and 0.012, respectively, by ANCOVA).

Conclusion

The SD-OCT data revealed marginal differences in some foveal layers at peripheral locations and indicated that structural differences might exist between individuals with amblyopia and visually normal control subjects. However, the differences were scattered and represented no identifiable pattern. More studies with large samples and precise locations of the retinal layers must be performed to extend the present results.

High-Resolution Imaging of the Optic Nerve and Retina in Optic Nerve Hypoplasia

PURPOSE

To investigate the optic nerve and macular morphology in patients with optic nerve hypoplasia (ONH) using spectral-domain optical coherence tomography (SD OCT).

DESIGN

Prospective, cross-sectional, observational study.

SUBJECTS

A total of 16 participants with ONH (10 female and 6 male; mean age, 17.2 years; 6 bilateral involvement) and 32 gender-, age-, ethnicity-, and refraction-matched healthy controls.

METHODS

High-resolution SD OCT (Copernicus [Optopol Technology S.A., Zawiercie, Poland], 3 μm resolution) and handheld SD OCT (Bioptigen Inc [Research Triangle Park, NC], 2.6 μm resolution) devices were used to acquire horizontal scans through the center of the optic disc and macula.

MAIN OUTCOME MEASURES

Horizontal optic disc/cup and rim diameters, cup depth, peripapillary retinal nerve fiber layer (RNFL), and thickness of individual retinal layers in participants with ONH and in controls.

RESULTS

Patients with ONH had significantly smaller discs (P < 0.03 and P < 0.001 compared with unaffected eye and healthy controls, respectively), horizontal cup diameter (P < 0.02 for both), and cup depth (P < 0.02 and P < 0.01, respectively). In the macula, significantly thinner RNFL (nasally), ganglion cell layer (GCL) (nasally and temporally), inner plexiform layer (IPL) (nasally), outer nuclear layer (ONL) (nasally), and inner segment (centrally and temporally) were found in patients with ONH compared with the control group (P < 0.05 for all comparisons). Continuation of significantly thicker GCL, IPL, and outer plexiform layer in the central retinal area (i.e., foveal hypoplasia) was found in more than 80% of patients with ONH. Clinically unaffected fellow eyes of patients with ONH showed mild features of underdevelopment. Visual acuity and presence of septo-optic dysplasia were associated with changes in GCL and IPL. Sensitivity and specificity for the detection of ONH based on disc and retinal optical coherence tomography (OCT) parameters were >80%.

CONCLUSIONS

Our study provides evidence of retinal changes in ONH. In addition to thinning of retina layers mainly involving the RNFL and GCL, signs reminiscent of foveal hypoplasia were observed in patients with ONH. Optic nerve and foveal parameters measured using OCT showed high sensitivity and specificity for detecting ONH, demonstrating their useful for clinical diagnosis.

Choroidal thickness of children's eyes with anisometropic and strabismic amblyopia

PURPOSE

To compare the choroidal thickness of children's eyes with amblyopia due to strabismus or anisometropia to the fellow eye and age-matched controls.

METHODS

Forty patients with anisometropic amblyopia, 40 patients with strabismic amblyopia, and 40 age-matched controls were included in this cross-sectional study. Choroidal thickness was measured via the enhanced-depth imaging technique of spectral domain optical coherence tomography in all patients and controls. Choroidal thickness was measured at subfoveal area and at 500 μm intervals to the nasal and temporal to the fovea up to 2000 μm. Measurements were compared between the three groups.

RESULTS

The mean ages were 7.9 ± 2.6 years (range, 4-13 years) in the anisometropic group, 9.0 ± 3.7 (range 4-15 years) years in the strabismic group, and 8.4 ± 2.6 years (range 4-15 years) in the control group. The mean subfoveal choroidal thickness in the anisometropic group was 362 ± 82 μm in the amblyopic eyes and 301 ± 54 μm in the fellow eyes; in the strabismic group, 413 ± 82 μm in the amblyopic eyes and 316 ± 54 μm in the fellow eyes. The mean subfoveal choroidal thickness was 310 ± 78 μm in control eyes. The subfoveal choroids of both anisometropic and strabismic amblyopic eyes were significantly thicker than that of the fellow eyes of the corresponding groups and the control eyes (P < 0.05 for all).

CONCLUSIONS

The subfoveal choroid of eyes with anisometropic and strabismic amblyopia is significantly thicker than that of the fellow eye and the age-matched controls.

Meta-analysis of retinal changes in unilateral amblyopia using optical coherence tomography

PURPOSE

To evaluate retinal changes in unilateral amblyopia using optical coherence tomography.

METHODS

Major literature databases were searched for relevant studies. The primary outcome parameters were foveal minimum thickness (FMT), mean foveal thickness (MFT), mean macular thickness (MMT), and peripapillary retinal nerve fiber layer (pRNFL) thickness. Efficacy estimates were evaluated by standardized mean difference (SMD) and 95% confidence interval (CI) for changes in the outcomes of interest.

RESULTS

This meta-analysis included 28 clinical trials involving 408 patients .The FMT, MFT, and MMT in the amblyopic eyes was significantly greater than that of the fellow eyes (SMD = 0.22, 95% CI 0.05-0.39, p = 0.011; SMD = 0.21, 95% CI 0.06-0.36, p = 0.005; SMD = 0.21, 95% CI 0.08-0.35, p = 0.002, respectively). Only FMT was significantly increased in the amblyopia group compared with the normal control group (SMD = 0.38, 95% CI 0.19-0.57, p&lt;0.00001). Analysis showed that pRNFL thickness in the amblyopic eyes was thicker than in the fellow eyes (SMD = 0.13, 95% CI 0.02-0.24, p = 0.016). We did not find that those outcome measures differ between strabismic and anisometropic amblyopia.

CONCLUSIONS

The pooled evidence indicates that the amblyopic process may involve the retina, especially the macula. A thicker foveola was found in the amblyopic eyes when compared with visually normal control eyes. Further larger-scale studies especially involving emmetropic normal subjects will be valuable.

Macular and retinal nerve fiber thickness in recovered and persistent amblyopia

The aim of this study was to investigate the presence of increased macular or retinal nerve fiber layer thickness (RNFLT) in amblyopic eyes, find if the increased macular or RNFLT is related to the lack of response in amblyopic eyes, and to explore whether the increased central macular thickness (CMT) in amblyopic eyes is purely related to the hyperopia. This is a prospective descriptive study. CMT and peripapillary RNFLT were measured by spectral-domain optical coherence tomography to evaluate 60 patients with unilateral-treated amblyopia (median age 11.00 year). Patients were divided into two groups: 33 patients in recovered amblyopia group and 27 patients in persistent amblyopia group. The mean CMT in the recovered group was 247.31 (±23.4) versus 246.8 (±32.7) µm (p = 0.95) for the persistent group. The mean peripapillary RNFLT was 99.13 (±12.1) versus 99.9 (±14.9) µm (p = 0.85) for the persistent group. In anisometropic amblyopia, there was no significant difference in CMT and RNFLT in either group. Also there was no relation between the type of refractive error and CMT or RNFLT. There was no significant difference in CMT and RNFLT in amblyopic eyes for both the recovered amblyopia group and the persistent amblyopia group to explain the lack of response in persistent amblyopic eyes. Additionally there was no relation between the type of refractive error and CMT or peripapillary RNFLT.

Subfoveal Choroidal Thickness and Axial Length in Preschool Children with Hyperopic Anisometropic Amblyopia

PURPOSE

To investigate the relationship between subfoveal choroidal thickness and axial length in Japanese preschool children with hyperopic anisometropic amblyopia.

METHODS

Twenty-four children between the age of 3 and 6 years exhibiting hyperopic anisometropic amblyopia were examined. Differences in spherical equivalent between the two eyes were over 1.5 D in all children. Twenty-four eyes in 12 children without anisometropia and amblyopia were examined as age-matched normal controls. Subfoveal choroidal thickness was measured by using enhanced depth imaging optical coherence tomography. Axial length was measured with noncontact optical biometer.

RESULTS

The spherical equivalent ranged from +3.50 to +7.25 D in amblyopic eyes and from +0.75 to +3.50 D in fellow eyes. The subfoveal choroidal thickness was significantly greater in the amblyopic eyes than that in the fellow eyes (407.3 ± 54.2 μm versus 357.7 ± 54.3 μm, Paired t-test, p < 0.05). The axial length in the amblyopic eyes was significantly shorter than that in the fellow eyes (21.16 ± 0.64 mm versus 22.08 ± 0.72 mm, Paired t-test, p < 0.05). The mean choroidal thickness of the fellow eyes in hyperopic anisometropic amblyopia was greater than that in age matched normal children, although this difference did not reach statistical significance (326.0 ± 62.1 μm, p = 0.07). The subfoveal choroidal thickness in amblyopic children was negatively correlated with their axial length (r = -0.50, p < 0.01).

CONCLUSION

The subfoveal choroidal thickness of amblyopic children abnormally increased and the thicker subfoveal choroid is mildly correlated with their shorter axial length. The anomalous subfoveal choroidal thicknesses in our amblyopic children may reflect a delay in emmetropization.

Anterior visual pathways in amblyopia: quantitative assessment with diffusion tensor imaging

PURPOSE

To detect the abnormalities of the anterior visual pathways in children with amblyopia with diffusion tensor imaging.

METHODS

Ten children with unilateral amblyopia, 5 children with bilateral amblyopia, and 10 control children were treated using diffusion tensor imaging scanning in this institutional practice. Fractional anisotropy and mean diffusivity values were analyzed using diffusion tensor imaging in the prechiasmatic and chiasmatic regions. Fractional anisotropy and mean diffusivity values of the amblyopic groups were compared with the values of the control group using the Kruskal–Wallis test. The Mann–Whitney U test was used to evaluate pairwise differences between groups.

RESULTS

When compared with the control group, prechiasmatic fractional anisotropy values were significantly decreased in both affected and sound fellow eyes in the unilateral amblyopic group (P = .019 and .013), but not in the bilateral amblyopic group (P = .221). Mean diffusivity values were significantly greater in the sound fellow eye in the unilateral amblyopic group in the prechiasmatic region (P = .001 and .049).

CONCLUSION

Diffusion tensor imaging showed a significant decrease in fractional anisotropy and an increase in mean diffusivity values in the unilateral amblyopic group in both affected and sound fellow eyes. These findings may reflect axonal underdevelopment in anterior pathways, particularly in the unilateral amblyopic group.

One Year of Pediatric Ophthalmology and Strabismus Research in Review

PURPOSE

To highlight recent advances in amblyopia and strabismus.

DESIGN

Literature review.

METHODS

A literature search of articles published in the English language was performed in PubMed or MEDLINE between May 2012 and April 2013 using the terms amblyopia or strabismus. Articles deemed relevant were selected.

RESULTS

The review highlights articles that increase our understanding of strabismus and amblyopia as well as newer treatment strategies.

CONCLUSIONS

The review highlights some new information and possible future advances in amblyopia and strabismus.

The relationship between anisometropia and amblyopia

This review aims to disentangle cause and effect in the relationship between anisometropia and amblyopia. Specifically, we examine the literature for evidence to support different possible developmental sequences that could ultimately lead to the presentation of both conditions. The prevalence of anisometropia is around 20% for an inter-ocular difference of 0.5D or greater in spherical equivalent refraction, falling to 2-3%, for an inter-ocular difference of 3D or above. Anisometropia prevalence is relatively high in the weeks following birth, in the teenage years coinciding with the onset of myopia and, most notably, in older adults starting after the onset of presbyopia. It has about one-third the prevalence of bilateral refractive errors of the same magnitude. Importantly, the prevalence of anisometropia is higher in highly ametropic groups, suggesting that emmetropization failures underlying ametropia and anisometropia may be similar. Amblyopia is present in 1-3% of humans and around one-half to two-thirds of amblyopes have anisometropia either alone or in combination with strabismus. The frequent co-existence of amblyopia and anisometropia at a child's first clinical examination promotes the belief that the anisometropia has caused the amblyopia, as has been demonstrated in animal models of the condition. In reviewing the human and monkey literature however it is clear that there are additional paths beyond this classic hypothesis to the co-occurrence of anisometropia and amblyopia. For example, after the emergence of amblyopia secondary to either deprivation or strabismus, anisometropia often follows. In cases of anisometropia with no apparent deprivation or strabismus, questions remain about the failure of the emmetropization mechanism that routinely eliminates infantile anisometropia. Also, the chronology of amblyopia development is poorly documented in cases of 'pure' anisometropic amblyopia. Although indirect, the therapeutic impact of refractive correction on anisometropic amblyopia provides strong support for the hypothesis that the anisometropia caused the amblyopia. Direct evidence for the aetiology of anisometropic amblyopia will require longitudinal tracking of at-risk infants, which poses numerous methodological and ethical challenges. However, if we are to prevent this condition, we must understand the factors that cause it to develop.