Oxygen saturation in central retinal vein occlusion

Hypertension, hyperlipidaemia and thrombophilia as the most common risk factors for retinal vein occlusion in patients under 50 years

BACKGROUND AND AIMS

Cardiovascular (CV) diseases are the most common risk factors (RFs) for retinal vein occlusion (RVO) development in general. The aim of this study was to identify the most frequent causes of RVO in patients under 50.

METHODS

We retrospectively evaluated a group of patients with RVO under 50 years. The parameters of interest included age and sex, RVO type, presence of arterial hypertension (HT), hyperlipidaemia (HLD), diabetes mellitus (DM), congenital thrombophilic disorder (TD), obstructive sleep apnoea syndrome (OSAS), thyroid eye disease (TED), use of hormone contraception (HC) or hormone replacement therapy (HRT), glaucoma and other potential RFs. Patients with central RVO (CRVO), hemi-central RVO (HRVO), branch RVO (BRVO), impending CRVO and combined arterial-venous (AV) occlusion were included.

RESULTS

The group consisted of 110 eyes of 103 patients. CV disease was the most common systemic abnormality. 55.3% patients had HT, 17.5% had HLD. TD was the third most frequent RF (12.6%). The cohort also included patients with DM (6.8%), glaucoma (6.8%) and women using HC/HRT (26.2% of female patients). There were isolated cases of RVO due to retinal vasculitis, intense exercise, antiphospholipid syndrome and COVID-19 pneumonia. None of the patients had OSAS, TED or a haemato-oncological disease. The etiology remained unexplained in 20.4% patients. No difference was observed in RF occurrence between patients with CRVO and HRVO and those with BRVO.

CONCLUSION

The most common systemic abnormality in our cohort was CV disease, especially HT and HLD. The risk factors for central, hemi-central and branch RVOs were similar.

Distinctive Imaging Characteristics of Retinal and Cerebral Vessels between Central and Branch Retinal Vein Occlusion by MRI and AI-Based Image Analyzer

Retinal vessels have been good predictive and prognostic imaging biomarkers for systemic or eye diseases. Numerous studies have shown that the two retinal vein occlusion entities may correlate with cardiovascular and cerebrovascular events or primary open-angle glaucoma. This study aims to investigate if there is a disparity in the correlations between branch RVO (BRVO) and central RVO (CRVO) with systemic disorders or POAG, thus explaining the pathogenic difference between BRVO and CRVO. This retrospective case-control study enrolled 59 RVO subjects (118 eyes), including 25 CRVO and 34 BRVO subjects, who received routine eye and brain MRI examinations. The geometric characteristics of the caliber of the retinal and cerebral blood vessels and the optic nerve subarachnoid space width (ONSASW) were measured. Multivariable logistic regression analysis showed that ONSASW at 3 mm behind the globe (p = 0.044) and the relative retinal venular calibers (p = 0.031) were independent risk factors for the CRVO-affected eyes group in comparison with the BRVO-affected eyes group after adjusting for age, duration of hypertension, BMI, and IOP. In the CRVO-affected eyes, narrower relative retinal arteriolar calibers (p = 0.041) and wider relative venular calibers (p = 0.011) were independent risk factors compared with the CRVO-contralateral normal eyes when adjusting for IOP. We concluded that BRVO may be more associated with cerebrovascular diseases, and CRVO may be correlated with primary angle glaucoma. The geometric characteristics difference between the retinal and cerebrovascular may explain the pathological difference between CRVO and BRVO.

Oxygen-saturation-related functional parameter as a biomarker for diabetes mellitus-extraction method and clinical validation

Purpose: To develop a computational method for oxygen-saturation-related functional parameter analysis of retinal vessels based on traditional color fundus photography, and to explore their characteristic alterations in type 2 diabetes mellitus (DM). Methods: 50 type 2 DM patients with no-clinically detectable retinopathy (NDR) and 50 healthy subjects were enrolled in the study. An optical density ratio (ODR) extraction algorithm based on the separation of oxygen-sensitive and oxygen-insensitive channels in color fundus photography was proposed. With precise vascular network segmentation and arteriovenous labeling, ODRs were acquired from different vascular subgroups, and the global ODR variability (ODR_v) was calculated. Student's t-test was used to analyze the differences of the functional parameters between groups, and regression analysis and receiver operating characteristic (ROC) curves were used to explore the discrimination efficiency of DM patients from healthy subjects based on these functional parameters. Results: There was no significant difference in the baseline characteristics between the NDR and healthy normal groups. The ODRs of all vascular subgroups except the micro venule were significantly higher (p<0.05, respectively) while ODR_v was significantly lower (p<0.001) in NDR group than that in healthy normal group. In the regression analysis, the increased ODRs except micro venule and decreased ODR_v were significantly correlated with the incidence of DM, and the C-statistic for discrimination DM with all ODR is 0.777 (95% CI 0.687-0.867, p<0.001). Conclusion: A computational method to extract the retinal vascular oxygen-saturation-related optical density ratios (ODRs) with single color fundus photography was developed, and increased ODRs and decreased ODR_v of retinal vessels could be new potential image biomarkers of DM.

Acute uveitic phase of Vogt-Koyanagi-Harada disease: optic nerve head swelling, ocular blood flow and retinal oxygen metabolism

OBJECTIVES

To investigate the association of optic nerve head (ONH) swelling in the acute uveitic phase of Vogt-Koyanagi-Harada (VKH) disease with blood flow velocity in the choroid and ONH and oxygen saturation and diameter of retinal vessels.

METHODS

In this prospective study, 25 patients (50 eyes) were studied. Thirteen patients (26 eyes) had ONH swelling and 12 patients (24 eyes) had no ONH swelling. Laser speckle flowgraphy (LSFG) and retinal oximetry measurements were performed at presentation.

RESULTS

In the ONH, mean blur rate (MBR)-vessel, representing blood flow velocity in retinal vessels, was significantly lower in the eyes affected by ONH swelling, while choroidal MBR was not significantly different. Eyes with ONH swelling had a significantly lower oxygen saturation in retinal venules, a significantly higher arteriovenous oxygen saturation difference and a significantly smaller calibre of retinal arterioles compared with eyes without ONH swelling. There were significant positive correlations between the MBR-vessel of the ONH and venular oxygen saturation and calibre of retinal arterioles. In addition, MBR-vessel of the ONH had a significant negative correlation with arteriovenous oxygen saturation difference.

CONCLUSIONS

The occurrence of ONH swelling in the acute uveitic phase of VKH disease is associated with lower retinal blood flow velocity and smaller calibre of retinal arterioles as well as lower oxygen saturation in retinal venules and higher arteriovenous difference in oxygen saturation.

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

Purpose

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

Methods

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

Results

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

Conclusions

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

A Baseline Study of Oxygen Saturation in Parafoveal Vessels Using Visible Light Optical Coherence Tomography

The retinal macula is at the center of our visual field, and thus pathological damage in the macula significantly impacts an individual's quality of life. The parafoveal vessels form the inner retina provide oxygen perfusion, and the measurement of parafoveal oxygen saturation (sO₂) can evaluate macular metabolism and provide pathophysiological insight. In this paper, for the first time, we present a baseline study of microvascular oxygen saturation (sO₂) in perifoveal macular region using visible light optical coherence tomography (VIS-OCT) on normal eyes. The arterial and venous sO₂ from all eyes was 92.1 ± 7.1 (vol %) and 48.4 ± 5.0 (vol %) (mean ± SD), respectively. Arteriovenous sO₂ difference was 43.8 ± 9.5 (vol %). Marginal correlation was found between venous sO₂ and intraocular pressure (IOP) among eyes. No significant correlation was found between sO₂ and vessel topological features, including length, diameter, and distance to fovea. This baseline study could serve as a benchmark for the future sO₂ investigation of retinal macular pathologies.

Lack of predictive value of retinal oxygen saturation for visual outcome after angiostatic treatment of branch retinal vein occlusion

PURPOSE

Previous studies have shown that the retinal oxygen saturation in central retinal vein occlusion treated with anti-VEGF compound has no predictive value for visual outcome after 12 months. It is of interest to evaluate whether this conclusion is similar for patients with branch retinal vein occlusion among whom only some patients are treated.

METHODS

Retinal oxygen saturation, visual acuity and central retinal thickness were studied at the time of referral and after six and 12 months in 111 patients successively referred to the Department of Ophthalmology, Aarhus University Hospital, with a venous occlusion affecting branches peripheral from the central retinal venule. The predictive value of the oxygen saturation at referral was investigated in treated and untreated patients.

RESULTS

Seventy-three patients with visual acuity between 35 and 70 ETDRS letters at referral were treated with intravitreal injection of anti-VEGF compound. Over 12 months, the venous oxygen saturation improved in parallel with central retinal thickness and visual acuity but had no predictive value for visual outcome. In 12 untreated patients with visual acuity >70 ETDRS letters, younger age and high oxygen saturation at the time of referral were positive predictors for the visual outcome after 12 months.

CONCLUSION

Oxygen saturation, visual acuity and central retinal thickness improve in parallel during treatment of branch retinal vein occlusion with intravitreal anti-VEGF medication. Retinal oximetry at referral cannot predict visual acuity after 12 months in treated patients but may perhaps become a tool for predicting the visual prognosis in a subgroup of patients where treatment is omitted because of a too high visual acuity at the time of diagnosis of the disease.

Functional imaging of human retina using integrated multispectral and laser speckle contrast imaging

A novel integration of retinal multispectral imaging (MSI), retinal oximetry and laser speckle contrast imaging (LSCI) is presented for functional imaging of retinal blood vessels that could potentially allow early detection or monitoring of functional changes. We designed and built a cost-effective, scalable, retinal imaging instrument that integrates structural and functional retinal imaging techniques, including MSI, retinal oximetry and LSCI. Color fundus imaging was performed with 470 nm, 550 nm and 600 nm wavelength light emitting diode (LED) illumination. Retinal oximetry was performed using 550 nm and 600 nm LED illumination. LSCI of blood flow was performed using 850 nm laser diode illumination at 82 frames per second. LSCI can visualize retinal and choroidal vasculature without requiring exogenous contrast agents and can provide time-resolved information on blood flow, generating a cardiac pulse waveform from retinal vasculature. The technology can rapidly acquire structural MSI images, retinal oximetry and LSCI blood flow information in a simplified clinical workflow without requiring patients to move between instruments. Results from multiple modalities can be combined and registered to provide structural as well as functional information on the retina. These advances can reduce barriers for clinical adoption, accelerating research using MSI, retinal oximetry and LSCI of blood flow for diagnosis, monitoring and elucidating disease pathogenesis.

Clinical implications of retinal oximetry in retinal vein occlusion: a review

PURPOSE

To review retinal vein occlusion (RVO) and its relationship with retinal oxygen saturation via automated retinal oximetry in eyes with RVO.

METHODS

A literature review was performed in PubMed and Medline until October 2021 utilizing specific keywords and cross-matched reference lists.

RESULTS

This review found RVO to be associated with risk factors including age, hypertension, cardiovascular and metabolic conditions, male gender, and glaucoma. These may be attributed to a breakdown of regulatory processes in the retina. Retinal venous oxygen saturation (SvO₂ ) and arteriovenous difference in eyes with central RVO have been found to be reduced and elevated, respectively. The literature indicates variable and contradictory findings in regard to branch RVO and retinal oxygenation. Additionally, ischaemic eyes have been found to have elevated retinal arterial oxygen saturation; however, the literature reports variable results regarding SvO₂ levels. Medications have been suggested to increase SvO₂ in RVO eyes, which may represent an important mechanism for disease management. Ranibizumab is currently the most studied drug for retinal oxygenation in RVO and has been suggested to increase SvO₂ in RVO eyes. In comparison, dexamethasone was found to decrease SvO₂ .

CONCLUSION

The current literature on retinal oxygenation in ischaemic subtypes of RVO and in drug therapies is minimal, and further work is required to expand upon our understanding of how ischaemia and drugs influence retinal oxygenation and clinical outcomes.

Advances in Retinal Oximetry

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

Functional and structural ophthalmic imaging using noncontact multimodal photoacoustic remote sensing microscopy and optical coherence tomography

Early diagnosis of ocular diseases improves the understanding of pathophysiology and aids in accurate monitoring and effective treatment. Advanced, multimodal ocular imaging platforms play a crucial role in visualization of ocular components and provide clinicians with a valuable tool for evaluating various eye diseases. Here, for the first time we present a non-contact, multiwavelength photoacoustic remote sensing (PARS) microscopy and swept-source optical coherence tomography (SS-OCT) for in-vivo functional and structural imaging of the eye. The system provides complementary imaging contrasts of optical absorption and optical scattering, and is used for simultaneous, non-contact, in-vivo imaging of murine eye. Results of vasculature and structural imaging as well as melanin content in the retinal pigment epithelium layer are presented. Multiwavelength PARS microscopy using Stimulated Raman scattering is applied to enable in-vivo, non-contact oxygen saturation estimation in the ocular tissue. The reported work may be a major step towards clinical translation of ophthalmic technologies and has the potential to advance the diagnosis and treatment of ocular diseases.

Multimodal Imaging of Microvascular Abnormalities in Retinal Vein Occlusion

The technologies of ocular imaging modalities such as optical coherence tomography (OCT) and OCT angiography (OCTA) have progressed remarkably. Of these in vivo imaging modalities, recently advanced OCT technology provides high-resolution images, e.g., histologic imaging, enabling anatomical analysis of each retinal layer, including the photoreceptor layers. Recently developed OCTA also visualizes the vascular networks three-dimensionally, which provides better understanding of the retinal deep capillary layer. In addition, ex vivo analysis using autologous aqueous or vitreous humor shows that inflammatory cytokine levels including vascular endothelial growth factor (VEGF) are elevated and correlated with the severity of macular edema (ME) in eyes with retinal vein occlusion (RVO). Furthermore, a combination of multiple modalities enables deeper understanding of the pathology. Regarding therapy, intravitreal injection of anti-VEGF drugs provides rapid resolution of ME and much better visual improvements than conventional treatments in eyes with RVO. Thus, the technologies of examination and treatment for managing eyes with RVO have progressed rapidly. In this paper, we review the multimodal imaging and therapeutic strategies for eyes with RVO with the hope that it provides better understanding of the pathology and leads to the development of new therapies.

Current use of the automatic retinal oximetry. Review

PURPOSE

To inform about possible use of the automatic retinal oximetry for the retinal oxygen saturation measurement in the eye and systemic diseases.

METHODS

We performed a literature review dealing with issues of retinal oxygen saturation monitoring by dual non-invasive retinal oximetry Oxymap T1 (Oxymap ehf. Reykjavík, Iceland).

RESULTS

We have found two main strains writing our paper on retinal oxygen saturation eye diseases. The first section concerns diseases created by having hypoxia as its main pathological factor  - for example diabetes mellitus and retinal vein occlusion. The second group deals with atrophy as the main pathological mechanism which is typical for decreasing retinal oxygen consumption - for example glaucoma or retinitis pigmentosa (the second one named is not included in our work). Oximetry in systemic diseases creates a relatively new chapter of this branch with a very big potential of interdisciplinary cooperation for the future. It is possible the cooperation will not only include diabetologists but also neurologists (for example, in diseases like sclerosis multiplex or Devics, Alzheimers and Parkinsons disease) and haematologists (retinal oxygen saturation changes in patients with different rheological attributes of blood).

CONCLUSION

Retinal oxygen saturation measuring by automatic retinal oximetry is a relatively new method with scientifically confirmed high reproducibility of results. Currently it is the only experimental method with vast potentials not only in the realm of the possibility of observing eye diseases (diabetic retinopathy, retinal vein occlusion or glaucoma) but also in developing interdisciplinary cooperation with diabetologists, neurologists and haematologists.

Automatic analysis of normative retinal oximetry images

Retinal oximetry is an important screening tool for early detection of retinal pathologies due to changes in the vasculature and also serves as a useful indicator of human-body-wide vascular abnormalities. We present an automatic technique for the measurement of oxygen saturation in retinal arterioles and venules using dual-wavelength retinal oximetry images. The technique is based on segmenting an optic-disc-centered ring-shaped region of interest and subsequent analysis of the oxygen saturation levels. We show that the two dominant peaks in the histogram of the oxygen saturation levels correspond to arteriolar and venular oxygen saturations from which the arterio-venous saturation difference (AVSD) can be calculated. For evaluation, we use a normative database of Asian Indian eyes containing 44 dual-wavelength retinal oximetry images. Validations against expert manual annotations of arterioles and venules show that the proposed technique results in an average arteriolar oxygen saturation (SatO₂) of 87.48%, venular SatO₂ of 57.41%, and AVSD of 30.07% in comparison with the expert ground-truth average arteriolar SatO₂ of 89.41%, venular SatO₂ of 56.32%, and AVSD of 33.09%, respectively. The results exhibit high consistency across the dataset indicating that the automated technique is an accurate alternative to the manual procedure.

Higher aqueous levels of matrix metalloproteinases indicated visual impairment in patients with retina vein occlusion after anti-VEGF therapy

PURPOSE

To investigate the levels of matrix metalloproteinases (MMPs) in aqueous humour of patients with retinal vein occlusion (RVO) and the relationship between intraocular MMP levels and retinal lesion and visual prognosis.

MATERIALS AND METHODS

52 RVO patients, including 23 with central retinal vein occlusion (CRVO) and 29 with branch retinal vein occlusion (BRVO) and 20 participants with senile cataract were enrolled in this study. Retinal lesions were examined by fundus colour photography, fluorescein fundus angiography and optical coherence tomographic angiography. Sixty microliters of aqueous humour were collected during intravitreal anti-Vascular Endothelial Growth Factor (VEGF) injection or cataract surgery. The aqueous levels of MMP-1, MMP-2, MMP-7, MMP-9 and MMP-10 were measured using the Luminex xMAP multiplex assay. The relationship between MMP levels and clinical presentations was analysed by Pearson correlation test.

RESULTS

The aqueous humour levels of MMP-1, MMP-2, MMP-7 and MMP-9, but not MMP10 in RVO patients were significantly higher than those in people with cataract after adjusting for age. Further analysis of RVO subgroups showed that the aqueous humour level of MMP2 in CRVO was significantly higher than that in BRVO. The aqueous humour levels of MMP-1 and MMP-2 were positively correlated with superficial capillary plexus vessel density (SVD), whereas the aqueous humour levels of MMP-1 and MMP-7 were negatively correlated with visual improvement following treatment. No correlation between aqueous humour levels of MMP and disease duration and central retinal thickness was observed.

CONCLUSIONS

RVO eyes had significantly higher intraocular levels of MMP-1, MMP-2, MMP-7 and MMP-9 than cataract eyes and the level of MMP2 appears to be related to the area of occlusion. Intraocular levels of MMP may positively affect SVD and negatively impact visual function in RVO.

Fundus-simulating phantom for calibration of retinal vessel oximetry devices

Retinal vessel oxygen supply is important for retinal tissue metabolism. Commonly used retinal vessel oximetry devices are based on dual-wavelength spectral measurement of oxyhemoglobin and deoxyhemoglobin. However, there is no traceable standard for reliable calibration of these devices. In this study, we developed a fundus-simulating phantom that closely mimicked the optical properties of human fundus tissues. Microchannels of precisely controlled topological structures were produced by soft lithography to simulate the retinal vasculature. Optical properties of the phantom were adjusted by adding scattering and absorption agents to simulate different concentrations of fundus pigments. The developed phantom was used to calibrate the linear correlation between oxygen saturation (SO₂) level and optical density ratio in a dual-wavelength oximetry device. The obtained calibration factors were used to calculate the retinal vessel SO₂ in both eyes of five volunteers aged between 24 and 27 years old. The test results showed that the mean arterial and venous SO₂ levels after phantom calibration were coincident with those after empirical value calibration, indicating the potential clinical utility of the produced phantom as a calibration standard.

The Relationship Between Retinal Vessel Oxygenation and Spatial Distribution of Retinal Nonperfusion in Retinal Vascular Diseases

Purpose

We study the relationship between retinal vessel oxygenation and the spatial distribution of retinal nonperfusion using ultrawide field angiography in eyes with retinal vascular diseases.

Methods

This prospective single center study recruited 57 eligible eyes from 44 patients with retinal vascular diseases. Retinal oximetry measurements were obtained using the Oxymap T1 device to determine the arteriovenous (AV) difference. Retinal nonperfusion was measured from ultrawide field angiography images taken with the Optos 200TX system and superimposing the images with the concentric rings template to determine the area and distribution of retinal nonperfusion.

Results

Seven (12.3%) eyes had a diagnosis of a branch or hemiretinal vein occlusion, 24 (42.1%) with central retinal vein occlusion and 26 (45.6%) with diabetic retinopathy (11 [19.3%] nonproliferative and 15 [26.3%] proliferative diabetic retinopathy). The correlation between the total area of retinal nonperfusion with the AV difference controlling for age was not statistically significant (R = -0.103, P = 0.449). However, when analyzing the correlation of AV difference with the area of retinal nonperfusion in the posterior pole controlling for age and peripheral nonperfusion, this was significant (R = -0.295, P = 0.029). This was not significant for the area of retinal nonperfusion in the periphery while controlling for posterior pole nonperfusion and age (R = 0.124, P = 0.368).

Conclusions

Retinal nonperfusion has a negative correlation with AV difference measured on retinal oximetry. This correlation is significant in the posterior pole, but not in the peripheral retina.

Retinal oxygen saturation as a non-invasive estimate for mixed venous oxygen saturation and cardiac output

PURPOSE

To investigate the correlation between retinal vessel oxygen saturation and mixed venous oxygen saturation (SvO_2-mixed ) and cardiac output (CO).

METHODS

Retinal arterial (SaO_2-retinal ) and venous (SvO_2-retinal ) oxygen saturation were measured non-invasively with dual-wavelength retinal oximetry in subjects receiving invasive measurements of SvO_2-mixed and CO through right heart catheterization. Correlations were analysed using Spearman's rank correlation coefficients and linear regression models.

RESULTS

Fourteen patients (median age 62.7 years, range: 21-77) were included in the analysis. When adjusted for age, SvO_2-retinal showed a positive correlation with SvO_2-mixed (β = 0.80, p = 0.003). Retinal arteriovenous oxygen saturation difference was significantly correlated with the inverse of CO (Spearman's ρ = 0.59, p = 0.026).

CONCLUSION

This pilot study provides proof of concept for the use of retinal oximetry as a non-invasive tool to assess systemic cardiovascular function.

Development of a novel noninvasive system for measurement and imaging of the arterial phase oxygen density ratio in the retinal microcirculation

PURPOSE

This study was conducted in order to develop a novel noninvasive system for measurement and imaging of the arterial oxygen density ratio (ODR) in the retinal microcirculation.

METHODS

We developed a system composed of two digital cameras with two different filters, which were attached to a fundus camera capable of simultaneously obtaining two images. Actual measurements were performed on healthy volunteer eyes (n = 61). A new algorithm for ODR measurement and pixel-level imaging of erythrocytes was constructed from these data. The algorithm was based on the morphological closing operation and the line convergence index filter. For system calibration, we compared and verified the ODR values in arterioles and venules that were specified in advance for 56 eyes with reproducibility. In 10 additional volunteers, ODR measurements and imaging of the arterial phase in the retinal microcirculation corresponding to changes in oxygen saturation of the peripheral arteries at normal breathing and breath holding were performed.

RESULTS

Estimation of incident light to erythrocytes and pixel-level ODR calculation were achieved using the algorithm. The mean ODR values of arterioles and venules were 0.77 ± 0.060 and 1.02 ± 0.067, respectively. It was possible to separate these regions, calibrate at the pixel level, and estimate the arterial phase. In each of the 10 volunteers, changes in the arterial phase ODR corresponding to changes in oxygen saturation of the peripheral arteries were observed before and after breath holding on ODR images. The mean ODR in 10 volunteers was increased by breath holding (p < 0.05).

CONCLUSIONS

We developed a basic system for arterial phase ODR measurement and imaging of the retinal microcirculation. With further validation and development, this may provide a useful tool for evaluating retinal oxygen metabolism in the retinal microcirculation.

Retinal oximetry and systemic arterial oxygen levels

PURPOSE

Continuous peripheral pulse oximetry for monitoring adequacy of oxygenation is probably the most important technological advance for patients' monitoring and safety in the last decades. Pulse oximetry has the disadvantage of measuring the peripheral circulation, and the only mean to measure oxygen content of the central circulation is by invasive technology. Determination of blood oxyhaemoglobin saturation in the retinal vessels of the eye can be achieved noninvasively through spectrophotometric retinal oximetry which provides access to the central nervous system circulation. The aim of the thesis was to determine whether retinal oximetry technique can be applied for estimation of the central nervous system circulation which until now has only been possible invasively. This was achieved by measuring oxyhaemoglobin saturation in three adult subject study groups: in people with central retinal vein occlusion (CRVO) to observe local tissue hypoxia, in patients with severe chronic obstructive pulmonary disease (COPD) on long-term oxygen therapy to observe systemic hypoxaemia and in healthy subjects during hyperoxic breathing to observe systemic hyperoxemia. In addition, the fourth study that is mentioned was performed to test whether retinal oximetry is feasible for neonates.

METHODS

Retinal oximetry in central retinal vein occlusion: Sixteen subjects with central retinal vein occlusion participated in the study. The oxyhaemoglobin saturation of the central retinal vein occlusion affected eye was compared with the fellow unaffected eye. Retinal oximetry in healthy people under hyperoxia: Thirty healthy subjects participated in the study, and the oxyhaemoglobin saturation of retinal arterioles and venules was compared between normoxic and hyperoxic breathing. Retinal oximetry in severe chronic obstructive pulmonary disease: Eleven patients with severe chronic obstructive pulmonary disease participated in the study. Retinal oximetry measurements were made with and without their daily supplemental oxygen therapy. Retinal arteriolar oxyhaemoglobin saturation when inspiring ambient air was compared with blood samples from the radial artery and finger pulse oximetry and healthy controls. The healthy control group was assembled from our database for comparison of oxyhaemoglobin saturation of retinal arterioles and venules during the ambient air breathing. The retinal oximeter is based on a conventional fundus camera and a specialized software. A beam splitter coupled with two high-resolution digital cameras allows for simultaneous acquisition of retinal images at separative wavelengths for calculation of oxyhaemoglobin saturation. In addition, retinal images of 28 full-term healthy neonates were obtained with scanning laser ophthalmoscope combined with modified Oxymap analysis software for calculation of the optical density ratio and vessel diameter RESULTS: Retinal oximetry in central retinal vein occlusion: Mean retinal venous oxyhaemoglobin saturation was 31 ± 12% in CRVO eyes and 52 ± 11% in unaffected fellow eyes (mean ± SD, n = 14, p < 0.0001). The arteriovenous oxygen difference (AV-difference) was 63 ± 11% in CRVO eyes and 43 ± 7% in fellow eyes (p < 0.0001). The variability of retinal venous oxyhaemoglobin saturation was considerable within and between eyes affected by CRVO. There was no difference in oxyhaemoglobin saturation of retinal arterioles between the CRVO eyes and the unaffected eyes (p = 0.49). Retinal oximetry in healthy people under hyperoxia: During hyperoxic breathing, the oxyhaemoglobin saturation in retinal arterioles increased to 94.5 ± 3.8% as compared with 92.0 ± 3.7% at baseline (n = 30, p < 0.0001). In venules, the mean oxyhaemoglobin saturation increased to 76.2 ± 8.0% from 51.3 ± 5.6% (p < 0.0001) at baseline. The AV-difference was markedly lower during hyperoxic breathing as compared with the normoxic breathing (18.3 ± 9.0% versus 40.7 ± 5.7%, p < 0.0001). Retinal oximetry in severe chronic obstructive pulmonary disease: During ambient air breathing, chronic obstructive pulmonary disease subjects had significantly lower oxyhaemoglobin saturation than healthy controls in both retinal arterioles (87.2 ± 4.9% versus 93.4 ± 4.3%, p = 0.02, n = 11) and venules (45.0 ± 10.3% versus 55.2 ± 5.5%, p = 0.01) but the AV-difference was not markedly different (p = 0.17). Administration of their prescribed oxygen therapy significantly increased the oxyhaemoglobin saturation in retinal arterioles (87.2 ± 4.9% to 89.5 ± 6.0%, p = 0.02) but not in venules (45.0 ± 10.3% to 46.7 ± 12.8%, p = 0.3). Retinal oximetry values were slightly lower than finger pulse oximetry (mean percentage points difference = -3.1 ± 5.5) and radial artery blood values (-5.0 ± 5.4). Retinal oximetry study in neonates: The modified version of the retinal oximetry instrument estimated the optical density ratio in retinal arterioles to be 0.256 ± 0.041 that was significantly different from the 0.421 ± 0.089 in venules (n = 28, p < 0.001, paired t-test). The vascular diameter of retinal arterioles was markedly narrower than of venules (14.1 ± 2.7 and 19.7 ± 3.7 pixels, p < 0.001).

CONCLUSION

The results of this thesis indicate that spectrophotometric retinal oximetry is sensitive to both local and systemic changes in oxyhaemoglobin saturation. Retinal oxyhaemoglobin saturation values are slightly lower than radial artery blood sample and finger pulse oximetry values. The discrepancies between the different modalities are expected to derive from countercurrent exchange between central retinal artery and vein within the optic nerve but calibration issues cannot be excluded as contributing to this difference. Despite these differences, the findings indicate the potential of retinal oximetry for noninvasive real-time measurements of oxyhaemoglobin saturation in central nervous system vessels. Following calibration upgrade and technological improvement, verification retinal oximetry may potentially be applied to critically ill and anaesthesia care patients. The study on combined scanning laser ophthalmoscope and retinal oximetry supports the feasibility of the technique for oximetry analysis in newly born babies.

Handheld Retinal Oximetry in Healthy Young Adults

Purpose

The objective of this study was to measure the relative retinal oxygen saturation with a prototype, mobile handheld oximeter in upright and supine position and to compare these measurements to the gold standard Oxymap T1 retinal oximeter in upright position. A handheld oximeter is needed for measurements of infants with retinopathy of prematurity as well as acutely injured and bedridden adults.

Methods

Healthy volunteers (age 18–35) were recruited at the Leiden University Medical Center. Retinal images were acquired with the handheld oximeter and the Oxymap T1. Both cameras are dual-wavelength oximeters and acquire images with wavelengths of 570 and 600 nm. Retinal oxygen saturation values were determined for both the handheld camera and the Oxymap T1.

Results

Twenty-one subjects (age 25 ± 2 years) were included. In upright position, the oxygen saturation for the arterioles was 92.2% to 4.9% vs. 95.5% ± 4.2% and for the venules 57.9% ± 10.2% vs. 57.7% ± 6.4% for the handheld camera and Oxymap T1, respectively. The oxygen saturation was higher in the arterioles than the venules for both cameras (P < 0.05). In supine position, measured with the handheld oximeter, the oxygen saturation in the arterioles was 92.3% ± 5.8% and 59.2% ± 6.1% in the venules.

Conclusions

Performance of the prototype, mobile handheld oximeter Corimap camera compares well with the Oxymap T1, with a slightly larger standard deviation in oxygen saturation measurements, both in upright and supine patients.

Translation Relevance

To date, to our knowledge, no oximeters are available for handheld use and for measurement in supine position in infants and bedridden adults. Here we tested such an oximeter and show that its performance compares well with that of the gold standard Oxymap T1 in healthy adults.

Design of a portable phantom device to simulate tissue oxygenation and blood perfusion

We propose a portable phantom system for calibration and validation of medical optical devices in a clinical setting. The phantom system comprises a perfusion module and an exchangeable tissue-simulating phantom that simulates tissue oxygenation and blood perfusion. The perfusion module consists of a peristaltic pump, two liquid storage units, and two pressure suppressors. The tissue-simulating phantom is fabricated by a three-dimensional (3D) printing process with microchannels embedded to simulate blood vessels. Optical scattering and absorption properties of biologic tissue are simulated by mixing graphite powder and titanium dioxide powder with clear photoreactive resin at specific ratios. Tissue oxygen saturation (StO₂) and blood perfusion are simulated by circulating the mixture of blood and intralipid at different oxygenation levels and flow rates. A house-made multimodal imaging system that combines multispectral imaging and laser speckle imaging are used for non-invasive detection of phantom oxygenation and perfusion, and the measurements are compared with those of a commercial Moor device as well as numerical simulation. By acquiring multimodal imaging data from one phantom and applying the calibration factors in different settings, we demonstrate the technical feasibility to calibrate optical devices for consistent measurements. By simulating retina tissue vasculature and acquiring functional images at different tissue oxygenation and blood perfusion levels, we demonstrate the clinical potential to simulate tissue anomalies. Our experiments imply the clinical potential of a portable, low-cost, and traceable phantom standard to calibrate and validate medical optical devices for improved performance.

Correlation Between Ischemic Index of Retinal Vein Occlusion and Oxygen Saturation in Retinal Vessels

PURPOSE

To evaluate the relationship between the ischemic index and the oxygen saturation in retinal vessels in patients with retinal vein occlusion.

DESIGN

Prospective, cross-sectional study.

METHODS

We performed a prospective study. The cohort consisted of 43 eyes of 43 patients with retinal vein occlusions (RVO), 23 of whom had central retinal vein occlusions (CRVO) and 20 who had branch retinal vein occlusions (BRVO). We evaluated the retinal vessel saturation using an automatic retinal oximetry device. The retinal ischemic index (ISI) was determined using ultra-widefield fluorescein angiography.

RESULTS

Mean arterial saturation (±SD) was 100% ± 11%, mean vein saturation was 52% ±13%, and mean A-V difference was 48% ± 16% in eyes with BRVO. The average ISI in the same group was 0.48 (range 0-1). There was no statistically significant correlation between the retinal ischemic index and retinal saturation in the BRVO group. The affected eye in the CRVO group had a mean arterial saturation of 101% ± 6%, vein saturation of 44% ± 11 % and A-V difference of 58% ± 10%. The average ISI in the CRVO group was 0.54 (range 0-1). A statistically significant negative correlation between ISI and vein saturation was found in the CRVO group (r = -0.686; P =.0003). A significant positive correlation between ISI and the A-V difference was found in the CRVO group (r = 0.893; P <.0001).

CONCLUSIONS

Oxygen saturation in the retinal vein and the arteriovenous difference correlated with the ischemic index in CRVO patients. No correlation was found for BRVO patients.

Non-invasive assessment of cerebral oxygenation: A comparison of retinal and transcranial oximetry

BACKGROUND

To investigate the correlation between cerebral (SO2-transcranial), retinal arterial (SaO2-retinal) and venous (SvO2-retinal) oxygen saturation as measured by near-infrared spectroscopy (NIRS) and retinal oximetry respectively.

METHODS

Paired retinal and cerebral oxygen saturation measurements were performed in healthy volunteers. Arterial and venous retinal oxygen saturation and diameter were measured using a non-invasive spectrophotometric retinal oximeter. Cerebral oxygen saturation was measured using near-infrared spectroscopy. Correlations between SO2-transcranial and retinal oxygen saturation and diameter measurements were assessed using Pearson correlation coefficients. Lin's concordance correlation coefficient (CCC) and Bland-Altman analysis were performed to evaluate the agreement between SO2-transcranial as measured by NIRS and as estimated using a fixed arterial:venous ratio as 0.3 x SaO2-retinal + 0.7 x SvO2-retinal. The individual relative weight of SaO2-retinal and SvO2-retinal to obtain the measured SO2-transcranial was calculated for all subjects.

RESULTS

Twenty-one healthy individuals aged 26.4 ± 2.2 years were analyzed. SO2-transcranial was positively correlated with both SaO2-retinal and SvO2-retinal (r = 0.44, p = 0.045 and r = 0.43, p = 0.049 respectively) and negatively correlated with retinal venous diameter (r = -0.51, p = 0.017). Estimated SO2-transcranial based on retinal oximetry showed a tolerance interval of (-13.70 to 14.72) and CCC of 0.46 (95% confidence interval: 0.05 to 0.73) with measured SO2-transcranial. The average relative weights of SaO2-retinal and SvO2-retinal to obtain SO2-transcranial were 0.31 ± 0.11 and 0.69 ± 0.11, respectively.

CONCLUSION

This is the first study to show the correlation between retinal and cerebral oxygen saturation, measured by NIRS and retinal oximetry. The average relative weight of arterial and venous retinal oxygen saturation to obtain the measured transcranial oxygen saturation as measured by NIRS, approximates the established arterial:venous ratio of 30:70 closely, but shows substantial inter-individual variation. These findings provide a proof of concept for the role of retinal oximetry in evaluating cerebral oxygenation.

Retinal Vessel Oxygen Saturation and Vessel Diameter in Inactive Graves Ophthalmopathy

PURPOSE

To investigate whether inactive thyroid-associated ophthalmopathy (TAO) affects retinal oxygen saturation and/or vessel diameter.

METHODS

Via an observational case-control study, retinal circulation was measured in patients with inactive TAO (mild, moderate, and severe) and normal subjects by retinal oximetry. Complete ophthalmologic examination, including noncontact tonometry and Hertel exophthalmometry, was performed; history of smoking and dysthyroid disease were recorded. Analysis of variance or the Kruskal-Wallis test was used to compare oximetry values between TAO and controls. Simple linear regression was used to analyze the correlation of Hertel, smoking, and intraocular pressure with oximetry values.

RESULTS

Seventy-six eyes were enrolled: 19 controls, and 17 mild, 21 moderate, and 19 severe inactive TAO. Retinal oxygen saturation did not change significantly in inactive TAO versus controls; arteriole saturation: severe, 95.7% ± 7.0%; moderate, 93.2% ± 3.9%; mild, 90.3% ± 4.8%; and controls, 93.1% ± 6.4%; vein saturation: severe, 57.4% ± 7.1%; moderate, 59.0% ± 7.0,; mild, 56.3% ± 7.9%; and controls, 58.5% ± 6.5%; arteriovenous saturation: severe, 38.3% ± 8.0%; moderate, 34.2% ± 7.1%; mild, 33.9% ± 6.8%; and controls, 34.6% ± 5.9%. However, retinal venous diameter with severe TAO (137.3 ± 12.5 μm) significantly decreased in comparison with controls (148.8 ± 10.2 μm, p = 0.017). Otherwise, no significant change in vessel diameter was found between TAO and controls. No statistically significant correlations were found between Hertel values or intraocular pressure and oximetry values. However, there was a positive significant correlation between smoking and arteriovenous oxygen saturation (p = 0.017, β = 4.61).

CONCLUSIONS

In inactive TAO versus controls, retinal oxygen saturation fluctuated and could be affected by smoking; however, the retinal venous diameter only decreased significantly for severe TAO. This implies that TAO may affect retinal circulation; this effect could be accelerated by smoking.

A method for volumetric retinal tissue oxygen tension imaging

PURPOSE

Inadequate retinal oxygenation occurs in many vision-threatening retinal diseases, including diabetic retinopathy, retinal vascular occlusions, and age-related macular degeneration. Therefore, techniques that assess retinal oxygenation are necessary to understand retinal physiology in health and disease. The purpose of the current study is to report a method for the three-dimensional (3D) imaging of retinal tissue oxygen tension (tPO₂) in rats.

METHODS

Imaging was performed in Long Evans pigmented rats under systemic normoxia (N = 6) or hypoxia (N = 3). A vertical laser line was horizontally scanned on the retina and a series of optical section phase-delayed phosphorescence images were acquired. From these images, phosphorescence volumes at each phase delay were constructed and a 3D retinal tPO₂ volume was generated. Retinal tPO₂ volumes were quantitatively analyzed by generating retinal depth profiles of mean tPO₂ (M_tPO2) and the spatial variation of tPO₂ (SV_tPO2). The effects of systemic condition (normoxia/hypoxia) and retinal depth on M_tPO2 and SV_tPO2 were determined by mixed linear model.

RESULTS

Each 3D retinal tPO₂ volume was approximately 500 × 750 × 200 μm (horizontal × vertical × depth) and consisted of 45 en face tPO₂ images through the retinal depth. M_tPO2 at the chorioretinal interface was significantly correlated with systemic arterial oxygen tension (P = 0.007; N = 9). There were significant effects of both systemic condition and retinal depth on M_tPO2 and SV_tPO2, such that both were lower under hypoxia than normoxia and higher in the outer retina than inner retina (P < 0.001).

CONCLUSION

For the first time, 3D imaging of retinal tPO₂ was demonstrated, with potential future application for assessment of physiological alterations in animal models of retinal diseases.

Preoperative and postoperative measurements of retinal vessel oxygen saturation in patients with different grades of cataracts

PURPOSE

To assess the retinal vessel blood oxygen saturation preoperatively and postoperatively in patients with different grades of cataracts.

METHODS

This study included 40 eyes from 40 patients with senile cataracts ranging from grade 1 to 3. All 40 eyes received standard phacoemulsification and intraocular lens implantation. The mean retinal arterial and venular oxygen saturation (SO₂ ) levels were measured using the Oxymap system prior to, immediately after and 3 months after cataract surgery. The arterial-venular difference (AVD) was calculated as the difference between the arterial and venular saturations. Linear regression analysis was performed to identify the relationship between the lens density grades and changes in the measured retina vessel SO₂ .

RESULTS

The mean age of the subjects was 64.1 ± 7.8 years, and 14 (35%) were male. The mean SO₂ level increased in the retinal arterioles from 90.5 ± 7.8% to 95.0 ± 6.7% immediately postoperatively and to 98.3 ± 5.3% at 3 months after surgery (p < 0.001). The mean venular saturation levels were 43.4 ± 9.7%, 58.0 ± 8.5% and 61.9 ± 6.5% at baseline, immediately after and at 3 months postoperatively, respectively, with a statistically significant increase after surgery (p < 0.001). The mean AVD significantly decreased from 47.1 ± 8.9% before surgery to 37.1 ± 7.3% immediately after and 36.4 ± 6.4% 3 months after surgery (p < 0.02). A linear regression model indicated that denser lens opacity was significantly associated with a larger amplitude of the venular SO₂ level and the AVD after cataract surgery: change of venous SO₂  = 5.934-0.191 × Age + 13.534 × Lens Grade (age: p = 0.586; lens grade: p < 0.001); change in AVD = -7.438-0.037 × Age + 7.744 × Lens Grade (age: p = 0.917; lens grade: p = 0.031).

CONCLUSIONS

Retinal arterial and venular oxygen saturation levels are significantly increased after cataract surgery, and increase rates vary with the preoperative grading of lens density.

Evaluating ocular blood flow

Studies have shown that vascular impairment plays an important role in the etiology and pathogenesis of various ocular diseases including glaucoma, age-related macular degeneration, diabetic retinopathy, and retinal venous occlusive disease. Thus, qualitative and quantitative assessment of ocular blood flow (BF) is a topic of interest for early disease detection, diagnosis, and management. Owing to the rapid improvement in technology, there are several invasive and noninvasive techniques available for evaluating ocular BF, with each of these techniques having their own limitations and advantages. This article reviews these important techniques, with a particular focus on Doppler Fourier domain optical coherence tomography (OCT) and OCT-angiography.

Robot-assisted retinal vein cannulation in an in vivo porcine retinal vein occlusion model

PURPOSE

To evaluate the feasibility of robot-assisted retinal vein cannulation for retinal vein occlusion.

METHODS

Prospective experimental study performed in in vivo porcine eyes. A standard three port pars plana vitrectomy was followed by laser-induced branch retinal vein occlusion. Consequently, a retinal vein cannulation with the help of a surgical robot and a microneedle was performed. Complete success was defined as a stable intravenous position of the needle tip confirmed by blood washout for at least 3 min. Secondary outcomes were the occurrence of intra-operative complications and technical failures.

RESULTS

Cannulation was successful in 15 of 18 eyes with a complete success rate (duration of infusion of more than 3 min) of 73% after exclusion of two eyes from analysis due to failure in establishing a blood clot. There were no technical failures regarding the robotic device. The intravessel injections of ocriplasmin in two of two eyes led to a clot dissolution. In a subset of five eyes, a second cannulation attempt at the border of the optic disc resulted in a stable intravessel position and infusion during 362 (±138) seconds.

CONCLUSION

Robot-assisted retinal vein cannulation with prolonged infusion time is technically feasible. Human experiments are required to analyse the clinical benefit of this new therapy.

Retinal oximetry in patients with ischaemic retinal diseases

The retinal oximeter is a new tool for non-invasive measurement of retinal oxygen saturation in humans. Several studies have investigated the associations between retinal oxygen saturation and retinal diseases. In the present systematic review, we examine whether there are associations between retinal oxygen saturation and retinal ischaemic diseases. We used PubMed and Embase to search for retinal oxygen saturation and retinal ischaemic diseases. Three separate searches identified a total of 79 publications. After two levels of manual screening, 10 studies were included: six about diabetic retinopathy (DR) and four about retinal vein occlusion. No studies about retinal artery occlusion were included. In diabetes, all studies found that increases in retinal venous oxygen saturation (rvSatO₂ ) were associated with present as well as increasing levels of DR. Four of six studies also found increased retinal arterial oxygen saturation (raSatO₂ ) in patients with DR. In patients with central retinal vein occlusion (CRVO), all studies found that rvSatO₂ was reduced, but raSatO₂ remained unchanged. Branch retinal vein occlusion was not associated with changes in retinal oxygen saturation, but this was based on a single study. In conclusion, DR is associated with increased rvSatO₂ and might also be related to increased raSatO₂ . Central retinal vein occlusion (CRVO) is correlated with increased rvSatO₂ but unrelated to raSatO₂ . Prospective studies are needed to expand these findings. These would tell whether retinal oximetry could be a potential tool for screening or a biomarker of treatment outcome in patients with ischaemic retinal diseases.

Retinal oximetry during treatment of retinal vein occlusion by ranibizumab in patients with high blood pressure and dyslipidemia

INTRODUCTION

In the present study, we examined retinal vascular oxygen saturation in patients with retinal vein occlusion (RVO), high blood pressure (HBP) and dyslipidemia, before and during intravitreal vascular endothelial growth factor (VEGF) injection (ranibizumab).

METHODS

We retrospectively reviewed the medical records of six patients with visual acuity (VA) reduced by macular edema (ME) secondary to RVO with HBP and dyslipidemia, who underwent intravitreal anti-VEGF injection between October 2014 and February 2015 in the department of ophthalmology of François-Quesnay Hospital at Mantes-la-Jolie (France). The main inclusion criterion was the presence of RVO with ME and decreased VA. The primary endpoint was improvement of retinal venous oxygen saturation in patients with RVO before and 3 months after intravitreal ranibizumab injection. Secondary outcomes were improvement of retinal arterial oxygen saturation, improvement of best-corrected visual acuity (BCVA) on the Early Treatment Diabetic Retinopathy Study (ETDRS) scale, regression of ME measured by the central macular thickness (CMT) in nm and studying the correlation between blood pressure (BP) and retinal venous oxygen saturation before and after ranibizumab.

RESULTS

Six eyes of six patients were included. Before treatment, the mean (standard deviation [SD]) of the retinal venous saturation (%) was 38.1±14.2. Three months after the injections, the mean (SD) of the retinal venous saturation (%) increased statistically significantly 49.2±11 (P=0.03).

CONCLUSION

In this study, retinal venous oxygen saturation in patients with RVO, HBP and dyslipidemia was partially normalized during intravitreal ranibizumab treatment.

Photoacoustic imaging of the eye: A mini review

The eye relies on the synergistic cooperation of many different ocular components, including the cornea, crystalline lens, photoreceptors, and retinal neurons, to precisely sense visual information. Complications with a single ocular component can degrade vision and sometimes cause blindness. Immediate treatment and long-term monitoring are paramount to alleviate symptoms, restore vision, and cure ocular diseases. However, successful treatment requires understanding ocular pathological mechanisms, precisely detecting and monitoring the diseases. The investigation and diagnosis of ocular diseases require advanced medical tools. In this mini review, we discuss non-invasive photoacoustic (PA) imaging as a potential research tool and medical screening device. In the research setting, PA imaging can provide valuable information on the disease progression. In the clinical setting, PA imaging can potentially aid in disease detection and treatment monitoring.

Retinal vessel oxygen saturation in a healthy young Chinese population

PURPOSE

To measure retinal vessel oxygen saturation in a healthy young Chinese population and to determine the effects of multiple factors (gender, age, dioptre, vessel diameter and ocular perfusion pressure - OPP) on retinal oxygen saturation.

METHODS

A total of 126 healthy Chinese individuals aged from 19 to 30 were included in this study. A retinal oximeter (Oxymap T1) was used to measure retinal vessel oxygen saturation by retinal imaging at two different wavelengths. The mean retinal vessel oxygen saturation (Sat_O2 ) of arterioles, venules and arteriovenous (AV) difference overall and in four separate quadrants were measured. Intra-ocular pressure, blood pressure, finger pulse oximetry value, vessel diameter and dioptre were also measured. The correlations between OPP and dioptre, OPP and vessel diameter, and dioptre and vessel diameter were analysed. And the effects of multiple factors on the retinal oxygen saturation were analysed.

RESULTS

The mean oxygen saturation was 93.2 ± 6.3% in the retinal arterioles, 60.4 ± 5.3% in venules and 32.9 ± 6.4% in AV difference. The temporal quadrants had lower measurements of arteriolar and venular oxygen saturation and AV difference compared with nasal quadrants (p < 0.001). The oxygen saturation of the arterioles, venules and AV difference were unaffected by any unique factor. Arteriolar and venular retinal oxygen saturation correlated negatively with the product of dioptre and OPP. Arteriolar retinal oxygen saturation correlated positively with the product of dioptre and vessel diameter.

CONCLUSIONS

This study provided a normal reference of Sat_O2 in healthy young Chinese individuals. It was a reflection of the normal state of retinal oxygen metabolism affected by several factors.

DYNAMIC AND STATIC RETINAL VESSEL ANALYSES IN PATIENTS WITH MACULAR EDEMA SECONDARY TO RETINAL VEIN OCCLUSION

PURPOSE

To investigate the vasomotor responses and diameter of retinal vessels in patients with macular edema secondary to retinal vein occlusion (RVO).

METHODS

A total of 18 eyes of 18 RVO patients were consecutively included and compared with age- and sex-matched controls. Participants underwent an examination with optical coherence tomography and dynamic and static retinal vessel analyses using the Dynamic Vessel Analyzer.

RESULTS

Dynamic vessel analysis in RVO showed mean maximum venous and arterial dilation of 2.22 ± 1.23% and 1.87 ± 1.41%, respectively, as compared with 5.05 ± 2.75% (P = 0.001) and 3.95 ± 1.44% (P = 0.0004), respectively, in controls. Static retinal vessel analysis in RVO revealed a mean arteriovenous ratio (AVR) of 0.74 ± 0.09 versus 0.90 ± 0.04 (P < 0.0001) in controls. Mean AVRs of occluded and nonoccluded quadrants in RVO were 0.71 ± 0.13 and 0.85 ± 0.19, respectively; in the corresponding quadrants of controls, mean AVRs were 0.90 ± 0.19 (P < 0.0001) and 0.86 ± 0.17 (P = 0.89), respectively. In branch RVO patients, mean AVRs of occluded versus nonoccluded quadrants were 0.70 ± 0.06 and 0.90 ± 0.22 (P = 0.002), respectively.

CONCLUSION

In patients with macular edema secondary to RVO, dynamic vessel analysis showed an impairment of both venous and arterial motility and/or reactivity and static vessel analysis showed a reduced AVR indicating a general enlargement of the retinal venous network. Moreover, in branch RVO patients, static analysis demonstrated that retinal vessels could actually be uninvolved by the occlusive process in areas spared by disease.

Retinal vessel oxygen saturation and vessel diameter in high myopia

PURPOSE

To investigate changes in retinal vessel oxygen saturation and diameter in high myopia.

METHODS

Relative oxygen saturation was measured in the retinal blood vessels of 54 participants with high myopia and compared to a control group of 54 individuals with emmetropia with the Oxymap T1 retinal oximeter. The participants with high myopia were further divided into two groups according to the grade of myopic retinopathy: Group A (grade < M2 ) and Group B (grade ≥ M2 ). One-way anova was used to analyse the mean saturation and diameter of retinal arterioles and venules and the mean difference in arterio-venous saturation among the four groups. Further analysis of multiple comparisons was performed with the Bonferroni test. Linear regression was used to analyse the correlation of ocular perfusion pressure or best corrected visual acuity with other variables.

RESULTS

For all of the high myopia patients, retinal arteriole saturation (92.3 ± 5.6%) and the difference in arterio-venous saturation (30.8 ± 5.0%) were significantly lower than in normal individuals (96.0 ± 5.8%, 35.4 ± 6.2%; p = 0.006, p < 0.001, respectively). In Group A, only the difference in arterio-venous saturation (31.0 ± 4.7%) was significantly lower than in the control group (p = 0.011). In Group B, retinal arteriole saturation (92.2 ± 5.3%) and the difference in arterio-venous saturation (30.7 ± 5.3%) were also lower than the control group (p = 0.02, p = 0.001, respectively). Both retinal arteriole diameter and retinal venule diameter were narrower than in participants with high myopia than the control group (p < 0.001). No statistically significant correlations were found between ocular perfusion pressure or best corrected visual acuity with any other variables.

CONCLUSIONS

The study demonstrated decreased retinal arteriole saturation and decreased difference in arterio-venous saturation as well as narrowing retinal vessel diameter in highly myopic eyes. Further studies are needed to determine if such changes play a role in the development of high myopia and its complications or occur as a consequence of tissue remodelling during axial elongation.

Study of retinal vessel oxygen saturation in ischemic and non-ischemic branch retinal vein occlusion

AIM

To explore how oxygen saturation in retinal blood vessels is altered in ischemic and non-ischemic branch retinal vein occlusion (BRVO).

METHODS

Fifty BRVO eyes were divided into ischemic (n=26) and non-ischemic (n=24) groups, based on fundus fluorescein angiography. Healthy individuals (n=52 and n=48, respectively) were also recruited as controls for the two groups. The mean oxygen saturations of the occluded vessels and central vessels were measured by oximetry in the BRVO and control groups.

RESULTS

In the ischemic BRVO group, the occluded arterioles oxygen saturation (SaO2-A, 106.0%±14.3%), instead of the occluded venule oxygen saturation (SaO2-V, 60.8%±9.4%), showed increases when compared with those in the same quadrant vessels (SaO2-A, 86.1%±16.5%) in the contralateral eyes (P<0.05). The oxygen saturations of the central vessels showed similar trends with those of the occluded vessels. In the non-ischemic BRVO group, the occluded and central SaO2-V and SaO2-A showed no significant changes. In both the ischemic and non-ischemic BRVOs, the central SaO2-A was significantly increased when compared to healthy individuals.

CONCLUSION

Obvious changes in the occluded and central SaO2-A were found in the ischemic BRVO group, indicating that disorders of oxygen metabolism in the arterioles may participate in the pathogenesis of ischemic BRVO.

Normative Values of Retinal Oxygen Saturation in Rhesus Monkeys: The Beijing Intracranial and Intraocular Pressure (iCOP) Study

OBJECTIVE

To study the normal values of the retinal oxygen saturation in Rhesus monkeys and to evaluated repeatability and reproducibility of retinal oxygen saturation measurements.

METHODS

Eighteen adult Rhesus macaque monkeys were included in this experimental study. An Oxymap T1 retinal oximeter (Oxymap, Reykjavik, Iceland) was used to perform oximetry on all subjects. Global arterial (SaO2) and venous oxygen saturation (SvO2), arteriovenous difference in SO2 were measured. In the first examination, each eye was imaged three times. At the following two examinations, each eye was imaged once. All examinations were finished in one month. P values were calculated to evaluate the difference between the measurements during three visits by performing an ANOVA. Intra-visit and inter-visit intraclass correlation coefficient (ICC) was determined.

RESULTS

At baseline, the average SaO2 and SvO2 were 89.48 ± 2.64% and 54.85 ± 2.18%, respectively. The global A-V difference was 34.63 ± 1.91%. The difference between the three visits was not significant (p>0.05). The highest A-V difference in SO2 and lowest saturations were found in the inferotemporal quadrant. Intra-session and inter-visit repeatability were both high. For all oxygen saturation parameters, the ICC values of the intra-session repeatability ranged between 0.92 and 0.96. As found previously, a relatively high ICC value for inter-visit repeatability also was found for all oxygen saturation measurements, ranging between 0.86 and 0.94, with the lowest values in the infero-nasal quadrant.

CONCLUSIONS

Our study is the first to describe retinal SO2 in healthy Rhesus monkeys. In normal monkey eyes, the reproducibility and repeatability of retinal oximetry oxygen saturation measurements were high in the retinal arterioles and venules. Our results support that Oxymap T1 retinal oximetry is a suitable and reliable technique in monkey studies.

Retinal oxygen distribution and the role of neuroglobin

The retina is the tissue layer at the back of the eye that is responsible for light detection. Whilst equipped with a rich supply of oxygen, it has one of the highest oxygen demands of any tissue in the body and, as such, supply and demand are finely balanced. It has been suggested that the protein neuroglobin (Ngb), which is found in high concentrations within the retina, may help to maintain an adequate supply of oxygen via the processes of transport and storage. We construct mathematical models, formulated as systems of reaction-diffusion equations in one-dimension, to test this hypothesis. Numerical simulations show that Ngb may play an important role in oxygen transport, but not in storage. Our models predict that the retina is most susceptible to hypoxia in the regions of the photoreceptor inner segment and inner plexiform layers, where Ngb has the potential to prevent hypoxia and increase oxygen uptake by 30-40 %. Analysis of a simplified model confirms the utility of Ngb in transport and shows that its oxygen affinity ([Formula: see text] value) is near optimal for this process. Lastly, asymptotic analysis enables us to identify conditions under which the piecewise linear and quadratic approximations to the retinal oxygen profile, used in the literature, are valid.

Analysis of retinal vessels in eyes with retinitis pigmentosa by retinal oximeter

PURPOSE

To study changes in the oxygen saturation and calibre of retinal vessels in the eyes of patients with retinitis pigmentosa (RP).

METHOD

In this cross-sectional study, 63 eyes of 63 patients with RP and 14 eyes of 14 healthy subjects were enrolled and examined at Toyama University Hospital. Retinal oximetry measurements were performed using a spectrophotometric oximetry unit (Oxymap T1, Reykjavik, Iceland), which is coupled to a fundus camera base (TRC-50DX; Topcon Corporation, Tokyo, Japan). Retinal arterioles of 28 eyes and retinal venules of 35 eyes with adequate image quality were selected for analyses. We calculated the residual areas of visual field (I-4e and V-4e isopters) in each eye and determined the correlations between the data of retinal vessels and the areas of visual field in the RP eyes.

RESULTS

The mean oxygen saturation of the venules in the RP eyes (60.2 ± 6.5%) was significantly higher than that in healthy eyes (54.6 ± 6.3%, p = 0.0089). The oxygen saturation of the venules in the RP eyes correlated significantly with the residual areas of the visual field of V-4e (r = -0.50; p = 0.0030). The mean calibres of the retinal arterioles (91.1 ± 9.1 μm) and retinal venules (116 ± 13 μm) of the RP eyes were significantly narrower than those of the healthy eyes (115 ± 11 μm and 152 ± 15 μm, respectively, p < 0.01). The retinal arteriolar and venular calibres in the RP eyes correlated with the residual areas of the visual field of V-4e (r = 0.43; p = 0.033 and r = 0.45; p = 0.011, respectively).

CONCLUSIONS

The severity of RP was correlated with increased oxygen saturations in the retinal venules and decreased retinal vessel calibres.

Noninvasive assessment of retinal vascular oxygen content among normal and diabetic human subjects: a study using hyperspectral computed tomographic imaging spectroscopy

PURPOSE

This pilot study was aimed to demonstrate the clinical feasibility of using hyperspectral computed tomographic spectroscopy to measure blood oxygen content in human retinal vessels.

METHODS

All procedures were performed under a University of Southern California Institutional Review Board-approved protocol and after obtaining informed consent. Fifty-seven subjects with and without diabetic retinopathy were dilated for standard fundus photography. Fundus photographs and retinal vascular oxygen measurements (oximetry) were made using a custom-made hyperspectral computed tomographic imaging spectrometer coupled to a standard fundus camera. Oximetry measurements were made along arteries (Aox) and veins (Vox) within vessel segments that were 1 to 2 disk diameters from the optic disk.

RESULTS

For all control subjects (n = 45), mean Aox and Vox were 93 ± 7% and 65 ± 5% (P = 0.001), respectively. For all diabetic subjects (n = 12), mean Aox and Vox were 90 ± 7% and 68 ± 5% (P = 0.001), respectively. In subjects with proliferative diabetic retinopathy, Aox was significantly lower, and Vox was significantly higher than other groups (85 ± 4% and 71 ± 4%, respectively; P = 0.04, analysis of variance). There was a highly significant difference in the arteriovenous difference between subjects with proliferative diabetic retinopathy and those in the control group (14 vs. 26%, P = 0.003).

CONCLUSION

Hyperspectral computed tomographic spectroscopy is a clinically feasible method for measurement and analysis of vascular oxygen content in retinal health and disease. This study uses the techniques relevant to oximetry; however, the breadth of spectral data available through this method may be applicable to study other anatomical and functional features of the retina in health and disease.

Venous oxygen saturation is reduced and variable in central retinal vein occlusion

PURPOSE

To estimate the presence and variability of retinal hypoxia in patients with central retinal vein occlusion (CRVO).

METHOD

Hemoglobin oxygen saturation was measured in retinal vessels of both eyes in 14 patients with unilateral CRVO. The noninvasive spectrophotometric retinal oximeter is based on a fundus camera and simultaneously captures two images at 570 nm and 600 nm wavelengths. Five of the patients were followed with repeated retinal oximetry images over time.

RESULTS

The mean oxygen saturation in retinal venules was 31 % ±12 % in CRVO eyes and 52 % ±11 % in unaffected fellow eyes (mean ±SD, n = 14, p < 0.0001). The arteriovenous difference was 63 % ±11 % in eyes with CRVO and 43 % ±7 % in fellow eyes (p < 0.0001). The variability of retinal venous oxygen saturation was substantial within and between eyes affected by CRVO. Venular oxygen saturation improved with treatment and over time in all five patients that were followed.

CONCLUSION

CRVO eyes are hypoxic compared to fellow eyes and arteriovenous difference in hemoglobin oxygen saturation is increased. This is consistent with tissue hypoxia resulting from reduced blood flow. Further studies are needed to understand the correlation between hypoxia, severity of disease and prognosis.

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.