Retinal Oximetry and Vessel Diameter Measurements With a Commercially Available Scanning Laser Ophthalmoscope in Diabetic Retinopathy

The retinal oxygen metabolism and hemodynamics as a substitute for biochemical tests to predict nonproliferative diabetic retinopathy

Predicting the occurrence of nonproliferative diabetic retinopathy (NPDR) using biochemical parameters is invasive, which limits large-scale clinical application. Noninvasive retinal oxygen metabolism and hemodynamics of 215 eyes from 73 age-matched healthy subjects, 90 diabetic patients without DR, 40 NPDR, and 12 DR with postpanretinal photocoagulation were measured with a custom-built multimodal retinal imaging device. Diabetic patients underwent biochemical examinations. Two logistic regression models were developed to predict NPDR using retinal and biochemical metrics, respectively. The predictive model 1 using retinal metrics incorporated male gender, insulin treatment condition, diastolic duration, resistance index, and oxygen extraction fraction presented a similar predictive power with model 2 using biochemical metrics incorporated diabetic duration, diastolic blood pressure, and glycated hemoglobin A1c (area under curve: 0.73 vs. 0.70; sensitivity: 76% vs. 68%; specificity: 64% vs. 62%). These results suggest that retinal oxygen metabolic and hemodynamic biomarkers may replace biochemical parameters to predict the occurrence of NPDR .

Functional OCT reveals anisotropic changes of retinal flicker-evoked vasodilation

The purpose of this study is to verify the effect of anisotropic property of retinal biomechanics on vasodilation measurement. A custom-built optical coherence tomography (OCT) was used for time-lapse imaging of flicker stimulation-evoked vessel lumen changes in mouse retinas. A comparative analysis revealed significantly larger (18.21%) lumen dilation in the axial direction compared to the lateral (10.77%) direction. The axial lumen dilation predominantly resulted from the top vessel wall movement toward the vitreous direction, whereas the bottom vessel wall remained stable. This observation indicates that the traditional vasodilation measurement in the lateral direction may result in an underestimated value.

Association between Fundus Tessellation and Contrast Sensitivity in Myopic Eyes

PURPOSE

To assess the association of fundus tessellation with contrast sensitivity, Quality of Vision questionnaire, and other factors at five years postcorneal refractive surgery.

METHODS

This is a cross-sectional study. Both eyes of 98 subjects (196 eyes) who received femtosecond laser in situ keratomileusis (FS-LASIK) or small incision lenticular extraction (SMILE) five years prior were enrolled in this study. Fundus tessellation was imaged using wide-angle fundus photographs and graded into four categories with the assistance of the ETDRS grid. Photopic and mesopic contrast sensitivity were measured under the best correction. The Quality of Vision (QoV) questionnaire was used to assess visual symptoms.

RESULTS

Fundus tessellation was classified as follows: 19 eyes were grade 0 (9.7%), 28 eyes were grade 1 (14.3%), 59 eyes were grade 2 (30.1%), and 90 eyes were grade 3 (45.9%). Higher degrees of fundus tessellation were associated with lower photopic contrast sensitivity, a significant difference was observed at spatial frequencies of 6cpd (p = 0.030, grade 1 >grade 3 p = 0.011). Higher degrees of fundus tessellation were also associated with lower mesopic contrast sensitivity, a significant difference was observed at spatial frequencies of 18cpd (p = 0.011, grade 0 >grade 3 p = 0.012). The preoperative degree of myopia was positively associated with fundus tessellation grade (p < 0.001). However, in linear mixed-effect model analysis, no significant influence of parameters (contrast sensitivity, preoperative myopia, and QoV scores) upon different tessellation grades was found (p > 0.05).

CONCLUSIONS

Patients with moderate and high myopia were more likely to have higher grades of fundus tessellation. Higher degree of fundus tessellation associates with lower contrast sensitivity. Patients with moderate and high myopia should be concerned with retinal-choroidal changes. Contrast sensitivity could be a clinical sign for progression of tessellation and used to screen for early retinal-choroidal changes to prevent pathologic myopia.

OCT-Angiography of deep and superficial retinal vascular density changes in diabetes without diabetic retinopathy

PURPOSE

To analyze the vascular density values (VD) in the superficial and the deep retinal network and in all perimacular sectors of diabetic eyes without clinical retinopathy, comparing them to that of healthy, nondiabetic control eyes. We investigated the factors that may influence these values.

METHODS

This was a cross-sectional study including diabetics without diabetic retinopathy and healthy control subjects. All subjects underwent OCTA examination (RTVue-XR Avanti; Optovue, Fremont, CA, USA). The scanned area was 3×3mm and centered on the fovea. In the superficial and deep capillary plexus, we evaluated the total VD, parafoveal, temporal, nasal, superior and inferior sectors, as well as the superior and inferior hemi-sector VD. The sectors with the greatest area under the curve (AUC) were determined. We evaluated the correlation between certain risk factors and VD values.

RESULTS

VD values in all sectors of the superficial capillary plexus layer and deep capillary plexus layer in the diabetic group were significantly lower than in the control group. Among all the parameters, the superficial total VD and superficial parafoveal VD had the greatest AUC (0.992 and 0.991 respectively). The sector with the greatest AUC was the temporal sector in both the SCP (0.990) and DCP (0.976). Age, creatinine clearance and hyperlipemia correlated with vascular density.

CONCLUSIONS

Superficial and deep retinal VD are both decreased in diabetic patients without diabetic retinopathy. Our results suggest that OCTA might be a promising tool for diabetic retinopathy screening. Quantitative microvascular changes might precede clinical damage.

Retinal Vessel Caliber Measurement Bias in Fundus Images in the Presence of the Central Light Reflex

Purpose

To investigate the agreement between a fundus camera and a scanning laser ophthalmoscope in retinal vessel caliber measurements and to identify whether the presence of the central light reflex (CLR) explains potential discrepancies.

Methods

For this cross-sectional study, we obtained fundus camera and scanning laser ophthalmoscope images from 85 eyes of 85 healthy individuals (aged 50-65 years) with different blood pressure status. We measured the central retinal artery equivalent (CRAE) and central retinal artery vein equivalent (CRVE) with the Knudtson-Parr-Hubbard algorithm and assessed the CLR using a semiautomatic grading method. We used Bland-Altman plots, 95% limits of agreement, and the two-way mixed effects intraclass correlation coefficient for consistency [ICC(3,1)] to describe interdevice agreement. We used multivariable regression to identify factors associated with differences in between-device measurements.

Results

The between-device difference in CRAE (9.5  µm; 95% confidence interval, 8.0-11.1  µm) was larger than the between-device difference in CRVE (2.9  µm; 95% confidence interval, 1.3-4.5  µm), with the fundus camera yielding higher measurements (both P < 0.001). The 95% fundus camera-scanning laser ophthalmoscope limits of agreement were -4.8 to 23.9  µm for CRAE and -12.0 to 17.8  µm for CRVE. The corresponding ICCs(3,1) were 0.89 (95% confidence interval, 0.83-0.92) and 0.91 (95% confidence interval, 0.86-0.94). The between-device CRAE difference was positively associated with the presence of a CLR (P = 0.002).

Conclusions

Fundus cameras and scanning laser ophthalmoscopes yield correlated but not interchangeable caliber measurements. The CLR induces bias in arteriolar caliber in fundus camera images, compared with scanning laser ophthalmoscope images.

Translational Relevance

Refined measurements could yield better estimates of the association between retinal vessel caliber and ophthalmic or systemic disease.

Ophthalmic imaging in diabetic retinopathy: A review

Retinal imaging has been a key tool in the diagnosis, evaluation, management and documentation of diabetic retinopathy (DR) and diabetic macular oedema (DMO) for many decades. Imaging technologies have rapidly evolved over the last few decades, yielding images with higher resolution and contrast with less time, effort and invasiveness. While many retinal imaging technologies provide detailed insight into retinal structure such as colour reflectance photography and optical coherence tomography (OCT), others such as fluorescein or OCT angiography and oximetry provide dynamic and functional information. Many other novel imaging technologies are in development and are poised to further enhance our evaluation of patients with DR.

Alterations in Retinal Vascular and Oxygen Metrics in Treated and Untreated Proliferative Diabetic Retinopathy: A Case Report

Proliferative diabetic retinopathy (PDR) is a vision-threatening complication of diabetes. Panretinal photocoagulation (PRP) and anti-vascular endothelial growth factor (anti-VEGF) are approved treatment modalities aimed at regressing neovascularization. Data are lacking about abnormalities in retinal vascular and oxygen metrics before and after combination treatments. A 32-year-old Caucasian male diagnosed with PDR in the right eye was treated by a combination of PRP and multiple anti-VEGF treatments over a 12-month period. The subject underwent optical coherence tomography (OCT) angiography, Doppler OCT, and retinal oximetry before treatment and at 12 months, which was 6 months following the last treatment. Measurements of vascular metrics (vessel density [VD] and mean arterial and venous diameters [DA, DV]) and oxygen metrics (total retinal blood flow [TRBF], inner retinal oxygen delivery [DO2], metabolism [MO2], and extraction fraction [OEF]) were obtained. Both before and after treatments, VD, TRBF, MO2, and DO2 were below the normal lower confidence limits. Additionally, DV and OEF were decreased after treatments. Alterations in retinal vascular and oxygen metrics were reported for the first time in untreated and treated PDR. Future studies are warranted to evaluate the clinical value of these metrics in PDR.

Incidence, progression and regression of diabetic retinopathy in a northeastern Chinese population

AIM

To determine the incidence, progression and regression of diabetic retinopathy (DR), with corresponding risk factors, in a northeastern Chinese population of patients with type 2 diabetes.

METHODS

Among 2006 patients who completed baseline examinations in 2012-2013 and underwent re-examination after a mean interval of 21.2 months, 1392 patients with gradable fundus photographs for both baseline and follow-up examinations were included. Incidence was defined as new development of any DR among patients without DR at baseline. An increase of ≥2 scales (concatenating Early Treatment Diabetic Retinopathy Study levels of both eyes) in eyes with DR at baseline was defined as progression, while a reduction of ≥2 scales was defined as regression.

RESULTS

The age- and sex-standardised incidence, progression and regression were 5.8% (95% CI 4.7% to 6.9%), 26.8% (95% CI 24.8% to 28.8%) and 10.0% (95% CI 8.6% to 11.3%), respectively. In addition to poor blood glucose control, wider central retinal venular equivalent was associated with both incidence (relative risk (RR) 2.17, 95% CI 1.09 to 4.32, for ≥250 µm vs <210 µm) and progression (RR 2.00, 95% CI 1.02 to 3.96, for ≥250 µm vs <210 µm). Patients without insulin therapy (RR 0.64, 95% CI 0.43 to 0.97) and patients with wider central retinal arteriolar equivalent (RR 1.14, 95% CI 1.02 to 1.26, per 10 µm increase) were likely to exhibit DR regression.

CONCLUSION

We determined the incidence, progression and regression of DR among northeastern Chinese patients with type 2 diabetes. Retinal vessel diameters, in addition to blood glucose level, influence the natural evolution of DR.

Retinal Oxygen Delivery and Metabolism Response to Hyperoxia During Bilateral Common Carotid Artery Occlusion in Rats

Purpose

The purpose of the current study was to test the hypothesis that responses of total retinal blood flow (TRBF), inner retinal oxygen delivery (DO₂), metabolism (MO₂), and extraction fraction (OEF) to hyperoxia are higher after minutes of bilateral common carotid artery occlusion (BCCAO) as compared to days of BCCAO.

Methods

Twenty-eight rats were subjected to BCCAO for 30 minutes (n = 12), 1 day (n = 8), or 3 days (n = 8). Eight of the 12 rats were also evaluated at baseline, prior to BCCAO. During room air breathing (RA) and 100% O₂ inspiration (hyperoxia), blood flow and phosphorescence lifetime imaging were performed to measure TRBF and vascular O₂ contents, respectively. DO₂, MO₂, and OEF were calculated from these measurements.

Results

After 30 minutes or 3 days of BCCAO, TRBF did not differ between RA and hyperoxia conditions (P ≥ 0.14) but decreased under hyperoxia after 1 day (P = 0.01). Compared to RA, DO₂ and MO₂ were increased under hyperoxia after 30 minutes of BCCAO (P ≤ 0.02). Additionally, MO₂ was decreased under hyperoxia after 1 day of BCCAO (P = 0.04). OEF was decreased under hyperoxia compared to RA (P < 0.001). Under hyperoxia, TRBF and DO₂ were reduced after all BCCAO durations compared to baseline (P ≤ 0.04), whereas MO₂ did not differ from baseline after 30 minutes of BCCAO (P = 1.00).

Conclusions

The findings indicate that hyperoxia introduced minutes after ischemia can reduce DO₂ impairments and potentially return MO₂ to approximately normal values. This information contributes to the knowledge of the effect of supplemental oxygen intervention on TRBF, DO₂, MO₂, and OEF outcomes after variable durations of ischemia.

Retinal Oxygen Delivery and Extraction in Ophthalmologically Healthy Subjects With Different Blood Pressure Status

Purpose

To compare retinal oxygen delivery (DO2) and oxygen extraction (VO2) in ophthalmologically healthy subjects with different blood pressure (BP) status.

Methods

In this case-control study, we prospectively included 93 eyes of 93 subjects (aged 50-65 years) from a Dutch cohort (n = 167,000) and allocated them to four groups (low BP, normal BP [controls], treated arterial hypertension [AHT], untreated AHT). We estimated vascular calibers from fundus images and fractal dimension from optical coherence tomography angiography scans. We combined calibers, fractal dimension, BP, and intraocular pressure measurements in a proxy of retinal blood flow (RBF), using a Poiseuille-based model. We measured arterial and venous oxygen saturations (SaO2, SvO2) with a scanning laser ophthalmoscope. We calculated the DO2 and VO2 from the RBF, SaO2, and SvO2. We compared the DO2 and VO2 between groups and investigated the DO2-VO2 association.

Results

DO2 and VO2 were different between groups (P = 0.009, P = 0.036, respectively). In a post hoc analysis, the low BP group had lower DO2 than the untreated AHT group (P = 4.9 × 10-4). The low BP group and the treated AHT group had a lower VO2 than the untreated AHT group (P = 0.021 and P = 0.034, respectively). There was a significant DO2-VO2 correlation (Robs = 0.65, bobs = 0.51, P = 2.4 × 10-12). After correcting for shared measurement error, the slope was not significant.

Conclusions

The DO2 and VO2 were altered in ophthalmologically healthy subjects with different BP status. Future studies could elucidate whether these changes can explain the increased risk of ophthalmic pathologies in those subjects.

Translational Relevance

Understanding the baseline interplay between BP, retinal perfusion, and oxygenation allows for improved evaluation of retinal disease manifestation.

Retinal vascular oxygen saturation in response to a less extensive laser treatment in proliferative diabetic retinopathy

PURPOSE

The purpose of this study was to evaluate the association between retinal laser burden and vascular oxygen saturation in patients with proliferative diabetic retinopathy (PDR) treated with different extent of retinal laser.

METHODS

The study was a prospective, interventional study of patients with treatment-naïve PDR. Patients were treated with navigated retinal laser (Navilas^® , OD-OS GmbH, Teltow, Germany) in different doses. Retinal oximetry was obtained at baseline (BL) prior to laser and after 6 months (M6). Patients were divided into three groups according to total laser spots applied: <1500 spots (Group 1), 1500-2000 spots (Group 2), and >2000 spots (Group 3).

RESULTS

We included 33 eyes of 28 patients with treatment-naïve PDR. The groups did not differ according in BL characteristics. Between BL and M6, retinal arteriolar oxygen saturation did not change but retinal venular oxygen saturation (median with interquartile range) decreased in Groups 1 and 2 (1: 65.5 ± 8.8% versus 60.5 ± 9.5%, p = 0.04; 2: 65.3 ± 7.3% versus 63.0 ± 13.5%, p = 0.04). Focal retinal venular oxygen saturation, located to quadrants with retinal neovascularization, decreased in Group 2 from BL to M6 (67.5 ± 13.3% versus 61.5 ± 8.8%, p = 0.04). Retinal venular diameter decreased from BL to M6 in Group 1 (174.5 ± 15.3 μm versus 165.1 ± 28.7 μm, p = 0.01).

CONCLUSIONS

In this study of patients with treatment-naïve PDR, we showed that a less extensive laser treatment caused a reduction in retinal venular oxygen saturation and diameter 6 months after treatment. Our results suggest that less extensive laser treatment may be sufficient to improve the retinal metabolic environment conducive to PDR regression.

Impacts of Systemic Hypertension on the Macular Microvasculature in Diabetic Patients Without Clinical Diabetic Retinopathy

Purpose

To identify the impact of hypertension (HTN) on macular microvasculature in type 2 diabetes (T2DM) patients without clinical diabetic retinopathy.

Methods

In this retrospective cross-sectional study, subjects were divided into three groups: controls (control group), patients with T2DM (DM group), and patients with both T2DM and HTN (DM + HTN group). The vessel length density (VD) was compared among the groups. Linear regression analyses were performed to identify factors associated with VD.

Results

The VD in the control, DM, and DM + HTN groups was 20.43 ± 1.16, 19.50 ± 1.45, and 18.19 ± 2.06 mm⁻¹, respectively (P < 0.001). The best-corrected visual acuity (B = −9.30; P = 0.002), duration of T2DM (B = −0.04; P = 0.020), HTN (B = −0.51; P = 0.016), signal strength (B = 1.12; P < 0.001), and ganglion cell–inner plexiform layer thickness (B = 0.06; P < 0.001) were significant factors affecting VD in patients with T2DM. Additionally, the hemoglobin A1c (HbA1c) (B = −0.49; P = 0.016) was significantly associated with VD in patients with both T2DM and HTN.

Conclusions

Patients with T2DM had impaired macular microvasculature, and patients with T2DM with HTN exhibited greater impairment of the microvasculature than did patients with T2DM only. Additionally, physicians should be aware that the macular microvasculature would be more vulnerable to hyperglycemic damage under ischemic conditions by HTN.

Relating Retinal Vascular Oxygen Saturation and Microvasculature Morphology at Progressive Stages of Diabetic Retinopathy

Purpose

Diabetic retinopathy (DR) is a common cause of vision loss in working age adults and presents changes in retinal vessel oxygenation and morphology. The purpose of this study was to test the hypothesis that there is an association of retinal vessel oxygen saturation with vessel density (VD) and tortuosity in DR.

Methods

Ninety-five subjects were classified in the following groups: nondiabetic control (N = 25), no DR (N = 28), mild nonproliferative DR (NPDR; N = 21), moderate to severe NPDR (N = 14), or treated proliferative DR (PDR; N = 7). Retinal oximetry was performed to measure arterial and venous oxygen saturation (SO_2A and SO_2V) and calculate oxygen extraction fraction (OEF). Optical coherence tomography angiography (OCTA) was performed for measurements of VD and vessel tortuosity index (VTI).

Results

There were statistically significant differences in SO_2A and SO_2V among groups (P< 0.004). SO_2A and SO_2V were higher in the PDR group compared to the control group and SO_2V was also higher in the moderate to severe NPDR group. VD differed significantly among groups (P = 0.003), whereas VTI was not significantly different (P = 0.22). Compared to the control group, VD was lower in moderate to severe NPDR and PDR groups. VD was also lower in the PDR group than that in the no DR group (P = 0.03). There was a significant correlation of VTI with SO_2V (r = 0.32, P = 0.002) and OEF (r = −0.35, P = 0.001).

Conclusions

Retinal vessel morphology, oxygenation, and tissue oxygen extraction were associated with each other in a cohort of subjects with and without DR.

Translational Relevance

The findings of this study have the potential to improve clinical management of DR by providing better understanding of human disease pathophysiology and propelling future studies to identify multiple image-based biomarkers for improved disease diagnosis and monitoring.

Advances in Retinal Oximetry

Similar to other organs, the retina relies on tightly regulated perfusion and oxygenation. Previous studies have demonstrated that retinal blood flow is affected in a variety of eye and systemic diseases, including diabetic retinopathy, age-related macular degeneration, and glaucoma. Although measurement of peripheral oxygen saturation has become a standard clinical measurement through the development of pulse oximetry, developing a noninvasive technique to measure retinal oxygen saturation has proven challenging, and retinal oximetry technology currently remains inadequate for reliable clinical use. Here, we review current strategies and approaches, as well as several newer technologies in development, and discuss the future of retinal oximetry.

Effect of ECG-gating Retinal Photographs on Retinal Vessel Caliber Measurements in Subjects with and without Type 2 Diabetes

Purpose/Aim of this study: Retinal vessel caliber is an independent risk marker of cardiovascular disease risk. However, variable mechanical delays in capturing retinal photographs and cardiac cycle-induced retinal vascular changes have been shown to reduce the accuracy of retinal vessel caliber measurements, but this has only ever been investigated in healthy subjects. This cross-sectional study is the first study to investigate this issue in type 2 diabetes. The aim of this study was to determine whether ECG-gating retinal photographs reduce the variability in retinal arteriolar and venular caliber measurements in controls and type 2 diabetes.Materials and Methods: Fifteen controls and 15 patients with type 2 diabetes were arbitrarily recruited from Westmead Hospital, Sydney, Australia. A mydriatic fundoscope connected to our novel ECG synchronization unit captured 10 ECG-gated (at the QRS) and 10 ungated digital retinal photographs of the left eye in a randomized fashion, blinded to study participants. Two independent reviewers used an in-house semi-automated software to grade single cross-sectional vessel diameters across photographs, between 900 and 1800 microns from the optic disc edge. The coefficient of variation compared caliber variability between retinal arterioles and venules.Results: Our ECG synchronization unit reported the smallest time delay (33.1 ± 48.4 ms) in image capture known in the literature. All 30 participants demonstrated a higher reduction in retinal arteriolar (ungated: 1.02, 95%CI 0.88-1.17% vs ECG-gated: 0.39, 95%CI 0.29-0.49%, p < .0001) than venular (ungated 0.62, 95%CI 0.53-0.73% vs ECG-gated: 0.26, 95%CI 0.19-0.35%, p < .0001) coefficient of variation by ECG-gating photographs. Intra-observer repeatability and inter-observer reproducibility analysis reported high interclass correlation coefficients ranging from 0.80 to 0.86 and 0.80 to 0.93 respectively.Conclusion: ECG-gating photographs at the QRS are recommended for retinal vessel caliber analysis in controls and patients with type 2 diabetes as they refine measurements.

Retinal oxygen saturation changes progressively over time in diabetic retinopathy

PURPOSE

According to cross-sectional studies, oxygen saturation is elevated in retinal vessels in diabetic patients. We evaluated how retinal oxygenation (metabolic marker), vessel diameters and retinopathy grade (structural markers) change over time in diabetic patients.

DESIGN

Prospective cohort study following screening in a hospital setting.

METHODS

Retinal oximetry images were acquired in 214 patients with the Oxymap T1 oximeter. Imaging was repeated after a median of 3.0 years (range 0.76-6.8 years). Oxygen saturation and vessel diameters were measured in the right eye. Semiquantitative grading of retinopathy according to international guidelines and red lesion count were performed on fundus photographs.

RESULTS

Retinopathy grade according to the international semiquantitative grading system was unchanged. Arteriolar saturation increased by 0.75±0.15 percentage points per year of follow-up (p<0.0001). Venular saturation increased by 1.74±0.26 percentage points per year (p<0.0001) and arteriovenous difference decreased by 0.99±0.20 percentage points per year (p<0.0001). Arteriolar diameters decreased by 2.7±8.5μm (p<0.0001) between visits and venular diameters decreased by 2.4±9.1μm (p = 0.0002). Median increase in red lesion count between visits was 2 lesions (range -128 to 212 lesions, p<0.0001). The change in red lesion count and change in diameters did not correlate with the length of follow-up (p>0.44).

CONCLUSIONS

Oxygen saturation in larger retinal vessels can increase and arteriovenous difference can decrease over time in diabetic patients without any observable changes in retinopathy grade. The results suggest that changes in retinal oxygen saturation may precede progression of diabetic retinopathy or that oxygen saturation is more sensitive to disease progression than retinopathy grade.

Densitometric Profiles of Optic Disc Hemorrhages in the Ocular Hypertension Treatment Study

PURPOSE

The origin of blood in glaucoma-related disc hemorrhages (DH) remains unknown. A prior clinic-based study of primary open-angle glaucoma (POAG)-related DH showed that they had grayscale pixel intensities more similar to blood from retinal macroaneurysms and adjacent retinal arterioles than to blood from retinal vein occlusions or adjacent retinal venules, suggesting an arterial source. Here we assessed the densitometric profile of DH from fundus photographs in the Ocular Hypertension Treatment Study (OHTS).

DESIGN

Retrospective cross-sectional study of prospectively collected images.

METHODS

Stereo disc photographs of 161 DH events from 83 OHTS participants (mean age [standard deviation (SD)]: 65.6 [9.2] years; 46.6% female; 13.0% black race) were imported into ImageJ to measure densitometry differences (adjacent arterioles minus DH [ΔA] or venules minus DH [ΔV]). Their size as percentage of disc area, ratio of length to midpoint width, and location relative to the disc margin were also analyzed. We performed t tests to compare ΔA and ΔV, analysis of variance to compare ΔA and ΔV across DH recurrent events, and multivariable linear regression to identify determinants of ΔA and ΔV.

RESULTS

Mean (SD) ΔA and ΔV were -2.2 (8.7) and -11.4 (9.7) pixel intensity units, respectively (P < .001). ΔA and ΔV each did not differ significantly across recurrence of DH (P ≥ .92) or between DH events with and without POAG (P ≥ .26).

CONCLUSIONS

OHTS DH had densitometric measurements more similar in magnitude to adjacent arterioles than venules, supporting an arterial origin for DH. Vascular dysregulation may contribute to disc hemorrhage formation in ocular hypertension.

Assessment of retinal vascular oxygenation and morphology at stages of diabetic retinopathy in African Americans

BACKGROUND

Diabetic retinopathy (DR) is a microvascular complication of diabetes and a leading cause of blindness in working-age adults. The likelihood of visual impairment associated with DR is two-fold higher in the African-American (AA) compared to non-Hispanic white. Although alterations in retinal vessel oxygenation and morphology have been reported in DR, there is limited knowledge about these vascular changes in AA subjects. The purpose of the current study was to investigate alterations in retinal vascular oxygen saturation (SO2), vessel diameter (D) and tortuosity at severity stages of DR in AA subjects.

METHODS

A nested case-control study of 56 AA subjects was conducted. Right eyes were grouped as non-diabetic (ND) (N = 26), no clinical DR (NDR) (N = 19), or moderate/severe non-proliferative DR (NPDR) (N = 11). Imaging was performed using a commercially available scanning laser ophthalmoscope. Images were analyzed to determine retinal arterial and venous SO2 (SO2A and SO2V), diameter (DA and DV), and vessel tortuosity index (VTI) (VTIA and VTIV).

RESULTS

SO2V and DV were higher in NPDR compared to ND and NDR groups (P < 0.05). There were no significant differences in SO2A and DA among ND, NDR, and NPDR groups (P > 0.8). Maximum VTIA was higher in diabetics (NDR and NPDR) compared to non-diabetics (P < 0.03). There was no significant difference in maximum VTIV among the 3 groups (P = 0.5).

CONCLUSIONS

The findings advance our understanding of DR pathophysiology in the AA population and may propel identification of race-specific retinal vascular biomarkers for improved diagnosis and monitoring of DR.

Effects of Prolonged Type 2 Diabetes on the Inner Retinal Layer and Macular Microvasculature: An Optical Coherence Tomography Angiography Study

Purpose: To identify the effects of prolonged type 2 diabetes (T2DM) on macular microcirculation and the inner retinal layer in diabetic eyes without clinical diabetic retinopathy (DR). Methods: 97, 92, and 57 eyes in the control, patients with T2DM < 10 years (DM group one), and patients with T2DM ≥ 10 years (DM group two) were enrolled. The ganglion cell-inner plexiform layer (GC-IPL) thickness and superficial vessel density (VD) were compared. Linear regression analyses were performed to identify factors associated with VD in T2DM patients. Results: GC-IPL thicknesses in the control, DM group one, and DM group two were 84.58 ± 0.89, 83.49 ± 0.70, and 79.04 ± 0.96 μm, respectively (p < 0.001). The VDs of the full area were 20.32 ± 0.15, 19.46 ± 0.17, and 18.46 ± 0.23 mm⁻¹ (p < 0.001). Post-hoc analyses revealed that the VDs of the full area was significantly different in the control vs. DM group one (p = 0.001), control vs. DM group two (p < 0.001), and DM group one vs. DM group two (p = 0.001). Multivariate linear regression analyses revealed that DM duration (p = 0.037), visual acuity (p = 0.013), and GC-IPL thickness (p < 0.001) were significantly associated with the VD of T2DM patients. Conclusions: We confirmed GC-IPL thinning and decreased superficial VD in the macular areas using OCTA in T2DM patients. Patients with T2DM ≥ 10 years exhibited significantly more severe macular microcirculation impairment compared to patients with T2DM < 10 years and normal controls.

Short-time effect of intravitreal injections on retinal vascular oxygenation and vessel diameter in patients with diabetic macular oedema or neovascular age-related macular degeneration

PURPOSE

To investigate the short-time effect of intravitreal injections (IVI) of the vascular endothelial growth factor inhibitors ranibizumab and aflibercept on retinal arterial and venous oxygen saturation (SO2a and SO2v), arteriovenous oxygen saturation difference (AVD) and vessel diameter (VDa and VDv) in patients with diabetic macular oedema (DME) and patients with choroidal neovascularization (CNV) due to age-related macular degeneration.

METHODS

Uncontrolled prospective observational study in 100 eyes. Retinal vessel oxygen saturation and diameters were assessed using a retinal oximeter before and minutes after IVI of ranibizumab or aflibercept.

RESULTS

40 eyes with CNV and 34 eyes with DME were included in the analysis. At baseline, SO2a and SO2v were significantly higher in DME (p = 0.043 and p = 0.009, respectively). After IVI, SO2a significantly decreased in CNV and DME eyes by 2.6% (p = 0.016) and 4.6% (p = 0.002) and SO2v decreased by 14.0% (p = 0.004) and 12.4% (p = 0.017), respectively. However, a significant increase in AVD was only found in CNV (15.7%, p = 0.001). VDa decreased significantly only in DME by 5.7% (p = 0.010). No medication-specific disease effect was found and vice versa.

CONCLUSIONS

The observed changes can be interpreted as signs of increased metabolic demand during the physiological stress after an IVI. The abnormal arterial constriction and the abolished increase in AVD seen only in eyes with DME indicate an impairment of vascular autoregulation and oxygen distribution and a reduced neuroretinal metabolism in the diabetic retina with a significant impact on inner retinal oxygen consumption shortly after IVI.

Influence of cataract light scatters on retinal vessel oxygen saturation

BACKGROUND

The aim of our study was to compare metabolic (oxygen saturation; %) and anatomical parameters (retinal vessel diameter; μm) of retinal vessel oximetry (RO) in patients with cataract formation against those of healthy controls with clear lenses.

METHODS

A total of 96 eyes of 62 subjects were examined: 51 eyes from 33 cataract patients (mean age: 64.8y) were compared to 45 eyes from 29 controls with clear lenses (mean age: 61.5y). RO was performed with the oxygen saturation measurement tool from the RVA (IMEDOS Systems UG). The oxygen saturation in all major peripapillary retinal arterioles (A-SO₂ ) and venules (V-SO₂ ) was measured, and their difference (A-V SO₂ ) was calculated. In addition, the corresponding diameter in retinal arterioles (D-A) and venules (D-V) was evaluated. Cataract formation was graded according to the Lens Opacities Classification System III (LOCS III). Oximetry data were compared with the grade of cataract formation within both groups. For statistical evaluation, anova-based linear mixed-effects models were calculated (spss^® , pairwise comparisons: Bonferroni-corrected; p < 0.05).

RESULTS

Cataract eyes showed significantly lower A-SO₂ and A-V SO₂ values (mean ± SD 92.52 ± 9.80% and 28.56 ± 9.80%), when compared to healthy controls (95.47 ± 4.48% and 34.8 ± 7.08%; p = 0.046 and p = 0.001). Within the cataract group, cortical opacities showed significant interactions with the A-SO₂ , V-SO₂ and the A-V SO₂ parameters (p = 0.027; p = 0.002; and p = 0.026, respectively).

CONCLUSIONS

These data indicate that the cataract-induced light scatter influences optical retinal oxygen measurements. Cortical opacities showed the highest influence on RO measurement when compared to nuclear opacification, nuclear colour and posterior cataract formation.

Deep spectral learning for label-free optical imaging oximetry with uncertainty quantification

Measurement of blood oxygen saturation (sO2) by optical imaging oximetry provides invaluable insight into local tissue functions and metabolism. Despite different embodiments and modalities, all label-free optical-imaging oximetry techniques utilize the same principle of sO2-dependent spectral contrast from haemoglobin. Traditional approaches for quantifying sO2 often rely on analytical models that are fitted by the spectral measurements. These approaches in practice suffer from uncertainties due to biological variability, tissue geometry, light scattering, systemic spectral bias, and variations in the experimental conditions. Here, we propose a new data-driven approach, termed deep spectral learning (DSL), to achieve oximetry that is highly robust to experimental variations and, more importantly, able to provide uncertainty quantification for each sO2 prediction. To demonstrate the robustness and generalizability of DSL, we analyse data from two visible light optical coherence tomography (vis-OCT) setups across two separate in vivo experiments on rat retinas. Predictions made by DSL are highly adaptive to experimental variabilities as well as the depth-dependent backscattering spectra. Two neural-network-based models are tested and compared with the traditional least-squares fitting (LSF) method. The DSL-predicted sO2 shows significantly lower mean-square errors than those of the LSF. For the first time, we have demonstrated en face maps of retinal oximetry along with a pixel-wise confidence assessment. Our DSL overcomes several limitations of traditional approaches and provides a more flexible, robust, and reliable deep learning approach for in vivo non-invasive label-free optical oximetry.

Relationship between retinal vessel tortuosity and oxygenation in sickle cell retinopathy

Background

Reduced retinal vascular oxygen (O₂) content causes tissue hypoxia and may lead to development of vision-threatening pathologies. Since increased vessel tortuosity is an early sign for some hypoxia-implicated retinopathies, we investigated a relationship between retinal vascular O₂ content and vessel tortuosity indices.

Methods

Dual wavelength retinal oximetry using a commercially available scanning laser ophthalmoscope was performed in both eyes of 12 healthy (NC) and 12 sickle cell retinopathy (SCR) subjects. Images were analyzed to quantify retinal arterial and venous O₂ content and determine vessel tortuosity index (VTI) and vessel inflection index (VII) in circumpapillary regions. Linear mixed model analysis was used to determine the effect of disease on vascular O₂ content, VTI and VII, and relate vascular O₂ content with VTI and VII. Models accounted for vessel type, fellow eyes, age and mean arterial pressure.

Results

Retinal arterial and venous O₂ content were lower in SCR (O_2A = 11 ± 4 mLO₂/dL, O_2V = 7 ± 2 mLO₂/dL) compared to NC (O_2A = 18 ± 3 mLO₂/dL, O_2V = 13 ± 3 mLO₂/dL) subjects (p < 0.001). As expected, O₂ content was higher in arteries (15 ± 5 mLO₂/dL) than veins (10 ± 4 mLO₂/dL) (p < 0.001), but not different between eyes (OD: 12 ± 5 mLO₂/dL; OS:13 ± 5 mLO₂/dL) (p = 0.3). VTI was not significantly different between SCR (0.18 ± 0.07) and NC (0.15 ± 0.04) subjects, or between arteries (0.18 ± 0.07) and veins (0.16 ± 0.04), or between eyes (OD: 0.18 ± 0.07, OS:0.17 ± 0.05) (p ≥ 0.06). VII was significantly higher in SCR (10 ± 2) compared to NC subjects (8 ± 1) (p = 0.003). VII was also higher in veins (9 ± 2) compared to arteries (8 ± 5) (p = 0.04), but not different between eyes (OD: 9 ± 2; OS: 9 ± 2) (p = 0.2). There was an inverse linear relationship between vascular O₂ (13 ± 5 mLO₂/dL) content and VII (9 ± 2) (β = -0.5; p = 0.02).

Conclusions

The findings augment knowledge of relationship between retinal vascular oxygenation and morphological changes and potentially contribute to identifying biomarkers for assessment of retinal hypoxia due to SCR and other retinopathies.

Correlation between Diabetic Retinopathy Severity and Oxygen Metabolism in Patients with Diabetic Macular Edema during Treatment with Intravitreal Aflibercept

PURPOSE

To attribute data on changes in diabetic retinopathy (DR) severity during treatment of diabetic macular edema (DME) with vascular endothelial growth factor inhibitors (anti-VEGF), this study aimed to (1) examine the correlation between oxygen saturations in retinal vessels and the number of DR lesions on ultra-wide field color fundus photographs prior to anti-VEGF treatment and (2) compare changes in oxygen saturations in retinal vessels with changes in the number of DR lesions after a loading dose of three monthly intravitreal injections of 2.0 mg of aflibercept.

METHODS

This 3-month prospective study included 37 eyes of patients with DME and varying severity of peripheral DR lesions. DR lesions were graded on wide field images and retinal oxygen saturations were evaluated by retinal oximetry. Patients were then treated with three monthly intravitreal injections of 2 mg aflibercept and wide field imaging and retinal oximetry were repeated 4 weeks after the last injection. Patients with proliferative DR or previous panretinal photocoagulation were excluded.

RESULTS

Baseline retinal arteriolar oxygen saturation increased with increasing DR severity and numbers of microaneurysms, hemorrhages, and cotton wool spots (p = 0.03, 0.01, 0.03, and <0.001), while no correlation between the severity of DR lesions and retinal venular oxygen saturation was found. After treatment with intravitreal aflibercept, the severity of DR lesions significantly reduced, while retinal arteriolar and venular oxygen saturation as well as the arteriolar-venular difference remained unchanged (95.5 vs. 95.8%, p = 0.44; 62.9 vs. 64.5%, p = 0.08; 32.5 vs. 31.4%, p = 0.33).

CONCLUSION

This study demonstrated that structural DR lesions correlate with retinal arteriolar oxygen saturation in patients with DME prior to anti-VEGF treatment and that improvement in the severity of DR lesions can occur without corresponding changes in retinal oxygen metabolism during intravitreal therapy. Our results suggest that DR severity on color fundus photographs should be interpreted with caution once intravitreal therapy is initiated.

Relating glaucomatous visual field loss to retinal oxygen delivery and metabolism

PURPOSE

To test the hypothesis that visual field (VF) loss is associated with decreased retinal blood flow, oxygen delivery and metabolism, and with increased retinal oxygen extraction fraction (OEF) in glaucomatous individuals.

METHODS

Glaucomatous subjects underwent automated perimetry, dual wavelength scanning laser ophthalmoscopy and Doppler optical coherence tomography imaging in order to measure mean deviation, central retinal arterial and venous diameter equivalents (CRAE and CRVE), oxygen saturation levels and total retinal blood flow (TRBF), respectively. Retinal oxygen delivery, metabolism and extraction fraction were derived from measurements of oxygen saturation and blood flow.

RESULTS

Twenty eyes of 14 subjects were included in the study. Mean deviation was on average -13.76 ± 8.70 dB. Mean CRAE and CRVE were 126 ± 28 and 191 ± 35 μm, respectively. Mean TRBF and oxygen delivery were 34.3 ± 11.7 μl/min and 6.5 ± 2.6 μl O₂ /min, respectively. Mean oxygen metabolism and extraction fraction were 2.1 ± 0.94 μl O₂ /min and 0.34 ± 0.15, respectively. Visual field (VF) loss was associated with reduced CRAE and CRVE, TRBF and oxygen delivery and associated with increased OEF.

CONCLUSION

Combined measurements of retinal blood flow and oxygen saturation in glaucomatous individuals suggest VF loss is associated with impaired oxygen delivery and augmented OEF.

Imaging Retinal Activity in the Living Eye

Retinal function has long been studied with psychophysical methods in humans, whereas detailed functional studies of vision have been conducted mostly in animals owing to the invasive nature of physiological approaches. There are exceptions to this generalization, for example, the electroretinogram. This review examines exciting recent advances using in vivo retinal imaging to understand the function of retinal neurons. In some cases, the methods have existed for years and are still being optimized. In others, new methods such as optophysiology are revealing novel patterns of retinal function in animal models that have the potential to change our understanding of the functional capacity of the retina. Together, the advances in retinal imaging mark an important milestone that shifts attention away from anatomy alone and begins to probe the function of healthy and diseased eyes.

Retinal vessel oxygen saturation and vessel diameter in healthy individuals during high-altitude exposure

PURPOSE

To assess changes of retinal vessel oxygen saturation and vessel diameter in healthy individuals during high-altitude exposure.

METHODS

Retinal oxygen saturation and vessel diameter were obtained at sea level (SL, 40 m) and high altitude (HA, 3681 m) on 17 healthy individuals from Beijing (six males, 28.06 ± 8.06 years) using Oxymap T1 and then compared with 21 residents from Yushu (10 males, 28.63 ± 6.00 years). Systemic and ocular parameters were also measured before and after high-altitude exposure. Data were presented as mean ± SD and analysed using paired and independent Student t-test with significance accepted at p < 0.05.

RESULTS

Short-term high-altitude exposure of Beijing Group significantly affected all the systemic and ocular parameters, as well as retinal oxygen saturation and vessel diameter ranging from overall quadrant to different quadrants, other than retinal venous oxygen saturation and retinal arterial diameter. However, these changes were not evident in those permanently living at HA. Pearson's correlation analysis revealed correlations between retinal oxygen saturation and systemic and ocular parameters (all p < 0.05). The multivariate linear regression analysis indicated that retinal arterial oxygen saturation was significantly associated with arterial peripheral arterial oxygen saturation (SpO₂ ) and subfoveal choroidal thickness.

CONCLUSION

Short-term exposure to HA induces retinal microcirculation disturbance and auto-regulatory response in healthy individuals, which is probably attributed to arterial SpO₂ and endothelial dysfunction under hypoxic conditions.

Retinal Oxygen Delivery and Metabolism in Healthy and Sickle Cell Retinopathy Subjects

Purpose

Reduction in inner retinal oxygen delivery (DO2) can cause retinal hypoxia and impair inner retinal oxygen metabolism (MO2), leading to vision loss. The purpose of the current study was to establish measurements of DO2 and MO2 in healthy subjects and test the hypothesis that DO2 and MO2 are reduced in sickle cell retinopathy (SCR) subjects.

Methods

Dual wavelength retinal oximetry and Doppler optical coherence tomography were performed in 12 healthy control and 12 SCR subjects. Images were analyzed to measure retinal arterial and venous oxygen content (O2A and O2V), venous diameter (DV), and total retinal blood flow (TRBF). Retinal arteriovenous oxygen content difference (O2AV), DO2, MO2, and oxygen extraction fraction (OEF) were calculated according to the following equations: O2AV = O2A - O2V; DO2 = TRBF * O2A; MO2 = TRBF * O2AV; OEF = MO2/DO2.

Results

Retinal DV and TRBF were higher in the SCR group as compared to the control group, whereas, O2A, O2V, and O2AV were lower in SCR group as compared to the control group. DO2, MO2, and OEF were not significantly different between control and SCR groups. MO2 and DO2 were linearly related, such that higher MO2 was associated with higher DO2. There was an inverse relationship between TRBF and OEF, such that lower TRBF was associated with higher OEF.

Conclusions

Increased blood flow compensated for decreased oxygen content, thereby maintaining DO2, MO2, and OEF at predominately lower stages of SCR. Quantitative assessment of these parameters has the potential to advance knowledge and improve diagnostic evaluation of retinal ischemic conditions.

Retinal Vessel Diameter Changes in Relation to Dark Adaptation and Acute Hyperglycemia

The purpose of this experimental clinical study was to assess the effects of dark adaptation and acute changes in glycemia on retinal vessel diameters in men. The study included 14 patients (mean age 63 years, range 48–74 years) with type 2 diabetes mellitus and minimal or no diabetic retinopathy. Retinal vessel diameters were assessed using infrared photography before and after dark adaptation, first while fasting and then at peak hyperglycemia during an oral glucose tolerance test (OGTT). Dark adaptation was accompanied by retinal vasodilatation, both during fasting (mean glycemia 7.6 ± 1.7 mM) and postprandial hyperglycemia (15.7 ± 4.2 mM). When fasting, the increase in vein diameter during dark adaptation was 2.0% after 20 min (P=0.018) and 2.9% after 40 min (P=0.010). When subjects were hyperglycemic, the increase during dark adaptation was 2.8% for retinal vein diameters (P=0.027) and 2.0% for retinal artery diameters after 20 min (P=0.002) and 1.7% for retinal artery diameters after 40 min (P=0.022). For identical conditions of light/dark adaptation, retinal vessels were dilated when subjects were fasting compared to postprandial hyperglycemia. Thus, darkness and fasting were both associated with retinal vasodilation in this short-term experiment in patients with type 2 diabetes. Future studies should determine whether both the stimuli of vasodilation lead to retinal hyperperfusion, which would support that they may be involved in the aggravation of diabetic retinopathy.

Comparative Study of Optical Coherence Tomography Angiography and Phase-Resolved Doppler Optical Coherence Tomography for Measurement of Retinal Blood Vessels Caliber

PURPOSE

To compare the accuracy of Doppler optical coherence tomography (DOCT) and OCT angiography (OCTA) for measuring retinal blood vessel caliber at different flow rates.

METHODS

A research-grade 1060-nm OCT system with 3.5-μm axial resolution in retinal tissue and 92,000 A scan/s image acquisition rate was used in this study. DOCT and OCTA measurements were acquired both from a flow phantom and in vivo from retinal blood vessels in six male Brown Norway rats. The total retinal blood flow (TRBF) was modified from baseline to 70% and 20% of baseline by reducing the ocular perfusion pressure (OPP). The retinal blood vessel caliber (RBVC) was measured from OCTA and DOCT images. The caliber measurements were conducted by two separate graders using a custom MATLAB-based image processing algorithm.

RESULTS

The RBVC measured with OCTA and DOCT for normal blood flow rates were not significantly different (56.69 ± 12.17 and 57.17 ± 9.46 μm, P = 0.27, respectively). However, significant differences were detected when TRBF was reduced to 70% (55.69 ± 11.56 vs. 50.62 ± 8.85 μm, P < 0.01) and 20% (50.29 ± 9.29 vs. 44.88 ± 7.13 μm, P < 0.01) of baseline.

CONCLUSIONS

Reduced TRBF resulted in inaccuracy of the RBVC measurements with DOCT in both the phantom and animal study. This result suggests that OCTA is a more accurate tool for RBVC evaluation when applied to retinal diseases associated with reduced TRBF, such as glaucoma and diabetic retinopathy.

TRANSLATIONAL RELEVANCE

Results from this study are directly applicable to clinical studies of retinal blood flow measured with OCTA and DOCT.

Optical coherence tomography angiography discerns preclinical diabetic retinopathy in eyes of patients with type 2 diabetes without clinical diabetic retinopathy

AIMS

To investigate changes in retinal vascular plexuses and choriocapillaris in patients with type 2 diabetes mellitus (DM2) without diabetic retinopathy (DR) and healthy controls using optical coherence tomography angiography (OCTA).

METHODS

A total of 71 DM2 and 67 healthy control subjects were included. All subjects underwent OCTA examination (RTVue-XR Avanti; Optovue, Fremont, CA, USA). Average vessel density in superficial capillary plexus (SCP), deep capillary plexus (DCP) and choriocapillaris, parafoveal vessel density in SCP and DCP, FAZ area (mm²) in SCP, microaneurysms and capillary nonperfusion were taken into analysis.

RESULTS

Parafoveal vessel density in both SCP and DCP decreased in the eyes without clinical DR compared to normal controls (p < 0.001). Diabetic patients with no signs of DR also had a significant reduction in average vessel density of SCP, DCP and choriocapillaris (p < 0.001, p < 0.001 and p = 0.006, respectively). No significant difference was found in FAZ area of SCP between DM2 eyes and healthy controls (p = 0.253). The average vessel density of SCP and DCP is not correlated with HbA1c or serum creatinine in DM2 patients. Microaneurysms seen in OCTA but not in fundus examination were found in 8 out of the 71 (11.3%) diabetic eyes, and capillary nonperfusion was noted in 18 of 71 diabetic eyes.

CONCLUSIONS

We demonstrated that OCTA can identify preclinical DR before the manifestation of clinically apparent retinopathy in diabetic eyes. DM2 patients without DR have SCP, DCP and choriocapillaris impairment. Our results suggested that OCTA might be a promising tool for regular screening of diabetic eyes for DR.