Intersubject variability of foveal cone photoreceptor density in relation to eye length

Modeling Human Macular Cone Photoreceptor Spatial Distribution

Purpose

The purpose of this study was to investigate the spatial distribution of human cone photoreceptors and examine cone density differences between the retinal meridians and quadrants.

Method

Using adaptive optics scanning laser ophthalmoscopy, the maculae were imaged in 17 eyes of 11 subjects with normal chorioretinal health aged 54 to 72 years. We measured cone density at 325 points within the central 10 degrees radius of the retina. Cone density spatial distributions along the primary retinal meridians and in four macular quadrants (superior-nasal, superior-temporal, inferior-temporal, and inferior-nasal) were analytically modeled using the polynomial function to assess the meridional and quadrantal difference.

Results

The mean and 95% confidence interval for the prediction of cone density along the primary retinal meridians was modeled with a 7-degree one-variable polynomial (R2 = 0.9761, root mean squared error [RMSE] = 0.0585). In the 4 retinal quadrants, cone density distribution was described by a 2-variable polynomial with X degree 3 and Y degree 4 (R² = 0.9834, RMSE = 0.0377). The models suggest no statistically significant difference between medians and between quadrants. However, cone density difference at corresponding spatial locations in different areas can be up to 25.6%. The superior-nasal region has more areas with high cone density, followed by quadrants of inferior-nasal, inferior-temporal, and superior-temporal.

Conclusions

Analytical modeling provides comprehensive knowledge of cone distribution across the entire macula. Although modeling analysis suggests no statistically significant difference between medians and between quadrants, the remarkable cone density discrepancies in certain regions should be accounted for in applications requiring sensitive detection of cone variation.

Intervisit Reproducibility of Foveal Cone Density Metrics

Purpose

To assess longitudinal reproducibility of metrics of foveal density (peak cone density [PCD], cone density centroid [CDC], and 80th percentile centroid area) in participants with normal vision.

Methods

Participants (n = 19; five male and 14 female) were imaged at two time points (average interval of 3.2 years) using an adaptive optics scanning light ophthalmoscope (AOSLO). Foveally centered regions of interest (ROIs) were extracted from AOSLO montages. Cone coordinate matrices were semiautomatically derived for each ROI, and cone mosaic metrics were calculated.

Results

On average, there were no significant changes in cone mosaic metrics between visits. The average ± SD PCD was 187,000 ± 20,000 cones/mm2 and 189,000 ± 21,700 cones/mm2 for visits 1 and 2, respectively (P = 0.52). The average ± SD density at the CDC was 183,000 ± 19,000 cones/mm2 and 184,000 ± 20,800 cones/mm2 for visits 1 and 2, respectively (P = 0.78). The average ± SD 80th percentile isodensity contour area was 15,400 ± 1800 µm2 and 15,600 ± 1910 µm2 for visits 1 and 2, respectively (P = 0.57).

Conclusions

Foveal cone mosaic density metrics were highly reproducible in the cohort examined here, although further study is required in more diverse populations.

Translational Relevance

Determination of the normative longitudinal changes in foveal cone topography is key for evaluating longitudinal measures of foveal cone topography in patients with progressive retinal dystrophies.

Multimodal in-vivo maps as a tool to characterize retinal structural biomarkers for progression in adult-onset Stargardt disease

Purpose

To characterize retinal structural biomarkers for progression in adult-onset Stargardt disease from multimodal retinal imaging in-vivo maps.

Methods

Seven adult patients (29-69 years; 3 males) with genetically-confirmed and clinically diagnosed adult-onset Stargardt disease and age-matched healthy controls were imaged with confocal and non-confocal Adaptive Optics Scanning Light Ophthalmoscopy (AOSLO), optical coherence tomography (OCT), fundus infrared (FIR), short wavelength-autofluorescence (FAF) and color fundus photography (CFP). Images from each modality were scaled for differences in lateral magnification before montages of AOSLO images were aligned with en-face FIR, FAF and OCT scans to explore changes in retinal structure across imaging modalities. Photoreceptors, retinal pigment epithelium (RPE) cells, flecks, and other retinal alterations in macular regions were identified, delineated, and correlated across imaging modalities. Retinal layer-thicknesses were extracted from segmented OCT images in areas of normal appearance on clinical imaging and intact outer retinal structure on OCT. Eccentricity dependency in cell density was compared with retinal thickness and outer retinal layer thickness, evaluated across patients, and compared with data from healthy controls.

Results

In patients with Stargardt disease, alterations in retinal structure were visible in different image modalities depending on layer location and structural properties. The patients had highly variable foveal structure, associated with equally variable visual acuity (-0.02 to 0.98 logMAR). Cone and rod photoreceptors, as well as RPE-like structures in some areas, could be quantified on non-confocal split-detection AOSLO images. RPE cells were also visible on dark field AOSLO images close to the foveal center. Hypo-reflective gaps of non-waveguiding cones (dark cones) were seen on confocal AOSLO in regions with clinically normal CFP, FIR, FAF and OCT appearance and an intact cone inner segment mosaic in three patients.

Conclusion

Dark cones were identified as a possible first sign of retinal disease progression in adult-onset Stargardt disease as these are observed in retinal locations with otherwise normal appearance and outer retinal thickness. This corroborates a previous report where dark cones were proposed as a first sign of progression in childhood-onset Stargardt disease. This also supports the hypothesis that, in Stargardt disease, photoreceptor degeneration occurs before RPE cell death.

Evaluation of objective and subjective binocular ocular refraction with looking in type

BACKGROUND

This study aimed to compare the results of the Chronos binocular/monocular refraction system, that measures objective and subjective ocular refraction in one unit, to objective findings obtained from a conventional autorefractometer and a conventional subjective ocular refraction using a trial-frame in real space.

METHODS

Twenty-eight healthy volunteers (21.2 ± 1.5 years old) were included in this study. Objective ocular refraction was measured using two tests: the Chronos binocular/monocular refraction system under binocular conditions and a conventional autorefractometer under monocular conditions. Subjective ocular refraction was measured using three tests: Chronos binocular/monocular refraction system under binocular, monocular conditions, and trial-frame in the real space under monocular conditions. The measurement distance was set to 5.0 m for each test. All ocular refractions were converted into spherical equivalents (SEs).

RESULTS

The objective SE was significantly more negative with Chronos binocular/monocular refraction system under binocular condition (- 4.08 ± 2.76 D) than with the conventional autorefractometer under monocular condition (- 3.85 ± 2.66 D) (P = 0.002). Although, the subjective SE was significantly more negative with Chronos binocular/monocular refraction system under binocular condition (- 3.55 ± 2.67 D) than with the trial-frame in the real space under monocular condition (- 3.33 ± 2.75 D) (P = 0.002), Chronos binocular/monocular refraction system under monocular condition (- 3.17 ± 2.57 D) was not significantly different from that in trial-frame in real space under monocular condition (P = 0.33).

CONCLUSION

These findings suggest that the Chronos binocular/monocular refraction system, which can complete both objective and subjective ocular refraction tests in a single unit, is suitable for screening ocular refraction, although it produces slightly more myopic results. Furthermore, subjective ocular refraction testing accuracy in Chronos binocular/monocular refraction system can be equivalent to trial-frame in real-space testing by switching from binocular to monocular condition.

The effect of sampling window size on topographical maps of foveal cone density

Purpose

To characterize the effect of sampling window size on maps of foveal cone density derived from adaptive optics scanning light ophthalmoscope (AOSLO) images of the cone mosaic.

Methods

Forty-four AOSLO-derived montages of the foveal cone mosaic (300 x 300µm) were used for this study (from 44 individuals with normal vision). Cone photoreceptor coordinates were semi-automatically identified by one experienced grader. From these coordinates, cone density matrices across each foveal montage were derived using 10 different sampling window sizes containing 5, 10, 15, 20, 40, 60, 80, 100, 150, or 200 cones. For all 440 density matrices, we extracted the location and value of peak cone density (PCD), the cone density centroid (CDC) location, and cone density at the CDC.

Results

Across all window sizes, PCD values were larger than those extracted at the CDC location, though the difference between these density values decreased as the sampling window size increased (p<0.0001). Overall, both PCD (r=-0.8099, p=0.0045) and density at the CDC (r=-0.7596, p=0.0108) decreased with increasing sampling window size. This reduction was more pronounced for PCD, with a 27.8% lower PCD value on average when using the 200-cone versus the 5-cone window (compared to only a 3.5% reduction for density at the CDC between these same window sizes). While the PCD and CDC locations did not occur at the same location within a given montage, there was no significant relationship between this PCD-CDC offset and sampling window size (p=0.8919). The CDC location was less variable across sampling windows, with an average per-participant 95% confidence ellipse area across the 10 window sizes of 47.56µm² (compared to 844.10µm² for the PCD location, p<0.0001).

Conclusion

CDC metrics appear more stable across varying sampling window sizes than PCD metrics. Understanding how density values change according to the method used to sample the cone mosaic may facilitate comparing cone density data across different studies.

Comparison between Two Adaptive Optics Methods for Imaging of Individual Retinal Pigmented Epithelial Cells

The Retinal Pigment Epithelium (RPE) plays a prominent role in diseases such as age-related macular degeneration, but imaging individual RPE cells is challenging due to their high absorption and low autofluorescence emission. The RPE lies beneath the highly reflective photoreceptor layer (PR) and contains absorptive pigments, preventing direct backscattered light detection when the PR layer is intact. Here, we used near-infrared autofluorescence adaptive optics scanning laser ophthalmoscopy (NIRAF AOSLO) and transscleral flood imaging (TFI) in the same healthy eyes to cross-validate these approaches. Both methods revealed a consistent RPE mosaic pattern and appeared to reflect a distribution of fluorophores consistent with findings from histological studies. Interestingly, even in apparently healthy RPE, we observed dynamic changes over months, suggesting ongoing cellular activity or alterations in fluorophore distribution. These findings emphasize the value of NIRAF AOSLO and TFI in understanding RPE morphology and dynamics.

Reduced contrast sensitivity function is correlated with changes to cone photoreceptors in simple high myopia

Purpose

To investigate the contrast sensitivity function (CSF) changes in simple high myopia (SHM) and evaluate the correlations between these changes with the early changes in the retinal microstructure.

Methods

This prospective study comprised 81 subjects, 20 with emmetropia (EM), 26 with low myopia and moderate myopia (LM/MM), and 35 with SHM. The area under the log CSF curve (AULCSF) and the cut-off spatial frequency (Cut-off SF) were employed as measures of CSF. Adaptive optics (AO) was employed to quantify the cone density, spacing, and regularity. The thickness and blood flow of the retinal sublayers were determined from vertical and horizontal optical coherence tomography angiography (OCTA) A-scans. Swept-source optical coherence tomography (SS-OCT) was employed to analyze the choroidal thickness (CT) and choroidal vascularity using a custom algorithm. Differences in the retinal and choroidal parameters, cone distribution, AULCSF, and Cut-off SF were compared among the three groups. Multivariate linear mixed models were used to elucidate the associations between photoreceptor morphological alterations, retinal and choroidal parameters, and AULCSF.

Results

The AULCSF and Cut-off SF were significantly lower in the SHM group compared to the EM and LM groups (p < 0.05). The SHM group had less cone density, larger cone spacing, and lower cone regularity than the EM and LM/MM groups (p < 0.05). Moreover, the thickness of the inner segment of photoreceptors (IS), retinal pigment epithelium (RPE) layer and choroid were reduced, and the outer segment of photoreceptors (OS) was thicker in the SHM group compared to the EM and LM/MM groups (all p < 0.05). A longer axial length (AL) was correlated with decreased AULCSF, cone density, and cone spacing (r = -0.800 to 0.752, all p < 0.050). Additionally, decreased CSF was correlated with lower cone density (r = 0.338, p = 0.035).

Conclusion

Decreased contrast sensitivity was observed in patients with SHM and cone density was significantly correlated with reduced AUCSF.

Age-Related Macular Degeneration, a Mathematically Tractable Disease

A progression sequence for age-related macular degeneration onset may be determinable with consensus neuroanatomical nomenclature augmented by drusen biology and eye-tracked clinical imaging. This narrative review proposes to supplement the Early Treatment of Diabetic Retinopathy Study (sETDRS) grid with a ring to capture high rod densities. Published photoreceptor and retinal pigment epithelium (RPE) densities in flat mounted aged-normal donor eyes were recomputed for sETDRS rings including near-periphery rich in rods and cumulatively for circular fovea-centered regions. Literature was reviewed for tissue-level studies of aging outer retina, population-level epidemiology studies regionally assessing risk, vision studies regionally assessing rod-mediated dark adaptation (RMDA), and impact of atrophy on photopic visual acuity. The 3 mm-diameter xanthophyll-rich macula lutea is rod-dominant and loses rods in aging whereas cone and RPE numbers are relatively stable. Across layers, the largest aging effects are accumulation of lipids prominent in drusen, loss of choriocapillary coverage of Bruch's membrane, and loss of rods. Epidemiology shows maximal risk for drusen-related progression in the central subfield with only one third of this risk level in the inner ring. RMDA studies report greatest slowing at the perimeter of this high-risk area. Vision declines precipitously when the cone-rich central subfield is invaded by geographic atrophy. Lifelong sustenance of foveal cone vision within the macula lutea leads to vulnerability in late adulthood that especially impacts rods at its perimeter. Adherence to an sETDRS grid and outer retinal cell populations within it will help dissect mechanisms, prioritize research, and assist in selecting patients for emerging treatments.

Seasonal and Annual Change in Physiological Ocular Growth of 7- to 11-Year-Old Norwegian Children

Purpose

To investigate seasonal and annual change in physiological eye growth in Norwegian school children.

Methods

Measurements of ocular biometry, non-cycloplegic spherical equivalent autorefraction (SER), and choroidal thickness (ChT) were obtained for 92 children (44 females) aged 7 to 11 years at four time points over a year (November 2019-November 2020). Seasons (3- and 5-month intervals) were classified as winter (November-January), winter-spring (January-June), and summer-autumn (June-November). Cycloplegic SER was obtained in January and used to group children. The seasonal and annual changes were tested with a linear mixed-effects model (P values were adjusted for multiple comparisons).

Results

All the children experienced annual ocular growth, irrespective of SER, but less so during the summer-autumn. The baseline SER was lower (P < 0.001), axial length (AL) was longer (P < 0.038), and choroids were thicker in 10- to 11-year-old than 7- to 8-year-old mild hyperopes (P = 0.002). Assuming mild hyperopes (n = 65) experience only physiological eye growth, modeling revealed seasonal and annual increases in AL across sex and age (P < 0.018), with less change during the summer-autumn than winter-spring. The 7- to 8-year-olds had a larger decrease annually and over winter-spring in SER (P ≤ 0.036) and in ChT over winter-spring than the 10- to 11-year-olds (P = 0.006).

Conclusions

There were significant seasonal and annual changes in AL in children who had physiological eye growth irrespective of age within this cohort. Annual changes in SER and seasonal choroidal thinning were only observed in 7- to 8-year-old children. This indicates continued emmetropization in 7- to 8-year-olds and a transition to maintaining emmetropia in 10- to 11-year-olds.

Cone Density Is Correlated to Outer Segment Length and Retinal Thickness in the Human Foveola

Purpose

Assessment of the relationship between in vivo foveolar cone density, cone outer segment length (OSL), and foveal retinal thickness (RT).

Methods

Foveolar cone density maps covering the central ±300 µm of the retina were derived from adaptive optics scanning laser ophthalmoscopy images. The corresponding maps of foveal cone OSL and RT were derived from high-resolution optical coherence tomography volume scans. Alignment of the two-dimensional maps containing OSL and RT with the cone density map was achieved by placing the location of maximum OSL on the cone density centroid (CDC).

Results

Across 10 participants (27 ± 9 years; 6 female), cone density at the CDC was found to be between 147,038 and 215,681 cones/mm². The maximum OSL and minimum RT were found to lie between 31 and 40, and 193 and 226 µm, respectively. A significant correlation was observed between cone density at the CDC and maximum OSL (P = 0.001), as well as the minimal RT (P < 0.05). Across all participants, the best fit for the relationship between normalized cone density and normalized OSL within the central 300 µm was given by a quadratic function.

Conclusions

Using optical coherence tomography-derived measurements of OSL enables to estimate CDC cone density and two-dimensional foveal cone density maps for example in patient eyes unsuitable for adaptive optics imaging. Furthermore, the observation of a fixed relationship between the normalized OSL and cone density points to a conserved mechanism shaping the foveal pit.

Potential application of foveal structural measurements in treatment decision for retinopathy of prematurity: an OCT-based study

PURPOSE

To investigate foveal changes in infants with ROP not requiring treatment(nROP) and ROP infants needing treatment (tROP) using a handheld SD-OCT device.

METHOD

We performed horizontal SD-OCT scans through the fovea in 156 eyes of 81 infants diagnosed with ROP. Foveal immaturity indices including the presence of inner retinal layers (IRL), absence of foveal outer nuclear layers widening (ONL) and attenuation of hyperreflective outer segment layers (OS), presence and type of cystoid macular edema (CME), epiretinal membrane (ERM), foveal pit depth (FPD), foveal pit width (FPW) and central foveal thickness (CFT) were calculated. The multivariate logistic regression model was used to predict the need for treatment based on OCT measurements.

RESULTS

The shape of the foveolar pit was not significantly different among tROP and nROP groups (P-value = 0.287, Chi-square test). IRL extrusion was incomplete in both tROP and nROP groups (P-value = 0.0.41, Chi-square test). Nevertheless, the presence of thicker IRL was more frequent in the nROP group in comparison with the tROP group (100% vs.64.8%, P-value = 0.001). CME was observed in 29% of eyes in the tROP group and 40% of eyes in the nROP group; however, this difference was not statistically significant (P-value = 0.32, Chi-square test). ERM was detected in 15 (75%) and 84 (65.6%) eyes in the nROP and tROP groups, respectively (P-value = 0.39, Chi-square test). Multivariate logistic regression analyses showed that the need for treatment was significantly associated with gestational age (GA), CFT and FPD (P-values 0.001 and 0.002 respectively).

CONCLUSIONS

This study demonstrated GA, foveal pit depth and the central foveal thickness could accurately predict the need for treatment with sensitivity, specificity, and diagnostic accuracy of 97%, 65% and 91.7% respectively.

Reduced Cone Density Is Associated with Multiple Sclerosis

Purpose

Multiple sclerosis (MS) is an inflammatory neurodegenerative disease of the central nervous system. Recent evidence suggests that degeneration of the inner layers of the retina occurs in MS. This study aimed to examine whether there are outer retinal changes in patients living with MS.

Design

This was a single center, cross-sectional study.

Participants

Sixteen patients with MS and 25 controls (volunteers without diagnosed MS) were recruited for the study.

Methods

We acquired volumetric spectral domain-OCT scans of the macula and a circular scan around the optic nerve head (ONH). We also captured adaptive optics (AO) images at 0° (centered on the foveola), 2°, 4°, and 6° temporal to the fovea.

Main Outcome Measures

We calculated the thickness of the different retinal layers in the macula and around the ONH using the inbuilt software of the OCT. We evaluated changes in cone photoreceptors by calculating cone density and spacing by the inbuilt AO automatic segmentation algorithm with manual correction. We compared patients with and without optic neuritis and controls.

Results

We found significant thinning of the inner retina and a thickening of the outer retina in the eye with a history of optic neuritis (eyes of patients with MS with a history of optic neuritis; mean difference [MD]: -11.13 ± 3.61 μm, P = 0.002 and MD: 2.86 ± 0.89 μm, P = 0.001; respectively). We did not observe changes in retinal layers without optic neuritis in eyes of patients with MS without a history of optic neuritis. However, regional differences were detected in the peripapillary retinal nerve fiber layer. Analyzing AO images revealed a significantly lower cone outer-segment density at all eccentricities in all patients compared with control eyes (P < 0.05), independent of optic neuritis history.

Conclusions

Our results showed that all MS cases were associated with decreased cone densities. Future longitudinal studies will help to elucidate whether this is a specific and sensitive method to detect and monitor the development and progression of MS.

Financial Disclosures

Proprietary or commercial disclosure may be found after the references.

Adaptive Optics Imaging of Inherited Retinal Disease

The human retina is amenable to direct, noninvasive visualization using a wide array of imaging modalities. In the ∼140 years since the publication of the first image of the living human retina, there has been a continued evolution of retinal imaging technology. Advances in image acquisition and processing speed now allow real-time visualization of retinal structure, which has revolutionized the diagnosis and management of eye disease. Enormous advances have come in image resolution, with adaptive optics (AO)-based systems capable of imaging the retina with single-cell resolution. In addition, newer functional imaging techniques provide the ability to assess function with exquisite spatial and temporal resolution. These imaging advances have had an especially profound impact on the field of inherited retinal disease research. Here we will review some of the advances and applications of AO retinal imaging in patients with inherited retinal disease.

Modeling rod and cone photoreceptor cell survival in vivo using optical coherence tomography

Many retinal diseases involve the loss of light-sensing photoreceptor cells (rods and cones) over time. The severity and distribution of photoreceptor loss varies widely across diseases and affected individuals, so characterizing the degree and pattern of photoreceptor loss can clarify pathophysiology and prognosis. Currently, in vivo visualization of individual photoreceptors requires technology such as adaptive optics, which has numerous limitations and is not widely used. By contrast, optical coherence tomography (OCT) is nearly ubiquitous in daily clinical practice given its ease of image acquisition and detailed visualization of retinal structure. However, OCT cannot resolve individual photoreceptors, and no OCT-based method exists to distinguish between the loss of rods versus cones. Here, we present a computational model that quantitatively estimates rod versus cone photoreceptor loss from OCT. Using histologic data of human photoreceptor topography, we constructed an OCT-based reference model to simulate outer nuclear layer thinning caused by differential loss of rods and cones. The model was able to estimate rod and cone loss using in vivo OCT data from patients with Stargardt disease and healthy controls. Our model provides a powerful new tool to quantify photoreceptor loss using OCT data alone, with potentially broad applications for research and clinical care.

Quantitative analysis of retinal microvascular structure and correlation with ocular parameters: A normative database of pediatric OCT angiography

PURPOSE

To analyze the vascular structure of the healthy pediatric population using optical coherence tomography angiography (OCTA), and to investigate its relationship with ocular parameters.

MATERIALS AND METHOD

This cross-sectional, prospective study included 142 eyes of 71 healthy subjects aged 8-18 years. All patients underwent a complete ocular assessment, biometric evaluation and OCTA analysis. Anterior chamber depth, axial length, corneal thickness values were also reported and their relationship with OCTA measurements was evaluated.

RESULTS

There was no statistical difference between girls and boys in terms of demographic characteristics (p>0.05). Neither vascular density in the deep and superficial capillary plexus nor FAZ area was significantly related to the ocular parameters or age of any patient in the study.

CONCLUSIONS

Despite the relatively small number of participants, this study may represent normative data for the Turkish pediatric population. It was also shown that vascular density and FAZ area in childhood are not affected by age, axis length, anterior chamber depth, cornea and foveal thickness.

Twenty-five years of clinical applications using adaptive optics ophthalmoscopy [Invited]

Twenty-five years ago, adaptive optics (AO) was combined with fundus photography, thereby initiating a new era in the field of ophthalmic imaging. Since that time, clinical applications of AO ophthalmoscopy to investigate visual system structure and function in both health and disease abound. To date, AO ophthalmoscopy has enabled visualization of most cell types in the retina, offered insight into retinal and systemic disease pathogenesis, and been integrated into clinical trials. This article reviews clinical applications of AO ophthalmoscopy and addresses remaining challenges for AO ophthalmoscopy to become fully integrated into standard ophthalmic care.

The age-related pattern of inner retinal thickening is affected by myopia development and progression

The longitudinal effect of myopic eye growth on each individual retinal layer has not been described to date on an established non-human primate (NHP) model of myopia. We evaluated the changes experienced by the overall and individual central and mid-peripheral retinal thickness profiles in marmosets (Callithrix jacchus) induced with myopia continuously for 5.5 months compared to controls using spectral-domain optical coherence tomography. Cycloplegic refractive state (Rx), vitreous chamber depth (VCD) and retinal thickness were measured at baseline and after 3 and 5.5 months on thirteen marmosets: eight animals with lens-induced myopia and five untreated controls. The overall and individual retinal layer thickness in the central and mid-peripheral retina were obtained and compared between groups. Regression models were used to explore the extent to which VCD or Rx changes could predict the thickness changes observed. While the retinas of control marmosets thickened significantly over 5.5 months, marmosets with lens-induced myopia experienced less retinal thickening and thinning at times, mostly in the inner neuroretinal layers and the ganglion cell-inner plexiform layer. The regression models suggest that 90% of the growth and refractive changes observed could be predicted by the thickness changes in the near to mid peripheral retina. This study confirms the longitudinal effect that myopia has on the inner retina of a NHP model during the early stages of myopia development. The observed myopia-driven differences in inner retina thickness templates might represent early biomarkers of myopia progression and associated complications.

Seasonal Variation in Diurnal Rhythms of the Human Eye: Implications for Continuing Ocular Growth in Adolescents and Young Adults

Purpose

To investigate the diurnal rhythms in the human eye in winter and summer in southeast Norway (latitude 60°N).

Methods

Eight measures (epochs) of intraocular pressure, ocular biometry, and optical coherence tomography were obtained from healthy participants (17–24 years of age) on a mid-winter's day (n = 35; 6 hours of daylight at solstice) and on a day the following summer (n = 24; 18 hours of daylight at solstice). Participants wore an activity monitor 7 days before measurements. The epochs were scheduled relative to the individual's habitual wake and sleep time: two in the day (morning and midday) and six in the evening (every hour until and 1 hour after sleep time). Saliva was collected for melatonin. A linear mixed-effects model was used to determine significant diurnal variations, and a sinusoid with a 24-hour period was fitted to the data with a nonlinear mixed-effects model to estimate rhythmic statistics.

Results

All parameters underwent significant diurnal variation in winter and summer (P < 0.002). A 1-hour phase advance was observed for melatonin and ocular axial length in the summer (P < 0.001). The degree of change in axial length was associated with axial length phase advance (R² = 0.81, P < 0.001) and choroidal thickening (R² = 0.54, P < 0.001) in summer.

Conclusions

Diurnal rhythms in ocular biometry appear to be synchronized with melatonin secretion in both winter and summer, revealing seasonal variation of diurnal rhythms in young adult eyes. The association between axial length and seasonal changes in the phase relationships between ocular parameters and melatonin suggests a between-individual variation in adaptation to seasonal changes in ocular diurnal rhythms.

Automatic quantification of cone photoreceptors in adaptive optics scanning light ophthalmoscope images using multi-task learning

Adaptive optics scanning light ophthalmoscope (AO-SLO) can directly image the cone photoreceptor mosaic in the living human retina, which offers a potentially great tool to detect cone-related ocular pathologies by quantifying the changes in the cone mosaic. However, manual quantification is very time-consuming and automation is highly desirable. In this paper, we developed a fully automatic method based on multi-task learning to identify and quantify cone photoreceptors. By including cone edges in the labels as the third dimension of the classification, our method provided more accurate and reliable results than the two previously reported methods. We trained and validated our network in an open data set consisting of over 200,000 cones, and achieved a 99.20% true positive rate, 0.71% false positive rate, and 99.24% Dice's coefficient on the test set consisting of 44,634 cones. All are better than the reported methods. In addition, the reproducibility of all three methods was also tested and compared, and the result showed the performance of our method was generally closer to the gold standard. Bland-Altman plots show that our method was more stable and accurate than the other two methods. Then ablation experiment was further done, and the result shows that multi-task learning is essential to achieving accurate quantifications. Finally, our method was also extended to segment the cones to extract the size information. Overall, the method proposed here demonstrated great performance in terms of accuracy and reliability, which can be used to efficiently quantify the subtle changes associated with the progression of many diseases affecting cones.

Radial and Tangential Retinal Magnifications as Functions of Visual Field Angle Across Spherical, Oblate, and Prolate Retinal Profiles

Purpose

To provide a tool for calculating radial and tangential retinal magnifications as functions of field angle and retinal shape and to articulate patterns of magnification across the retina for monocular and binocular combinations of prolate-, oblate-, and spherical-shaped retinas.

Methods

Formulae were derived to calculate radial and tangential retinal magnifications (mm/deg) from field angle (degrees), retinal asphericity (unitless conic constant), retinal vertex radius of curvature (mm), and nodal point position (mm). Monocular retinal magnifications were determined for eyes with prolate, spherical, and oblate retinas as functions of field angle. Bilateral differences in magnifications were examined for combinations of those eyes.

Results

Retinal shape substantially affects magnification profiles even for eyes with the same axial length. Greatest magnification changes across a retina and between eyes, as well as greatest increase in radial-tangential differences (distortion), occur with prolate retinas. Binocular magnification differences were smallest for oblate retinas. Nodal points anterior to the vertex center of curvature and oblate asphericity both cause field-dependent reductions in magnification relative to the fovea (barrel distortion), whereas nodal points posterior to vertex center of curvature and prolate asphericity cause the opposite (pincushion distortion). Retinal magnification differences due to eye shape are much greater than aniseikonia thresholds and chromatic differences in magnification. A spreadsheet tool implements the magnification calculations.

Conclusions

Local retinal magnifications as functions of field angle have substantial effects on objective applications (imaging retinal anatomy) and subjective experiences (aniseikonia) and quantify an ocular property that differs across eye shapes and refractive errors.

Translational Relevance

Methods are provided to customize the calculation of radial and tangential magnifications across the retina for individual eyes, which will bolster the multifactorial study of the effects of foveal and peripheral optics across eye shapes and refractive errors.

Longitudinal Evaluation of Changes in Retinal Architecture Using Optical Coherence Tomography in Achromatopsia

Purpose

This prospective study investigates longitudinal changes in retinal structure in patients with achromatopsia (ACHM) using optical coherence tomography (OCT).

Methods

Seventeen patients (five adults, 12 children) with genetically confirmed CNGA3- or CNGB3-associated ACHM underwent ocular examination and OCT over a follow-up period of between 2 and 9.33 years (mean = 5.7 years). Foveal tomograms were qualitatively graded and were segmented for quantitative analysis: central macular thickness (CMt), outer nuclear layer thickness (ONLt), and size of the foveal hyporeflective zone (vertical HRZ thickness: HRZt and horizontal HRZ width: HRZw) were measured. Data were analyzed using linear mixed regression models. Both age and visit were included into the models, to explore the possibility that the rate of disease progression depends on patient age.

Results

Fifteen of 17 patients (88%) showed longitudinal changes in retinal structure over the follow-up period. The most common patterns of progression was development of ellipsoid zone (EZ) disruption and HRZ. There was a significant increase in HRZt (P = 0.01) and HRZw (P = 0.001) between visits and no significant change in CMt and ONLt. Retinal parameters showed no difference in changes by genetic mutation (CNGA3 (n = 11), CNGB3 (n = 6)).

Conclusions

This study demonstrates clear longitudinal changes in foveal structure mainly in children, but also in adults with ACHM, over a long follow-up period. The longitudinal foveal changes suggest that treatment at an earlier age should be favored.

Retinal magnification factors at the fixation locus derived from schematic eyes with four individualized surfaces

Retinal magnification factors (RMFs) allow the conversion of angles to lengths in retinal images. In this work, we propose paraxial and non-paraxial RMF calculation methods that incorporate the individual topography and separation of the anterior and posterior surfaces of the cornea and crystalline lens, assuming homogeneous ocular media. Across 34 eyes, the two RMF methods differ by 0.1% on average, due to surface tilt, decenter, and lack of rotational symmetry in the non-paraxial modeling, which results in up to 2.2% RMF variation with retinal meridian. Differences with widely used individualized RMF calculation methods are smallest for eyes with ∼24 mm axial length, and as large as 7.5% in a 29.7 mm long eye (15D myope). To better model the capture of retinal images, we propose the tracing of chief rays, instead of the scaling of posterior nodal or principal distances often used in RMF definitions. We also report that RMF scale change is approximately proportional to both refractive error and axial separation between the ophthalmoscope's exit pupil and the eye's entrance pupil, resulting in RMF changes as large as 13% for a 1cm displacement in a 15D myopic eye. Our biometry data shows weak correlation and statistical significance between surface radii and refractive error, as well as axial length, whether considering all eyes in the study, or just the high myopes, defined as those with refractive error sphere equivalent ≤ -4D. In contrast, vitreous thicknesses show a strong correlation (r ≤ -0.92) and significance (p ≤ 10-13) with refractive error when considering all eyes or just high myopes (r ≤ -0.95; p ≤ 10-5). We also found that potential RMF change with depth of cycloplegia and/or residual accommodation is smaller than 0.2%. Finally, we propose the reporting of individual ocular biometry data and a detailed RMF calculation method description in scientific publications to facilitate the comparison of retinal imaging biomarker data across studies.

The relation of the multifocal electroretinographic response to macular layer volume

PURPOSE

To determine the association of the multifocal electroretinographic (mfERG) response amplitude with the volumes of the inner, postreceptor, and photoreceptor retinal layers in the region stimulated by each mfERG element.

METHODS

Sixteen healthy, young adult control subjects were studied. Each of the 103 hexagonal elements of the standard, scaled mfERG were aligned, where possible, with patches of retina imaged using optical coherence tomography. Stimuli falling on the fovea and on the optic nerve head were excluded. Linear mixed-effects modeling was then used to derive estimated coefficients (voltage/volume) for the mfERG response throughout the full 80 ms standard epoch. The resulting predicted response amplitudes originating in each layer were then compared to pharmacologically "dissected" mfERGs obtained from other studies in monkey eyes.

RESULTS

Across the duration of the response, the amplitude of the modeled contribution from (1) the inner retina was small-to-modest, (2) the postreceptor retina was larger and contained two prominent peaks, and (3) the photoreceptor response was the largest and most closely paralleled the overall (i.e., intact) response, including late-appearing oscillations. The significance of each layer's contribution was greatest when the absolute amplitude of that layer's response was largest. The contribution of the inner retina was maximally significant in the interval between the prominent troughs and peaks of the intact response. The contributions of the postreceptor and photoreceptor responses were maximally significant at the prominent troughs and peaks of the intact response.

CONCLUSIONS

The results of the model were in good overall agreement with previous interpretations of the cellular contributions to the mfERG. There was also fair agreement with pharmacologically dissected monkey mfERG responses. Thus, the estimations of the contributions of the retinal layers to the mfERG so produced appeared plausible.

Impact of Reference Center Choice on Adaptive Optics Imaging Cone Mosaic Analysis

Purpose

Foveal center marking is a key step in retinal image analysis. We investigated the discordance between the adaptive optics (AO) montage center (AMC) and the foveal pit center (FPC) and its implications for cone mosaic analysis using a commercial flood-illumination AO camera.

Methods

Thirty eyes of 30 individuals (including 15 healthy and 15 patients with rod–cone dystrophy) were included. Spectral-domain optical coherence tomography was used to determine the FPC, and flood-illumination AO imaging was performed with overlapping image frames to create an AO montage. The AMC was determined by averaging the (0,0) coordinates in the four paracentral overlapping AO image frames. Cone mosaic measurements at various retinal eccentricities were compared between corresponding retinal loci relative to the AMC or FPC.

Results

AMCs were located temporally to the FPCs in 14 of 15 eyes in both groups. The average AMC–FPC discordance was 0.85° among healthy controls and 0.33° among patients with rod-cone dystrophy (P < 0.05). The distance of the AMC from the FPC was a significant determinant of the cone density (β estimate = 218 cells/deg²/deg; 95% confidence interval [CI], 107–330; P < 0.001) and inter-cone distance (β estimate = 0.28 arcmin/deg; 95% CI, 0.15–0.40; P < 0.001), after adjustment for age, sex, axial length, spherical equivalent, eccentricity, and disease status.

Conclusions

There is a marked mismatch between the AMC and FPC in healthy eyes that may be modified by disease process such as rod–cone dystrophy. We recommend users of AO imaging systems carefully align the AO montage with a foveal anatomical landmark, such as the FPC, to ensure precise and reproducible localization of the eccentricities and regions of interest for cone mosaic analysis.

The Impact of Axial Eye Growth on Foveal Avascular Zone Measurements in Children

SIGNIFICANCE

Foveal avascular zone (FAZ) area is a frequently used biomarker in diseases impacting the retinal vasculature in pediatric populations. Variation in axial length between individuals results in differences in lateral image scale, which affect the accuracy of FAZ area measurements. Accordingly, changes in axial length over time within individual children would affect estimates of FAZ area change.

PURPOSE

This study aimed to quantify how changes in axial length over time affect estimates of FAZ area change using optical coherence tomography angiography (OCT-A) images.

METHODS

Twenty pediatric participants (<18 years old) and 40 adult participants were imaged on Optovue's Avanti system (Fremont, CA) and had axial length measurements acquired at two time points. The FAZ was segmented twice using the OCT-A image at each time point. Foveal avascular zone area was estimated at both time points using the assumed/fixed axial length of the OCT-A device (unscaled) and using the participant's axial length (scaled). Changes in FAZ area over time were compared between the pediatric and adult groups using both unscaled and scaled data.

RESULTS

The average ± standard deviation follow-up time was 3.35 ± 1.66 years for the pediatric group and 2.90 ± 1.65 years for the adult group. Using unscaled data, FAZ area seemed to decrease between visits in the pediatric group (P = .004), whereas the FAZ area increased between visits in the adult group (P = .003). When correctly scaled data were used, the FAZ area still increased between visits for the adult group (P < .001), although the FAZ area no longer showed a significant change between visits for the pediatric group (P = .37). When comparing the normalized FAZ area change across visits between unscaled and scaled data, a significant difference was found between the adult and pediatric groups (P < .001).

CONCLUSIONS

Scaled data should be used when measuring FAZ area in pediatric populations, especially in longitudinal studies.

Geometrical scaling of the developing eye and photoreceptors and a possible relation to emmetropization and myopia

In this study the role of vergence in relation to age-dependent scaling of eye and photoreceptor parameters is studied. The underlying hypothesis is that the size and packing of outer segments is matched to the pupil size outdoors in photopic conditions. Vergence is analysed in relation to the angular spectrum of waves being incident using age-dependent data from the literature for the actual geometry and density of photoreceptor cones and rods. This approach is used to derive simple relations for the angular confinement of light along outer segments. Only with a small photopic pupil can leakage and crosstalk for both central and peripheral photoreceptors be entirely ruled out due to the finite length of the outer segments. A limiting 3 mm pupil size is found for children in the school age. Larger pupils will increase the likelihood of leakage and crosstalk that may therefore impact on emmetropization. This study has introduced a new paradigm in myopia research by considering vergence across the 3-D retina as being matched to the angular spectrum of waves being incident from the eye pupil. Emmetropization suggests a delicate balance between photoreceptor outer segment length and density in relation to pupil size. Only when balanced will leakage and crosstalk between adjacent outer segments be effectively suppressed thereby ensuring the highest possible light capture efficiency by visual pigments in the outer segments whether an image is formed on the retina or not.

The retinal and perceived locus of fixation in the human visual system

Due to the dramatic difference in spatial resolution between the central fovea and the surrounding retinal regions, accurate fixation on important objects is critical for humans. It is known that the preferred retinal location (PRL) for fixation of healthy human observers rarely coincides with the retinal location with the highest cone density. It is not currently known, however, whether the PRL is consistent within an observer or is subject to fluctuations and, moreover, whether observers' subjective fixation location coincides with the PRL. We studied whether the PRL changes between days. We used an adaptive optics scanning laser ophthalmoscope to project a Maltese cross fixation target on an observer's retina and continuously imaged the exact retinal location of the target. We found that observers consistently use the same PRL across days, regardless of how much the PRL is displaced from the cone density peak location. We then showed observers small stimuli near the visual field location on which they fixated, and the observers judged whether or not the stimuli appeared in fixation. Observers' precision in this task approached that of fixation itself. Observers based their judgment on both the visual scene coordinates and the retinal location of the stimuli. We conclude that the PRL in a normally functioning visual system is fixed, and observers use it as a reference point in judging stimulus locations.

Fixational eye movements in passive versus active sustained fixation tasks

Human fixational eye movements are so small and precise that high-speed, accurate tools are needed to fully reveal their properties and functional roles. Where the fixated image lands on the retina and how it moves for different levels of visually demanding tasks is the subject of the current study. An Adaptive Optics Scanning Laser Ophthalmoscope (AOSLO) was used to image, track and present a variety of fixation targets (Maltese cross, disk, concentric circles, Vernier and tumbling-E letter) to healthy subjects. During these different passive (static) or active (discriminating) tasks under natural eye motion, the landing position of the target on the retina was tracked in space and time over the retinal image directly with high spatial (<1 arcmin) and temporal (960 Hz) resolution. We computed both the eye motion and the exact trajectory of the fixated target's motion over the retina. We confirmed that compared to passive tasks, active tasks elicited a partial inhibition of microsaccades, leading to longer drift periods compensated by larger corrective saccades. Consequently, the overall fixation stability during active tasks was on average 57% larger than during passive tasks. The preferred retinal locus of fixation was the same for each task and did not coincide with the location of the peak cone density.

Theoretical versus empirical measures of retinal magnification for scaling AOSLO images

The adaptive optics scanning light ophthalmoscope (AOSLO) allows cellular resolution imaging of the living retina. The accuracy of many quantitative measurements made from these images requires accurate estimates of the lateral scale of the images. Here, we used trial lenses, which are known to affect the relative magnification of the retinal image, to compare empirical measures of image scale with theoretical estimates from a four-surface optical model. The theoretical optical model overestimated the empirically determined change in image scale in 70% of the subjects examined, albeit to varying degrees. While the origin for the differences between subjects is not known, residual accommodation during imaging likely contributes to this variability in retinal magnification. These data provide an opportunity to derive improved lateral scaling error estimates for structural metrics extracted from AOSLO retinal images.

Cone photoreceptor density in the Copenhagen Child Cohort at age 16-17 years

PURPOSE

To examine cone density in relation to gestational and morphological parameters in the Copenhagen Child Cohort (CCC2000).

METHODS

The macula was imaged using adaptive optics in 1,296 adolescents aged 16-17 years. Axial length and distance visual acuity were determined. Absolute and angular cone photoreceptor density were analysed for an 80 × 80-pixel area, 2 degrees temporal to the fovea. Association with axial length was analysed with linear regression. Correlation with visual acuity was described with a Pearson correlation coefficient. Associations of cone density with gestational parameters, maternal smoking, sex and age were analysed using multiple regression adjusted for axial length.

RESULTS

Mean absolute cone density was 30,007 cones/mm² (SD ± 3,802) and mean angular cone density was 2,383 cones/deg² (SD ± 231). Peri- and postnatal parameters, sex and age had no statistically significant effect on cone density (p > 0.05). Absolute cone density decreased with longer axial length (-2,855 cones/mm² per mm or -9.7% per mm, p < 0.0001). For angular density, which included a correction for the geometrical enlargement of the eye with axial length, a decrease with axial length was detectable, but it was small (-20 cones/deg² per mm or -0.84% per mm, p = 0.009).

CONCLUSIONS

The decrease in cone density per unit solid angle with increasing axial length was small, less than 1 percent per mm, indicating that expansion of the posterior pole during the development of refraction takes place without a clinically significant loss of cones. Perinatal parameters, within the spectrum presented by the study population, had no detectable effect on cone density.

Human gaze is systematically offset from the center of cone topography

The small physical depression of the human retina, the fovea, is the retinal locus of prime visual resolution, achieved by a peaking topography of the light-sensitive cone photoreceptor outer segments^1-3 and a post-receptor wiring scheme preserving high-density sampling.^4,5 Humans dynamically direct their gaze such that the retinal images of objects of interest fall onto the foveola, the central one-degree diameter of the fovea,^6-8 but it is yet unclear whether a relationship between the individual photoreceptor topography at this location and visual fixation behavior exists.^9,10 By employing adaptive optics in vivo imaging and micro-stimulation,^11-13 we created topographical maps of the complete foveolar cone mosaics in both eyes of 20 healthy participants while simultaneously recording the retinal location of a fixated visual object in a psychophysical experiment with cellular resolution. We found that the locus of fixation was systematically shifted away from the topographical center toward a naso-superior quadrant on the retina, about 5 min of arc of visual angle on average, with a mirror symmetrical trend between fellow eyes. In cyclopean view, the topographical centers were superior to the fixated target, corresponding to areas in the visual field usually more distant^14,15 and thus containing higher spatial frequencies. Given the large variability in foveal topography between individuals, and the surprising precision with which fixation is repeatedly directed to just a small bouquet of cones in the foveola, these findings demonstrate a finely tuned, functionally relevant link between the development of the cellular mosaic of photoreceptors and visual behavior.

Promises and pitfalls of evaluating photoreceptor-based retinal disease with adaptive optics scanning light ophthalmoscopy (AOSLO)

Adaptive optics scanning light ophthalmoscopy (AOSLO) allows visualization of the living human retina with exquisite single-cell resolution. This technology has improved our understanding of normal retinal structure and revealed pathophysiological details of a number of retinal diseases. Despite the remarkable capabilities of AOSLO, it has not seen the widespread commercial adoption and mainstream clinical success of other modalities developed in a similar time frame. Nevertheless, continued advancements in AOSLO hardware and software have expanded use to a broader range of patients. Current devices enable imaging of a number of different retinal cell types, with recent improvements in stimulus and detection schemes enabling monitoring of retinal function, microscopic structural changes, and even subcellular activity. This has positioned AOSLO for use in clinical trials, primarily as exploratory outcome measures or biomarkers that can be used to monitor disease progression or therapeutic response. AOSLO metrics could facilitate patient selection for such trials, to refine inclusion criteria or to guide the choice of therapy, depending on the presence, absence, or functional viability of specific cell types. Here we explore the potential of AOSLO retinal imaging by reviewing clinical applications as well as some of the pitfalls and barriers to more widespread clinical adoption.

Assessing Interocular Symmetry of the Foveal Cone Mosaic

Purpose

To test the hypothesis that foveal cone topography is symmetrical between contralateral eyes.

Methods

We used adaptive optics scanning light ophthalmoscopy to acquire images of the foveal cone mosaic in each eye of 58 subjects with normal vision (35 female, 23 male). Cones were semiautomatically identified over a 300 × 300-µm foveal area. From these cone coordinates, maps of cone density were derived, and we extracted estimates of peak cone density from each map. Mosaic regularity was assessed using Voronoi cell area regularity (VCAR). Average roundness and average area of the 70%, 75%, 80%, 85%, and 90% of peak density isodensity contours were evaluated.

Results

The average peak cone density for right eyes was 180,286 cones/mm² (n = 49) and for left eyes was 182,397 cones/mm² (n = 45), with a mean absolute difference of 6363 cones/mm² (n = 43). Peak density, cone spacing, VCAR, and average area within the isodensity contours of fellow eyes were not significantly different (P = 0.60, P = 0.83, P = 0.30, and P = 0.39, respectively). However, the average roundness of the isodensity contours was 2% more circular in the right eyes than in the left eyes (P = 0.02).

Conclusions

There is interocular symmetry of peak foveal cone density, mosaic regularity, and area encompassing the most densely packed cells in subjects with normal vision. The origin and significance of the observed interocular difference in average roundness of the isodensity contours are unclear.

Cone parameters in different vision levels from the adaptive optics imaging

To investigate the relationship between visual resolution and cone parameters in eyes with different levels of best corrected visual acuity (BCVA).Seventeen eyes of 10 volunteers with BCVA of 20/12.5 or better (group 1) and 16 eyes of 10 volunteers with BCVA of 20/16 (group 2) were investigated in the study. Images of the cone photoreceptors at 1.5° from the fovea were obtained using an adaptive optics (AO) retinal camera. The BCVA was obtained following a subjective refraction using a standardized logMAR visual acuity chart.The mean cone density (29,570.96 ± 2489.94 cells/mm2) at 1.5° from the fovea in group 1 (BCVA ≥ 20/12.5, n = 17) was significantly greater (P < .001) than that (22,963.59 ± 2987.92 cells/mm2) in group 2 (BCVA = 20/16, n = 16). The cone spacing at 1.5° from the fovea in group 1 was 6.45 ± 0.28 μm (mean ± SD), which was significantly smaller (P < .001) than 7.36 ± 0.50 μm (mean ± SD) in group 2. In the stepwise regression analysis, greater angular cone density (odds ratio [OR], 4.48; P = .005) and smaller angular cone spacing (OR, 0.60; P = .007) at 1.5° from the fovea were significantly associated with the better BCVA.The greater cone density and smaller cone spacing at the parafovea were found in eyes with BCVA of 20/12.5 or better, as compared to that in eyes with BCVA of 20/16. Knowledge of cone distribution for different BCVA levels may be beneficial for different clinical conditions.

The Fovea in Retinopathy of Prematurity

Purpose

Because preterm birth and retinopathy of prematurity (ROP) are associated with poor visual acuity (VA) and altered foveal development, we evaluated relationships among the central retinal photoreceptors, postreceptor retinal neurons, overlying fovea, and VA in ROP.

Methods

We obtained optical coherence tomograms (OCTs) in preterm born subjects with no history of ROP (none; n = 61), ROP that resolved spontaneously without treatment (mild; n = 51), and ROP that required treatment by laser ablation of the avascular peripheral retina (severe; n = 22), as well as in term born control subjects (term; n = 111). We obtained foveal shape descriptors, measured central retinal layer thicknesses, and demarcated the anatomic parafovea using automated routines. In subsets of these subjects, we obtained OCTs eccentrically through the pupil (n = 46) to reveal the fiber layer of Henle (FLH) and obtained adaptive optics scanning light ophthalmograms (AO-SLOs) of the parafoveal cones (n = 34) and measured their spacing and distribution.

Results

Both VA and foveal depth decreased with increasing ROP severity (term, none, mild, severe). In severe subjects, foveae were broader than normal and the parafovea was significantly enlarged compared to every other group. The FLH was thinner than normal in mild (but not severe) subjects. VA was associated with foveal depth more than group. Density of parafoveal cones did not differ significantly among groups.

Conclusions

Foveal structure is associated with loss of VA in ROP. The preserved FLH in severe (relative to mild) eyes suggests treatment may help cone axon development. The significantly larger parafovea and increased outer nuclear layer (ONL) thickness in ROP hint that some developmental process affecting the photoreceptors is not arrested in ROP but rather is supranormal.

Adaptive optics for high-resolution imaging

Adaptive optics (AO) is a technique that corrects for optical aberrations. It was originally proposed to correct for the blurring effect of atmospheric turbulence on images in ground-based telescopes and was instrumental in the work that resulted in the Nobel prize-winning discovery of a supermassive compact object at the centre of our galaxy. When AO is used to correct for the eye's imperfect optics, retinal changes at the cellular level can be detected, allowing us to study the operation of the visual system and to assess ocular health in the microscopic domain. By correcting for sample-induced blur in microscopy, AO has pushed the boundaries of imaging in thick tissue specimens, such as when observing neuronal processes in the brain. In this primer, we focus on the application of AO for high-resolution imaging in astronomy, vision science and microscopy. We begin with an overview of the general principles of AO and its main components, which include methods to measure the aberrations, devices for aberration correction, and how these components are linked in operation. We present results and applications from each field along with reproducibility considerations and limitations. Finally, we discuss future directions.

The Influence of Axial Length Upon the Retinal Ganglion Cell Layer of the Human Eye

Purpose

Variation in retinal thickness with eye size complicates efforts to estimate retinal ganglion cell number from optical coherence tomography (OCT) measures. We examined the relationship among axial length, the thickness and volume of the ganglion cell layer (GCL), and the size of the optic chiasm.

Methods

We used OCT to measure GCL thickness over 50 degrees of the horizontal meridian in 50 healthy participants with a wide range of axial lengths. Using a model eye informed by individual biometry, we converted GCL thickness to tissue volume per square degree. We also measured the volume of the optic chiasm for 40 participants using magnetic resonance imaging (MRI).

Results

There is a positive relationship between GCL tissue volume and axial length. Given prior psychophysical results, we conclude that increased axial length is associated with increased retinal ganglion cell size, decreased cell packing, or both. We characterize how retinal ganglion cell tissue varies systematically in volume and spatial distribution as a function of axial length. This model allows us to remove the effect of axial length from individual difference measures of GCL volume. We find that variation in this adjusted GCL volume correlates well with the size of the optic chiasm.

Conclusions

Our results provide the volume of ganglion cell tissue in the retina, adjusted for the presumed effects of axial length upon ganglion cell size and/or packing. The resulting volume measure accounts for individual differences in the size of the optic chiasm, supporting its use to characterize the post-retinal visual pathway.

Translational Relevance

Variations in ametropia can confound clinical measures of retinal features. We present a framework within which the thickness and volume of retinal structures can be measured and corrected for the effects of axial length.

Three-dimensional assessment of para- and perifoveal photoreceptor densities and the impact of meridians and age in healthy eyes with adaptive-optics optical coherence tomography (AO-OCT)

An adaptive optics optical coherence tomography (AO-OCT) system is used to assess sixty healthy eyes of thirty subjects (age 22 to 75) to evaluate how the outer retinal layers, foveal eccentricity and age effect the mean cone density. The cone mosaics of different retinal planes (the cone inner segment outer segment junction (IS/OS), the cone outer segment combined with the IS/OS (ISOS+), the cone outer segment tips (COST), and the full en-face plane (FEF)) at four main meridians (superior, nasal, inferior, temporal) and para- and perifoveal eccentricities (ecc 2.5° and 6.5°) were analyzed quantitatively. The mean overall cone density was 19,892/mm² at ecc 2.5° and 13,323/mm² at ecc 6.5°. A significant impact on cone density was found for eccentricity (up to 6,700/mm² between ecc 2.5° and 6.5°), meridian (up to 3,700/mm² between nasal and superior meridian) and layer (up to 1,400/mm² between FEF and IS/OS). Age showed only a weak negative effect. These factors as well as inter-individual variability have to be taken into account when comparing cone density measurements between healthy and pathologically changed eyes, as their combined effect on density can easily exceed several thousand cones per mm² even in parafoveal regions.

Carbonic anhydrase inhibition in X-linked retinoschisis: An eye on the photoreceptors

The retinoschisin protein is encoded on the short arm of the X-chromosome by RS1, is expressed abundantly in photoreceptor inner segments and in bipolar cells, and is secreted as an octamer that maintains the structural integrity of the retina. Mutations in RS1 lead to X-linked retinoschisis (XLRS), a disease characterized by the formation of cystic spaces between boys' retinal layers that frequently present in ophthalmoscopy as a "spoke-wheel" pattern on their maculae and by progressively worsening visual acuity (VA). There is no proven therapy for XLRS, but there is mixed evidence that carbonic anhydrase inhibitors (CAIs) produce multiple beneficial effects, including improved VA and decreased volume of cystic spaces. Consequently, linear mixed-effects (LME) models were used to evaluate the effects of CAI therapy on VA and central retinal thickness (CRT, a proxy for cystic cavity volume) in a review of 19 patients' records. The mechanism of action of action of CAIs is unclear but, given that misplaced retinoschisin might accumulate in the photoreceptors, it is possible-perhaps even likely-that CAIs act to benefit the function of photoreceptors and the neighboring retinal pigment epithelium by acidification of the extracellular milieu; patients on CAIs have among the most robust photoreceptor responses. Therefore, a small subset of five subjects were recruited for imaging on a custom multimodal adaptive optics retinal imager for inspection of their parafoveal cone photoreceptors. Those cones that were visible, which numbered far fewer than in controls, were enlarged, consistent with the retinoschisin accumulation hypothesis. Results of the LME modeling found that there is an initial benefit to both VA and CRT in CAI therapy, but these wane, in both cases, after roughly two years. That said, even a short beneficial effect of CAIs on the volume of the cystic spaces may give CAI therapy an important role as pretreatment before (or immediately following) administration of gene therapy.

Altered spatial summation optimizes visual function in axial myopia

This study demonstrates significant differences between the area of complete spatial summation (Ricco's area, RA) in eyes with and without non-pathological, axial myopia. Contrast thresholds were measured for six stimuli (0.01-2.07 deg²) presented at 10º eccentricity in 24 myopic subjects and 20 age-similar non-myopic controls, with RA estimated using iterative two-phase regression analysis. To explore the effects of axial length-induced variations in retinal image size (RIS) on the measurement of RA, refractive error was separately corrected with (i) trial lenses at the anterior focal point (near constant inter-participant RIS in mm), and (ii) contact lenses (RIS changed with axial length). For spectacle corrected measurements, RA was significantly larger in the myopic group, with a significant positive correlation also being observed between RA and measures of co-localised peripheral ocular length. With contact lens correction, there was no significant difference in RA between the groups and no relationship with peripheral ocular length. The results suggest RA changes with axial elongation in myopia to compensate for reduced retinal ganglion cell density. Furthermore, as these changes are only observed when axial length induced variations in RIS are accounted for, they may reflect a functional adaptation of the axially-myopic visual system to an enlarged RIS.

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.

The optic nerve head, lamina cribrosa, and nerve fiber layer in non-myopic and myopic children

The purpose of this study was to evaluate the optic nerve head, lamina cribrosa, retina, and choroid in school age children using spectral domain optical coherence tomography (SD-OCT) and to assess these structural parameters in relation to age, axial length, and refractive error. Healthy children, ages 11.15 ± 2.62 years (range 6-15 years, n = 53), underwent cycloplegic autorefraction, biometry, and SD-OCT imaging in both eyes. Images were analyzed using custom written programs in MATLAB, after adjustment for lateral magnification. Peripapillary retinal nerve fiber layer (RNFL) thickness, retinal and choroidal thicknesses, Bruch's membrane opening (BMO) area, minimum rim width (MRW), and anterior lamina cribrosa surface depth (ALCSD) were determined and analyzed with age, axial length, and refraction. Results show that axial length increased and refractive error became more myopic with increasing age (R2 = 0.25, β = 0.18, P < 0.0001 and R2 = 0.27, β = -0.37, P < 0.0001, respectively). Minimum foveal thickness and central 1 mm retinal thickness increased with increasing age (R2 = 0.15, β = 2.38, P < 0.01 and R2 = 0.11, β = 3.16, P < 0.05, respectively). Age-adjusted raw values for peripapillary RNFL thickness decreased with increasing axial length (R2 = 0.11, β = -3.18, P < 0.05); however, this relationship was not present when image magnification was corrected (R2 = 0.07, β = 2.72, P = 0.09). BMO area increased with myopic refractive error (R2 = 0.16, β = -0.10, P < 0.01). Age-adjusted vertical cup-to-disc ratio decreased with increasing axial length and myopic refractive error (R2 = 0.12, β = -0.05, P < 0.05 and R2 = 0.11, β = 0.03, P = 0.05, respectively). Mean MRW, mean ALCSD, and peripapillary choroidal thickness were not associated with age, axial length, or refraction. Mean MRW was significantly thinner in eyes with deeper ALCS (R2 = 0.41, β = -0.83, P < 0.0001). These findings provide normal values for retinal and optic nerve head parameters in school age children, and also suggest that ocular remodeling occurs in some structures in school age children with normal eye growth and during early stages of myopia development.

Inter- and Intra-individual Variations in Foveal Outer Nuclear Layer Thickness and Their Associations with Clinical Characteristics in a Healthy Chinese Population

Purpose

To evaluate foveal outer nuclear layer (ONL) thickness and the difference thereof between bilateral eyes and their possible associations with clinical characteristics in a healthy Chinese population.

Materials and Methods

Normal subjects were enrolled. Generalized linear models were used to assess the associations of foveal ONL thickness with sex, age, and spherical equivalents (SEs) and the associations of the difference in foveal ONL thickness between bilateral eyes with sex, age, and difference in SEs between bilateral eyes.

Results

Totally, 304 subjects were included. The average foveal ONL thickness was 103.19 ± 14.25 (range 70-151) μm in the right eye and 103.90 ± 14.63 (range 69-155) μm in the left eye. The mean difference in foveal ONL thickness between right and left eyes was -0.71 ± 4.36 (range -13 to +12) μm. Men had slightly greater foveal ONL thickness values in both right and left eyes compared with women (both P < 0.05); however, some women had a thicker foveal ONL than that of men (85/198 vs. 46/106 in the right eye; 79/198 vs. 52/106in the left eye). Age and SEs were not associated with foveal ONL thickness in either eye (all P > 0.05). Sex, age, and difference in SEs between bilateral eyes were not associated with the difference in foveal ONL thickness between bilateral eyes (all P > 0.05).

Conclusions

Foveal ONL thickness showed wide variation in a normal Chinese population but little difference between bilateral eyes. Both these parameters could not be adjusted by sex, age, SEs, or the SEs difference between bilateral eyes. Thus, in those diseases involving only one eye, the difference or ratio of foveal ONL thickness between the affected eye and normal fellow eye may reflect the actual degree of the disease, rather than the foveal ONL thickness in the affected eye alone.

Seven Myths on Crowding and Peripheral Vision

Crowding has become a hot topic in vision research, and some fundamentals are now widely agreed upon. For the classical crowding task, one would likely agree with the following statements. (1) Bouma's law can be stated, succinctly and unequivocally, as saying that critical distance for crowding is about half the target's eccentricity. (2) Crowding is predominantly a peripheral phenomenon. (3) Peripheral vision extends to at most 90° eccentricity. (4) Resolution threshold (the minimal angle of resolution) increases strongly and linearly with eccentricity. Crowding increases at an even steeper rate. (5) Crowding is asymmetric as Bouma has shown. For that inner-outer asymmetry, the peripheral flanker has more effect. (6) Critical crowding distance corresponds to a constant cortical distance in primary visual areas like V1. (7) Except for Bouma's seminal article in 1970, crowding research mostly became prominent starting in the 2000s. I propose the answer is "not really" or "not quite" to these assertions. So should we care? I think we should, before we write the textbook chapters for the next generation.

Geometry of the Retinal Nerve Fibers From Emmetropia Through to High Myopia at Both the Temporal Raphe and Optic Nerve

Purpose

The geometry of retinal nerve fibers may be altered with myopia, a known risk factor for glaucoma. Recent developments in high resolution imaging have enabled direct visualization of nerve fiber bundles at the temporal raphe with clinical hardware, providing evidence that this area is sensitive to glaucomatous damage. Here, we test the hypothesis that nerve fiber geometry is altered by myopia, both at the temporal raphe and surrounding the optic nerve head.

Methods

Seventy-eight healthy individuals participated, with refractive errors distributed between emmetropia and high myopia (+0 to -13 DS). Custom high-density OCT scans were used to visualize RFNL bundle trajectory at the temporal raphe. A standard clinical OCT protocol was used to assess papillary minimum rim width (MRW) and peripapillary retinal nerve fiber layer (RNFL) thickness.

Results

Measures of raphe shape-including position, orientation, and width-did not depend significantly on axial length. In 7.5% of subjects, the raphe was rotated sufficiently that inversion of structure-function mapping to visual field space is predicted in the nasal step region. Low concordance to ISNT and related rules was observed in myopia (e.g., for RNFL, 8% of high axial myopes compared with 67% of emmetropes). Greater robustness to refractive error was observed for the IT rule.

Conclusions

High density OCT scans enabled visualization of marked interindividual variation in temporal raphe geometry; however, these variations were not well predicted by degree of myopia as represented by axial length. That said, degree of myopia was associated with abnormal thickness profiles for the papillary and peripapillary nerve fiber layer.

Photoreceptor Degeneration is Correlated With the Deterioration of Macular Retinal Sensitivity in High Myopia

Purpose

To investigate structural changes in the retinal outer layers and choroid using adaptive optics (AO) and optical coherence tomography (OCT) in eyes with myopia, and to correlate the changes with decreased macular light sensitivity (MLS).

Methods

This prospective study included 27 subjects with emmetropia and low myopia (EM/LM), 25 with moderate myopia (MM), and 25 with high myopia (HM). Microperimetry was used to quantify MLS in each subject, while AO and OCT images of fundus were analyzed to quantify cone density and regularity and thickness of outer retinal sublayers and choroid. Differences of MLS, cone distribution, and chorioretinal thicknesses were compared among the three groups, and the associations among photoreceptor morphological alterations, MLS, and other parameters were analyzed.

Results

In HM, the MLS, cone density and regularity, and thicknesses of the myoid and ellipsoid zone (MEZ), Henle fiber layer and outer nuclear layer, interdigitation zone and RPE/Bruch complex, and choroid were lower than in EM/LM. Decreased MLS was correlated with lower cone density and regularity, and thinner MEZ and choroid in the inner region, and with lower cone density, thinner MEZ and choroid, and longer axial length in the outer region. Multivariate regression showed that better MLS was correlated with thicker MEZ in the inner region and with higher cone density in the outer region.

Conclusions

Altered cone distribution and outer retinal thickness, especially cone density and MEZ thickness, were significantly correlated with decline of MLS in HM, which may help to evaluate and monitor visual impairment in HM.