Age-Related Decline of Retinal Oxygen Extraction in Healthy Subjects

Purpose

To investigate the age-dependence of total retinal blood flow and total retinal oxygen extraction in healthy subjects and determine their possible correlations with structural optical coherence tomography (OCT) parameters.

Methods

This observational cross-sectional study consisted of 68 healthy subjects (mean ± SD age, 45.6 ± 16.3 years; 47% female). Total retinal oxygen extraction was calculated based on measurement of total retinal blood flow using bi-directional Doppler OCT and measurement of oxygen saturation using spectroscopic reflectometry. Retinal nerve fiber layer thickness was measured using OCT, and the total number of retinal ganglion cells was estimated based on a previous published model. Correlation of these parameters with age was studied and the association between structural OCT parameters and hemodynamic vascular parameters was calculated.

Results

Both structural and vascular parameters showed a significant decline with increasing age. The correlation coefficients were between r = -0.25 and r = -0.41. Furthermore, structural and vascular parameters were significantly correlated with each other. The strongest association was found between the level of total retinal oxygen extraction and the number of retinal ganglion cells (r = 0.75, P < 0.001).

Conclusions

We showed that there was an age-related decline of retinal oxygen extraction. Levels of retinal oxygen extraction are correlated to retinal nerve fiber layer thickness and number of retinal ganglion cells. Our data partially explain the wide inter-individual variability in retinal blood flow values in healthy subjects. Longitudinal studies are required to study the time course of vascular and neuronal loss in humans.

Characterization of dry eye disease in a mouse model by optical coherence tomography and fluorescein staining

A custom-built ultrahigh-resolution optical coherence tomography (UHR-OCT) system and fluorescein staining were employed for investigation of a scopolamine induced dry eye mouse model. Acquired data was used to evaluate common and complementary findings of the two modalities. Central corneal thickness as measured by UHR-OCT increased significantly over the study period of 24 hours, from 89.0 ± 3.57 µm to 92.2 ± 4.07 µm. Both techniques were able to show corneal lesions with a large range of severity. Localized fluorescein staining was detected in 5% and diffuse staining in 45% of cases where no epithelial damage was visible with OCT. However, OCT revealed stromal defects in 6% and endothelial defects in 18% of the cases, which could not be visualized via fluorescein staining. Thus, while fluorescein staining widely detected defects of the corneal surface in a mouse model of dry eye disease, OCT non-invasively revealed additional information about defect depth and involvement of particular layers.

Ultrahigh-resolution anterior segment optical coherence tomography for analysis of corneal microarchitecture during wound healing

PURPOSE

To employ ultrahigh-resolution (UHR) optical coherence tomography (OCT) for investigation of the early wound healing process in corneal epithelium.

METHODS

A custom-built UHR-OCT system assessed epithelial healing in human keratoconic cornea after epi-off crosslinking (CXL) procedure and a wound healing model in rabbits with iatrogenic corneal injury. 3D OCT data sets enhanced obtaining epithelial thickness maps and evaluation of reepithelization stage. Accompanying changes in deeper corneal microarchitecture were analysed.

RESULTS

The mean central corneal thickness in 40 eyes with keratoconus at baseline was 482.7 ± 38.2 μm, while mean central epithelial thickness (CET) was 43.8 ± 6.4 μm. At the final visit 20 ± 5 days post-CXL procedure, CET was 35.0 ± 5.8 μm, significantly thinner after reepithelization (p < 0.001). Surgical success was assessed at the final visit through the demarcation line (DL), identified at 43.7 ± 13.5% stromal depth. In rabbits, the mean CET in 20 eyes at baseline was 35.9 ± 2.6 μm. In rabbits that revealed complete wound closure (10/20 eyes) at the last study day at 72 hr, CET was significantly thinner compared to baseline (30.4 ± 2.8 μm versus 35.4 ± 2.9 μm, p = 0.005). An intra-stromal landmark indicating early keratocyte apoptosis was measured at 30.0 ± 5.1% stromal depth. Epithelial thickness maps showed the time-course of corneal healing.

CONCLUSION

Ultrahigh-resolution (UHR)-OCT provided precise assessment of epithelial wound and its healing by 3D-mapping. In addition, microarchitectural changes in the cornea in early phases of epithelial healing were revealed.

Regulation of Choroidal Blood Flow During Isometric Exercise at Different Levels of Intraocular Pressure

Purpose

There is evidence that choroidal blood flow (ChBF) is regulated in a complex way during changes in ocular perfusion pressure (OPP). We hypothesized that ChBF regulates better in response to changes in mean arterial pressure (MAP) than in intraocular pressure (IOP).

Methods

Eighteen volunteers (mean age, 26 years) were recruited for a randomized, three-way crossover design. MAP was varied via isometric exercise. IOP was either kept normal or elevated by 10 or 20 mm Hg by using a suction cup. Subfoveal ChBF was measured continuously for 8 minutes with laser Doppler flowmetry and OPP was calculated as 2/3*MAP-IOP. For data analysis, values from all subjects were pooled according to either IOP or MAP values, and correlation analyses were done.

Results

When data were grouped according to IOP, no correlation was observed between ChBF and MAP, but ChBF was lower the higher the IOP (P < 0.001). When data were grouped according to MAP, a significant correlation was found between ChBF and IOP (P < 0.001). When data were pooled according to IOP, the correlation between ChBF and OPP was weaker (P < 0.05). The OPP at which ChBF significantly increased from baseline was 61.3% ± 4.9% without suction cup, 65.2% ± 3.5% when IOP was increased by 10 mm Hg, and slightly lower when IOP was increased by 20 mm Hg (56.3% ± 4.8%, P = 0.07), but this effect did not reach the level of significance.

Conclusions

The present study provides further evidence that the regulation of ChBF during changes in OPP is controlled by complex mechanisms in humans and has less capacity to adapt to IOP elevation than to MAP increase.

Automatic assessment of tear film and tear meniscus parameters in healthy subjects using ultrahigh-resolution optical coherence tomography

Many different parameters exist for the investigation of tear film dynamics. We present a new tear meniscus segmentation algorithm which automatically extracts tear meniscus area (TMA), height (TMH), depth (TMD) and radius (TMR) from UHR-OCT measurements and apply it to a data set including repeated measurements from ten healthy subjects. Mean values and standard deviations are 0.0174 ± 0.007 mm², 0.272 ± 0.069 mm, 0.191 ± 0.049 mm and 0.309 ± 0.123 mm for TMA, TMH, TMD and TMR, respectively. A significant correlation was found between all respective tear meniscus parameter pairs (all p < 0.001, all Pearson's r ≥ 0.657). Challenges, limitations and potential improvements related to the data acquisition and the algorithm itself are discussed. The automatic segmentation of tear meniscus measurements acquired with UHR-OCT might help in a clinical setting to further understand the tear film and related medical conditions like dry eye disease.

Automated segmentation of dermal fillers in OCT images of mice using convolutional neural networks

We present a system for automatic determination of the intradermal volume of hydrogels based on optical coherence tomography (OCT) and deep learning. Volumetric image data was acquired using a custom-built OCT prototype that employs an akinetic swept laser at ~1310 nm with a bandwidth of 87 nm, providing an axial resolution of ~6.5 μm in tissue. Three-dimensional data sets of a 10 mm × 10 mm skin patch comprising the intradermal filler and the surrounding tissue were acquired. A convolutional neural network using a u-net-like architecture was trained from slices of 100 OCT volume data sets where the dermal filler volume was manually annotated. Using six-fold cross-validation, a mean accuracy of 0.9938 and a Jaccard similarity coefficient of 0.879 were achieved.

Optic nerve head blood flow regulation during changes in arterial blood pressure in patients with primary open-angle glaucoma

PURPOSE

Abnormal autoregulation of optic nerve head blood flow (ONHBF) has been postulated to play an important role in primary open-angle glaucoma (POAG). We used laser Doppler flowmetry (LDF) to estimate quantitatively the ONHBF and compared ONHBF autoregulation between glaucoma patients and healthy controls during isometric exercise.

METHODS

Forty patients with POAG and 40 healthy age- and sex-matched subjects underwent three periods of isometric exercise, each consisting of 2 min of handgripping. Optic nerve head blood flow (ONHBF) was measured continuously using LDF. Systemic blood pressure, intraocular pressure and ocular perfusion pressure were assessed in all participants.

RESULTS

Isometric exercise was associated with an increase in ocular perfusion pressure during all handgripping periods in both groups (p < 0.001). However, there was no change in ONHBF in either group. Three of the glaucoma patients and two of the healthy subjects showed a consistent 10% decrease in blood flow during isometric exercise, in spite of an increase in their blood pressure. This difference between groups was not significant (p = 0.61). Four other glaucoma subjects showed a consistent increase in blood flow of more than 10% during isometric exercise, whereas this was not seen in healthy subjects (p = 0.035).

CONCLUSION

This study suggests that abnormal ONHBF autoregulation is more often seen in patients with POAG than healthy control subjects. The relationship to the glaucoma disease process is currently unknown and requires further investigation.

Persistence of Efficacy of 0.1% Cyclosporin A Cationic Emulsion in Subjects with Severe Keratitis Due to Dry Eye Disease: A Nonrandomized, Open-label Extension of the SANSIKA Study

PURPOSE

Results from a 6-month double-masked and a 6-month open-label study (SANSIKA) established the efficacy and safety of once-daily 0.1% cyclosporin A cationic emulsion (CsA CE) in severe keratitis due to dry eye disease (DED). This article presents results from the Post-SANSIKA study, a 24-month extension of SANSIKA assessing the sustained efficacy of CsA CE after treatment discontinuation.

METHODS

Time to relapse (corneal fluorescein staining [CFS] score ≥4 [modified Oxford scale]) was assessed after treatment discontinuation in patients from the SANSIKA study who had CFS improvement from a score of 4 to ≤2 after 6 or 12 months of treatment with CsA CE.

FINDINGS

Of 62 patients who achieved a CFS score ≤2 at the end of the SANSIKA study, 38 did not relapse and 24 (39%) relapsed during the 24-month period after CsA CE discontinuation; the latter (relapse) group comprised 35% of patients initially treated with CsA CE for 12 months in SANSIKA versus 47% of those treated for 6 months only. Patients spent the most time during the extension study at CFS scores of 1 or 2 (median duration of 8.5 weeks and 14.7 weeks per year, respectively), indicating marked improvement, and less time at scores of 3, 4, or 5 (median time, 2.0 weeks, 0 weeks, and 0 weeks per year). Of 23 patients eligible for safety analysis (ie, patients who received the study treatment at least once), 12 (52.2%) reported a total of 26 ocular adverse events (AEs). Among these, 5 ocular AEs, reported in 5 patients (21.7%), were considered related to study treatment: 3 events of mild instillation site pain in 3 patients (13.0%) and eye discharge and foreign body sensation, each reported in 1 patient (4.3%). Only 1 systemic AE (nasal congestion), reported in 1 patient (4.3%), was considered related to study treatment. None of the AEs led to treatment discontinuation.

IMPLICATIONS

The majority of patients who discontinued CsA CE after experiencing DED improvement in the SANSIKA study did not experience a relapse in this 24-month follow-up study; these patients spent the most time at CFS scores consistent with marked improvement. CsA CE had a favorable safety/tolerability profile over 2 years. Treatment for up to 12 months with CsA CE provides sustained improvements in patients with severe keratitis due to DED. EudraCT registration no. 2012-002066-12.

Correlation of retinal neurodegeneration with measures of peripheral autonomic neuropathy in type 1 diabetes

Purpose

To evaluate the relationship of neuroretinal layer thickness with sensitive measures of cardiovascular autonomic neuropathy in diabetic patients with non‐proliferative diabetic retinopathy (NPDR).

Methods

Twenty‐seven eyes of 27 patients with type 1 diabetes presenting with mild‐to‐moderate NPDR were compared to 27 healthy control (HC) eyes matched for age and gender. The total macular volume (TMV) and the volumes of individual neurosensory layers in the macula were analysed from spectral domain optical coherence tomography using automated layer segmentation. Cardiovascular autonomic regulation was assessed by short‐term power spectrum analysis of heart rate variability (HRV) before, during and after an orthostatic challenge.

Results

The patients had an age of 46 ± 12 years and diabetes since 28 ± 9 years. Diastolic and mean arterial pressure was lower in the patients compared to HCs. TMV (r = 0.58, p = 0.002), inner plexiform layer volume (IPLV; r = 0.39, p = 0.047) and inner nuclear layer volume (INLV; r = 0.60, p = 0.001) were associated with reduced recovery of low‐frequency (LF) spectral power of HRV after orthostatic load in diabetic patients but not in HCs. The response of LF spectral power during the orthostatic manoeuvre was blunted in patients compared to HCs (p = 0.02). Diabetes duration was negatively associated with TMV and INLV, whereas IPLV was significantly reduced in eyes with moderate NPDR compared to HCs.

Conclusion

The results indicate a correlation between inner retinal tissue loss and diminished autonomic regulation in type 1 diabetic patients with mild‐to‐moderate NPDR. The observed changes can be interpreted as congruent early signs of retinal and systemic neuropathy in diabetes.

Method comparison of two non-invasive dual-wavelength spectrophotometric retinal oximeters in healthy young subjects during normoxia

PURPOSE

Spectrophotometric retinal oximetry is a non-invasive technology for measuring oxygen saturation in arterioles and venules (SaO₂ , SvO₂ ). We compared two commercially available systems: the Oxymap T1 (Oxymap ehf., Reykjavik, Iceland) and the Dynamic Vessel Analyzer (DVA, Imedos, Jena, Germany).

METHODS

Twenty healthy adults were included after giving informed consent. Two measurement cycles 30 min apart, including Oxymap T1, DVA, arterialized capillary blood draw of the earlobe (ScO₂ ) and peripheral oxygen saturation using finger pulse oximetry (SpO₂ ) were scheduled.

RESULTS

SaO₂ (p > 0.0004) but not SvO₂ (p < 0.05) was statistically significantly different between the retinal oximeters used. Agreement between devices using repeated SO₂ measurements resulted in a standard deviation (SD) of differences of 3.5% in retinal arterioles and 4.8% in venules. Bland-Altman plot using the mean of a participant's two measurements from each device showed an average mean difference of 4.4% (95% confidence limits of agreement: -8.6 to 17.4) and -3.3% (95% confidence limits of agreement: -28.8 to 22.2) for SaO₂ and SvO₂ , respectively. Comparison of mean SaO₂ and SvO₂ with mean ScO₂ and SpO₂ indicated that SO₂ measurements were generally higher in ScO₂ and SpO₂ .

CONCLUSION

This study shows very good repeatability for both devices, which is consistent with the literature. However, it does not show sufficient concordance between SaO₂ measurements from both devices, indicating that patients should be followed by one device only. Differences in absorbance wavelengths used and image post-processing may explain the differences.

Distinguishing Keratoconic Eyes and Healthy Eyes Using Ultrahigh-Resolution Optical Coherence Tomography-Based Corneal Epithelium Thickness Mapping

PURPOSE

To find differences in epithelial thickness (ET) maps of eyes with keratoconus (KC) and healthy eyes.

DESIGN

Institutional cross-sectional study.

METHODS

In this study 40 keratoconic eyes and 76 healthy eyes were scanned using a custom-built ultrahigh-resolution optical coherence tomography system. Automated segmentation ET maps with 17 subsectors were calculated (central, temporal inferior, temporal superior, nasal inferior, and nasal superior area). The thinnest point of the epithelium (minET), the thickest point of the epithelium (maxET), and the thinnest point diagonally opposing the thickest point (ET_max/op) were additional parameters. Ratios were calculated as follows: minET/diagonally opposing point (R₁), maxET/diagonally opposing point (R₂), inferior temporal area/superior nasal area (R_TI/NS), and inferior/superior hemisphere (R_I/S). Furthermore, collected parameters were analyzed regarding their diagnostic accuracy (area under the curve; AUC).

RESULTS

Statistically significant differences were as follows: central ET, 46.25 ± 2.56/50.91 ± 1.66; minET, 38.50 ± 2.10/46.79 ± 1.27; ET_max/op, 47.14 ± 2.45/49.60 ± 1.57; temporal inferior area: 43.93 ± 2.95/51.04 ± 1.51 (all mean ± standard deviation, μm); R₁, 0.76 ± 0.09/0.93 ± 0.04; R₂, 1.08 ± 0.04/1.21 ± 0.16; R_TI/NS, 0.85 ± 0.08/1.02 ± 0.04; R_I/S: 0.92 ± 0.07/0.99 ± 0.02. AUC values were R₁: 0.979 (confidence interval [CI]: 0.957-1.000), R_TI/NS: 0.977 (CI: 0.951-1.000), and minET: 0.928 (CI: 0.880-0.977).

CONCLUSIONS

Epithelial thickness maps could clearly visualize different ET patterns. Parameters with the highest potential of diagnostic discrimination between eyes with KC and healthy eyes were, in descending order, R₁, R_TI/NS, and minET. Consequently, epithelial thickness irregularity and asymmetry seem to be the most promising diagnostic factor in terms of discriminating between keratoconic eyes and healthy eyes.

Effect of changing from preserved prostaglandins to preservative-free tafluprost in patients with glaucoma on tear film thickness

PURPOSE

Long-term glaucoma therapy with preservative-containing eye drops may impact ocular surface health. This study was performed to investigate whether a switch from preserved topical prostaglandin therapy to preservative-free tafluprost therapy improves precorneal tear film thickness in patients with glaucoma or ocular hypertension.

METHODS

A total of 30 patients who were under topical preservative-containing prostaglandin monotherapy for at least 6 months were included. Patients were then switched from preserved prostaglandin therapy to unpreserved tafluprost drops once daily. Tear film thickness was measured at baseline and 4 and 12 weeks after therapy change with an ultrahigh-resolution optical coherence tomography system. Furthermore, clinical measures of ocular surface disease were determined and symptoms were assessed using the Dry Eye-Related Quality-of-Life Score.

RESULTS

After switching to unpreserved tafluprost, tear film thickness significantly increased from 4.7 ± 0.5 to 5.0 ± 0.6 µm 4 weeks after therapy change and still tended to be increased after 12 weeks (4.8 ± 0.7 µm). Breakup time significantly increased from 5.1 ± 2.3 to 7.2 ± 3.4 s and to 10.1 ± 3.6 s after therapy change. In addition, a significant decrease in corneal staining score from 1.8 ± 0.7 to 1.4 ± 0.8 after 4 weeks and to 0.7 ± 0.7 after 12 weeks treatment was observed. Switching to preservative-free drops reduced Dry Eye-Related Quality-of-Life Score from 11.4 ± 11.0 to 5.7 ± 6.4 and to 4.7 ± 7.5.

CONCLUSION

Our data show that switching to preservative-free tafluprost leads to an increase in tear film thickness, breakup time, and an improvement of Dry Eye-Related Quality-of-Life Score. Our results therefore indicate that a switch to unpreserved tafluprost is beneficial for ocular surface health in patients under long-term preserved prostaglandin eye drops.

[Structural endpoints for glaucoma studies]

BACKGROUND

Structural endpoints have been discussed as surrogate endpoints for the approval of neuroprotective drugs in glaucoma.

OBJECTIVE

Is the evidence strong enough to establish structural endpoints as surrogate endpoints?

MATERIAL AND METHODS

Review of current understanding between structure and function in glaucoma.

RESULTS

The introduction of optical coherence tomography has revolutionized imaging in glaucoma patients. Clinically either the nerve fiber layer thickness can be measured along a circle centered in the optic nerve head or the ganglion cell layer thickness can be assessed in the macular region, the latter being quantified in combination with other inner retinal layers. On a microscopic level there is a strong correlation between structural and functional loss but this relation can only partially be described with currently available clinical methods. This is particularly true for longitudinal course of the disease in glaucoma patients. Novel imaging techniques that are not yet used clinically may have the potential to increase our understanding between structure and function in glaucoma but further research in this field is required.

CONCLUSION

The current evidence does not allow the establishment of structural endpoints as surrogate endpoints for phase 3 studies in glaucoma. Neuroprotective drugs have to be approved on the basis of visual field data because this is the patient-relevant endpoint. Structural endpoints can, however, play an important role in phase 2 and proof of concept studies.

Assessment of choroidal blood flow using laser speckle flowgraphy

Background/aims

There is considerable interest in novel techniques to quantify choroidal blood flow (CBF) in humans. In the present study, we investigated a novel technique to measure CBF based on laser speckle flowgraphy (LSFG) in healthy subjects.

Methods

This study included 31 eyes of 31 healthy, non-smoking subjects aged between 19 and 74 years. A commercial LSFG instrument was used to measure choroidal vessel diameter (CVD) and relative flow volume (RFV) in choroidal vessels that were identified on fundus photos, an approach that was used previously only for retinal vessels. The reproducibility and the effect of isometric exercise on these parameters were investigated. The latter was compared with measurement of subfoveal CBF using laser Doppler flowmetry (LDF).

Results

Intraclass correlation coefficients for CVD and RFV were higher than 0.8 indicating excellent reproducibility. During isometric exercise, we observed an increase in ocular perfusion pressure of approximately 60% (P<0.001). The increase in RFV and CBF was lower, but also highly significant versus baseline (at minute 6 of isometric exercise: RFV 10.5%±4.2%, CBF 8.3%±3.6%; P<0.001 each) indicating choroidal autoregulation.

Conclusion

LSFG may be a novel approach to study blood flow in choroidal vessels. Data are reproducible and show good agreement with LDF data.

Trial registration number

NCT02102880, Results.

Vitamin D and Age-Related Macular Degeneration

In recent years, the relationship between vitamin D and health has received growing attention from the scientific and medical communities. Vitamin D deficiencies have been repeatedly associated with various acute and chronic diseases, including age-related macular degeneration (AMD). Its active metabolite, 1α,25-dihydoxy vitamin D, acts as a modulator of cell proliferation, differentiation and apoptosis, and cumulative data from experimental and observational studies suggest that relatively a lower vitamin D status could be a potential risk factor for the development of early and/or late AMD. Herein, we made a narrative review of the mechanisms linking a potential role of vitamin D with the current concepts of AMD pathophysiology.

Optic nerve head and retinal blood flow regulation during isometric exercise as assessed with laser speckle flowgraphy

The aim of the present study was to investigate regulation of blood flow (BF) in the optic nerve head (ONH) and a peripapillary region (PPR) during an isometric exercise-induced increase in ocular perfusion pressure (OPP) using laser speckle flowgraphy (LSFG) in healthy subjects. For this purpose, a total of 27 subjects was included in this study. Mean blur rate in tissue (MT) was measured in the ONH and in a PPR as well as relative flow volume (RFV) in retinal arteries (ART) and veins (VEIN) using LSFG. All participants performed isometric exercise for 6 minutes during which MT and mean arterial pressure were measured every minute. From these data OPP and pressure/flow curves were calculated. Isometric exercise increased OPP, MT_ONH and MT_PRR. The relative increase in OPP (78.5 ± 19.8%) was more pronounced than the increase in BF parameters (MT_ONH: 18.1 ± 7.7%, MT_PRR: 21.1 ± 8.3%, RFV_ART: 16.5 ±12.0%, RFV_VEIN: 17.7 ± 12.4%) indicating for an autoregulatory response of the vasculature. The pressure/flow curves show that MT_ONH, MT_PRR, RFV_ART, RFV_VEIN started to increase at OPP levels of 51.2 ± 2.0%, 58.1 ± 2.4%, 45.6 ± 1.9% and 45.6 ± 1.9% above baseline. These data indicate that ONHBF starts to increase at levels of approx. 50% increase in OPP: This is slightly lower than the values we previously reported from LDF data. Signals from the PPR may have input from both, the retina and the choroid, but the relative contribution is unknown. In addition, retinal BF appears to increase at slightly lower OPP values of approximately 45%. LSFG may be used to study ONH autoregulation in diseases such as glaucoma.

Trial Registration: ClinicalTrials.gov NCT02102880

Retinal oxygen extraction in individuals with type 1 diabetes with no or mild diabetic retinopathy

AIMS/HYPOTHESIS

The aim of this study was to compare retinal oxygen extraction in individuals with diabetes with no or mild non-proliferative diabetic retinopathy and healthy age- and sex-matched volunteers.

METHODS

A total of 24 participants with type 1 diabetes and 24 healthy age- and sex-matched volunteers were included in this cross-sectional study. Retinal oxygen extraction was measured by combining total retinal blood flow measurements using a custom-built bi-directional Doppler optical coherence tomography system with measurements of oxygen saturation using spectroscopic reflectometry. Based on previously published mathematical modelling, the oxygen content in retinal vessels and total retinal oxygen extraction were calculated.

RESULTS

Total retinal blood flow was higher in diabetic participants (46.4 ± 7.4 μl/min) than in healthy volunteers (40.4 ± 5.3 μl/min, p = 0.002 between groups). Oxygen content in retinal arteries was comparable between the two groups, but oxygen content in retinal veins was higher in participants with diabetes (0.15 ± 0.02 ml O₂/ml) compared with healthy control participants (0.13 ± 0.02 ml O₂/ml, p < 0.001). As such, the arteriovenous oxygen difference and total retinal oxygen extraction were reduced in participants with diabetes compared with healthy volunteers (total retinal oxygen extraction 1.40 ± 0.44 vs 1.70 ± 0.47 μl O₂/min, respectively, p = 0.03).

CONCLUSIONS/INTERPRETATION

Our data indicate early retinal hypoxia in individuals with type 1 diabetes with no or mild diabetic retinopathy as compared with healthy control individuals. Further studies are required to fully understand the potential of the technique in risk stratification and treatment monitoring.

TRIAL REGISTRATION

ClinicalTrials.gov NCT01843114.

Factors Determining Flicker-Induced Retinal Vasodilation in Healthy Subjects

PURPOSE

The purpose of this study was to analyze factors determining retinal arterial and venous responses to stimulation with diffuse luminance flicker in healthy subjects.

METHODS

We retrospectively analyzed results obtained in 374 healthy subjects who had previously participated in clinical studies in our department. A total of 153 subjects underwent a protocol in which flicker stimulation was delivered through the fundus camera at 8 Hz (protocol 1), separating measurement and stimulation light depending on the wavelength, and 221 subjects underwent a protocol in which diffuse luminance flicker was delivered at 12.5 Hz with high modulation depth (protocol 2). We investigated whether sex, systemic blood pressure, baseline vessel size, blood plasma concentration of fasting glucose and hematocrit, and serum concentration of cholesterol, triglycerides, creatinine and C-reactive protein influenced the retinal vascular response to flicker stimulation.

RESULTS

Flicker responses in arteries and veins were more pronounced in protocol 2 than in protocol 1 (P < 0.001, each). In both of the protocols the vascular response to stimulation with diffuse luminance flicker was larger in smaller vessels (P between 0.001 and 0.016). In protocol 2 the retinal arterial flicker response was negatively associated with cholesterol serum levels (P = 0.033); in protocol 1, only a tendency toward this effect was observed (P = 0.056).

CONCLUSIONS

The present analysis indicates that retinal arterial and venous responses to stimulation with diffuse luminance flicker depend on the way the stimulation is delivered through the fundus camera. In addition, the flicker response varied with vessel size, that is, the smaller the vessel width, the larger the flicker response. Finally, our data indicate that, even within the normal range, higher cholesterol serum levels are associated with lower hyperemic flicker responses.

Effect of different lubricant eye gels on tear film thickness as measured with ultrahigh-resolution optical coherence tomography

PURPOSE

To compare the effect of a single drop of different lubricant eye gels on tear film thickness (TFT) as measured with ultrahigh-resolution optical coherence tomography (UHR-OCT) in patients with mild-to-moderate dry eye disease (DED).

METHODS

The study followed a randomized, single-masked, observer-blinded parallel group design. Patients received a single dose of either unpreserved trehalose 3% + hyaluronic acid 0.15% (TH), hyaluronic acid 0.2% (HA) or polyethylene glycol 0.4% + propylene glycol 0.3% (PP) eye drops. In total, 60 patients were included and TFT was measured using a custom-built UHR-OCT system.

RESULTS

The mean TFT at baseline was 3.5 ± 0.7 μm. There was a significant difference in the time-course of TFT between the three groups (p = 0.001 between groups). Ten minutes after instillation, TFT increased by 155.8 ± 86.6%, 65.7 ± 71.5% and 33.4 ± 19.6% in the PP, TH and HA group, respectively (p < 0.001 between groups). Thirty minutes after instillation, the effect of all three different agents on TFT was comparable. Sixty and 120 min after administration, a significant increase in TFT was only seen for the TH group, but not for the other products (60 min: p < 0.021 between groups; 120 min: p < 0.037 between groups).

CONCLUSION

In conclusion, we observed differences in TFT after administration of the lubricant gels. Ten minutes after instillation, a pronounced increase in TFT was observed in all groups. As compared to the other products, the combination of trehalose 3% + hyaluronic acid 0.15% offers a significantly longer increase in TFT indicating for a longer residence time.

Measurements of Retinal Perfusion Using Laser Speckle Flowgraphy and Doppler Optical Coherence Tomography

Purpose

This study evaluated the validity of retinal perfusion measurements using laser speckle flowgraphy (LSFG) by means of in vitro experiments and direct comparison with dual-beam Doppler optical coherence tomography (D-OCT) in a healthy Caucasian population.

Methods

The flow velocity of scattering solution pumped through a glass capillary was measured at 17 different flow velocities (range, 0.5-47 mm/s) using LSFG. The flow within the glass capillary was produced by a computer-controlled infusion pump. In vivo, three consecutive LSFG scans were obtained in 20 eyes of 20 healthy Caucasian subjects before and after pharmacological pupil dilation. Relative flow volume (RFV), the primary output parameter of LSFG, was comparatively validated relative to absolute measurements of retinal blood flow and velocity as obtained from D-OCT.

Results

In the in vitro experiments, RFV was found to saturate at a level of approximately 700 arbitrary units (au) or 23.5 mm/s of actual velocity. In vivo, RFV was in significant agreement with absolute blood flow measurements as obtained from D-OCT in arteries (r = 0.69, P = 0.001) and veins (r = 0.74, P < 0.001). However, linear regression analysis revealed significant positive zero offset values for RFV of 223.4 and 282.7 au in arteries and veins, respectively.

Conclusions

Measurements of RFV were successfully obtainable, reproducible, and not influenced by pharmacological pupil dilation. Nevertheless, our data revealed flaws in the LSFG method of measuring retinal perfusion in Caucasians. Adjustment to the technique is required to address apparent issues with RFV, especially saturation effects with higher arterial flow rates. The present dataset may provide a valuable tool to do so. (Clinicaltrials.gov number NCT02582411).

Estimating Retinal Blood Flow Velocities by Optical Coherence Tomography

Importance

While optical coherence tomography (OCT) angiography has been considered to evaluate retinal capillary blood flow instead of fluorescein angiography, the reflectance pattern of blood vessels on structural OCT might also provide retinal capillary flow data in the absence of fluorescein angiography. This potential has been insufficiently explored, despite promising data concerning a possible relationship between the reflectance pattern of blood vessels and their perfusion velocity in a laboratory setting.

Objective

To evaluate the potential of retinal blood flow velocity estimation by structural OCT.

Design, Setting, and Participants

Cross-sectional observational study conducted from June to November 2015 at a tertiary clinical referral center. Sixty arteries (the superior and inferior temporal arteries) from 30 eyes of 30 patients (17 female, 13 male) were included in the study.

Main Outcomes and Measures

Based on the intraluminal contrast patterns of retinal arteries on OCT, 3 independent graders categorized the blood flow velocities as low, medium, or high. These results and the results from a software-based intraluminal contrast analysis were compared with the retinal blood flow velocities measured by video fluorescein angiography.

Results

Among the 30 eyes of 30 patients (mean [SD] age, 72.6 [12.3] years; 17 female, 13 male), 15 were controls without retinal occlusion, 6 had a branch retinal artery occlusion, and 9 had a central retinal artery occlusion. When discriminating between low flow velocities and medium or high flow velocities, the graders' sensitivity ranged from 88.2% to 100% (grader 1: 88.2%; 95% CI, 63.6%-98.5%; grader 2: 88.2%; 95% CI, 63.6%-98.5%; and grader 3: 100%; 95% CI, 69.8%-100%) and their specificity ranged from 97.6% to 100% (grader 1: 100%; 95% CI, 87.7%-100%; grader 2: 97.6%; 95% CI, 87.4%-99.9%; and grader 3: 100%; 95% CI, 87.7%-100%). The κ coefficients of the comparison between the 3 graders and the angiography were 0.77 (95% CI, 0.60-0.93; P < .001), 0.64 (95% CI, 0.44-0.83; P < .001), and 0.87 (95% CI, 0.74-0.99; P < .001). In the computer-based assessment, the contrast reduction of the intraluminal pattern could be numerically expressed in a specific coefficient in the model (I2, describing the angular change of the backscattering intensity in the model), which presented nonoverlapping intervals between low flow velocities and medium or high flow velocities (mean [SD] I2, 0.3 [5.3], 20.4 [6.4], and 21.7 [4.0], respectively).

Conclusions and Relevance

This study suggests that a low retinal blood flow velocity reflects in a visually distinct contrast reduction of the intraluminal pattern of retinal vessels on OCT. Larger studies are required to assess the clinical benefits.

Ultrahigh-resolution OCT imaging of the human cornea

We present imaging of corneal pathologies using optical coherence tomography (OCT) with high resolution. To this end, an ultrahigh-resolution spectral domain OCT (UHR-OCT) system based on a broad bandwidth Ti:sapphire laser is employed. With a central wavelength of 800 nm, the imaging device allows to acquire OCT data at the central, paracentral and peripheral cornea as well as the limbal region with 1.2 µm x 20 µm (axial x lateral) resolution at a rate of 140 000 A-scans/s. Structures of the anterior segment of the eye, not accessible with commercial OCT systems, are visualized. These include corneal nerves, limbal palisades of Vogt as well as several corneal pathologies. Cases such as keratoconus and Fuchs's endothelial dystrophy as well as infectious changes caused by diseases like Acanthamoeba keratitis and scarring after herpetic keratitis are presented. We also demonstrate the applicability of our system to visualize epithelial erosion and intracorneal foreign body after corneal trauma as well as chemical burns. Finally, results after Descemet's membrane endothelial keratoplasty (DMEK) are imaged. These clinical cases show the potential of UHR-OCT to help in clinical decision-making and follow-up. Our results and experience indicate that UHR-OCT of the cornea is a promising technique for the use in clinical practice, but can also help to gain novel insight in the physiology and pathophysiology of the human cornea.

Psychophysical Vision Simulation of Diffractive Bifocal and Trifocal Intraocular Lenses

PURPOSE

The visual performance of monofocal, bifocal, and trifocal intraocular lenses was evaluated by human individuals using a vision simulator device. This allowed investigation of the visual impression after cataract surgery, without the need actually to implant the lenses.

METHODS

The randomized, double-masked, three-way cross-over study was conducted on 60 healthy male and female subjects aged between 18 and 35 years. Visual acuity (Early Treatment Diabetic Retinopathy Study; ETDRS) and contrast sensitivity tests (Pelli-Robson) under different lighting conditions (luminosities from 0.14-55 cd/m², mesopic to photopic) were performed at different distances.

RESULTS

Visual acuity tests showed no difference for corrected distance visual acuity data of bi- and trifocal lens prototypes (P = 0.851), but better results for the trifocal than for the bifocal lenses at distance corrected intermediate (P = 0.021) and distance corrected near visual acuity (P = 0.044). Contrast sensitivity showed no differences between bifocal and trifocal lenses at the distant (P = 0.984) and at the near position (P = 0.925), but better results for the trifocal lens at the intermediate position (P = 0.043). Visual acuity and contrast sensitivity showed a strong dependence on luminosity (P < 0.001).

CONCLUSIONS

At all investigated distances and all lighting conditions, the trifocal lens prototype often performed better, but never worse than the bifocal lens prototype.

TRANSLATIONAL RELEVANCE

The vision simulator can fill the gap between preclinical lens development and implantation studies by providing information of the perceived vision quality after cataract surgery without implantation. This can reduce implantation risks and promotes the development of new lens concepts due to the cost effective test procedure.

Measurement of Retinal Vascular Caliber From Optical Coherence Tomography Phase Images

PURPOSE

To compare retinal vessel calibers extracted from phase-sensitive optical coherence tomography (OCT) images with vessel calibers as obtained from the Retinal Vessel Analyzer (RVA).

METHODS

Data from previously published studies in 13 healthy subjects breathing room air (n = 214 vessels) and 7 subjects breathing 100% oxygen (n = 101 vessels) were used. Vessel calibers from OCT phase images were measured vertically along the optical axis by three independent graders. The data from RVA fundus images were corrected for magnification to obtain absolute values.

RESULTS

The average vessel diameter as obtained from OCT images during normoxia was lower than from RVA images (83.8 ± 28.2 μm versus 86.6 ± 28.0 μm, P < 0.001). The same phenomenon was observed during 100% oxygen breathing (OCT: 81.0 ± 22.4 μm, RVA: 85.5 ± 26.0 μm; P = 0.001). Although the agreement between the two methods was generally high, the difference in individual vessels could be as high as 40%. These differences were neither dependent on absolute vessel size nor preferably found in specific subjects. Interobserver differences between OCT evaluators were much lower than differences between the techniques.

CONCLUSIONS

Extracting vessel calibers from OCT phase images may be an attractive approach to overcome some of the problems associated with fundus imaging. The source of differences in vessel caliber between the two methods remains to be investigated. In addition, it remains unclear whether OCT-based vessel caliber measurement is superior to fundus camera-based imaging in risk stratification for systemic or ocular disease. (ClinicalTrials.gov numbers, NCT00914407, NCT02531399.).

An Exploratory Microdialysis Study to Assess the Ocular Pharmacokinetics of Ciprofloxacin Eye Drops in Rabbits

PURPOSE

Despite the high frequency of use, data regarding the ocular in vivo pharmacokinetics (PK) of topically applied antibiotics are sparse. This study seeks to investigate the PK of the widely used fluoroquinolone ciprofloxacin by means of in vivo microdialysis.

METHODS

Twelve New Zealand white rabbits were included in the experiments. Under general anesthesia, microdialysis probes were implanted in the anterior chamber and the posterior segment of the same eye. After a period of 90 min after implantation, one drop of ciprofloxacin was administered onto the ocular surface. Microdialysis samples of the anterior chamber and the vitreous were collected every 30 min for 6 h. Relative recovery was assessed by retrodialysis to calculate absolute concentration values. Samples were analyzed using HPLC.

RESULTS

In the anterior chamber, the maximum total drug concentration (Cmax) amounted to 0.373 ± 0.281 μg/mL and was reached (Tmax) after 116 ± 36 min. Calculated area under the concentration-time curve AUC(0-n) for ciprofloxacin in the anterior chamber was 78.8 ± 47.1 μg min/mL. For the vitreous, Cmax was 0.02 ± 0.03 μg/mL and maximum drug concentration was reached 107 ± 60 min after topical administration. AUC(0-n) for ciprofloxacin in the vitreous was 0.269 ± 0.370 μg min/mL.

CONCLUSION

Microdialysis is a suitable method to assess in vivo pharmacokinetic profiles in the anterior chamber and in the vitreous. In the anterior chamber, maximum drug concentration was reached ∼2 h after single drug administration. Although the drug concentration in the vitreous was considerably lower, time course of drug concentration was comparable.

Relation of retinal blood flow and retinal oxygen extraction during stimulation with diffuse luminance flicker

Cerebral and retinal blood flow are dependent on local neuronal activity. Several studies quantified the increase in cerebral blood flow and oxygen consumption during activity. In the present study we investigated the relation between changes in retinal blood flow and oxygen extraction during stimulation with diffuse luminance flicker and the influence of breathing gas mixtures with different fractions of O₂ (FiO₂; 100% 15% and 12%). Twenty-four healthy subjects were included. Retinal blood flow was studied by combining measurement of vessel diameters using the Dynamic Vessel Analyser with measurements of blood velocity using laser Doppler velocimetry. Oxygen saturation was measured using spectroscopic reflectometry and oxygen extraction was calculated. Flicker stimulation increased retinal blood flow (57.7 ± 17.8%) and oxygen extraction (34.6 ± 24.1%; p < 0.001 each). During 100% oxygen breathing the response of retinal blood flow and oxygen extraction was increased (p < 0.01 each). By contrast, breathing gas mixtures with 12% and 15% FiO₂ did not alter flicker–induced retinal haemodynamic changes. The present study indicates that at a comparable increase in blood flow the increase in oxygen extraction in the retina is larger than in the brain. During systemic hyperoxia the blood flow and oxygen extraction responses to neural stimulation are augmented. The underlying mechanism is unknown.

Nonneovascular Age-Related Macular Degeneration

The discovery of several genetic variants associated with an increased risk for age-related macular degeneration (AMD) has led to a completely new understanding of AMD. In addition to the known modifiable risk factors, genetic risk factors may also help to assess the risk to progress to nonneovascular AMD. Recently published primary studies have indicated that genetic risk analysis may be valuable in the selection of the currently available antioxidant therapy. So far, the best evidence for preventing progression to nonneovascular AMD comes from the Age-Related Eye Disease Studies (AREDS) I and II. These studies indicate that high doses of antioxidants can reduce the risk of progression to the advanced form of the disease. However, the recent evaluation of the addition of either lutein and zeaxanthin, or ω-3 long-chain polyunsaturated fatty acids, or both, to the established AREDS I formulation did not significantly reduce the risk of developing advanced AMD. There is clearly a large unmet medical need for new therapeutic options for nonneovascular AMD. The modulation of the complement cascade is - despite initially disappointing outcomes obtained with blocking complement factor 5 - currently the most promising approach to the treatment of nonneovascular AMD.

Effect of hyaluronic acid on tear film thickness as assessed with ultra-high resolution optical coherence tomography

PURPOSE

The aim of this study was to assess the effect of a single drop of hyaluronic acid on tear film thickness (TFT) in healthy subjects.

METHODS

Sixteen healthy subjects (eight male/eight female) aged between 20 and 36 years were included in this randomized, double-masked placebo-controlled study. One eye received a single dose of hyaluronic acid (Olixia pure(®) ; Croma Pharma, Korneuburg, Austria) eye drops, and the fellow eye received physiologic saline solution as placebo control. The study eye was chosen randomly. TFT as measured with a custom-built Fourier-domain optical coherence tomography (FD-OCT) system was the main outcome variable and measured before and every 10 min until 1 hr after topical administration.

RESULTS

Baseline TFT was 4.8 ± 0.5 μm in the study eye and 5.0 ± 0.4 μm in the control eyes. Hyaluronic acid significantly increased TFT (p = 0.008 versus placebo) with a maximum effect 10 min after instillation (13.9 ± 11.9%). Post hoc analysis revealed that an increase in TFT was seen until 30 min after administration compared to placebo. Data in the placebo group show high reproducibility with an intraclass correlation coefficient of 0.93 and a coefficient of variation of 5.4 ± 3.3%.

CONCLUSION

The data of this study indicate that hyaluronic acid increases TFT for as long as 30 min in healthy subjects. In addition, our data provide evidence that our custom-built OCT system is capable of measuring residence time of lubricants on the ocular surface.

Blood flow velocity vector field reconstruction from dual-beam bidirectional Doppler OCT measurements in retinal veins

In this paper, we demonstrate the possibility to reconstruct the actual blood flow velocity vector field in retinal microvessels from dual-beam bidirectional Doppler optical coherence tomography measurements. First, for a better understanding of measured phase patterns, several flow situations were simulated on the basis of the known dual beam measurement geometry. We were able to extract the vector field parameters that determine the measured phase pattern, allowing for the development of an algorithm to reconstruct the velocity vector field from measured phase data. In a next step, measurements were performed at a straight vessel section and at a venous convergence; the obtained phase data were evaluated by means of the new approach. For the straight vessel section, the reconstructed flow velocity vector field yielded a parabolic flow. For the venous convergence, however, the reconstructed vector field deviated from a parabolic profile, but was in very good accordance with the simulated vector field for the given vessel geometry. The proposed algorithm allows predictions of the velocity vector field. Moreover, the algorithm is also sensitive to directional changes of the flow velocity as small as <1°, thereby offering insight in the flow characteristics of the non-Newtonian fluid blood in microvessels.

Nutritional supplements in age-related macular degeneration

Age-related macular degeneration (AMD) is the most frequent cause of blindness in the Western World. While with new therapies that are directed towards vascular endothelial growth factor (VEGF), a potentially efficient treatment option for the wet form of the disease has been introduced, a therapeutic regimen for dry AMD is still lacking. There is evidence from several studies that oral intake of supplements is beneficial in preventing progression of the disease. Several formulations of micronutrients are currently available. The present review focuses on the role of supplements in the treatment and prevention of AMD and sums up the current knowledge about the most frequently used micronutrients. In addition, regulatory issues are discussed, and future directions for the role of supplementation in AMD are highlighted.

Pharmacotherapy of glaucoma

Glaucoma is a group of diseases involving the optic nerve and associated structures, which is characterized by progressive visual field loss and typical changes of the optic nerve head (ONH). The only known treatment of the disease is reduction of intraocular pressure (IOP), which has been shown to reduce glaucoma progression in a variety of large-scale clinical trials. Nowadays, a relatively wide array of topical antiglaucoma drugs is available, including prostaglandin analogues, carbonic anhydrase inhibitors, beta-receptor antagonists, adrenergic agonists, and parasympathomimetics. In clinical routine, this allows for individualized treatment taking risk factors, efficacy, and safety into account. A major challenge is related to adherence to therapy. Sustained release devices may help minimize this problem but are not yet available for clinical routine use. Another hope arises from non-IOP-related treatment concepts. In recent years, much knowledge has been gained regarding the molecular mechanisms that underlie the disease process in glaucoma. This also strengthens the hope that glaucoma therapy beyond IOP lowering will become available. Implementing this concept with clinical trials remains, however, a challenge.

Antioxidative capacity of a dietary supplement on retinal hemodynamic function in a human lipopolysaccharide (LPS) model

PURPOSE

Beneficial effects of dietary supplements in age-related macular degeneration (AMD) are related to antioxidative properties. In the Age-Related Eye Disease Study 1 (AREDS 1), a reduced progression to late stage AMD was found using vitamin C, E, zinc, and β-carotene. We showed previously that the AREDS 1 formulation restores the O2-induced retinal vasoconstrictor response of retinal vessels in a human endotoxin (lipopolysaccharide [LPS]) model.

METHODS

We hypothesized that the abnormal O2-induced retinal red blood cell (RBC) flow response can be modulated by a different formulation (vitamin C, E, and zinc, lutein/zeaxanthin, selenium, taurine, Aronia extract, and omega-3 free fatty acids). A total of 43 healthy subjects was included in this randomized, double masked, placebo-controlled parallel group study. The reactivity of retinal arterial and venous diameter, RBC velocity, and flow to 100% O2 breathing was investigated in the absence and presence of 2 ng/kg LPS. Between the two study days was a 14-day period of daily dietary supplement intake.

RESULTS

The decrease in retinal arterial diameter, RBC velocity, and flow during 100% O2 breathing was diminished significantly after LPS infusion. Dietary supplement intake for 14 days almost restored the response of retinal hemodynamic parameters to 100% O2 after LPS administration. This effect was significant for retinal arterial diameter (P = 0.03 between groups), and RBC velocity and flow (each P < 0.01 between groups).

CONCLUSIONS

The present data indicate restoring of the RBC flow response to 100% O2 after LPS administration. This is likely due to an amelioration of endothelial dysfunction resulting from oxidative stress, a factor involved in AMD pathophysiology. (ClinicalTrials.gov number, NCT00914576.).

Flicker-induced retinal vasodilatation is not dependent on complement factor H polymorphism in healthy young subjects

Purpose

The complement factor H (CFH) tyrosine 402 histidine (Y402H, rs1061170) variant is known to be significantly associated with age-related macular degeneration (AMD). Whether this genetic variant may impact retinal blood flow regulation is largely unknown. This study investigated whether flicker-induced vasodilation, an indicator for the coupling between neural activity and blood flow, is altered in subjects carrying the rs1061170 risk allele.

Methods

One hundred healthy subjects (aged between 18 and 45 years) were included in this study. Retinal blood flow regulation was tested by assessing retinal vessel calibres in response to stimulation with diffuse flicker light. Retinal vascular flicker responses were determined with a Dynamic Vessel Analyzer (DVA). In addition, genotyping for rs1061170 was performed.

Results

Eighteen subjects were homozygous for the risk allele C, 50 were homozygous for the ancestral allele T, and 31 subjects were heterozygous (CT). One subject had to be excluded from data evaluation, as no genetic analysis could be performed due to technical difficulties. Baseline diameters of retinal arteries (p = 0.39) and veins (p = 0.64) were comparable between the three groups. Flicker-induced vasodilation in both retinal arteries (p = 0.38) and retinal veins (p = 0.62) was also comparable between the three studied groups.

Conclusions

Our data indicate that homozygous healthy young carriers of the C risk allele at rs1061170 do not show abnormal flicker-induced vasodilation in the retina. This suggests that the high-risk genetic variant of CFH polymorphism does not impact neuro-vascular coupling in healthy subjects.

Regulation of retinal oxygen metabolism in humans during graded hypoxia

Animal experiments indicate that the inner retina keeps its oxygen extraction constant despite systemic hypoxia. For the human retina no such data exist. In the present study we hypothesized that systemic hypoxia does not alter inner retinal oxygen extraction. To test this hypothesis we included 30 healthy male and female subjects aged between 18 and 35 years. All subjects were studied at baseline and during breathing 12% O₂ in 88% N₂ as well as breathing 15% O₂ in 85% N₂. Oxygen saturation in a retinal artery (SO₂art) and an adjacent retinal vein (SO₂vein) were measured using spectroscopic fundus reflectometry. Measurements of retinal venous blood velocity using bidirectional laser Doppler velocimetry and retinal venous diameters using a Retinal Vessel Analyzer (RVA) were combined to calculate retinal blood flow. Oxygen and carbon dioxide partial pressure were measured from earlobe arterialized capillary blood. Retinal blood flow was increased by 43.0 ± 23.2% (P < 0.001) and 30.0 ± 20.9% (P < 0.001) during 12% and 15% O₂ breathing, respectively. SO₂art as well as SO₂vein decreased during both 12% O₂ breathing (SO₂art: -11.2 ± 4.3%, P < 0.001; SO₂vein: -3.9 ± 8.5%, P = 0.012) and 15% O₂ breathing (SO₂art: -7.9 ± 3.6%, P < 0.001; SO₂vein: -4.0 ± 7.0%, P = 0.010). The arteriovenous oxygen difference decreased during both breathing periods (12% O2: -28.9 ± 18.7%; 15% O₂: -19.1 ± 16.7%, P < 0.001 each). Calculated oxygen extraction did, however, not change during our experiments (12% O₂: -2.8 ± 18.9%, P = 0.65; 15% O₂: 2.4 ± 15.8%, P = 0.26). Our results indicate that in healthy humans, oxygen extraction of the inner retina remains constant during systemic hypoxia.

Retinal oxygen metabolism during normoxia and hyperoxia in healthy subjects

PURPOSE

To characterize retinal metabolism during normoxia and hyperoxia in healthy subjects.

METHODS

Forty-six healthy subjects were included in the present study, and data of 41 subjects could be evaluated. Retinal vessel diameters, as well as oxygen saturation in arteries and veins, were measured using the Dynamic Vessel Analyzer. In addition, retinal venous blood velocity was measured using bidirectional laser Doppler velocimetry, retinal blood flow was calculated, and oxygen and carbon dioxide partial pressures were measured from arterialized capillary blood from the earlobe. Measurements were done during normoxia and during 100% oxygen breathing.

RESULTS

Systemic hyperoxia caused a significant decrease in retinal venous diameter (-13.0% ± 4.5%) and arterial diameter (-12.1% ± 4.0%), in retinal blood velocity (-43.4% ± 7.7%), and in retinal blood flow (-57.0% ± 5.7%) (P < 0.001 for all). Oxygen saturation increased in retinal arteries (+4.4% ± 2.3%) and in retinal veins (+19.6% ± 6.2%), but the arteriovenous oxygen content difference significantly decreased (-29.4% ± 19.5%) (P < 0.001 for all). Blood oxygen tension in arterialized blood showed a pronounced increase from 90.2 ± 7.7 to 371.3 ± 92.7 mm Hg (P < 0.001). Calculated oxygen extraction in the eye decreased by as much as 62.5% ± 9.5% (P < 0.001).

CONCLUSIONS

Our data are compatible with the hypothesis that during 100% oxygen breathing a large amount of oxygen, consumed by the inner retina, comes from the choroid, which is supported by previous animal data. Interpretation of oxygen saturation data in retinal arteries and veins without quantifying blood flow is difficult. (ClinicalTrials.gov number, NCT01692821.).

Gender differences in ocular blood flow

Gender medicine has been a major focus of research in recent years. The present review focuses on gender differences in the epidemiology of the most frequent ocular diseases that have been found to be associated with impaired ocular blood flow, such as age-related macular degeneration, glaucoma and diabetic retinopathy. Data have accumulated indicating that hormones have an important role in these diseases, since there are major differences in the prevalence and incidence between men and pre- and post-menopausal women. Whether this is related to vascular factors is, however, not entirely clear. Interestingly, the current knowledge about differences in ocular vascular parameters between men and women is sparse. Although little data is available, estrogen, progesterone and testosterone are most likely important regulators of blood flow in the retina and choroid, because they are key regulators of vascular tone in other organs. Estrogen seems to play a protective role since it decreases vascular resistance in large ocular vessels. Some studies indicate that hormone therapy is beneficial for ocular vascular disease in post-menopausal women. This evidence is, however, not sufficient to give any recommendation. Generally, remarkably few data are available on the role of sex hormones on ocular blood flow regulation, a topic that requires more attention in the future.