Retinal microcirculation in patients with diabetes mellitus: dynamic and morphological analysis of perifoveal capillary network

CAPILLARY PLEXUS ANOMALIES IN DIABETIC RETINOPATHY ON OPTICAL COHERENCE TOMOGRAPHY ANGIOGRAPHY

PURPOSE

To analyze the foveal microvasculature in eyes with diabetic retinopathy (DR) using optical coherence tomography angiography (OCTA) and fluorescein angiography (FA).

METHODS

In this retrospective study of 20 eyes of 14 patients with DR imaged using OCTA and FA, clinical features of DR such as microaneurysms, capillary nonperfusion areas, and intraretinal microvascular abnormalities were analyzed.

RESULTS

In the superficial plexus, a rarefaction of capillaries with capillary nonperfusion areas was present in all eyes. Some of these nonperfused areas were not detected on FA and were better delimited on OCTA. Conversely, in the deep plexus, capillary nonperfusion areas were seen only in 35% (7/20) of eyes, whereas DR led to an alteration of the normal capillary vortex pattern in all eyes. Only 62% of microaneurysms visualized on FA were detected by OCTA (P = 0.02). Intraretinal microvascular abnormalities were well detected by both FA and OCTA.

CONCLUSION

Optical coherence tomography angiography allowed detecting DR anomalies in both superficial and deep capillary plexus in all eyes. The ability of OCTA to detect microaneurysms was lower than that of FA although its accuracy for assessing capillary nonperfusion was better and may enable a proper grading of DR progression.

CORRELATION OF FOVEAL AVASCULAR ZONE SIZE WITH FOVEAL MORPHOLOGY IN NORMAL EYES USING OPTICAL COHERENCE TOMOGRAPHY ANGIOGRAPHY

PURPOSE

To analyze the foveal avascular zone (FAZ) in normal eyes using optical coherence tomography angiography.

METHODS

Prospective noncomparative case series. The parafoveal region of 70 eyes from 67 healthy subjects was imaged using optical coherence tomography angiography to visualize the superficial and deep capillary plexuses and correlated with standard macular volume scans using spectral domain optical coherence tomography to determine foveal morphology.

RESULTS

In all 70 eyes imaged, 2 vascular plexuses could be detected within the retina: a superficial plexus within the ganglion cell layer and a deep plexus within the inner nuclear layer. A measurable FAZ was visualized in both plexuses in all imaged eyes. The FAZ area was variable in the study population with a mean of 0.266 mm² ± 0.097 mm² in the superficial plexus (range: 0.071 mm²-0.527 mm²) and a mean of 0.495 mm² ± 0.227 mm² in the deep plexus (range: 0.160 mm²-0.795 mm²). The FAZ area was significantly larger in the deep plexus (P < 0.0001) compared with superficial plexus. The FAZ area in both plexuses correlated inversely with central macular thickness and central macular volume (P < 0.0001). No significant correlation was found between superficial plexus FAZ area and age (P = 0.55) or sex (P = 0.34). In the same manner, no significant correlation was found between deep plexus FAZ area and age (P = 0.13) or sex (P = 0.13).

CONCLUSION

Optical coherence tomography angiography provides a noninvasive method to visualize and measure the superficial and deep plexus FAZ in a normal population. The FAZ can vary in size and shape, with the FAZ area significantly larger in the deep compared with the superficial plexus. Both superficial and deep FAZ area correlate inversely with foveal thickness and volume.

Optical Coherence Tomography Angiography in Retinal Diseases

Optical coherence tomography angiography (OCTA) is a new, non-invasive imaging system that generates volumetric data of retinal and choroidal layers. It has the ability to show both structural and blood flow information. Split-spectrum amplitude-decorrelation angiography (SSADA) algorithm (a vital component of OCTA software) helps to decrease the signal to noise ratio of flow detection thus enhancing visualization of retinal vasculature using motion contrast. Published studies describe potential efficacy for OCTA in the evaluation of common ophthalmologic diseases such as diabetic retinopathy, age related macular degeneration (AMD), retinal vascular occlusions and sickle cell disease. OCTA provides a detailed view of the retinal vasculature, which allows accurate delineation of microvascular abnormalities in diabetic eyes and vascular occlusions. It helps quantify vascular compromise depending upon the severity of diabetic retinopathy. OCTA can also elucidate the presence of choroidal neovascularization (CNV) in wet AMD. In this paper, we review the knowledge, available in English language publications regarding OCTA, and compare it with the conventional angiographic standard, fluorescein angiography (FA). Finally, we summarize its potential applications to retinal vascular diseases. Its current limitations include a relatively small field of view, inability to show leakage, and tendency for image artifacts. Further larger studies will define OCTA's utility in clinical settings and establish if the technology may offer a non-invasive option of visualizing the retinal vasculature, enabling us to decrease morbidity through early detection and intervention in retinal diseases.

Rapid high resolution imaging with a dual-channel scanning technique

A spatial shift between channels in a dual-beam raster-scan imaging system introduces a temporal separation between images from the two channels that can be much shorter than the frame rate of the system. The technique is demonstrated by measuring the velocity of erythrocytes in the retinal capillaries. We used an adaptive optics scanning laser ophthalmoscope and introduced a temporal separation between imaging channels of 4.7 ms. We imaged three subjects and measured changing capillary blood flow velocity at the pulse rate. Since the time shift between channels is easily and continuously adjustable, this method can be used to measure rapidly changing events in any raster scan system with little added complexity.

Association of Diabetic Macular Nonperfusion With Outer Retinal Disruption on Optical Coherence Tomography

IMPORTANCE

Diabetic macular nonperfusion leads to decreased perifoveal capillary blood flow, which in turn causes chronic ischemia of the retinal tissue. Using point-to-point correlation between spectral-domain optical coherence tomography (SD-OCT) and nonperfusion on fluorescein angiography, we observed that retinal capillary nonperfusion is associated with photoreceptor compromise on OCT. This study highlights a new concept of a possible contribution of the retinal deep capillary plexus to photoreceptor compromise in diabetic retinopathy in the absence of diabetic macular edema.

OBJECTIVE

To report outer retinal structural changes associated with enlargement of the foveal avascular zone and/or capillary nonperfusion in the macular area of diabetic patients.

DESIGN, SETTING, AND PARTICIPANTS

Retrospective observational cross-sectional study in 9 patients who were diagnosed as having diabetic retinopathy without diabetic macular edema and underwent fluorescein angiography and SD-OCT for diabetic retinopathy from July 8, 2014, to December 1, 2014, at a tertiary academic referral center. This analysis was conducted between December 2, 2014, and January 31, 2015.

MAIN OUTCOMES AND MEASURES

Outer retinal changes on SD-OCT in areas of macular ischemia.

RESULTS

The study included 13 eyes of 9 diabetic patients (4 men and 5 women aged 34-58 years) with a mean duration of diabetes mellitus of 14.5 years. Nine eyes showed outer retinal disruption revealed by SD-OCT that colocalized to areas of enlargement of the foveal avascular zone and macular capillary nonperfusion. Four fellow eyes with normal foveal avascular zones did not show any retinal changes on SD-OCT.

CONCLUSIONS AND RELEVANCE

Macular ischemia in diabetic patients can be associated with photoreceptor compromise. The presence of disruption of the photoreceptors on OCT in diabetic patients can be a manifestation of underlying capillary nonperfusion in eyes without diabetic macular edema. Ischemia at the deep capillary plexus may play an important role in these outer retinal changes.

Hypoxia and Dark Adaptation in Diabetic Retinopathy: Interactions, Consequences, and Therapy

In diabetes, retinal blood flow is compromised, and retinal hypoxia is likely to be further intensified during periods of darkness. During dark adaptation, rod photoreceptors in the outer retina are maximally depolarized and continuously release large amounts of the neurotransmitter glutamate-an energetically demanding process that requires the highest oxygen consumption per unit volume of any tissue of the body. In complete darkness, even more oxygen is consumed by the outer retina, producing a steep fall in the retinal oxygen tension curve which reaches a nadir at the depth of the mitochondrial-rich rod inner segments. In contrast to the normal retina, the diabetic retina cannot meet the added metabolic load imposed by the dark-adapted rod photoreceptors; this exacerbates retinal hypoxia and stimulates the overproduction of vascular endothelial growth factor (VEGF). The use of nocturnal illumination to prevent dark adaptation, specifically reducing the rod photoreceptor dark current, should ameliorate diabetic retinopathy.

Swept-source OCT angiography of the retinal vasculature using intensity differentiation-based optical microangiography algorithms

BACKGROUND AND OBJECTIVE

To demonstrate the feasibility of using a 1,050-nm swept-source optical coherence tomography (SS-OCT) system to achieve noninvasive retinal vasculature imaging in human eyes.

MATERIALS AND METHODS

Volumetric data sets were acquired using a 1-µm SS-OCT prototype that operated at a 100-kHz A-line rate. A scanning protocol designed to allow for motion contrast processing, referred to as OCT angiography or optical microangiography (OMAG), was used to scan an approximately 3 × 3–mm area in the central macular region of the retina within approximately 4.5 seconds. An intensity differentiation-based OMAG algorithm was used to extract three-dimensional retinal functional microvasculature information.

RESULTS

Intensity signal differentiation generated capillary-level resolution en face OMAG images of the retina. The parafoveal capillaries were clearly visible, thereby allowing visualization of the foveal avascular zone in healthy subjects.

CONCLUSION

The capability of OMAG to produce retinal vascular images was demonstrated using the 1-µm SS-OCT prototype. This technique has potential clinical value for studying retinal vasculature abnormalities.

Improved detectability of microcirculatory dynamics by laser speckle flowmetry

Mechanisms of renal autoregulation generate oscillations in arterial blood flow at several characteristic frequencies. Full-field laser speckle flowmetry provides a real-time imaging of superficial blood microcirculation. The possibility to detect changes in oscillatory dynamics is an important issue in biomedical applications. In this paper we show how laser power density affects quality of the recorded signal and improves detectability of temporal changes in microvascular perfusion.

A morphological study of the foveal avascular zone in patients with diabetes mellitus using optical coherence tomography angiography

PURPOSE

To investigate the area and radius of the foveal avascular zone (FAZ) in patients with diabetes mellitus (DM) using optical coherence tomography (OCT) angiography.

METHODS

Sixty-five patients with DM (113 eyes) and 62 healthy controls (85 eyes) were included in the study. All of the participants underwent examination with OCT angiography. The vertical radius (VR), horizontal radius (HR) and area of the FAZ were measured.

RESULTS

The differences in HR, VR and area between the control and DM groups were statistically significant (p = 0.01, 0.00 and 0.00, respectively). There were no statistically significant differences between the control group and the no-diabetic retinopathy (DR) group in HR or VR (p = 0.07 and 0.08, respectively), but there was a statistically significant difference in area (p = 0.04). The size of the FAZ was greater in patients with DR compared to the control group. The differences in HR, VR and area were statistically significant (p = 0.01, 0.00 and 0.00, respectively). In addition, the clinically significant macular oedema (CSME) group also had a larger FAZ area than the non-CSME group (p = 0.02).

CONCLUSIONS

Patients with DM had a larger FAZ, and patients with more severely damaged retinas had a much larger FAZ. OCT angiography is a new convenient and noninvasive method for studying the FAZ. This novel examination will yield considerable amounts of data that cannot be obtained using previous research methods.

Macular ischemia and outcome of vitrectomy for diabetic macular edema

PURPOSE

To investigate the effect of vitrectomy on the treatment of diabetic macular edema (DME) refractory to nonsurgical therapies and to determine the preoperative prognostic factors related to surgical outcomes.

METHODS

Seventy-seven eyes from 74 patients who had undergone vitrectomy and macular photocoagulation 2 weeks after vitrectomy for nontractional DME refractory to anti-vascular endothelial growth factor or steroid injection and/or macular grid/focal photocoagulation were included. The eyes were divided into the responsive group (group 1) and the unresponsive group (group 2) according to the postoperative changes in central subfield thickness (CST). The changes in best-corrected visual acuity (BCVA) and CST were compared. Potential preoperative predictors including the size of the foveal avascular zone (FAZ) were assessed.

RESULTS

In group 1 (N = 51), the mean logarithm of the minimum angle of resolution BCVAs at baseline was 0.60 ± 0.30; 6 months after vitrectomy, 0.47 ± 0.28 (vs. baseline, p < 0.001); 12 months after vitrectomy, 0.38 ± 0.28 (vs. baseline, p < 0.001). However, in group 2 (N = 26), no significant change in BCVA was noted. Among the preoperative factors, only the size of the FAZ showed a significant difference between the two groups (0.45 ± 0.17 mm(2) in group 1 vs. 0.59 ± 0.26 mm(2) in group 2; p = 0.020). The enlarged FAZ was also significantly correlated with worse BCVA at the postoperative 6-month follow-up (r = 0.256, p = 0.025).

CONCLUSIONS

Vitrectomy is an effective treatment modality for DME refractory to nonsurgical therapies, especially in cases without enlarged FAZ. Preoperative evaluation of the perfusion status of the macula seems helpful to selecting candidates for vitrectomy.

Assessment of perfused foveal microvascular density and identification of nonperfused capillaries in healthy and vasculopathic eyes

PURPOSE

To analyze the foveal microvasculature of young healthy eyes and older vasculopathic eyes, imaged using in vivo adaptive optics scanning light ophthalmoscope fluorescein angiography (AOSLO FA).

METHODS

AOSLO FA imaging of the superficial retinal microvasculature within an 800-μm radius from the foveal center was performed using simultaneous confocal infrared (IR) reflectance (790 nm) and fluorescence (488 nm) channels. Corresponding IR structural and FA perfusion maps were compared with each other to identify nonperfused capillaries adjacent to the foveal avascular zone. Microvascular densities were calculated from skeletonized FA perfusion maps.

RESULTS

Sixteen healthy adults (26 eyes; mean age 25 years, range, 21-29) and six patients with a retinal vasculopathy (six eyes; mean age 55 years, range, 44-70) were imaged. At least one nonperfused capillary was observed in five of the 16 healthy nonfellow eyes and in four of the six vasculopathic eyes. Compared with healthy eyes, capillary nonperfusion in the vasculopathic eyes was more extensive. Microvascular density of the 16 healthy nonfellow eyes was 42.0 ± 4.2 mm(-1) (range, 33-50 mm(-1)). All six vasculopathic eyes had decreased microvascular densities.

CONCLUSIONS

AOSLO FA provides an in vivo method for estimating foveal microvascular density and reveals occult nonperfused retinal capillaries. Nonperfused capillaries in healthy young adults may represent a normal variation and/or an early sign of pathology. Although limited, the normative data presented here is a step toward developing clinically useful microvascular parameters for ocular and/or systemic diseases.

The association between the foveal avascular zone and retinal thickness

PURPOSE

To investigate the association between the size and shape of the foveal avascular zone and retinal thickness in healthy subjects.

METHODS

In vivo imaging of the foveal microvasculature was performed on 32 subjects by using an adaptive optics scanning laser ophthalmoscope (AOSLO). Motion contrast maps of the AOSLO images were used to generate a montage revealing the foveal capillary network. Foveal avascular zone (FAZ) diameters along the horizontal (FAZH) and vertical (FAZV) meridians were measured on the montages. An asymmetry index (AI) of the FAZ was then computed as the ratio of the FAZH to FAZV. Retinal thickness was investigated by using spectral-domain optical coherence tomography (SDOCT). Inner retinal layer (INLFAZ) thickness and outer nuclear layer (ONLFAZ) thickness were measured at the edges of the FAZ on the horizontal and vertical SDOCT scans on the same eye.

RESULTS

The foveal capillary network was readily visualized in all subjects. As expected there was individual variation in the size and shape of the FAZ. Along the horizontal and vertical meridians, the mean±SD (μm) of the FAZ diameter was 607±217 and 574±155, respectively. The INLFAZ thickness was 68±9 and 66±9, and the ONLFAZ thickness was 103±13 and 105±11, respectively. The mean±SD of the AI was 1.03±0.27. The difference between FAZH and FAZV decreases with increasing FAZ area (P=0.004). Mean ONLFAZ was negatively correlated with FAZ effective diameter (P<0.0001). No significant correlation was found between mean INLFAZ and FAZ effective diameter (P=0.16).

CONCLUSIONS

Despite large individual variations in size and shape of the FAZ, the INLFAZ has a relatively constant thickness at the margins of the FAZ, suggesting the presence of retinal capillaries is needed to sustain an INLFAZ thickness greater than 60 μm. A smaller FAZ area is associated with a vertically elongated FAZ.

Correlation of morphological pattern of optical coherence tomography in diabetic macular edema with systemic risk factors in middle aged males

To study correlation of different optical coherence tomography (OCT) patterns of diabetic macular edema (DME) with systemic risk factors. Institutional cross-sectional double-masked non-interventional study with 330 eyes of middle-aged male type 2 diabetes patients with DME. Various systemic parameters were measured. Diffuse retinal thickening (DRT), cystoid and serous patterns of DME were identified on OCT. Comparison between DRT versus non-DRT and serous versus non-serous eyes was done in respect to systemic parameters. Correlation of serous and DRT pattern was tested with systemic parameters above and below specified values. Mean age was 54.4 ± 7.1 years. Mean duration of diabetes was 8.7 ± 4.2 years. Mean serum globulin level was significantly higher (p = 0.018) in serous compared to non-serous group. Prevalence of serous DME was significantly high in those with serum globulin level >3.5 gm/dl (prevalence ratio = 3.01, p = 0.040). Significant correlation of central macular thickness was observed with duration of diabetes (p = 0.002, r = 0.440).Visual acuity (logMAR) was correlated significantly with HbA1C (p = 0.031, r = 0.305). Increased serum globulin, a positive phase reactant of inflammation, was found significant independent risk factor for development of serous DME. This study did not identify any modifiable systemic factor for any of the OCT patterns in DME.

The oxygen saturation in retinal vessels from diabetic patients depends on the severity and type of vision-threatening retinopathy

PURPOSE

Diabetic retinopathy is characterized by morphological lesions in the retina secondary to disturbances in retinal blood flow which may influence the supply of oxygen to the retinal metabolism. Using retinal oximetry, it has been shown that the oxygen saturation is increased in retinal arterioles and venules from diabetic patients with retinopathy, but oxygenation before the development of retinopathy and possible differences in retinal oxygenation between diabetic maculopathy and proliferative diabetic retinopathy patients have not been evaluated.

METHODS

One-hundred and fifty-six consecutive patients referred for specialist evaluation of diabetic retinopathy, and eighty normal control persons were subjected to retinal oximetry of the larger retinal arterioles and venules. The diabetic patients were allocated to one of four groups with severity of retinopathy ranging from no retinopathy to vision-threatening retinopathy, and the oxygen saturation in arterioles and venules was compared between these groups.

RESULTS

Increasing severity of retinal changes from no retinopathy to diabetic maculopathy was accompanied with increasing oxygen saturation in retinal venules and decreasing oxygen extraction, whereas proliferative diabetic retinopathy showed increased oxygen saturation in both retinal arterioles and venules to result in a normal oxygen extraction.

CONCLUSION

Prospective observational and interventional studies are needed to show whether changes in retinal oxygen saturation precede or follow the development of diabetic retinopathy. Additionally, studies of regional variations in haemodynamic parameters in patients with vision-threatening diabetic retinopathy might improve the understanding of the pathophysiology of diabetic retinopathy. This is a precondition for improving the prevention and treatment of the disease.

Can optical coherence tomography replace fluorescein angiography in detection of ischemic diabetic maculopathy?

Background

We studied the association between (1) the retinal thickness, volume and structure assessed by optical coherence tomography (OCT), and (2) vascular changes around the fovea in FA to define the role of these methods in the detection of ischemic diabetic maculopathy.

Methods

This retrospective observational study included 51 eyes of 51 patients with severe non-proliferative and proliferative diabetic retinopathy, considering that they present advanced ischemic retinal changes. Based on the clinical examination and presence of leakage on FA, patients with clinically significant macular edema were excluded.

On FA, foveal avascular zone (FAZ) corresponds to capillary loss and reflects ischemic processes. Its outline and size were assessed according to the ETDRS standards. Patients were divided in two groups according to the severity of damage of the FAZ outline: ≤ grade 2 (n = 28) and ≥ grade 3 (n = 23).

As we expected ischemia-related damage, SD-OCT images were evaluated for retinal structure, volume, total thickness with division into the outer and inner retina (to assess the influence of two, non-overlapping blood sources) and ganglion cell layer thickness.

Results

The comparisons revealed that at least currently with described methods:

1. There is no significant association between FAZ outline and retinal volume, total thickness, and thickness of the outer and inner retina and ganglion cell layer.

2. There is no significant association between FAZ outline and following retinal structure characteristics: continuity of the external limiting membrane and inner segment/outer segment junction, and identification of the inner/outer retina boundary.

3. The identification of ganglion cell layer boundaries was significantly more difficult in more advanced FAZ outline grades.

4. FAZ size is not correlated with the retinal thickness and volume.

Conclusions

In patients with severe non-proliferative and proliferative diabetic retinopathy, it is not possible to predict FAZ outline/size based solely on the measurements assessing volume and thickness or retinal structure evaluation on OCT. Consequently, at present OCT cannot replace FA in the detection of ischemic diabetic maculopathy.

Predictive factors for the progression of diabetic macular ischemia

PURPOSE

To investigate the predictive factors for diabetic macular ischemia progression through the analysis of fluorescein angiography (FA) parameters.

DESIGN

Retrospective, longitudinal study.

METHODS

Data were collected from 79 eyes of 79 patients with type 2 diabetes mellitus. Macular ischemia severity was assessed using Early Treatment Diabetic Retinopathy Study (ETDRS) protocols and custom software used to quantify the foveal avascular zone (FAZ) area. Patients with ischemia grades "mild," "moderate," or "severe" and at least 2 macula-centered FA images over a minimum of 6 months were included. Main outcome measures were change in macular ischemia grades and FAZ enlargement rate (mm(2)/year).

RESULTS

The median FAZ areas in mild, moderate, and severe ischemia grades at baseline were 0.28, 0.37, and 0.73 mm(2), and significantly increased at the final FA (0.31, 0.41, and 1.23 mm(2)) (P = .001). The median duration of follow-up was 27.5, 31.0, and 24.0 months, and was not significantly different between groups. FAZ enlargement rates were higher in the more advanced ischemia grades--"severe" (0.073 mm(2) [10.4%]/year) compared to "mild" (0.021 mm(2) [7.50%]/year) (P = .02) or "moderate" (0.019 [5.13%] mm(2)/year) (P = .03). A greater ischemia severity grade was predictive for progression (odds ratio [OR] = 2.47, confidence interval [CI] = 1.21-5.05, P = .02). Macular ischemia progression itself was an independent predictive factor for visual acuity loss (OR = 4.60, CI = 1.54-13.7, P = .03).

CONCLUSIONS

The rate of FAZ enlargement ranges from 5%-10% of baseline FAZ area per year in eyes with established ischemia. A greater macular ischemia grade was independently predictive for progression, and diabetic macular ischemia progression itself was predictive of the loss of visual function.

Foveal avascular zone and its relationship to foveal pit shape

PURPOSE

To investigate the retinal microvasculature at the fovea and peripheral retina in humans using an adaptive optics scanning laser ophthalmoscope (AOSLO) and to examine the association of foveal avascular zone (FAZ) and foveal pit morphology.

METHODS

Retinal imaging of the foveal capillary network was performed on 11 subjects (15 eyes; age range 20 to 54 years) with an AOSLO. Standard deviation maps of the AOSLO images were generated from ∼10 to 30 frames, producing high-resolution maps delineating the complete capillary distribution of the retina. Foveal pit morphology was investigated in the same subjects by using a spectral domain optical coherence tomography. In an additional subject, only a relatively large retinal vasculature map was obtained using AOSLO.

RESULTS

A well-demarcated FAZ was seen in 11 subjects tested with foveal capillary imaging. There was considerable individual variation in the size and shape of the FAZ. The mean FAZ area and mean FAZ effective diameter were 0.33 mm(2) and 622 μm, respectively. Foveal thickness was found to be negatively correlated with the FAZ effective diameter.

CONCLUSIONS

The structure of the capillary network could be evaluated in the fovea and parafovea using our approach. We find that a smaller FAZ is associated with a narrower foveal pit opening and a thicker fovea.

Subclinical capillary changes in non-proliferative diabetic retinopathy

PURPOSE

To establish adaptive optics scanning laser ophthalmoscopy as a method to detect and characterize microscopic signs of diabetic retinopathy in capillaries and cone photoreceptors in the parafovea.

METHODS

Recently, adaptive optics scanning laser ophthalmoscope (AOSLO) has enabled noninvasive assessment of photoreceptors, capillaries, and leukocytes in the retinas of live human subjects. Repeated application of AOSLO imaging along with comparison to fluorescein angiography was used to track individual capillaries near the foveal avascular zone (FAZ) from one eye affected with severe non-proliferative diabetic retinopathy. Fluorescein angiography was used to identify clinical signs of diabetic retinopathy, such as microaneurysms and intraretinal microvascular abnormalities, and corresponding regions were imaged and assessed using the AOSLO. In addition, the structural integrity of photoreceptors and the spatial distribution of leukocytes around the parafoveal capillary network were quantitatively assessed.

RESULTS

Capillaries and cone photoreceptors were visualized using the AOSLO without the use of injected contrast agents. Although the majority of capillaries were stable over a period of 16 months, one capillary at the edge of the FAZ dropped out, leading to a small but significant increase in FAZ size. Longitudinal assessment of the capillaries also showed microaneurysm formation and disappearance as well as the formation of tiny capillary bends similar in appearance to intraretinal microvascular abnormalities. The leukocytes in the capillary network were found to preferentially travel through the same routes in all four visits, suggesting that these channels are robust against small changes to the surrounding capillaries. In this eye, cone photoreceptor spacing was increased in the fovea when compared with normal data but stable across all visits.

CONCLUSIONS

AOSLO imaging can be used to longitudinally track capillaries, leukocytes, and photoreceptors in diabetic retinopathy. Capillary changes that can be detected include dropout of individual capillaries as well as formation and disappearance of microaneurysms.

Vision, retinal thickness, and foveal avascular zone size after intravitreal bevacizumab for diabetic macular edema

INTRODUCTION

To investigate three monthly intravitreal bevacizumab (IVB) injections effects in chronic diabetic macular edema (DME).

METHODS

A prospective, noncomparative study in which inclusion criteria were; DME with central macular thickness (CMT) of at least 250 μm, and no treatment for diabetic retinopathy (DR) within 4 months before the first injection. All eyes received three monthly 1.25 mg IVB injections. CMT by optical coherence tomography, visual acuity (VA), foveal avascular zone (FAZ) greatest linear dimension (GLD), and area of FAZ by fundus fluorescein angiography were documented initially and 1 month after last injection. Outcomes (P<0.05 were significant) and correlations (r values) were analyzed.

RESULTS

A total of 29 eyes of 29 patients (group 1, 19 female, 10 male), aged 60.7±6.6 years were analyzed. The patients were split into two groups; group 2 included 15 mild-to-moderate nonproliferative DR, and group 3 included 14 more-severe DR. VA gain was significant in all groups (P<0.05). Mean CMT decrease was approximately 46, 36, and 55 μm in groups 1, 2, and 3, respectively (P<0.05 only in group 1). A 0.045-mm2 increase in FAZ area was obtained in group 1 (P<0.05). In group 2, an increase in GLD and area of FAZ was 0.048 mm and 0.058 mm2, respectively (P<0.05), whereas in group 3, FAZ enlargement was nonsignificant. VA and CMT were significantly correlated (r values=0.5-0.6), except for the final VA-final CMT in group 2. FAZ dimensions and other parameters (VA and CMT) were noncorrelated.

CONCLUSION

According to the authors' short-term results, three monthly IVB injections can be used for chronic DME regardless of VA, CMT, or FAZ dimensions, despite the FAZ enlargement encountered, especially in cases with milder DR.

Relationship between the foveal avascular zone and foveal pit morphology

PURPOSE

To assess the relationship between foveal pit morphology and size of the foveal avascular zone (FAZ).

METHODS

Forty-two subjects were recruited. Volumetric images of the macula were obtained using spectral domain optical coherence tomography. Images of the FAZ were obtained using either a modified fundus camera or an adaptive optics scanning light ophthalmoscope. Foveal pit metrics (depth, diameter, slope, volume, and area) were automatically extracted from retinal thickness data, whereas the FAZ was manually segmented by two observers to extract estimates of FAZ diameter and area.

RESULTS

Consistent with previous reports, the authors observed significant variation in foveal pit morphology. The average foveal pit volume was 0.081 mm(3) (range, 0.022 to 0.190 mm(3)). The size of the FAZ was also highly variable between persons, with FAZ area ranging from 0.05 to 1.05 mm(2) and FAZ diameter ranging from 0.20 to 1.08 mm. FAZ area was significantly correlated with foveal pit area, depth, and volume; deeper and broader foveal pits were associated with larger FAZs.

CONCLUSIONS

Although these results are consistent with predictions from existing models of foveal development, more work is needed to confirm the developmental link between the size of the FAZ and the degree of foveal pit excavation. In addition, more work is needed to understand the relationship between these and other anatomic features of the human foveal region, including peak cone density, rod-free zone diameter, and Henle fiber layer.

In diabetic eyes, multifocal ERG reflects differences in function between the nasal part and the temporal part of the macula

PURPOSE

The purpose of the present study was to compare retinal function between the perifoveal nasal and perifoveal temporal areas of diabetic eyes using multifocalERG (mfERG).

METHODS

We included 36 eyes from 27 patients with diabetes (age 58 ± 14 years; duration of diabetes 13 ± 9 years; HbA(1c) 7.1 ± 1.8%) and a control group with 18 eyes from 18 healthy subjects (age 57 ± 11 years). Retinal thickness was assessed with optical coherence tomography (OCT) in the perifoveal areas corresponding to the summed nasal and temporal inner and outer areas. MfERG amplitude and implicit time were recorded from corresponding areas.

RESULTS

Diabetic eyes showed lower mfERG amplitude in the nasal area than in the temporal area (14 ± 6 vs 17 ± 7 nV/deg(2); p < 0.0001) and longer implicit time (31 ± 3 vs 30 ± 3 ms; p = 0.005). In the control group, there were no significant differences between the two areas.

CONCLUSION

Diabetic eyes showed lower amplitude and longer implicit time in the nasal area than in the temporal, which might indicate that the nasal area is more vulnerable. These findings may be of importance for evaluation of diabetic maculopathy and outcome after laser treatment.

Noninvasive imaging of the foveal avascular zone with high-speed, phase-variance optical coherence tomography

PURPOSE

To demonstrate the application of phase-variance optical coherence tomography (pvOCT) for contrast agent-free in vivo imaging of volumetric retinal microcirculation in the human foveal region and for extraction of foveal avascular zone dimensions.

METHODS

A custom-built, high-speed Fourier-domain OCT retinal imaging system was used to image retinas of two healthy subjects and eight diabetic patients. Through the acquisition of multiple B-scans for each scan location, phase differences between consecutive scans were extracted and used for phase-variance contrast, identifying motion signals from within blood vessels and capillaries. The en face projection view of the inner retinal layers segmented out from volumetric pvOCT data sets allowed visualization of a perfusion network with the foveal avascular zone (FAZ). In addition, the authors presented 2D retinal perfusion maps with pseudo color-coded depth positions of capillaries.

RESULTS

Retinal vascular imaging with pvOCT provides accurate measurements of the FAZ area and its morphology and a volumetric perfusion map of microcapillaries. In this study using two images from each fundus fluorescein angiography (FA) and pvOCT, the measured average areas of the FAZ from two healthy subjects were below 0.22 mm(2), and each of eight diabetic patients had an enlarged FAZ area, larger than 0.22 mm(2). Moreover, the FAZ areas demonstrated a significant correlation (r = 0.91) between measurements from FA and pvOCT.

CONCLUSIONS

The high-speed pvOCT allows contrast agent-free visualization of capillary networks in the human foveal region that is analogous to fundus FA imaging. This could allow for noninvasive diagnosis and progression monitoring of diabetic retinopathy in clinical settings.

Macular microcirculation in patients with epiretinal membrane before and after surgery

BACKGROUND

To investigate blood flow velocity (BFV) in the perifoveal capillaries before and after vitreous surgery for patients with epiretinal membrane (ERM).

METHODS

Twenty-one eyes in patients with ERM and 16 eyes in healthy subjects were involved in this study. Fluorescein angiography was performed using a scanning laser ophthalmoscope and BFV was analyzed by the tracing method. Foveal thickness (FT) was measured by optical coherence tomography.

RESULTS

BFV was significantly slower in the ERM patients (1.04 ± 0.10 mm/s) than in the healthy subjects (1.49 ± 0.11 mm/s ) (p = 0.0010). BFV in the ERM patients 6 months after vitreous surgery (6 M) (1.21 ± 0.02 mm/s) significantly increased compared with BFV before surgery (0 M) (1.04 ± 0.10 mm/s) (p = 0.0061). BFV 1 year after vitreous surgery (1 Y) significantly increased (1.38 ± 0.02 mm/s) compared with BFV(6 M) (1.21 ± 0.02 mm/s) (p = 0.0235). FT was significantly greater in the ERM patients (351.7 ± 87.1 μm) than in the healthy subjects (158.9 ± 16.9 μm) (p = 0.0011). FT (6 M) significantly decreased (285.3 ± 36.9 μm) compared with FT before surgery (0 M) (351.7 ± 87.1 μm) (p = 0.0212). FT did not show significant differences between (6 M) and (1 Y). No significant correlation was found between BFV and FT before surgery.

CONCLUSIONS

Perifoveal capillary BFV in patients with ERM was slower than that in the healthy subjects, and significantly improved after vitreous surgery as time progressed. It can be said that perifoveal capillary BFV is related to the development and improvement of ERM in the long term.

Imaging retinal capillaries using ultrahigh-resolution optical coherence tomography and adaptive optics

PURPOSE

Ultrahigh-resolution optical coherence tomography (UHR-OCT) with adaptive optics (AO) provides micrometer-scale 3D resolution that is attractive for imaging the retinal microvasculature. Such imaging may be useful for early detection of pathologic changes as in diabetic retinopathy. Here the authors investigate this potential for detecting individual capillaries in healthy subjects.

METHODS

UHR-AO-OCT volumes centered on the fovea were acquired from seven subjects (age range, 25-61 years) with three preselected with no foveal avascular zone (FAZ). Images were compared with entoptic diagrams using the capillaries at the rim of the FAZ. Methods of comparison were testing for the presence of a FAZ, noting distinct features in the capillary pattern, and measuring the size of the FAZ. Additional analysis included measurements of capillary diameter and depth range with retinal eccentricity.

RESULTS

UHR-AO-OCT results are consistent with entoptic observations for all three methods of comparison. FAZ diameters measured by UHR-AO-OCT and entoptic imaging are strongly correlated (R(2) = 0.86). Average capillary diameter near the FAZ rim is 5.1 (4.6) ± 1.4 μm, with the value in parentheses accounting for axial image blur. This is consistent with histology (average, ~4.7 μm). Depth range of the capillaries increases monotonically with eccentricity (0°-1.25°) and is larger and more variable for subjects without FAZ.

CONCLUSIONS

UHR-AO-OCT permits observation of many of the capillaries proximal to the FAZ, including those of average size based on published histology. This supports the view that the vast majority of capillaries in the retina are likely detectable with UHR-AO-OCT.

High-resolution wide-field imaging of perfused capillaries without the use of contrast agent

Purpose:

Assessment of capillary abnormalities facilitates early diagnosis, treatment, and follow-up of common retinal pathologies. Injected contrast agents like fluorescein are widely used to image retinal capillaries, but this highly effective procedure has a few disadvantages, such as untoward side effects, inconvenience of injection, and brevity of the time window for clear visualization. The retinal function imager (RFI) is a tool for monitoring retinal functions, such as blood velocity and oximetry, based on intrinsic signals. Here we describe the clinical use of hemoglobin in red blood cells (RBCs) as an intrinsic motion-contrast agent in the generation of detailed noninvasive capillary-perfusion maps (nCPMs).

Patients and methods:

Multiple series of nCPM images were acquired from 130 patients with diabetic retinopathy, vein occlusion, central serous retinopathy, age-related macular degeneration, or metabolic syndrome, as well as from 37 healthy subjects. After registration, pixel value distribution parameters were analyzed to locate RBC motion.

Results:

The RFI yielded nCPMs demonstrating microvascular morphology including capillaries in exquisite detail. Maps from the same subject were highly reproducible in repeated measurements, in as much detail and often better than that revealed by the very best fluorescein angiography. In patients, neovascularization and capillary nonperfusion areas were clearly observed. Foveal avascular zones (FAZ) were sharply delineated and were larger in patients with diabetic retinopathy than in controls (FAZ diameter: 641.5 ± 82.3 versus 463.7 ± 105 μm; P < 0.001). Also visible were abnormal vascular patterns, such as shunts and vascular loops.

Conclusion:

Optical imaging of retinal capillaries in human patients based on motion contrast is noninvasive, comfortable, safe, and can be repeated as often as required for early diagnosis, treatment guidance, and follow up of retinal disease progression.

Hemorheology and microvascular disorders

The present review presents basic concepts of blood rheology related to vascular diseases. Blood flow in large arteries is dominated by inertial forces exhibited at high flow velocities, while viscous forces (i.e., blood rheology) play an almost negligible role. When high flow velocity is compromised by sudden deceleration as at a bifurcation, endothelial cell dysfunction can occur along the outer wall of the bifurcation, initiating inflammatory gene expression and, through mechanotransduction, the cascade of events associated with atherosclerosis. In sharp contrast, the flow of blood in microvessels is dominated by viscous shear forces since the inertial forces are negligible due to low flow velocities. Shear stress is a critical parameter in microvascular flow, and a force-balance approach is proposed for determining microvascular shear stress, accounting for the low Reynolds numbers and the dominance of viscous forces over inertial forces. Accordingly, when the attractive forces between erythrocytes (represented by the yield stress of blood) are greater than the shear force produced by microvascular flow, tissue perfusion itself cannot be sustained, leading to capillary loss. The yield stress parameter is presented as a diagnostic candidate for future clinical research, specifically, as a fluid dynamic biomarker for microvascular disorders. The relation between the yield stress and diastolic blood viscosity (DBV) is described using the Casson model for viscosity, from which one may be able determine thresholds of DBV where the risk of microvascular disorders is high.

Imaging of the parafoveal capillary network and its integrity analysis using fractal dimension

Using a spectral domain OCT system, equipped with a broadband Ti:sapphire laser, we imaged the human retina with 5 µm x 1.3 µm transverse and axial resolution at acquisition rate of 100 kHz. Such imaging speed significantly reduces motion artifacts. Combined with the ultra-high resolution, this allows observing microscopic retinal details with high axial definition without the help of adaptive optics. In this work we apply our system to image the parafoveal capillary network. We demonstrate how already on the intensity level the parafoveal capillaries can be segmented by a simple structural high pass filtering algorithm. This data is then used to quantitatively characterize the capillary network of healthy and diseased eyes. We propose to use the fractal dimension as index for capillary integrity of pathologic disorders.

Imaging of the parafoveal capillary network and its integrity analysis using fractal dimension

Using a spectral domain OCT system, equipped with a broadband Ti:sapphire laser, we imaged the human retina with 5 µm x 1.3 µm transverse and axial resolution at acquisition rate of 100 kHz. Such imaging speed significantly reduces motion artifacts. Combined with the ultra-high resolution, this allows observing microscopic retinal details with high axial definition without the help of adaptive optics. In this work we apply our system to image the parafoveal capillary network. We demonstrate how already on the intensity level the parafoveal capillaries can be segmented by a simple structural high pass filtering algorithm. This data is then used to quantitatively characterize the capillary network of healthy and diseased eyes. We propose to use the fractal dimension as index for capillary integrity of pathologic disorders.

Adaptive optics and the eye (super resolution OCT)

The combination of adaptive optics (AO) and optical coherence tomography (OCT) was first reported 8 years ago and has undergone tremendous technological advances since then. The technical benefits of adding AO to OCT (increased lateral resolution, smaller speckle, and enhanced sensitivity) increase the imaging capability of OCT in ways that make it well suited for three-dimensional (3D) cellular imaging in the retina. Today, AO-OCT systems provide ultrahigh 3D resolution (3 × 3 × 3 μm³) and ultrahigh speed (up to an order of magnitude faster than commercial OCT). AO-OCT systems have been used to capture volume images of retinal structures, previously only visible with histology, and are being used for studying clinical conditions. Here, we present representative examples of cellular structures that can be visualized with AO-OCT. We overview three studies from our laboratory that used ultrahigh-resolution AO-OCT to measure the cross-sectional profiles of individual bundles in the retinal nerve fiber layer; the diameters of foveal capillaries that define the terminal rim of the foveal avascular zone; and the spacing and length of individual cone photoreceptor outer segments as close as 0.5° from the fovea center.

Speed quantification and tracking of moving objects in adaptive optics scanning laser ophthalmoscopy

Microscopic features of the human retina can be resolved noninvasively using an adaptive optics scanning laser ophthalmoscope (AOSLO). We describe an improved method to track and quantify the speed of moving objects in AOSLO videos, which is necessary for characterizing the hemodynamics of retinal capillaries. During video acquisition, the objects of interest are in constant motion relative to the background tissue (object motion). The background tissue is in constant motion relative to the AOSLO, due to continuous eye motion during video recordings (eye motion). The location at which AOSLO acquires data is also in continuous motion, since the imaging source is swept in a raster scan across the retina (raster scanning). We show that it is important to take into consideration the combination of object motion, eye motion, and raster scanning for accurate quantification of object speeds. The proposed methods performed well on both experimental AOSLO videos as well as synthetic videos generated by a virtual AOSLO. These methods improve the accuracy of methods to investigate hemodynamics using AOSLO imaging.

Comprehensive in vivo micro-vascular imaging of the human eye by dual-beam-scan Doppler optical coherence angiography

Comprehensive angiography provides insight into the diagnosis of vascular-related diseases. However, complex microvascular networks of unstable in vivo organs such as the eye require micron-scale resolution in three dimensions and a high sampling rate to access a wide area as maintaining the high resolution. Here, we introduce dual-beam-scan Doppler optical coherence angiography (OCA) as a label-free comprehensive ophthalmic angiography that satisfies theses requirements. In addition to high resolution and high imaging speed, high sensitivity to motion for detecting tiny blood flow of microvessels is achieved by detecting two time-delayed signals with scanning of two probing beams separated on a sample. We present in vivo three-dimensional imaging of the microvasculature of the posterior part of the human eye. The demonstrated results show that this technique may be used for comprehensive ophthalmic angiography to evaluate the vasculature of the posterior human eye and to diagnose variety of vascular diseases.

Reduced retinal blood flow velocity in diabetic retinopathy

PURPOSE

The purpose of this study was to compare the retinal blood flow velocities of patients with diabetes and healthy control subjects. We used a novel device offering a noninvasive diagnostic of retinal function.

METHODS

Flow velocities in retinal arterioles and venules were quantitatively analyzed by retinal function imager scanning in 58 eyes of 42 patients with nonproliferative diabetic retinopathy and 51 eyes of 32 normal subjects. Group differences were assessed by the mixed-model effect.

RESULTS

Average velocity in arterial compartments (in mm/s) was 3.74 +/- 1.09 for the diabetic group and 4.19 +/- 0.99 for the control subjects. The average velocity of all segments, taking associated heart rate and individual segment widths into account, was 17% slower in the diabetic group (P < 0.0001). In both groups, average venous compartment velocity was lower than the arterial velocity (2.61 +/- 0.65 for the diabetic group; 3.03 +/- 0.59 for the control subjects). Individual vein velocities, taking heart rate and segment widths into account, was 17% slower, on average, in the diabetic group (P < 0.0001).

CONCLUSION

Our measurement showed significantly decreased flow velocities in the retinal arterioles and venules of patients with diabetes compared with healthy control subjects, supporting the view of abnormal vessel function in eyes with nonproliferative diabetic retinopathy.

Blood velocity pulse quantification in the human conjunctival pre-capillary arterioles

Axial red blood cell velocity pulse was quantified throughout its period by high speed video microcinematography in the human eye. In 30 conjunctival precapillary arterioles (6 to 12 microm in diameter) from 15 healthy humans, axial velocities ranged from 0.4 (the minimum of all the end diastolic values) to 5.84 mm/s (the maximum of all the peak systolic values). With the velocity pulse properly quantified, two parameters can be estimated: (1) the average velocity of the pulse during a cardiac cycle AVV (average velocity value) and (2) the magnitude of the pulsation using Pourcelot's resistive index RI. These parameters are important for the estimation of other hemodynamic parameters such as the average volume flow and the average shear stress. The results of this study revealed that the AVV in the human precapillary arterioles ranged between 0.52 and 3.26 mm/s with a mean value for all microvessels of 1.66 mm/s+/-0.11(SE). The RI ranged between 35.5% and 81.8% with a mean value of 53.1%+/-2.2. Quantitative information was obtained for the first time on the velocity pulse characteristics just before the human capillary bed.

Noninvasive visualization and analysis of parafoveal capillaries in humans

PURPOSE

To demonstrate a noninvasive method to visualize and analyze the parafoveal capillary network in humans.

METHODS

An adaptive optics scanning laser ophthalmoscope was used to acquire high resolution retinal videos on human subjects. Video processing tools that enhance motion contrast were developed and applied to the videos to generate montages of parafoveal retinal capillaries. The capillary network and foveal avascular zone (FAZ) were extracted using video and image analysis algorithms. The capillary densities in the zone immediately outside the FAZ were calculated and the variation in density as a function of direction was investigated. Extracted FAZ geometries were used to calculate area and effective diameters. The authors also compared their method against fluorescein angiography (FA) for one subject.

RESULTS

The parafoveal capillaries were clearly visible when the motion contrast in noninvasive videos was enhanced. There was a marked improvement in the contrast of the parafoveal capillaries when compared to the unprocessed videos. The average FAZ area was 0.323 mm(2), with an average effective diameter of 633 microm. There was no variation in capillary density near the FAZ in different directions.

CONCLUSIONS

Using motion cues to enhance vessel contrast is a powerful tool for visualizing the capillary network, in the absence of contrast agents. The authors demonstrate a tool to study the microcirculation of healthy subjects noninvasively.

Macular microcirculation in hypertensive patients with and without branch retinal vein occlusion

PURPOSE

Our purpose was to determine whether a reduction in blood flow velocity (BFV) in the perifoveal capillaries is involved in the pathogenesis of branch retinal vein occlusion (BRVO) in patients with hypertension.

METHODS

Subjects included hypertensive patients with (n = 12) and without (n = 16) BRVO and healthy volunteers (n = 16). Perifoveal BFV was measured by the tracing method using fluorescein angiography and a scanning laser ophthalmoscope. Logistic regression analysis was performed to assess factors that influenced the presence or absence of BRVO.

RESULTS

Mean BFV showed a significant decrease across the three groups (healthy controls: 1.49 +/- 0.11 mm/second; hypertensive patients without BRVO: 1.36 +/- 0.12 mm/second; hypertensive patients with BRVO: 1.16 +/- 0.24 mm/second; p(trend) < 0.001). Multivariate logistic regression analysis showed that BFV was a significant risk factor for the presence of BRVO.

CONCLUSIONS

Perifoveal capillary BFV is reduced in hypertensive patients with and without BRVO. It is possible that a decrease in BFV may be involved in the occurrence of BRVO. Measurement of perifoveal capillary BFV may be useful for investigating the pathogenesis and progression of BRVO.

Diabetic patients with retinopathy show increased retinal venous oxygen saturation

BACKGROUND

Longstanding diabetes mellitus results in a disturbed microcirculation. A new imaging oximeter was used to investigate the effect of this disturbance on retinal vessel oxygen saturation.

METHODS

The haemoglobin oxygen saturation was measured in the retinal arterioles and venules of 41 diabetic patients (65 +/- 12.3 years) with mild non-proliferative through proliferative diabetic retinopathy (DR). Twelve individuals (61.3 +/- 6.2 years, mean +/- standard deviation) without systemic or ocular disease were investigated as controls. Measurements were taken by an imaging oximeter (oxygen module by Imedos GmbH, Jena). This technique is based on the proportionality of the oxygen saturation and ratio of the optical density of the vessel at two wavelengths (548 nm and 610 nm).

RESULTS

Whereas there were no significant differences in the arterial oxygen saturation between controls and diabetic retinopathy at any stage, the venous oxygen saturation increased in diabetic patients with the severity of the retinopathy: controls 63 +/- 5%, mild non-proliferative DR 69 +/- 7%, moderate non-proliferative DR 70 +/- 5%, severe non-proliferative DR, 75 +/- 5%, and proliferative DR 75 +/- 8%.

CONCLUSIONS

The increase of retinal vessel oxygen saturation in diabetic retinopathy points to a diabetic microvascular alteration. This may be due to occlusions and obliterations in the capillary bead and the formation of arterio-venous shunt vessels. On the other hand, hyperglycaemia-induced endothelial dysfunction, with subsequent suppression of the endothelial NO-synthase and disturbance of the vascular auto-regulation, may contribute to retinal tissue hypoxia.

Ocular blood flow in diabetes and age-related macular degeneration

The 2 leading causes of blindness in adults in the industrialized nations, diabetic retinopathy and age-related macular degeneration, have been investigated thoroughly with respect to their pathogenesis. In recent years, it has been discovered that dysfunctional ocular microcirculation appears to play a part in the development of both diseases. In diabetic retinopathy, it has been shown that the disease is associated with early retinal vascular dysregulation. In the later states of the disease, retinal tissue hypoxia is a major trigger of sight-threatening neovascularization. In age-related macular degeneration, there is increasing evidence that reduced blood flow in the choroid is associated with the development and progression of the disease. Knowledge of the pathophysiological vascular states underlying these diseases is essential for the assessment and development of future therapies.

Regulation of retinal blood flow in health and disease

Optimal retinal neuronal cell function requires an appropriate, tightly regulated environment, provided by cellular barriers, which separate functional compartments, maintain their homeostasis, and control metabolic substrate transport. Correctly regulated hemodynamics and delivery of oxygen and metabolic substrates, as well as intact blood-retinal barriers are necessary requirements for the maintenance of retinal structure and function. Retinal blood flow is autoregulated by the interaction of myogenic and metabolic mechanisms through the release of vasoactive substances by the vascular endothelium and retinal tissue surrounding the arteriolar wall. Autoregulation is achieved by adaptation of the vascular tone of the resistance vessels (arterioles, capillaries) to changes in the perfusion pressure or metabolic needs of the tissue. This adaptation occurs through the interaction of multiple mechanisms affecting the arteriolar smooth muscle cells and capillary pericytes. Mechanical stretch and increases in arteriolar transmural pressure induce the endothelial cells to release contracting factors affecting the tone of arteriolar smooth muscle cells and pericytes. Close interaction between nitric oxide (NO), lactate, arachidonic acid metabolites, released by the neuronal and glial cells during neural activity and energy-generating reactions of the retina strive to optimize blood flow according to the metabolic needs of the tissue. NO, which plays a central role in neurovascular coupling, may exert its effect, by modulating glial cell function involved in such vasomotor responses. During the evolution of ischemic microangiopathies, impairment of structure and function of the retinal neural tissue and endothelium affect the interaction of these metabolic pathways, leading to a disturbed blood flow regulation. The resulting ischemia, tissue hypoxia and alterations in the blood barrier trigger the formation of macular edema and neovascularization. Hypoxia-related VEGF expression correlates with the formation of neovessels. The relief from hypoxia results in arteriolar constriction, decreases the hydrostatic pressure in the capillaries and venules, and relieves endothelial stretching. The reestablished oxygenation of the inner retina downregulates VEGF expression and thus inhibits neovascularization and macular edema. Correct control of the multiple pathways, such as retinal blood flow, tissue oxygenation and metabolic substrate support, aiming at restoring retinal cell metabolic interactions, may be effective in preventing damage occurring during the evolution of ischemic microangiopathies.

Relationship of Macular Microcirculation and Retinal Thickness with Visual Acuity in Diabetic Macular Edema

PURPOSE

To assess perifoveal capillary blood flow velocity (BFV), capillary occlusion, and retinal thickness at the central fovea in diabetic patients with or without clinically significant macular edema; to examine the relationships of these variables with visual acuity (VA); and to identify their contributions to visual outcome and diabetic macular edema.

DESIGN

Comparative cross-sectional prospective study.

PARTICIPANTS AND CONTROLS

Diabetic patients with clinically significant macular edema (CSME) (n = 22), matched diabetic patients without CSME (n = 22), and healthy volunteers (n = 16).

METHODS

Capillary BFV was measured by fluorescein angiography using a scanning laser ophthalmoscope and was analyzed by the tracing method. Severity of perifoveal capillary abnormalities was classified by the size and outline of the foveal avascular zone (FAZ) and extent of foveal capillary loss. Macular thickness was measured by optical coherence tomography. Each subject underwent a complete ophthalmic evaluation, and best-corrected VA (BCVA) was converted to the logarithm of the minimum angle of resolution scale.

MAIN OUTCOME MEASURES

Relationship of perifoveal capillary BFV, capillary occlusion, and foveal thickness with VA.

RESULTS

Best-corrected VA significantly differed among all 3 groups (P<0.0001). Best-corrected VA correlated negatively with BFV (r = -0.644, P<0.0001) among all subjects and positively with retinal thickness at the central fovea in diabetic patients with CSME (r = 0.640, P = 0.0013). There was a positive correlation between BCVA and severity in the size of the FAZ (r = 0.484, P = 0.0015), outline of the FAZ (r = 0.542, P = 0.0004), and extent of foveal capillary loss (r = 0.585, P = 0.0001) among all diabetic subjects. Multiple regression analysis showed that retinal thickness at the central fovea was the only variable that significantly predicted VA (standardized regression coefficient, 0.635; P = 0.0001).

CONCLUSIONS

Best-corrected VA was associated with perifoveal capillary BFV, severity of perifoveal capillary occlusion, and retinal thickness at the central fovea in diabetic patients, but the greatest contributing factor was only the retinal thickness.

Retinal arteriolar hemodynamic response to an acute hyperglycemic provocation in early and sight-threatening diabetic retinopathy

The aim was to quantify the magnitude of retinal arteriolar vascular reactivity to a hyperglycemic provocation in diabetic patients stratified by severity of retinopathy and in age-matched controls. The sample comprised 20 non-diabetic controls (Group 1), 19 patients with no clinically visible DR (Group 2), 18 patients with mild-to-moderate non-proliferative DR (Group 3) and 18 patients with diabetic macular edema (Group 4). Retinal hemodynamic measurements using the Canon Laser Blood Flowmeter (CLBF-100) were acquired before and 1 h after drinking a standardized oral glucose load drink. The magnitude of the retinal vascular response, as well as max:min velocity ratio and wall shear rate (WSR), was calculated and compared across groups. A significant change in blood glucose level was observed for all groups (p<0.05). The change in blood glucose elevation was significantly less in Group 1 compared to the other groups. No significant change in arteriolar diameter, blood velocity, blood flow, max:min velocity ratio and WSR was found in patients with diabetes and in age-matched subjects without diabetes. The results of this study indicate that retinal arteriolar blood flow is unaffected by acute elevation of blood glucose using an oral glucose load drink in patients with diabetes and in age-matched controls. This lack of a blood flow response to acute hyperglycemia in patients with early sight-threatening DR may be explained by a loss of retinal vascular reactivity.

Relationship between Macular Microcirculation and Progression of Diabetic Macular Edema

PURPOSE

To investigate the relationship between blood flow velocity in perifoveal capillaries and retinal thickness at the central fovea in diabetic patients with clinically significant macular edema to elucidate the pathogenesis of diabetic macular edema.

DESIGN

Comparative, cross-sectional, prospective study.

PARTICIPANTS

Participants included diabetic patients with clinically significant macular edema (n = 22), matched diabetic patients without clinically significant macular edema (n = 22), and healthy volunteers (n = 16).

METHODS

Perifoveal capillary blood flow velocity was measured with fluorescein angiography using a scanning laser ophthalmoscope and was analyzed by the tracing method. The severity of perifoveal capillary abnormalities among diabetic patients without clinically significant macular edema was classified by the size and outline of the foveal avascular zone and extent of foveal capillary loss at the baseline by the use of fluorescein angiograms. Retinal thickness at the central fovea was measured by optical coherence tomography.

MAIN OUTCOME MEASURES

Relationship between perifoveal capillary blood flow velocity and retinal thickness at the central fovea.

RESULTS

Perifoveal capillary blood flow velocity was significantly slower in the patients with clinically significant macular edema (0.94+/-0.09 mm/second) compared with the patients without clinically significant macular edema (1.22+/-0.20 mm/second) and the healthy volunteers (1.49+/-0.11 mm/second; P<0.0001). Capillary blood flow velocity negatively correlated with retinal thickness at the central fovea (r = -0.4018, P<0.0001). Capillary blood flow velocity varied among diabetic patients without clinically significant macular edema, and there were significant negative correlations between capillary blood flow velocity and the size of the foveal avascular zone (r = -0.52, P = 0.01), the outline of the foveal avascular zone (r = -0.44, P = 0.04), and the extent of foveal capillary loss (r = -0.51, P = 0.01).

CONCLUSIONS

The reduction of perifoveal capillary blood flow velocity may occur before the increase of retinal thickness at the central fovea in the diabetic patients. It is possible that long-term reduction of blood flow velocity increases retinal thickness of the macula and may play an important role in the development and the progression of diabetic macular edema.

Multifocal electroretinography and fluorescein angiography in retinal vein occlusion

PURPOSE

To interpret the responses of multifocal electroretinography (mfERG) of the central macula in retinal vein occlusion and correlate them with findings of fluorescein angiography, optical coherence tomography (OCT), and visual acuity.

METHODS

In a prospective observational case series, 25 patients with branch or hemiretinal vein occlusion were examined by means of mfERG, fluorescein angiography, OCT, and visual acuity.

RESULTS

In eyes with macular ischemia shown by fluorescein angiography, the implicit time was significantly longer than that in eyes without macular ischemia (P = 0.006; Mann-Whitney test). The amplitudes in the first-order kernel responses in the ischemic eyes were more reduced (P = 0.04; Mann-Whitney test), and the amplitudes in the central macula correlated well with visual acuity (P = 0.006; Pearson correlation). There was no significant correlation between the degree of retinal thickening as measured by OCT and the mfERG responses, although there was a trend toward reduced amplitudes in the maculae with central retinal thickening (P = 0.08; Pearson correlation).

CONCLUSION

Macular ischemia as measured by fluorescein angiography correlates well with the prolonged implicit time on the first-order kernel response in mfERG, but no significant correlation was detected between the responses of mfERG and the OCT findings. Further investigations will demonstrate if this can be used as a prognostic instrument in the selection of patients, who will most likely benefit from future treatments.

Tracing method in the assessment of retinal capillary blood flow velocity by fluorescein angiography with scanning laser ophthalmoscope

PURPOSE

To evaluate the feasibility of a new method (the tracing method) for measuring perifoveal capillary blood flow velocity (BFV).

METHODS

The BFV in the perifoveal capillaries was measured in 12 eyes of healthy subjects and 12 eyes of patients with clinically significant macular edema (CSME) by fluorescein angiography using a scanning laser ophthalmoscope by either the tracing method or the conventional method. A randomized crossover design was employed to assign the subjects to each method.

RESULTS

The number of capillaries recognized by the tracing method in healthy subjects and in patients with CSME was significantly higher than that recognized with the conventional method (P = 0.0134 and P = 0.0108, respectively). The number of fluorescent dots detected by the tracing method in healthy subjects and in patients with CSME was also significantly higher than that detected with the conventional method (P = 0.0002 and P = 0.0137, respectively). Accurate detection of the movement of fluorescent dots and analysis of BFV were possible with the tracing method. The BFV varied within each perifoveal capillary, and the mean BFV also varied according to capillary location in the macula.

CONCLUSION

The new tracing method is useful for analyzing the retinal capillary BFV in healthy subjects and in patients with CSME.

Semi-automated detection of the foveal avascular zone in fluorescein angiograms in diabetes mellitus

BACKGROUND

The foveal avascular zone (FAZ) is known to enlarge in diabetic retinopathy. In a preliminary study, the authors applied a region growing algorithm to fluorescein angiograms to detect the FAZ in a semi-automated fashion.

METHODS

The FAZ in 44 fluorescein angiograms of 44 eyes of 41 patients with diabetic retinopathy underwent manual outlining, then analysis with the region growing function of the ENVI image analysis software. The same algorithm was applied after median filtering of the images.

RESULTS

Correlation coefficient was 0.98 between the first two authors, 0.89 between the first author and semi-automated detection before median filtering and 0.91 after median filtering. Average surface areas however, were smaller with semi-automated detection (1951 pixels before and 2226 pixels after median filtering) than with manual detection (3032 pixels).

CONCLUSIONS

Semi-automated detection of the FAZ is possible, but refinements need to be made in angiogram quality and/or image pretreatment to improve results.