The retinal function imager and clinical applications

Measurement of retinal blood flow precision in the human eye with multimodal adaptive optics imaging

Impaired retinal blood flow (RBF) autoregulation plays a key role in the development and progression of several ocular diseases, including glaucoma and diabetic retinopathy. Clinically, reproducible RBF quantitation could significantly improve early diagnosis and disease management. Several non-invasive techniques have been developed but are limited for retinal microvasculature flow measurements due to their low signal-to-noise ratio and poor lateral resolution. In this study, we demonstrate reproducible vessel caliber and retinal blood flow velocity measurements in healthy human volunteers using a high-resolution (spatial and temporal) multimodal adaptive optics system with scanning laser ophthalmoscopy and optical coherence tomography.

Assessment of Blood Flow Velocity in Retinal Vasculitis Using the Retinal Function Imager-A Pilot Study

Background: This pilot study aimed to evaluate the Retinal Function Imager (RFI) for visualizing retinal vasculature and assessment of blood flow characteristics in patients with retinal vasculitis. The RFI is a non-invasive imaging device measuring the blood flow velocity (BFV) in secondary and tertiary retinal vessels using hemoglobin as an intrinsic motion-contrast agent. Methods: To test the feasibility of the RFI for patients with retinal vasculitis, capillary perfusion maps (nCPMs) were generated from 15 eyes of eight patients (five females; mean age: 49 ± 12 years) with a mean uveitis duration of 74 ± 85 months. Five of these patients had birdshot chorioretinopathy, and three had primarily non-occlusive venous retinal vasculitis of unknown origin. To reflect that the BFV may be more reduced in patients with prolonged disease, patients were classified into a short-term (uveitis duration: 8-15 months) and a long-term uveitis group (uveitis duration: 60-264 months). Data were compared with healthy controls (16 eyes of 11 patients; mean age 45 ± 12 years; 8 females). Results: The mean BFV in the controls was 3.79 ± 0.50 mm/s in the retinal arteries and 2.35 ± 0.44 mm/s in the retinal veins, which was significantly higher compared to the retinal vasculitis group. Patients revealed an arterial BFV of 2.75 ± 0.74 mm/s (p < 0.001) and a venous BFV of 1.75 ± 0.51 mm/s (p = 0.016). In the short-term group, a trend towards a decreased venular and arteriolar BFV was seen, while a significant reduction was observed in the long-term group. The patients' microvasculature anatomy revealed by the nCPMs appeared unevenly distributed and a lower number of blood vessels were seen, along with a lower degree of complexity of their branching patterns, when compared with controls. Conclusions: This study demonstrated a reduction in venular and arteriolar BFVs in patients with retinal vasculitis. BFV alterations were already observed in early disease stages and became more pronounced in progressed disease. Additionally, we showed that retinal microvasculature changes may be observed by nCPMs. Retinal imaging with the RFI may serve as a diagnostic and quantifying tool in retinal vasculitis.

Age-related alterations in retinal capillary function

PURPOSE

To determine age-related alterations in the retinal capillary function (RCF, the ability to transport blood flow) in healthy subjects.

METHODS

A total of 148 healthy subjects (aged 18 to 83 years) were enrolled, and one eye of each subject was imaged. Retinal blood flow (RBF) was measured using a Retinal Function Imager, and retinal capillary density (RCD, expressed as fractal dimension Dbox) was measured using optical coherence tomography angiography. RCF was defined as the ratio of RBF to RCD, representing the ability to transport blood flow. The relationship between RCF and age was analyzed. In addition, the cohort was divided into four groups (G1, <35 years, G2, 35-49 years, G3, 50-64 years, and G4, ≥65 years) for further analysis.

RESULTS

With all data, the relation between the RCF and age had a trend of a quadratic model (G1-4: r = 0.16, P = 0.14). After 35 years (i.e., G2-4), the relation had a trend between the RCF and age fitted into a negative linear model (r = -0.23, P = 0.05). Moreover, after 50 years (i.e., G3-4), the negative linear model became stronger (r = -0.37, P = 0.03). The average RCF was 2.24 ± 0.22 μl/s/Dbox in G4, significantly lower than that in G2 (2.65 ± 0.56 μl/s/Dbox, P = 0.018) and G3 (2.64 ± 0.70 μl/s/Dbox, P = 0.034), but did not reach a significant level compared to that in G1 (2.55 + 0.51 μl/s/Dbox, P = 0.056).

CONCLUSIONS

This is the first study to determine age-related alterations in the RCF in a healthy population. Decreased RCF in the older group may represent a characteristic pattern of normal aging.

Neuroprotection for Nonarteritic Central Retinal Artery Occlusion: Lessons from Acute Ischemic Stroke

Nonarteritic central retinal artery occlusion (NA-CRAO) is a variant of acute ischemic stroke (AIS) and is a cause of sudden severe loss of vision. There are guidelines by the American Heart Association and the American Stroke Association for the care of CRAO patients. This review explores the basis of retinal neuroprotection for CRAO and its potential for improving the outcome of NA-CRAO. Recently, there have been significant advances in research into the use of neuroprotection to treat retinal diseases, including retinal detachment, age-related macular degeneration, and inherited retinal diseases. Also, neuroprotective research in AIS has been extensive, and newer drugs tested, including Uric acid, Nerinetide, and Otaplimastat, with promising results. Progress in cerebral neuroprotection after AIS offers hope for retinal neuroprotection after CRAO; and a possibility of extrapolating research findings from AIS into CRAO. Combining neuroprotection and thrombolysis can extend the therapeutic window for NA-CRAO treatment and potentially improve outcomes. Experimented neuroprotection for CRAO includes Angiopoietin (Comp Ang1), KUS 121, Gene therapy (XIAP), and hypothermia. Efforts in the field of neuroprotection for NA-CRAO should focus on better imaging to delineate the penumbra after an acute episode of NA-CRAO (using a combination of high-definition optical coherence angiography and electrophysiology). Also, research should explore details of pathophysiologic mechanisms involved in NA-CRAO, allowing for further neuroprotective intervention, and closing the gap between preclinical and clinical neuroprotection.

Optical coherence tomography angiography analysis of changes in the foveal avascular zone in eyes with diabetic macular edema treated with intravitreal anti-vascular endothelial growth factor

Background

To analyze the changes in foveal avascular zone (FAZ) area, perimeter, and circularity in the superficial (SCP) and deep (DCP) capillary plexuses in eyes with diabetic macular edema (DME) treated with intravitreal anti-VEGF using optical coherence tomography angiography (OCTA).

Methods

This prospective observational study included 56 eyes from 32 patients with DME that received intravitreal anti-VEGF. OCTA images were obtained at baseline and 1, 3, and 6 months of follow-up. The outcome measures were FAZ area, perimeter, and circularity in both the SCP and DCP, as well as central subfield thickness (CST) and best-corrected visual acuity (BCVA).

Results

The mean number of intravitreal anti-VEGF injections received during the observation period was 4.60 ± 0.82 (range: 3–6). The FAZ area, perimeter, and circularity were statistically unchanged at all observation points in both the SCP (p = 0.772, p = 0.405, p = 0.157, respectively) and the DCP (p = 0.620, p = 0.769, p = 0.481, respectively). Despite having no change in the FAZ parameters, there was still a statistically significant decrease in CST (p < 0.001) as well as a statistically significant increase in BCVA (p = 0.004) during the observation period.

Conclusions

The FAZ area, perimeter, and circularity in the SCP and DCP as measured by OCTA remained stable during the first 6 months of intravitreal anti-VEGF therapy in eyes with DME. While there were no significant changes in the FAZ, treatment with intravitreal anti-VEGF still resulted in decreased CST and improved BCVA.

Subretinal fluid disturbs the retinal venous blood flow in central serous chorioretinopathy

The significance of subretinal fluid in the retinal blood flow is unclear. Here, we evaluated the association between subretinal fluid (SRF) and retinal blood flow in eyes with central serous chorioretinopathy (CSC) using a retinal functional imager (RFI) and optical coherence tomography angiography (OCTA). In this retrospective case–control study involving 26 eyes from 18 CSC patients and 25 eyes from 21 age- and sex-matched controls, we found that the CSC group showed significant differences from the control group in terms of the retinal venule blood flow velocity (3.60 ± 0.43 vs 3.96 ± 0.56 mm/s; p = 0.030), retinal venule blood flow rate (8.75 ± 2.67 vs 12.51 ± 7.12 nl/s; p = 0.040), and the diameter of retinal venules (118.26 ± 14.25 vs 126.92 ± 35.31 μm; p = 0.045). Linear regression analysis showed that SRF thickness accounted for a 36.9% reduction in venous BFR (p = 0.013). The difference in the O₂ saturation between retinal arteries and veins was greater in the CSC group. There was no correlation between SRF thickness and capillary densities in OCTA. Our findings suggest that disturbance in venous return and the associated altered oxygen may be significant changes in the retinal blood flow dynamics in eyes with SRF.

Comparative Assessment of Retinal Blood Flow Velocity Changes Following Brimonidine and Brinzolamide Administration Using Retinal Function Imaging

Purpose

Impaired ocular blood flow has been associated with the etiopathogenesis of glaucoma. Topical brimonidine lowers intraocular pressure, a major glaucoma risk factor. However, brimonidine's influence on retinal blood flow remains to be fully elucidated. Our aim was to compare the effect of topical brimonidine and brinzolamide administration on retinal blood flow velocity in second and third order vessels in healthy adults using the retinal function imager.

Methods

In 10 healthy probands between 23 and 32 years of age, one eye was randomly selected to receive 2 treatment rounds with 3 single doses of brimonidine 2 mg/mL and brinzolamide 10 mg/mL at 12-hour intervals each. The fellow eyes served as intra-individual controls. Immediately before the first drop and 2 hours after the last drop of each treatment round, all subjects were examined, including Goldmann tonometry, Pascal tonometry, assessment of retinal blood flow velocity using the retinal function imager, as well as blood pressure and pulse measurements.

Results

Intraocular pressure decreased significantly in treated eyes while remaining stable in control eyes, indicating reliable application of brimonidine and brinzolamide drops. In contrast, retinal blood flow velocities did not demonstrate any significant differences between groups after both treatment rounds.

Conclusions

Neither brimonidine nor brinzolamide appear to alter retinal blood flow velocity in a clinically relevant manner. The slight velocity changes detected in our study are likely physiologic fluctuations. Our findings do not support the rationale of a detrimental effect of topical brimonidine on ocular blood flow and hence brimonidine may be further administered for lowering intraocular pressure with the appropriate caution. However, our study is strongly limited by the small sample size and, thus, further research with larger cohorts of healthy volunteers and patients with glaucoma is needed to confirm the results.

Translational Relevance

The study provides information about the effect of the topically administered antiglaucoma medications brimonidine and brinzolamide on the ocular blood flow and its regulation. The findings indicate that beside the lowering of IOP there is no evidence for an additional effect on the development of glaucoma.

Retinal vessel changes in cerebrovascular disease

PURPOSE OF REVIEW

The retina is growingly recognized as a window into cerebrovascular and systemic vascular conditions. The utility of noninvasive retinal vessel biomarkers in cerebrovascular risk assessment has expanded due to advances in retinal imaging techniques and machine learning-based digital analysis. The purpose of this review is to underscore the latest evidence linking retinal vascular abnormalities with stroke and vascular-related cognitive disorders; to highlight modern developments in retinal vascular imaging modalities and software-based vasculopathy quantification.

RECENT FINDINGS

Longitudinal studies undertaken for extended periods indicate that retinal vascular changes can predict cerebrovascular disorders (CVD). Cerebrovascular ties to dementia provoked recent explorations of retinal vessel imaging tools for conceivable early cognitive decline detection. Innovative biomedical engineering technologies and advanced dynamic and functional retinal vascular imaging methods have recently been added to the armamentarium, allowing an unbiased and comprehensive analysis of the retinal vasculature. Improved artificial intelligence-based deep learning algorithms have boosted the application of retinal imaging as a clinical and research tool to screen, risk stratify, and monitor with precision CVD and vascular cognitive impairment.

SUMMARY

Mounting evidence supports the use of quantitative retinal vessel analysis in predicting CVD, from clinical stroke to neuroimaging markers of stroke and neurodegeneration.

Exploratory study of non-invasive, high-resolution functional macular imaging in subjects with diabetic retinopathy

AIM

To evaluate a high-resolution functional imaging device that yields quantitative data regarding macular blood flow and capillary network features in eyes with diabetic retinopathy (DR).

METHODS

Prospective, cross-sectional comparative case-series in which blood flow velocities (BFVs) and non-invasive capillary perfusion maps (nCPMs) in macular vessels were measured in patients with DR and in healthy controls using the Retinal Functional Imager (RFI) device.

RESULTS

A total of 27 eyes of 21 subjects were studied [9 eyes nonproliferative diabetic retinopathy (NPDR), 9 eyes proliferative diabetic retinopathy (PDR) and 9 controls]. All diabetic patients were type 2. All patients with NPDR and 5 eyes with PDR also had diabetic macular edema (DME). The NPDR group included eyes with severe (n=3) and moderate NPDR (n=6), and were symptomatic. A significant decrease in venular BFVs was observed in the macular region of PDR eyes when compared to controls (2.61±0.6 mm/s and 2.92±0.72 mm/s in PDR and controls, respectively, P=0.019) as well as PDR eyes with DME compared to NPDR eyes (2.36±0.51 mm/s and 2.94±1.09 mm/s in PDR with DME and NPDR, respectively, P=0.01).

CONCLUSION

The RFI, a non-invasive imaging tool, provides high-resolution functional imaging of the retinal microvasculature and quantitative measurement of BFVs in visually impaired DR patients. The isolated diminish venular BFVs in PDR eyes compared to healthy eyes and PDR eyes with DME in comparison to NPDR eyes may indicate the possibility of more retinal vein compromise than suspected in advanced DR.

Non-invasive measurement techniques for quantitative assessment of optic nerve head blood flow

Diseases of the optic nerve head involving changes in blood flow are common. However, the pathophysiology is not always fully understood. Several non-invasive methods for measuring optic nerve head blood flow are available, but currently no gold standard has been established. Methods for measuring blood flow in optic neuropathies including colour Doppler imaging, retinal function imager, optical coherence tomography angiography and laser speckle flowgraphy are reviewed. Ultrasound colour Doppler imaging is a fast measurement technique where several different parameters, especially the blood flow velocity, can be calculated. Though used for many years in ophthalmology, its use is not standardized and it requires significant observer skills. The retinal function imager is a direct method where the haemoglobin in erythrocytes is visualized and blood flow velocities in retinal vessels are calculated from a series of photos. The technique is not suitable for direct measurement of blood flow within the optic nerve head. Laser speckle flowgraphy uses a laser light which creates a light scatter pattern in the tissue. Particles moving in the area causes changes in the speckle pattern from which a relative blood flow can be estimated. It is, however, not known whether optic nerve head microcirculation is measurable with the technique. Optical coherence tomography angiography uses multiple scans to evaluate blood flow with good reproducibility but often problems with artefacts. The technique is continuously being refined and increasingly used in research as a tool for the study of blood flow in retinopathies and optic neuropathies. Most of the conducted studies are based on small sample sizes, but some of the methods show promising results in an optic nerve head blood flow research setting. Further and larger studies are required to provide standardized and comparable measurements before one or more of the methods can be considered clinical helpful in daily practice.

Clinical applications of the retinal functional imager: A brief review

The advances in treating blinding conditions often depends on the development of new techniques that allows early detection, treatment, and follow-up of the disease. Functional changes often precede structural changes in many retinal disorders. Therefore, detecting these changes helps in early diagnosis and management, with the intention of preventing permanent morbidity. The Retinal Functional Imager (RFI) is a non-invasive imaging system that allows us to assess the various functional parameters of the retina. The RFI quantitatively measures the retinal blood-flow velocity, oxygen saturation, metabolic demand and generates a non-invasive capillary perfusion map that provides details similar to a fluorescein angiography. All of these parameters correlate with the health of the retina, and are known to get deranged in retinal disease. This article is a brief review of published literature on the clinical utility of the RFI.