Current concepts of the effect of oxygen on the developing retina

Retinal oxygen: from animals to humans

This article discusses retinal oxygenation and retinal metabolism by focusing on measurements made with two of the principal methods used to study O₂ in the retina: measurements of PO₂ with oxygen-sensitive microelectrodes in vivo in animals with a retinal circulation similar to that of humans, and oximetry, which can be used non-invasively in both animals and humans to measure O₂ concentration in retinal vessels. Microelectrodes uniquely have high spatial resolution, allowing the mapping of PO₂ in detail, and when combined with mathematical models of diffusion and consumption, they provide information about retinal metabolism. Mathematical models, grounded in experiments, can also be used to simulate situations that are not amenable to experimental study. New methods of oximetry, particularly photoacoustic ophthalmoscopy and visible light optical coherence tomography, provide depth-resolved methods that can separate signals from blood vessels and surrounding tissues, and can be combined with blood flow measures to determine metabolic rate. We discuss the effects on retinal oxygenation of illumination, hypoxia and hyperoxia, and describe retinal oxygenation in diabetes, retinal detachment, arterial occlusion, and macular degeneration. We explain how the metabolic measurements obtained from microelectrodes and imaging are different, and how they need to be brought together in the future. Finally, we argue for revisiting the clinical use of hyperoxia in ophthalmology, particularly in retinal arterial occlusions and retinal detachment, based on animal research and diffusion theory.

Erythropoietin promotes retinal angiogenesis in a mouse model

This study aimed to investigate the effect and potential mechanisms of exogenous administration of recombinant human erythropoietin (rhEPO) on retinal angiogenesis in a mouse model of oxygen-induced retinopathy (OIR). Postnatal day 7 (P7) mice (n=132) were randomly assigned to one of six groups: Control group (n=22), OIR group (n=22), OIR + vehicle control group (n=22), OIR + rhEPO 10 IU group (n=22), OIR + rhEPO 50 IU group (n=22), and OIR + rhEPO 100 IU group (n=22). OIR was induced by exposing mice to 75±2% O2 for five days, followed by exposure to room air for a further five days. Animals in groups 3-6 (the OIR + vehicle control group and OIR + rhEPO 10 IU, 50 IU, and 100 IU groups) received an intraperitoneal injection of saline, or rhEPO 10 IU, 50 IU and 100 IU, respectively, which were administered daily from P7-P12. Immunofluorescent and hematoxylin-eosin staining were used to detect retinal neovascularization (RNV) in retinal whole mounts. Quantitative polymerase chain reaction and western blot analysis were used to detect the expression levels of vascular endothelial growth factor (VEGF), endothelial nitric oxide synthase (eNOS), and neuronal nitric oxide synthase (nNOS). RNV occurred in the OIR groups and was accompanied by dilated, twisted and occluded blood vessels. rhEPO treatment resulted in an increase in the number of newly formed and severely dilated vessels. rhEPO increased RNV in a dose-dependent manner, which was accompanied by an increase in the messenger RNA and protein expression of VEGF, eNOS and nNOS. Thus, exogenous use of rhEPO promotes the RNV in a mouse model of OIR and is accompanied by increased expression levels of VEGF, eNOS and nNOS.

Correlation between risk factors during the neonatal period and appearance of retinopathy of prematurity in preterm infants in neonatal intensive care units in Alexandria, Egypt

BACKGROUND

This study aimed to identify the main risk factors for development of retinopathy of prematurity (ROP) in neonatal intensive care units in Alexandria, Egypt, from January 2010 to January 2012.

METHODS

A prospective cohort study was undertaken in infants weighing < 1250 g and maternal postmenstrual age < 32 weeks if there was concern about prolonged exposure to oxygen. The main clinical outcomes were occurrence of any stage of ROP and in particular severe ROP. Perinatal variables considered were: birth weight, gestational age, gender, method of ventilation (nasal continuous airway pressure or intermittent mechanical ventilation), packed red blood cell and/or plasma transfusion, occurrence of sepsis, neonatal indirect hyperbilirubinemia, intraventricular hemorrhage, and patent ductus arteriosus. After obtaining informed consent from the parents, infants at risk were examined for ROP using indirect ophthalmoscopy, ie, RetCam II fundus photography.

RESULTS

The study included 152 infants of mean gestational age 31.02 weeks and mean birth weight 1.229 kg. Seventy-two cases (47.5%) were male and 80 cases (52.5%) were female. Of the cases screened, 100 (65.6%) had no ROP, 52 had ROP of any stage (34.4%), and 27 (18%) had stage 1, five (3.3%) had stage 2, 17 (11.5%) had stage 3, and three (1.6%) had stage 4 disease. No infants had stage 5 ROP. Of all our cases with ROP, 15 (28.6%) had prethreshold disease type 1 that required treatment, comprising 9.8% of all cases screened for ROP. Using stepwise logistic regression analysis, all risk factors studied were found to be significantly associated with the development of ROP, except for neonatal indirect hyperbilirubinemia. Severity of ROP was inversely proportional to birth weight and gestational age.

CONCLUSION

ROP occurred in 34.4% of all infants screened in the neonatal intensive care units at three obstetric hospitals in Alexandria. The main risk factors for development of threshold ROP by regression analysis were low birth weight, gestational age, method of ventilation, need for packed red blood cell and/or plasma transfusion, occurrence of sepsis, intraventricular hemorrhage, and patent ductus arteriosus but not neonatal indirect hyperbilirubinemia. We suggest that both immaturity and compromised pulmonary function are both important etiological factors in the development of ROP.

Ocular neovascularization

Retinal and choroidal vascular diseases constitute the most common causes of moderate and severe vision loss in developed countries. They can be divided into retinal vascular diseases, in which there is leakage and/or neovascularization (NV) from retinal vessels, and subretinal NV, in which new vessels grow into the normally avascular outer retina and subretinal space. The first category of diseases includes diabetic retinopathy, retinal vein occlusions, and retinopathy of prematurity, and the second category includes neovascular age-related macular degeneration (AMD), ocular histoplasmosis, pathologic myopia, and other related diseases. Retinal hypoxia is a key feature of the first category of diseases resulting in elevated levels of hypoxia-inducible factor-1 (HIF-1) which stimulates expression of vascular endothelial growth factor (VEGF), platelet-derived growth factor-B (PDGF-B), placental growth factor, stromal-derived growth factor-1 and their receptors, as well as other hypoxia-regulated gene products such as angiopoietin-2. Although hypoxia has not been demonstrated as part of the second category of diseases, HIF-1 is elevated and thus the same group of hypoxia-regulated gene products plays a role. Clinical trials have shown that VEGF antagonists provide major benefits for patients with subretinal NV due to AMD and even greater benefits are seen by combining antagonists of VEGF and PDGF-B. It is likely that addition of antagonists of other agents listed above will be tested in the future. Other appealing strategies are to directly target HIF-1 or to use gene transfer to express endogenous or engineered anti-angiogenic proteins. While substantial progress has been made, the future looks even brighter for patients with retinal and choroidal vascular diseases.

A predictive score for retinopathy of prematurity in very low birth weight preterm infants

AIMS

This study describes the development of a score based on cumulative risk factors for the prediction of severe retinopathy of prematurity (ROP) comparing the performance of the score against the birth weight (BW) and gestational age (GA) in order to predict the onset of ROP.

METHODS

A prospective cohort of preterm infants with BW≤ 1500 g and/or GA≤ 32 weeks was studied. The score was developed based on BW, GA, proportional weight gain from birth to the 6th week of life, use of oxygen in mechanical ventilation, and need for blood transfusions from birth to the 6th week of life. The score was established after linear regression, considering the impact of each variable on the occurrences of any stage and severe ROP. Receiver operating characteristic (ROC) curves were used to determine the best sensitivity and specificity values for the score. All variables were entered into an Excel spreadsheet (Microsoft) for practical use by ophthalmologists during screening sessions.

RESULTS

The sample included 474 patients. The area under the ROC curve for the score was 0.77 and 0.88 to predict any stage and severe ROP, respectively. These values were significantly higher for the score than for BW (0.71) and GA (0.69) when measured separately.

CONCLUSIONS

ROPScore is an excellent index of neonatal risk factors for ROP, which is easy to record and more accurate than BW and GA to predict any stage ROP or severe ROP in preterm infants. The scoring system is simple enough to be routinely used by ophthalmologists during screening examination for detection of ROP.

Role of Nrf2 in retinal vascular development and the vaso-obliterative phase of oxygen-induced retinopathy

In the initial stage of retinopathy of prematurity (ROP), hyperoxia causes retinal blood vessel obliteration. This is thought to occur in part through oxidative stress-induced apoptosis of endothelial cells. This study was designed to determine what role NF-E2-related factor 2 (Nrf2) plays in this process. Nrf2 is a transcription factor of the anti-oxidant response element that, if induced, may protect the retina from hyperoxia-induced oxidative stress. Nrf2 knockout mice (Nrf2-/-), Nrf2 wild type control mice (Nrf2+/+), and C57BL/6 mice were exposed to hyperoxia (75% O(2)) or normoxia from P7 through P12. Mice were sacrificed on P9 and P12 and the retinas were stained with GSA lectin-Cy3 to visualize retinal blood vessels. Hyperoxia exposed retinas were flat mounted and photographed, then the size of the avascular areas was determined. Additionally, retinas were cryopreserved after lectin staining and area analysis and then sectioned. Secondary or deep capillaries were then hand-counted in sections. In hyperoxia-treated mice, the avascular areas in Nrf2-/- P9 mice were significantly larger than those in Nrf2+/+ P9 mice (P = 0.01). However, there was no significant difference between Nrf2-/- and Nrf2+/+ mice at P12. Avascular areas at P12 were significantly smaller than that at P9 in Nrf2-/-, Nrf2+/+, and C57BL/6 mice (P = 0.0011, P = 0.009, and P = 0.001 respectively). The numbers of deep or secondary capillaries in air-reared Nrf2-/- mice were significantly decreased, when compared to Nrf2+/+ mice at P9 (P = 0.0082). On the other hand, there was no significant difference in deep capillary formation between air-reared Nrf2-/- and Nrf2+/+ mice at P12. Akt signaling activates Nrf2 and Akt was localized to retinal blood vessels in all animals and was increased in Nrf2+/+ and Nrf2-/- mice exposed to hyperoxia as compared to normoxia mice. Interestingly, during normal development this protection by Nrf2 occurs in a specific window of time that is also shared by angiogenesis. Hyperoxia treatment revealed a similar window of time where Nrf2 regulated anti-oxidant production was beneficial and contributed to the endothelial survival.

Retinal microvessels express less gamma-glutamyl transpeptidase than brain microvessels

In this investigation we localized and compared the level of gamma-glutamyl transpeptidase (GGTP) activity in retinal and brain preparations using histochemical, enzymatic and in situ hybridization assays. We compared GGTP distribution to another microvessel specific enzyme, alkaline phosphatase (AP). In the rat brain, GGTP activity was observed in microvessels and choroid plexus by a histochemical method. Similar studies in the rat retina revealed activity in the pigment epithelium but only a very weak reaction in microvessels. Histochemical staining for alkaline phosphatase was observed in both retinal and brain microvessels choroid plexus and pigment epithelium. Biochemical analysis verified that GGTP activity was significantly lower in retinal than brain microvessels, while alkaline phosphatase activity was similar in both types of microvessels. GGTP specific activity of bovine brain and retinal microvessels was 185 +/- 39 mUnits and 8.5 +/- 1.5 mUnits (p less than 0.001), respectively. By contrast, alkaline phosphatase specific activity in brain and retinal microvessels was 732 +/- 139 and 471 +/- 114 (p greater than 0.1), respectively. Choroid plexus and retinal pigment epithelium exhibited similar levels of GGTP and alkaline phosphatase. Differences in GGTP expression between retinal and brain microvessels were also observed on the mRNA level. In situ hybridization studies revealed that brain microvessels expressed four times more GGTP specific mRNA than retinal microvessels. We conclude that retinal microvessels do not express high levels of GGTP which may make them more vulnerable than brain microvessels to injuries mediated by leukotrienes and oxidative stress.

Hyperoxia inhibits several critical aspects of vascular development

Normal human retinal vascular development uses angiogenesis and vasculogenesis, both of which are interrupted in the vaso-obliteration phase of retinopathy of prematurity (ROP). Canine oxygen-induced retinopathy (OIR) closely resembles human ROP. Canine retinal endothelial cells (ECs) and angioblasts were used to model OIR and characterize the effects of hyperoxia on angiogenesis and vasculogenesis. Cell cycle analysis showed that hyperoxia reduced the number of G1 phase cells and showed increased arrest in S phase for both cell types. Migration of ECs was significantly inhibited in hyperoxia (P < 0.01). Hyperoxia disrupted the cytoskeleton of angioblasts but not ECs after 2 days. Differentiation of angioblasts into ECs (determined by acetylated low-density lipoprotein uptake) was evaluated after basic fibroblast growth factor treatment. Differentiation of angioblasts into pericytes was determined by smooth muscle actin expression after treatment with platelet-derived growth factor. Differentiation into ECs was significantly inhibited by hyperoxia (P < 0.0001). The percentage of CXCR4(+) cells (a marker for retinal vascular precursors) increased in both treatment groups after hyperoxia. These data show novel mechanisms of hyperoxia-induced disruption of vascular development.

Hyperoxia causes decreased expression of vascular endothelial growth factor and endothelial cell apoptosis in adult retina

Mice or humans with photoreceptor degenerations experience permeability and dropout of retinal capillaries. Loss of photoreceptors results in decreased oxygen usage and thinning of the retina with increased oxygen delivery to the inner retina. To investigate the possibility that increased tissue oxygen plays a role in the vascular damage, we exposed adult mice to hyperoxia, which also increases oxygen in the retina. After 1, 2, or 3 weeks of hyperoxia, there was a statistically significant decrease in retinal vascular density that was not reversible, and endothelial cell apoptosis was demonstrated by TUNEL staining. Mice exposed to hyperoxia and mice with photoreceptor degeneration both showed decreased expression of VEGF in the retina. After complete or near-complete degeneration of photoreceptors, there was increased expression of VEGF in RPE cells, which may explain the association of photoreceptor degeneration and neovascularization in or around the RPE. Increased expression of VEGF in photoreceptors of transgenic mice failed to prevent hyperoxia-induced retinal capillary dropout. These data suggest that increased oxygen in the retina, either by increased inspired oxygen or by photoreceptor degeneration, results in endothelial cell death and dropout of capillaries. Decreased expression of VEGF may be a contributing factor, but the situation may be more complicated for mature retinal vessels than it is for immature vessels, because VEGF replacement does not rescue mature retinal vessels, suggesting that other factors may also be involved.

Fluorescein angiography as a means of assessing retinal vascular pathology in oxygen-exposed newborn rats

Recently a new technique was reported in which examination of the retina of oxygen reared newborn rats was made using indirect fluorescence microscopy (Invest. Ophthal. Vis. Sci. 31:810). We have now improved this technique and systematically employed it as a means of studying oxygen-induced retinopathy in the newborn rat as an animal model for retinopathy of prematurity (ROP). Immediately after birth, litters of pigmented rats were exposed to an elevated oxygen atmosphere that varied in a cyclic fashion between 40% and 80% every 12 hours for 14 days. Rats raised simultaneously in room air served as controls. At the end of the treatment period, rats were deeply anesthetized, their pupils dilated, and 100 microliters of 3% sodium fluorescein was administered via tail venipuncture. After angiography, rats were awakened and returned to their mothers in room air for future assessment. Angiography was performed again at 4, 7, 38 or 56 days after oxygen treatment. Our results suggest that retinal pathology in the rat model parallels that of the human infant with ROP with respect to loss of patent retinal blood vessels in oxygen and subsequent abnormal vasoproliferation upon return to room air. Angiograms performed on room air-raised control rats at 14 days of age demonstrated our ability to resolve the smallest retinal capillaries. Angiograms conducted at the same age in oxygen-exposed rats revealed extensive areas of retinal non-perfusion and substantial arterio-venous shunting between adjacent primary vessels. If rats were maintained in room air for 4 or 7 days following oxygen exposure, a variety of pathologic signs became obvious including: 1) tortuosity of primary vessels mimicking human "plus disease", 2) frequent abnormal budding of tertiary vessels and capillaries, 3) retinal fluorescein leaks indicative of hemorrhages and 4) abnormal capillary tufts not evident at the time of removal from oxygen. By 38 and 56 days post-exposure, all four of these phenomena were still observed. Many of these pathologic phenomena could not have been observed with conventional methods currently used for retinal vascular assessment in small animals.

Hyperbaric oxygen tolerance in newborn mammals--hypothesis on mechanisms and outcome

Newborn mammals, compared to adults, are extremely resistant to the CNS effects of hyperbaric oxygenation (HBO) induced by excessive generation of reactive oxygen species. This tolerance to HBO may be related to either physiological responses or the chemical characteristics of the immature brain, including a low cerebral blood flow and energy metabolism, and a low concentration of polyunsaturated fatty acids. In adult mammals the main protective mechanism against CNS oxygen toxicity, besides endogenous antioxidants, is a transient HBO-induced cerebral vasoconstriction. How cerebral vasculature reacts to HBO in the immature brain is not known. We present indirect evidence suggesting that HBO in newborn rats induces a persistent cerebral vasoconstriction concurrently with a severe and maintained reduction in ventilation. It is speculated that the outcome of these physiologic responses to hyperoxic exposures may be: (a) extension of tolerance to both CNS and pulmonary oxygen poisoning; (b) creation of a profound hypoxic-ischemic condition in vulnerable neural structures; and (c) impairment of the circulatory and ventilatory responses to hypoxic stimuli on return to air with consequent development of a secondary hypoxic-ischemic condition. These hypothetical pre- and post-HBO events may set the stage for the development of some delayed neurological disorders, including the retinopathy of prematurity and the retardation of brain development in fetuses or prematurely-born infants subjected to oxygen therapy.

Variations in the severity of retinopathy seen in newborn rats supplemented with oxygen under different conditions of hyperbarism

The purposes of this experimental study were to evaluate the effects of oxygen supplementation delivered under hyperbaric conditions on the retinas of newborn rats and to determine the minimum and maximum levels of hyperbarism capable of protecting the retinal vessels from the toxic effects of oxygen without determining lethal effects in this experimental model. A control group of newborn rats were maintained with their mother for the first 12 days of life under room-air conditions. A second group of animals were exposed to a hyperbaric environment (+81 kPa) under normoxic conditions for the first 7 days of life and subsequently returned to normobaric conditions for the next 5 days. Examination of the retinal flat mounts from this latter group of animals revealed essentially normal vascular networks with only a modest degree of vasoconstriction. Two other litters of ratlings, with their mothers, were given supplemental oxygen at an FiO2 of 80% under a compression pressure of +101.25 kPa. In this group of animals, death of both of the mother rats from pulmonary edema occurred on the first day of treatment, and, in spite of immediate mother substitution, the newborn rats succumbed to the same complication. Five other groups, each containing two litters of newborn rats with their mothers, were exposed to FiO2s of 80% at hyperbaric levels ranging from +20.25- to +81.0 kPa for the first 7 days of life. On the eighth day, the FiO2s were reduced to 21%. After 5 days of room-air recovery, the animals were killed.(ABSTRACT TRUNCATED AT 250 WORDS)

Premature graduates of the newborn intensive care unit: a guide to followup

Approximately 40,000 infants weighing less than 1500 grams are born in the United States each year. Caring for these infants after their discharge from the newborn intensive care unit is complex and difficult. The survival, growth, routine health care maintenance, neurologic and developmental outcome and assessment, and long-term medical complications are reviewed.

Vitamin E and rabbit corneal endothelial cell survival

In this study, vitamin E (apha-tocopherol), a powerful antioxidant, is shown to double the survival time of isolated perfused rabbit corneal endothelium. The results are discussed and evidence concerning vitamin E in ocular tissue function is reviewed.

Effects of hyperbaric, normobaric and hypobaric oxygen supplementation on retinal vessels in newborn rats: a preliminary study

An experimental study was conducted on eight litters of newborn rats to evaluate the effects of supplemental oxygen administration on the retinal vasculature. The animals and their mothers were kept inside a pressure chamber and treated for the first 5 days of life. On the sixth day, they were removed and kept for five more days under room air and normobaric conditions. Three litters received continuous flow oxygen at 80% at a compression pressure of +81 kPa, one litter oxygen at 80% at a pressure of -39.5 kPa atms and three other litters received oxygen at 80% under normobaric conditions. The eighth litter was treated with room air oxygen at a compression pressure of +81 kPa. A severe retinopathy with marked retinal neovascularization was seen only in the newborn animals of the litters that received oxygen supplementation under normobaric or hypobaric conditions. Retinal vessels showed no pathological changes in the litters treated with hyperbaric normoxia or hyperoxia. It is possible to hypothesize that the prolonged period of oxygen supplementation failed to produce harmful effects on the retinal vasculature because the moderate hyperbarism caused mild retinal and choroidal vasoconstriction thus preventing excessive oxygen transport to the inner retina from the choroid during hyperoxia without inducing structural damage to the retinal tissue.

Vitamin E protects against retinopathy of prematurity through action on spindle cells

In the premature infant, exposure of the incompletely vascularized retina to increased oxygen tension can result in the development of a blinding disease, retinopathy of prematurity (ROP). Despite the judicious curtailment of oxygen, the incidence of ROP is on the increase due to the technological advances that have improved the survival of the very young preterm infant. Six clinical trials have documented the efficacy of vitamin E supplementation in suppressing the development of severe ROP, but the mechanism of this protection has remained unknown. This report proposes that spindle cells, mesenchymal precursors of the inner retinal capillaries, are the primary inducers of the neovascularization associated with ROP. Exposure of spindle cells to elevated oxygen tension increases their gap junction area. This early morphologic event immediately halts the normal vasoformative process and eventually triggers the neovascularization that is observed clinically 8-12 weeks later. Vitamin E supplementation above the deficient plasma levels of these infants suppresses gap junction formation and clinically reduces the severity without altering the total incidence of ROP.