Quantitative Comparison of Near-infrared Versus Short-wave Autofluorescence Imaging in Monitoring Progression of Retinitis Pigmentosa

Retinal Pigment Epithelium Pigment Granules: Norms, Age Relations and Pathology

The retinal pigment epithelium (RPE), which ensures the normal functioning of the neural retina, is a pigmented single-cell layer that separates the retina from the Bruch's membrane and the choroid. There are three main types of pigment granules in the RPE cells of the human eye: lipofuscin granules (LG) containing the fluorescent "age pigment" lipofuscin, melanoprotein granules (melanosomes, melanolysosomes) containing the screening pigment melanin and complex melanolipofuscin granules (MLG) containing both types of pigments simultaneously-melanin and lipofuscin. This review examines the functional role of pigment granules in the aging process and in the development of oxidative stress and associated pathologies in RPE cells. The focus is on the process of light-induced oxidative degradation of pigment granules caused by reactive oxygen species. The reasons leading to increased oxidative stress in RPE cells as a result of the oxidative degradation of pigment granules are considered. A mechanism is proposed to explain the phenomenon of age-related decline in melanin content in RPE cells. The essence of the mechanism is that when the lipofuscin part of the melanolipofuscin granule is exposed to light, reactive oxygen species are formed, which destroy the melanin part. As more melanolipofuscin granules are formed with age and the development of degenerative diseases, the melanin in pigmented epithelial cells ultimately disappears.

Nonlinear Reduction in Hyperautofluorescent Ring Area in Retinitis Pigmentosa

PURPOSE

To report baseline dimension of the autofluorescent (AF) ring in a large cohort of retinitis pigmentosa (RP) patients and to evaluate models of ring progression.

DESIGN

Cohort study.

PARTICIPANTS

Four hundred and forty-five eyes of 224 patients with clinical diagnosis of RP.

METHODS

Autofluorescent rings from near-infrared AF (NIRAF) and short-wavelength AF (SWAF) imaging modalities in RP eyes were segmented with ring area and horizontal extent extracted from each image for cross-sectional and longitudinal analyses. In longitudinal analysis, for each eye, ring area, horizontal extent, and natural logarithm of the ring area were assessed as the best dependent variable for linear regression by evaluating R2 values. Linear mixed-effects modeling was utilized to account for intereye correlation.

MAIN OUTCOME MEASURES

Autofluorescent ring size characteristics at baseline and ring progression rates.

RESULTS

A total of 439 eyes had SWAF imaging at baseline with the AF ring observed in 206 (46.9%) eyes. Mean (95% confidence interval) of ring area and horizontal extent were 7.85 (6.60 to 9.11) mm2 and 3.35 (3.10 to 3.60) mm, respectively. In NIRAF, the mean ring area and horizontal extent were 7.74 (6.60 to 8.89) mm2 and 3.26 (3.02 to 3.50) mm, respectively in 251 out of 432 eyes. Longitudinal analysis showed mean progression rates of -0.57 mm2/year and -0.12 mm/year in SWAF using area and horizontal extent as the dependent variable, respectively. When ln(Area) was analyzed as the dependent variable, mean progression was -0.07 ln(mm2)/year, which equated to 6.80% decrease in ring area per year. Similar rates were found in NIRAF (area: -0.59 mm2/year, horizontal extent: -0.12 mm/year and ln(Area): -0.08 ln(mm2)/year, equated to 7.75% decrease in area per year). Analysis of R2 showed that the dependent variable ln(Area) provided the best linear model for ring progression in both imaging modalities, especially in eyes with large overall area change.

CONCLUSIONS

Our data suggest that using an exponential model to estimate progression of the AF ring area in RP is more appropriate than the models assuming linear decrease. Hence, the progression estimates provided in this study should provide more accurate reference points in designing clinical trials in RP patients.

FINANCIAL DISCLOSURE(S)

Proprietary or commercial disclosure may be found in the Footnotes and Disclosures at the end of this article.

Lipofuscin, Its Origin, Properties, and Contribution to Retinal Fluorescence as a Potential Biomarker of Oxidative Damage to the Retina

Lipofuscin accumulates with age as intracellular fluorescent granules originating from incomplete lysosomal digestion of phagocytosed and autophagocytosed material. The purpose of this review is to provide an update on the current understanding of the role of oxidative stress and/or lysosomal dysfunction in lipofuscin accumulation and its consequences, particularly for retinal pigment epithelium (RPE). Next, the fluorescence of lipofuscin, spectral changes induced by oxidation, and its contribution to retinal fluorescence are discussed. This is followed by reviewing recent developments in fluorescence imaging of the retina and the current evidence on the prognostic value of retinal fluorescence for the progression of age-related macular degeneration (AMD), the major blinding disease affecting elderly people in developed countries. The evidence of lipofuscin oxidation in vivo and the evidence of increased oxidative damage in AMD retina ex vivo lead to the conclusion that imaging of spectral characteristics of lipofuscin fluorescence may serve as a useful biomarker of oxidative damage, which can be helpful in assessing the efficacy of potential antioxidant therapies in retinal degenerations associated with accumulation of lipofuscin and increased oxidative stress. Finally, amendments to currently used fluorescence imaging instruments are suggested to be more sensitive and specific for imaging spectral characteristics of lipofuscin fluorescence.

Primary versus Secondary Elevations in Fundus Autofluorescence

The method of quantitative fundus autofluorescence (qAF) can be used to assess the levels of bisretinoids in retinal pigment epithelium (RPE) cells so as to aid the interpretation and management of a variety of retinal conditions. In this review, we focused on seven retinal diseases to highlight the possible pathways to increased fundus autofluorescence. ABCA4- and RDH12-associated diseases benefit from known mechanisms whereby gene malfunctioning leads to elevated bisretinoid levels in RPE cells. On the other hand, peripherin2/RDS-associated disease (PRPH2/RDS), retinitis pigmentosa (RP), central serous chorioretinopathy (CSC), acute zonal occult outer retinopathy (AZOOR), and ceramide kinase like (CERKL)-associated retinal degeneration all express abnormally high fundus autofluorescence levels without a demonstrated pathophysiological pathway for bisretinoid elevation. We suggest that, while a known link from gene mutation to increased production of bisretinoids (as in ABCA4- and RDH12-associated diseases) causes primary elevation in fundus autofluorescence, a secondary autofluorescence elevation also exists, where an impairment and degeneration of photoreceptor cells by various causes leads to an increase in bisretinoid levels in RPE cells.

Measure of Visual Function

This chapter describes various methods of the assessment of visual function used for assessing disease progression and treatment response in patients with retinitis pigmentosa (RP). These methods include full-field stimulus testing (FST), near-infrared autofluorescence (NIR-AF), quantitative fundus autofluorescence (qAF), and quantitative near-infrared autofluorescence (qNIR-AF). This chapter will also outline the protocol for adaptive optics (AO) imaging of RP patients and cover how each of these methods is used for RP patients, with details including the expected findings, as evidenced by recent literature.

Fundus Photography Methodologies to Assess RP Patients

The development of fundus photography and imaging has improved our ability to diagnose and monitor inherited retinal degenerations. Nowadays, color fundus photography has become a staple in evaluating patients with retinitis pigmentosa (RP). Other important multimodal forms of fundus photography used today include red-free fundus photography, short-wavelength autofluorescence, and near-infrared autofluorescence. These photography methodologies provide valuable information on the natural history of disease progression, which in turn can lead to the identification of viable outcome measurements for current and future therapeutic trials. Further advances and developments in the field of fundus imaging will help in our understanding of RP and allied disorders.

Optical Coherence Tomography Angiography (OCTA) Findings in Retinitis Pigmentosa

Optical coherence tomography angiography (OCTA) is a noninvasive new imaging modality that can be used to diagnose and monitor progression of retinitis pigmentosa (RP). Cohorts and case series have shown correlation between OCTA findings and visual function and disease severity. Although an early use of the technology is promising, there are concerns about segmentation errors and artifacts. There is also a paucity of data on genotype and how that correlates with OCTA findings. Despite its limitations, OCTA remains a useful tool for clinicians managing retinitis pigmentosa patients.

Novel imaging techniques for hydroxychloroquine retinopathy

Hydroxychloroquine retinopathy is an increasingly recognized cause of iatrogenic, irreversible visual impairment due to the expanding use of hydroxychloroquine in combination with improvements in disease detection following advances in retinal imaging techniques. The prevalence of disease is estimated to be greater than 5% amongst individuals who have used the drug for 5 years or more. In addition to conventional imaging modalities, such as spectral-domain optical coherence tomography (OCT) and fundus autofluorescence (FAF), novel retinal imaging techniques such as en face OCT, OCT angiography, fluorescence lifetime imaging ophthalmoscopy, quantitative autofluorescence, and retromode imaging are capable of detecting structural changes in the retina. These novel retinal imaging techniques have shown promise in detecting earlier disease than is possible with current mainstream imaging modalities. Moreover, these techniques may identify disease progression as well as enabling functional correlation. In the future, these novel imaging techniques may further reduce the risk of visual loss from hydroxychloroquine retinopathy through the earlier detection of pre-clinical disease.

Association of Near-Infrared and Short-Wavelength Autofluorescence With the Retinal Sensitivity in Eyes With Resolved Central Serous Chorioretinopathy

Purpose

The purpose of this study was to compare the results of near-infrared autofluorescence (NIRAF) and short-wavelength autofluorescence (SWAF) imaging of eyes with resolved central serous chorioretinopathy (CSC) and to assess the retinal sensitivity (RS) in areas with abnormal autofluorescence (AF) using white-on-white (WW) and blue-on-yellow (BY) perimetries.

Methods

We examined 20 consecutive eyes with resolved CSC. We calculated the areas of abnormal AF detected by SWAF and NIRAF imaging as SWAF_area and NIRAF_area, respectively, and the number of measurement points within and outside abnormal SWAF and NIRAF regions were counted. The results of WW and BY perimetries were superimposed on the AF images, and the mean overall RS within and outside abnormal SWAF and NIRAF regions were calculated using both WW and BY perimetries (W-RSin_SWAF, W-RSout_SWAF, W-RSin_NIRAF, W-RSout_NIRAF, B-RSin_SWAF, B-RSout_SWAF, B-RSin_NIRAF, and B-RSout_NIRAF, respectively).

Results

The mean age of the participants was 54.1 years. The SWAF_area was significantly smaller than the NIRAF_area (P < 0.0001, Wilcoxon signed rank test). A χ2 test suggested a significant relationship between the number of measurement points within/outside abnormal SWAF and NIRAF regions (P < 0.0001). In the results of measurement by WW perimetry, there was a significant difference between W-RSin_NIRAF and W-RSout_NIRAF (P < 0.0001), but not between W-RSin_SWAF and W-RSout_SWAF (P = 0.060, Wilcoxon rank sum test). In contrast, on BY perimetry, there were significant differences between both B-RSin_SWAF and B-RSout_SWAF and between B-RSin_NIRAF and B-RSout_NIRAF (P < 0.0001).

Conclusions

NIRAF was useful for predicting impaired RS in eyes with resolved CSC.

Near-infrared fundus autofluorescence alterations correlate with swept-source optical coherence tomography angiography findings in patients with retinitis pigmentosa

Thirty-eight patients from 37 families with retinitis pigmentosa (RP) underwent macular 6 × 6-mm swept-source optical coherence tomography angiography (SS-OCTA) and 30° near-infrared fundus autofluorescence (NIR-FAF) acquisitions in one eye. Superficial vascular complex (SVC), deep capillary complex (DCC) and choriocapillaris (CC) angiograms were registered with NIR-FAF acquisitions to comparatively assess subjects with and without central area of preserved NIR-FAF (APA). On the subset of patients showing an APA, the vessel densities for SVC and DCC and flow deficits for CC were assessed in three directions (superior, inferior and temporal) from the fovea and compared to healthy 1:1 age-matched controls. Nine patients with no APA had evidence of severe central OCTA alterations at all levels, especially in the DCC. In the other 29 subjects presenting APA, all OCTA parameters were similar to healthy eyes within the APA, where the retina preserves its structural integrity. Outside the APA, both the DCC and CC were significantly reduced in all directions. These alterations are probably related to the outer retinal atrophy outside the APA. Comparing OCTA to other imaging modalities is helpful to determine the potential interest of OCTA findings as an outcome measure for disease status and progression.

Quantitative Fundus Autofluorescence in Rhesus Macaques in Aging and Age-Related Drusen

Purpose

To employ quantitative fundus autofluorescence (qAF) imaging in rhesus macaques to noninvasively assess retinal pigment epithelial (RPE) lipofuscin in nonhuman primates (NHPs) as a model of aging and age-related macular degeneration (AMD).

Methods

The qAF imaging was performed on eyes of 26 rhesus macaques (mean age 18.8 ± 8.2 years, range 4–27 years) with normal-appearing fundus or with age-related soft drusen using a confocal scanning laser ophthalmoscope with 488 nm excitation and an internal fluorescence reference. Eyes with soft drusen also underwent spectral-domain optical coherence tomography imaging to measure drusen volume and height of individual drusen lesions. The qAF levels were measured from the perifoveal annular ring (quantitative autofluorescence 8 [qAF8]) using the Delori grid, as well as focally over individual drusen lesions in this region. The association between qAF levels and age, sex, and drusen presence and volume were determined using multivariable regression analysis.

Results

Mean qAF levels increased with age (P < 0.001) and were higher in females (P = 0.047). Eyes with soft drusen exhibited reduced mean qAF compared with age-matched normal eyes (P = 0.003), with greater drusen volume showing a trend toward decreased qAF levels. However, qAF levels are focally increased over most individual drusen (P < 0.001), with larger drusen appearing more hyperautofluorescent (R² = 0.391, P < 0.001).

Conclusions

In rhesus macaques, qAF levels are increased with age and female sex, but decreased in eyes with soft drusen, similar to human AMD. However, drusen lesions appear hyperautofluorescent unlike those in humans, suggesting similarities and differences in RPE lipofuscin between humans and NHPs that may provide insight into drusen biogenesis and AMD pathogenesis.

Retinal Pigment Epithelium Atrophy in Recessive Stargardt Disease as Measured by Short-Wavelength and Near-Infrared Autofluorescence

Purpose

To compare the detection of retinal pigment epithelium (RPE) atrophy in short-wavelength (SW-AF) and near-infrared autofluorescence (NIR-AF) images in Stargardt disease (STGD1) patients.

Methods

SW-AF and NIR-AF images (115 eyes from 115 patients) were analyzed by two independent graders. Hypoautofluorescent (hypoAF) areas, indicative of RPE atrophy, were measured, and the two modalities were compared.

Results

Patients were segregated into four groups: nascent (6 [5%]), widespread (21 [18%]), discrete (55 [48%]), and chorioretinal atrophy (33 [29%]). The areas of hypoAF were larger in NIR-AF compared to SW-AF images in discrete (3.9 vs. 2.2 mm², P < 0.001) and chorioretinal atrophy (12.7 vs. 11.4 mm², P = 0.015). Similar findings were observed qualitatively in nascent and widespread atrophy patients. Using the area linear model (ALM), lesion area increased at similar rates in SW-AF and NIR-AF images of discrete atrophy (0.20 vs. 0.32 mm²/y, P = 0.275) and chorioretinal atrophy (1.30 vs. 1.74 mm²/y, P = 0.671). Using the radius linear model (RLM), the lesion effective radius also increased similarly in SW-AF and NIR-AF images in discrete (0.03 vs. 0.05 mm²/y, P = 0.221) and chorioretinal atrophy (0.08 vs. 0.10 mm²/y, P = 0.754) patients.

Conclusions

NIR-AF reveals a larger area of RPE atrophy in STGD1 patients compared to SW-AF images, but rates of lesion enlargement in the two modalities are similar.

Translational Relevance

Measurements of RPE atrophy by AF imaging are crucial for monitoring STGD1 disease progression and given our findings we advocate greater use of NIR-AF for patients.

Spectral-Domain Optical Coherence Tomography Is More Sensitive for Hydroxychloroquine-Related Structural Abnormalities Than Short-Wavelength and Near-Infrared Autofluorescence

Purpose

To analyze the appearance of structural abnormalities due to hydroxychloroquine (HCQ) toxicity by spectral-domain optical coherence tomography (SD-OCT) and short-wavelength autofluorescence (SW-AF) and near-infrared fundus autofluorescence (NIR-AF) imaging.

Methods

This retrospective cohort study included 88 eyes from 44 patients who had a history of or were currently taking HCQ. SD-OCT, SW-AF, and NIR-AF images were analyzed by two independent graders for the detection of HCQ-associated abnormalities.

Results

Sixty eyes (30 patients, 68%) presented with no abnormalities for either imaging modality. Twenty eyes (10 patients, 23%) presented with parafoveal abnormalities (ellipsoid zone attenuation and/or interdigitation zone continuity loss) in SD-OCT scans but with qualitatively normal SW-AF and NIR-AF images. Eight eyes (four patients, 9%) presented with bull's-eye maculopathy in SW-AF and NIR-AF images, with corresponding outer retinal structures disrupted parafoveally in SD-OCT scans ("flying saucer" sign). No patients presented with normal SD-OCT scans and concurrent abnormalities in SW-AF or NIR-AF images.

Conclusions

SD-OCT was more sensitive in detecting structural abnormalities than either SW-AF or NIR-AF imaging, suggesting its superiority as a screening imaging modality for HCQ toxicity. Maculopathy and abnormalities in the retinal pigment epithelium from HCQ toxicity can be appreciated in both SW-AF and NIR-AF images.

Translational Relevance

Although debate exists regarding the best imaging modalities for screening patients for potential HCQ toxicity, our study supports the use of SD-OCT over both SW-AF and NIR-AF imaging as a screening modality.

Fundus autofluorescence imaging

Fundus autofluorescence (FAF) imaging is an in vivo imaging method that allows for topographic mapping of naturally or pathologically occurring intrinsic fluorophores of the ocular fundus. The dominant sources are fluorophores accumulating as lipofuscin in lysosomal storage bodies in postmitotic retinal pigment epithelium cells as well as other fluorophores that may occur with disease in the outer retina and subretinal space. Photopigments of the photoreceptor outer segments as well as macular pigment and melanin at the fovea and parafovea may act as filters of the excitation light. FAF imaging has been shown to be useful with regard to understanding of pathophysiological mechanisms, diagnostics, phenotype-genotype correlation, identification of prognostic markers for disease progression, and novel outcome parameters to assess efficacy of interventional strategies in chorio-retinal diseases. More recently, the spectrum of FAF imaging has been expanded with increasing use of green in addition to blue FAF, introduction of spectrally-resolved FAF, near-infrared FAF, quantitative FAF imaging and fluorescence life time imaging (FLIO). This article gives an overview of basic principles, FAF findings in various retinal diseases and an update on recent developments.

Disease asymmetry and hyperautofluorescent ring shape in retinitis pigmentosa patients

Retinitis pigmentosa (RP) is described as a bilateral disease with inter-eye symmetry that presents on short-wavelength fundus autofluorescence (SW-AF) imaging with hyperautofluorescent (hyperAF) rings with an ellipsoid shape and regular borders. Nevertheless, both asymmetry and irregular ring morphologies are also observed. In this retrospective study of 168 RP patients, we characterize the degree of inter-eye asymmetry and frequency of irregular hyperAF ring morphologies according to mode of inheritance and disease-causing gene by using SW-AF imaging and spectral-domain optical coherence tomography (SD-OCT) scans. We observed that from 336 eyes, 290 (86%) presented with regular hyperAF rings and 46 (14%) presented with irregular shapes. From the 168 patients, 23 (14%) presented with asymmetric disease, with 16 (70%) of these patients also presenting with irregular ring shapes. Patients with autosomal dominant RP (adRP) had the highest proportion of irregular ring shapes (21%) and disease asymmetry (23%) in comparison to other modes of inheritance. Furthermore, both RP1 and RHO-adRP had the highest proportions of both disease asymmetry and irregular ring morphology. Our results suggest that in patients presenting with either irregular ring shapes or asymmetric disease, emphasis should be placed in targeted gene sequencing of genes known to cause adRP, such as RHO and RP1.

Near-infrared autofluorescence in young choroideremia patients

Purpose: To study near-infrared autofluorescence (NIR-AF) and short- wave autofluorescence (SW-AF) imaging modalities in young patients affected with choroideremia (CHM).Methods: NIR-AF and SW-AF images, Optical coherence tomography (OCT) and color fundus images were acquired from 3 young CHM patients (6 eyes) enrolled at the Regional Reference Center for Hereditary Retinal Degenerations of the Eye Clinic in Florence.Results: We studied 3 young CHM patients (6 eyes). The mean age of the patients was 17,3 years. Using NIR-AF, patient P1 was characterized by speckled hypo-autofluorescent areas at the posterior pole with a preserved central hyper-autofluorescence while patient P2 and P3 were characterized by a preserved NIR-AF signal only at the fovea. Using SW-AF, patient P1 was characterized by a normal macular autofluorescence and by a speckled FAF pattern involved the vascular arcades while patient P2 and P3 showed well-demarcated hypo-autofluorescence areas involving the posterior pole with a preserved macular autofluorescence. The differences between NIR-AF and SW-AF were more pronounced in advanced stages. In correspondence of preserved NIR-AF, the OCT examination showed regular and continuous outer retinal hyperreflective bands. We observed abnormal RPE/Bruch's membrane complex and EZ band externally to the NIR-AF signal area.Conclusions: NIR-AF imaging confirms an early RPE involvement allowing us to identify and to quantify the RPE pigment loss in choroideremia. For this reason, NIR-AF imaging can be useful for monitoring the progression of the disease and to study the effect of future treatments.

Lessons learned from quantitative fundus autofluorescence

Quantitative fundus autofluorescence (qAF) is an approach that is built on a confocal scanning laser platform and used to measure the intensity of the inherent autofluorescence of retina elicited by short-wavelength (488 nm) excitation. Being non-invasive, qAF does not interrupt tissue architecture, thus allowing for structural correlations. The spectral features, cellular origin and topographic distribution of the natural autofluorescence of the fundus indicate that it is emitted from retinaldehyde-adducts that form in photoreceptor cells and accumulate, under most conditions, in retinal pigment epithelial cells. The distributions and intensities of fundus autofluorescence deviate from normal in many retinal disorders and it is widely recognized that these changing patterns can aid in the diagnosis and monitoring of retinal disease. The standardized protocol employed by qAF involves the normalization of fundus grey levels to a fluorescent reference installed in the imaging instrument. Together with corrections for magnification and anterior media absorption, this approach facilitates comparisons with serial images and images acquired within groups of patients. Here we provide a comprehensive summary of the principles and practice of qAF and we highlight recent efforts to elucidate retinal disease processes by combining qAF with multi-modal imaging.

Multimodal structural disease progression of retinitis pigmentosa according to mode of inheritance

We analyze disease progression in retinitis pigmentosa (RP) according to mode of inheritance by quantifying the progressive decrease of the ellipsoid zone (EZ) line width on spectral domain optical coherence tomography (SD-OCT) and of the dimensions of the hyperautofluorescent ring on short-wave fundus autofluorescence (SW-FAF). In this retrospective study of 96 patients, average follow-up time was 3.2 ± 1.9 years. EZ line width declined at a rate of −123 ± 8 µm per year, while the horizontal diameter and ring area declined at rates of −131 ± 9 µm and −0.5 ± 0.05 mm² per year, respectively. Disease progression was found to be slowest for autosomal dominant RP and fastest for X-linked RP, with autosomal recessive RP progression rates between those of adRP and XLRP. EZ line width and ring diameter rates of disease progression were significantly different between each mode of inheritance. By using EZ line width and horizontal diameter as parameters of disease progression, our results confirm that adRP is the slowest progressing form of RP while XLRP is the fastest. Furthermore, the reported rates can serve as benchmarks for investigators of future clinical trials for RP and its different modes of inheritance.