Inner retinal oxygen delivery and metabolism in progressive stages of diabetic retinopathy.

Previous studies have reported increased retinal venous oxygen saturation and decreased retinal blood flow and oxygen metabolism in non-proliferative diabetic retinopathy (NPDR). The current study aimed to determine alterations in both inner retinal oxygen delivery (DO2) and metabolism (MO2) in proliferative DR (PDR) as well as at stages of NPDR. A total of 123 subjects participated in the study and were categorized into five groups: non-diabetic control (N = 32), diabetic with no diabetic retinopathy (NDR, N = 34), mild NPDR (N = 31), moderate to severe NPDR (N = 17), or PDR (N = 9). Multi-modal imaging was performed to measure oxygen saturation and blood flow, which were used for derivation of DO2 and MO2. There were significant associations of groups with DO2 and MO2. DO2 was lower in PDR and not significantly different in NDR and NPDR stages as compared to the non-diabetic control group. MO2 was decreased in PDR and moderate to severe NPDR as compared to the control group, and not significantly reduced in NDR and mild NPDR. The findings demonstrate reductions in both DO2 and MO2 in PDR and MO2 in moderate to severe NPDR, suggesting their potential as biomarkers for monitoring progression and treatment of DR.

Alterations in retinal pulse wave velocity under experimental ocular hypertension.

Impairments of blood flow and autoregulation have been implicated in diabetic retinopathy and glaucoma. Thus, identifying biomarkers of retinal vascular compliance and regulatory capacity is of potential value for understanding the pathophysiology and evaluating onset or progression of disease. Pulse wave velocity (PWV) represents the speed of the pulse-propagated pressure wave within blood vessels and has shown promise as a marker of vascular compliance. The purpose of the current study was to report a method for comprehensive assessment of retinal PWV based on spectral analysis of pulsatile intravascular intensity waveforms and determine alterations due to experimental ocular hypertension. Retinal PWV was linearly related to vessel diameter. Increased retinal PWV was associated with elevated intraocular pressure. Retinal PWV has the potential to serve as a vasoregulation biomarker for investigating vascular factors that contribute to the development of retinal diseases in animal models.

The accumulated oxygen deficit as an indicator of the ischemic retinal insult.

We here attempt to improve quantification of the ischemic retinal insult, that is, what is imposed on the retinal tissue by ischemia, especially in experimental models of ischemia. The ischemic retinal insult initiates the ischemic retinal injury (or outcome). Accordingly, it is reasonable to assume that the better the quantification of the insult, the better the correlation with, and thereby estimation of, the injury. The insult seldom has been quantified in terms of the relevant physiological factors, especially in connection with the rate of oxygen delivery (DO2). We here propose the accumulated oxygen deficit (AO2D) as an indicator of the ischemic retinal insult. We hypothesized that AO2D is correlated with the rate of oxygen metabolism measured 1 h after reperfusion following an episode of ischemia (MO2_1_Hr). Previously, we showed that MO2_1_Hr is related to the electroretinogram amplitude and the retinal thickness when they are measured seven days after reperfusion. We studied 27 rats, as well as 26 rats from our published data on retinal ischemia in which we had measurements of DO2 and duration of ischemia (T) of various levels and durations. We also measured DO2 in 29 rats treated with sham surgery. Ischemia was induced by either ipsilateral or bilateral common carotid artery occlusion or by ophthalmic artery occlusion, which gave a wide range of DO2. DO2 and MO2_1_Hr were evaluated based on three types of images: 1) red-free images to measure vessel diameters, 2) fluorescence images to estimate blood velocities by the displacement of intravascular fluorescent microspheres over time, and 3) phosphorescence images to quantify vascular oxygen tension from the phosphorescence lifetime of an intravascular oxygen sensitive phosphor. Loss of oxygen delivery (DO2L) was calculated as the difference between DO2 under normal/sham condition and DO2 during ischemia. AO2D, a volume of oxygen, was calculated as the product DO2L and T. Including all data, the linear relationship between AO2D and MO2_1_Hr was significant (R2 = 0.261, P = 0.0003). Limiting data to that in which T or DO2L was maximal also yielded significant relationships, and revealed that DO2L at a long duration of ischemia contributed disproportionately more than T to MO2_1_Hr. We discuss the potential of AO2D for quantifying the ischemic retinal insult, predicting the ischemic retinal injury and evaluating the likelihood of infarction.

Tailored lighting intervention (TLI) for improving sleep-wake cycles in older adults living with dementia.

Introduction: Sleep disturbance is a hallmark of Alzheimer's disease and related dementias, and caregiver stress caused by patients' nighttime wandering, injuries, and agitation are frequently at the root of decisions to move them to assisted living facilities, where typically dim institutional lighting can further exacerbate their sleep problems. This study explored the effects of a circadian-effective lighting intervention on actigraphic sleep measures and subjective assessments of sleep disturbance, depression, and sleep-disturbed behaviors. Methods: Fourteen older adult (≥60 years) participants (11 females, mean age = 84.1 [SD 8.9]), all diagnosed with moderate to severe dementia and sleep disturbance, were recruited from 3 assisted living and memory care facilities. Following a crossover, placebo-controlled design, 3 different lighting modes were used to deliver high levels of circadian stimulus to the participants' eyes for two 8-week intervention periods in a counter balanced order with a 4-week washout between the study's 2 conditions (dim light control vs. active intervention). Actigraphy and questionnaire data were collected over 7-day assessment periods that preceded (baseline weeks 1 and 9) and concluded (post-intervention week 9 and 22) the intervention periods. Actigraphic outcomes included sleep duration, sleep time, sleep efficiency, sleep start time, and sleep end time. Subjective assessments included the Cornell Scale for Depression in Dementia (CSDD), Pittsburgh Sleep Quality Index (PSQI), and Sleep Disorders Inventory (SDI) instruments. Results: Under the active condition, sleep duration significantly (p = 0.018) increased and sleep start time significantly (p = 0.012) advanced after the intervention compared to baseline. Also under the active condition, PSQI (p = 0.012), CSDD (p = 0.007), Sleep Disorders Inventory frequency (p = 0.015), and SDI severity (p = 0.015) scores were significantly lower after the intervention compared to baseline. Discussion: This study demonstrates that a circadian-effective lighting intervention delivering bright days and dark nights improves measures of sleep and mood in dementia patients living in controlled environments.

Impairments of retinal hemodynamics and oxygen metrics in ocular hypertension-induced ischemia-reperfusion.

Ischemia-reperfusion (I/R) is an established model for retinal neurodegeneration. However, there is limited knowledge of retinal physiological metrics and their relationships to retinal function and morphology in the I/R model. The purpose of the study was to test the hypotheses that retinal hemodynamic and oxygen metrics are impaired and associated with visual dysfunction, retinal thinning, and retinal ganglion cell (RGC) loss due to I/R injury. Intraocular pressure (IOP) was increased in one eye of 10 rats for 90 min followed by reperfusion. Fellow eyes served as controls. After one week of reperfusion, multimodal imaging was performed to quantify total retinal blood flow (TRBF) and retinal vascular oxygen contents. Retinal oxygen delivery (DO2) and metabolism (MO2) were calculated. Pattern-evoked electroretinography (PERG) and optical coherence tomography were performed to measure RGC function and retinal thicknesses, respectively. RGCs were counted from retina whole mounts. After one week of reperfusion, TRBF was lower in study eyes than in control eyes (p < 0.0003). Similarly, DO2 and MO2 were reduced in study eyes compared to control eyes (p < 0.003). PERG amplitude, TRT, IRT, ORT, and RGCs were also lower in study eyes (p ≤ 0.01). DO2 and MO2 were correlated with PERG amplitude, TRT, IRT, and ORT (r ≥ 0.6, p ≤ 0.005). The findings improve knowledge of physiological metrics affected by I/R injury and have the potential for identifying biomarkers of injury and outcomes for evaluating experimental treatments.

Retinal Vascular Physiology Biomarkers in a 5XFAD Mouse Model of Alzheimer's Disease.

BACKGROUND: Alzheimer's disease (AD) is a neurodegenerative disorder that affects the brain and retina and lacks reliable biomarkers for early diagnosis. As amyloid beta (Aß) manifestations emerge prior to clinical symptoms and plaques of amyloid may cause vascular damage, identification of retinal vascular biomarkers may improve knowledge of AD pathophysiology and potentially serve as therapeutic targets. The purpose of the current study was to test the hypothesis that retinal hemodynamic and oxygen metrics are altered in 5XFAD mice. METHODS: Thirty-two male mice were evaluated at 3 months of age: sixteen 5XFAD transgenic and sixteen wild-type mice. Spectral-domain optical coherence tomography, vascular oxygen tension, and blood flow imaging were performed in one eye of each mouse. After imaging, the imaged and fellow retinal tissues were submitted for histological sectioning and amyloid protein analysis, respectively. Protein analysis was also performed on the brain tissues. RESULTS: Retinal physiological changes in venous diameter and blood velocity, arterial and venous oxygen contents, coupled with anatomical alterations in the thickness of retinal cell layers were detected in 5XFAD mice. Moreover, an increase in Aß42 levels in both the retina and brain tissues was observed in 5XFAD mice. Significant changes in retinal oxygen delivery, metabolism, or extraction fraction were not detected. Based on compiled data from both groups, arterial oxygen content was inversely related to venous blood velocity and nerve fiber/ganglion cell layer thickness. CONCLUSIONS: Concurrent alterations in retinal hemodynamic and oxygen metrics, thickness, and tissue Aß42 protein levels in 5XFAD mice at 3 months of age corresponded to previously reported findings in human AD. Overall, these results suggest that this mouse model can be utilized for studying pathophysiology of AD and evaluating potential therapies.

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.

Assessment of retinal oxygen metabolism, visual function, thickness and degeneration markers after variable ischemia/reperfusion in rats.

After total retinal ischemia induced experimentally by ophthalmic vessel occlusion followed by reperfusion, studies have reported alterations in retinal oxygen metabolism (MO2), delivery (DO2), and extraction fraction (OEF), as well as visual dysfunction and cell loss. In the current study, under variable durations of ischemia/reperfusion, changes in these oxygen metrics, visual function, retinal thickness, and degeneration markers (gliosis and apoptosis) were assessed and related. Additionally, the prognostic value of MO2 for predicting visual function and retinal thickness outcomes was reported. Sixty-one rats were divided into 5 groups of ischemia duration (0 [sham], 60, 90, 120, or 180 min) and 2 reperfusion durations (1 h, 7 days). Phosphorescence lifetime and blood flow imaging, electroretinography, and optical coherence tomography were performed. MO2 reduction was related to visual dysfunction, retinal thinning, increased gliosis and apoptosis after 7-days reperfusion. Impairment in MO2 after 1-h reperfusion predicted visual function and retinal thickness outcomes after 7-days reperfusion. Since MO2 can be measured in humans, findings from analogous studies may find value in the clinical setting.

Variability of Retinal Oxygen Metrics in Healthy and Diabetic Subjects.

Purpose: Previous studies have reported alterations in total retinal blood flow (TRBF), oxygen delivery (DO2), oxygen metabolism (MO2), and oxygen extraction fraction (OEF) due to retinal diseases. The purposes of the current study were to determine variabilities and establish normal confidence intervals (CIs) for these metrics. Methods: A total of 22 healthy and 14 diabetic subjects participated in the study. Retinal vascular oxygen saturation (SO2) and TRBF were measured by oximetry and Doppler optical coherence tomography, respectively. DO2, MO2, and OEF were calculated from SO2 and TRBF measurements. Means, standard deviations (SDs), and CIs of metrics were determined in healthy subjects. Intra-visit variability was determined by the mean SDs of repeated measurements. Inter-visit variability was determined by the difference of measurements between two visits. Results: TRBF was 44 ± 15 µL/min (95% CI, 37-51) in healthy subjects. Intra-visit variabilities of TRBF were 5 µL/min and 6 µL/min in healthy and diabetic subjects, respectively. Inter-visit variability of TRBF was 3 µL/min in diabetic subjects. DO2, MO2, and OEF were 8.3 ± 2.9 µLO2/min (95% CI, 7.0-9.6), 3.2 ± 0.9 µLO2/min (95% CI, 2.8-3.6), and 0.40 ± 0.08 (95% CI, 0.36-0.43), respectively, in healthy subjects. Inter-visit variabilities of DO2, MO2, and OEF were 0.6 µLO2/min, 0.1 µLO2/min, and 0.03, respectively, in diabetic subjects. Conclusions: The findings established variabilities and normal baselines for TRBF, DO2, MO2, and OEF measurements in a small cohort of subjects. Translational Relevance: The variability and normal baselines of retinal oxygen metrics may be useful for diagnosing and monitoring patients with retinal diseases.

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

Purpose: Diabetic retinopathy (DR) is a common cause of vision loss in working age adults and presents changes in retinal vessel oxygenation and morphology. The purpose of this study was to test the hypothesis that there is an association of retinal vessel oxygen saturation with vessel density (VD) and tortuosity in DR. Methods: Ninety-five subjects were classified in the following groups: nondiabetic control (N = 25), no DR (N = 28), mild nonproliferative DR (NPDR; N = 21), moderate to severe NPDR (N = 14), or treated proliferative DR (PDR; N = 7). Retinal oximetry was performed to measure arterial and venous oxygen saturation (SO2A and SO2V) and calculate oxygen extraction fraction (OEF). Optical coherence tomography angiography (OCTA) was performed for measurements of VD and vessel tortuosity index (VTI). Results: There were statistically significant differences in SO2A and SO2V among groups (P ≤ 0.004). SO2A and SO2V were higher in the PDR group compared to the control group and SO2V was also higher in the moderate to severe NPDR group. VD differed significantly among groups (P = 0.003), whereas VTI was not significantly different (P = 0.22). Compared to the control group, VD was lower in moderate to severe NPDR and PDR groups. VD was also lower in the PDR group than that in the no DR group (P = 0.03). There was a significant correlation of VTI with SO2V (r = 0.32, P = 0.002) and OEF (r = -0.35, P = 0.001). Conclusions: Retinal vessel morphology, oxygenation, and tissue oxygen extraction were associated with each other in a cohort of subjects with and without DR. Translational Relevance: The findings of this study have the potential to improve clinical management of DR by providing better understanding of human disease pathophysiology and propelling future studies to identify multiple image-based biomarkers for improved disease diagnosis and monitoring.

Assessment of inner retinal oxygen metrics and thickness in a mouse model of inherited retinal degeneration.

The retinal degeneration 1 (rd1) mouse is a well-established model of inherited retinal degeneration, displaying photoreceptor degeneration and retinal vasculature damage. The purpose of the current study was to determine alterations in the rate of oxygen delivery from retinal circulation (DO2), the rate of oxygen extraction from the retinal circulation for metabolism (MO2), and oxygen extraction fraction (OEF) in rd1 mice. The study was performed in a total of 18 wild type (WT) and 10 rd1 mice at both 3-weeks and 12-weeks of age. Retinal arterial and venous oxygen contents (O2A and O2V) were measured using phosphorescence lifetime imaging. Total retinal blood flow (TRBF) was determined by fluorescence and red-free imaging. DO2 and MO2 were determined as TRBF × O2A and TRBF × (O2A-O2V), respectively. OEF was calculated as MO2/DO2. The thickness of individual retinal layers was measured from histology sections and inner retina (IR) and total retina (TR) thickness were calculated. TRBF, DO2 and MO2 were lower in rd1 mice compared to WT mice (P ≤ 0.001), whereas OEF was not significantly different between rd1 and WT mice (P = 0.4). TRBF and DO2 were lower at 3-weeks of age compared to 12-weeks of age (P ≤ 0.01), while MO2 was not significantly different between age groups (P = 0.4) and OEF was higher at 3-weeks of age compared to 12-weeks of age (P = 0.003). Additionally, the outer and inner retinal cell layer thicknesses were decreased in rd1 mice at 12-weeks of age compared to both age-matched WT mice and rd1 mice at 3-weeks of age (P ≤ 0.02). MO2 was directly correlated with both IR and TR thickness (R ≥ 0.50; P ≤ 0.03, N = 20). The findings indicate that the rate oxygen is supplied by the retinal circulation is decreased and the reduction in oxygen extracted for metabolism is related to retinal cell layer thinning in rd1 mice.

Control of retinal blood flow levels by selected combinations of cervical arterial ligations in rat.

The effect of various combinations of cervical arterial ligations (Combinations) on retinal blood flow (RBF) levels is not known in rats. We hypothesized: 1) No artery exists between the Circle of Willis and the eye, 2) Selective Combinations enable varying RBF levels between normal and zero, 3) In certain Combinations, the capillary bed of the head participates in supplying the eye. Twenty-six Combinations were studied in one eye of 20 Long-Evans rats under general anesthesia. RBF was quantitatively evaluated with our published imaging methods based on direct measurements of venous diameter and blood velocity from the displacement of fluorescent microspheres over time. For each Combination, one or more RBF values (runs) were measured. Data were obtained from 59 runs (2.9 ± 2.7 runs/rat). Levels of RBF ranged from normal to zero. An artery between the Circle of Willis and the eye was excluded. With some Combinations, flow traversed the capillary bed. Combinations were consolidated into five Groups based on the blood flow paths remaining after the ligations. A mixed linear model accounting for multiple measurements in the same eye demonstrated an effect of Group on RBF (P < 0.0005). By major source of ocular blood supply, the trend of RBF levels was: ipsilateral carotid artery > contralateral carotid artery > ipsilateral distal internal carotid artery retrograde from Circle of Willis. The findings advanced knowledge of the sources of blood supply to the rat eye and demonstrated a method of selective cervical arterial ligations for varying RBF levels with potential to impact future retinal ischemia research.

Retinal Oxygen Delivery, Metabolism, and Extraction Fraction during Long-Term Bilateral Common Carotid Artery Occlusion in Rats.

Retinal functional, biochemical, and anatomical changes have been previously reported in long-term experimental permanent bilateral common carotid artery occlusion (BCCAO). The purpose of the current study was to investigate progressive reductions in retinal oxygen metabolism (MO2) due to inadequate compensation by oxygen delivery (DO2) and extraction fraction (OEF) after BCCAO. Twenty-nine rats were subjected to BCCAO and were imaged after 3 hours, 3 days, 7 days, or 14 days. Six rats underwent a sham procedure. Phosphorescence lifetime and blood flow imaging were performed in both eyes to measure retinal oxygen contents and total retinal blood flow, respectively. DO2, MO2, and OEF were calculated from these measurements. Compared to the sham group, DO2 and MO2 were reduced after all BCCAO durations. OEF was increased after 3 hours and 3 days of BCCAO, but was not different from the sham group after 7 and 14 days. Between 3 and 7 days of BCCAO, DO2 increased, OEF decreased, and there was no significant difference in MO2. These findings may be useful to understand the pathophysiology of retinal ischemia.

Relation of Retinal Oxygen Measures to Electrophysiology and Survival Indicators after Permanent, Incomplete Ischemia in Rats.

Studies in experimental ischemia models by permanent bilateral common carotid artery occlusion (BCCAO) have reported reduced retinal electrophysiological function, coupled with inner retinal degeneration and gliosis. In the current study, we tested the hypothesis that long-term (up to 14 days) BCCAO impairs oxygen delivery (DO2), which affects oxygen metabolism (MO2) and extraction fraction (OEF), electrophysiological function, morphology, and biochemical pathways. Twenty-one rats underwent BCCAO (N = 12) or sham surgery (N = 9) and were evaluated in separate groups after 3, 7, or 14 days. Electroretinography (ERG), optical coherence tomography, blood flow and vascular oxygen tension imaging, and morphological and biochemical evaluations were performed in both eyes. Reduced ERG b-wave amplitudes and delayed implicit times were reported at 3, 7, and 14 days following BCCAO. Total retinal blood flow, MO2, and DO2 were reduced in all BCCAO groups. OEF was increased in both 3- and 7-day groups, while no significant difference was observed in OEF at 14 days compared to the sham group. At 14 days following BCCAO, total and inner retinal layer thickness was reduced, while the outer nuclear layer thickness and gliosis were increased. There was an increase in nuclei containing fragmented DNA at 3 days following BCCAO. The compensatory elevation in OEF following BCCAO did not meet the tissue demand, resulting in the subsequent reduction of MO2. The associations between retinal MO2, DO2, and retinal function were shown to be significant in the sequelae of persistent ischemia. In sum, measurements of DO2, MO2, and OEF may become useful for characterizing salvageable tissue in vision-threatening pathologies.