The effect of occluding the retinal and choroidal circulations on the electroretinogram of monkeys

Atropine Affects the Outer Retina During Inhibiting Form Deprivation Myopia in Guinea Pigs

PURPOSE

Atropine has been proven to be effective in retarding myopia progression. However, the underlying mechanism remains unknown. Our purpose was to detect morphological and functional changes caused by atropine during myopic inhibition.

METHOD

Twenty 2-week-old guinea pigs were randomly assigned to either the saline group (n = 10) or the atropine group (n = 10). Form-deprived myopia (FDM) and intravitreal injections were applied on the right eyes. The injections were given every 3 days, lasting for 2 weeks. The left eyes served as control. Ocular refraction, axial length, retinal, and choroidal thickness were collected at the start and the end of the experiment. Retinal function was evaluated via full-field electroretinogram (ERG) at the end of treatment.

RESULTS

The interocular differences (experimental eye minus control eye) of refraction error (RE), vitreous chamber depth (VCD), and axial length (AL) in the saline group were significantly greater than those in the atropine group (RE, VCD: P < .001, AL: P < .0001). The differences in choroidal thickness between the two groups did not reach statistical significance. However, a decreasing trend of choroidal thickness was observed in the saline group but not in the atropine group. Furthermore, the interocular differences of total retinal and outer retinal thickness in the atropine group were much thicker than in the saline group (P < .001 and P < .01, respectively). The treatment did not affect inner retinal thickness. In photopic ERG, the atropine-treated FDM eyes showed significantly greater a-wave amplitudes compared to the saline group.

CONCLUSION

During the process of inhibiting FDM, atropine showed an effect on the outer retina, most likely on the cones, in guinea pigs.

Changes in pulse waveforms in response to intraocular pressure elevation determined by laser speckle flowgraphy in healthy subjects

Background

The influences of intraocular pressure (IOP) elevations on the pulse waveform in the optic nerve head (ONH) were evaluated using laser speckle flowgraphy (LSFG) in normal subjects.

Methods

This prospective cross-sectional study was conducted at the Nagoya University Hospital. An ophthalmodynamometer was pressed on the sclera to increase the IOP by 20 mmHg or 30 mmHg for 1 min (experiment 1, 16 subjects) and by 30 mmHg for 10 min (experiment 2, 10 subjects). The mean blur rate (MBR) and the eight pulse waveform parameters determined using LSFG were measured before, immediately after and during an IOP elevation, and after the IOP returned to the baseline pressure.

Results

A significant elevation in the IOP and a significant reduction in the ocular perfusion pressure (OPP) were found after applying the ophthalmodynamometer (both, P < 0.001). The blowout score (BOS) reduced significantly (P < 0.001), and the flow acceleration index (FAI; P < 0.01) and resistivity index (RI; P < 0.001) increased significantly immediately after increasing the IOP by 20 or 30 mmHg (experiment 1). The BOS reduced significantly (P < 0.001), and the FAI (P < 0.01) and RI (P < 0.001) increased significantly after the IOP elevation by 30 mmHg in both experiment 2 and 1. However, the BOS and RI recovered significantly at time 10 compared to that in time 0 (immediately after IOP elevation) during the 10-min IOP elevation (P < 0.001 and P = 0.008, respectively).

Conclusions

In conclusion, the BOS, FAI, and RI of the pulse waveforms changed significantly with an acute elevation in the IOP. The change should be related to the larger difference between the maximum and minimum MBRs during the IOP elevation.

A Bioengineering Approach to Myopia Control Tested in a Guinea Pig Model

Purpose

To investigate the biocompatibility of an injectable hydrogel and its ability to control myopia progression in guinea pigs.

Methods

The study used a hydrogel synthesized from acrylated hyaluronic acid with a conjugated cell-binding peptide and enzymatically degradable crosslinker. Seven-day-old guinea pigs were first form deprived (FD) with diffusers for 1 week. One group was kept as an FD-only control; two groups received a sub-Tenon's capsule injection of either hydrogel or buffer (sham surgery) at the posterior pole of the eye. Form deprivation treatments were then continued for 3 additional weeks. Treatment effects were evaluated in terms of ocular axial length and refractive error. Safety was evaluated via intraocular pressure (IOP), visual acuity, flash electroretinograms (ERG), and histology.

Results

Both hydrogel and sham surgery groups showed significantly reduced axial elongation and myopia progression compared to the FD-only group. For axial lengths, net changes in interocular difference (treated minus control) were 0.04 ± 0.06, 0.02 ± 0.09, and 0.24 ± 0.08 mm for hydrogel, sham, and FD-only groups, respectively (P = 0.0006). Intraocular pressures, visual acuities, and ERGs of treated eyes were not significantly different from contralateral controls. Extensive cell migration into the implants was evident. Both surgery groups showed noticeable Tenon's capsule thickening.

Conclusions

Sub-Tenon's capsule injections of both hydrogel and buffer inhibited myopia progression, with no adverse effects on ocular health. The latter unexpected effect warrants further investigation as a potential novel myopia control therapy. That the hydrogel implant supported significant cell infiltration offers further proof of its biocompatibility, with potential application as a tool for drug and cell delivery.

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.

Intraoperative electrophysiological evaluations of macular function during peripheral scleral indentation

Scleral indentation is widely used to examine the peripheral fundus, however it can increase the intraocular pressure (IOP) to high levels which can then affect retinal function. We evaluated the effects of scleral indentation on the macular function electrophysiologically. Intraoperative focal macular electroretinograms (iFMERGs) were recorded with and without controlling the IOP in 7 eyes. Without IOP control, the IOP increased from 21.7 ± 4.9 to 92.7 ± 20.2 mmHg significantly (P = 0.020) and the amplitudes of the b-wave (from 6.29 ± 1.160 to 3.71 ± 1.98 uV, P = 0.007), on-photopic negative response (from 2.29 ± 0.99 to 0.72 ± 0.47 uV, on-PhNR, P = 0.005), and d-wave (from 2.57 ± 0.41 to 1.64 ± 0.69 uV, P = 0.007) decreased significantly soon after beginning the indentation. All values returned to the baseline levels after releasing the indentation. In the eyes with IOP controlled, the IOP and the amplitude of all components did not change significantly during and after the indentation except the on-PhNR amplitude which was significantly reduced during the indentation. The changes in the iFMERGs and macular function caused by scleral indentation were transient and reversible. The changes can be minimized by controlling the IOP.

Evidence for an enduring ischaemic penumbra following central retinal artery occlusion, with implications for fibrinolytic therapy

The rationale behind hyperacute fibrinolytic therapy for cerebral and retinal arterial occlusion is to rescue ischaemic cells from irreversible damage through timely restitution of tissue perfusion. In cerebral stroke, an anoxic tissue compartment (the "infarct core") is surrounded by a hypoxic compartment (the "ischaemic penumbra"). The latter comprises electrically-silent neurons that undergo delayed apoptotic cell death within 1-6 h unless salvaged by arterial recanalisation. Establishment of an equivalent hypoxic compartment within the inner retina following central retinal artery occlusion (CRAO) isn't widely acknowledged. During experimental CRAO, electroretinography reveals 3 oxygenation-based tissue compartments (anoxic, hypoxic and normoxic) that contribute 32%, 27% and 41% respectively to the pre-occlusion b-wave amplitude. Thus, once the anoxia survival time (≈2 h) expires, the contribution from the infarcted posterior retina is irreversibly extinguished, but electrical activity continues in the normoxic periphery. Inbetween these compartments, an annular hypoxic zone (the "penumbra obscura") endures in a structurally-intact but functionally-impaired state until retinal reperfusion allows rapid recovery from electrical silence. Clinically, residual circulation of sufficient volume flow rate generates the heterogeneous fundus picture of "partial" CRAO. Persistent retinal venous hypoxaemia signifies maximal extraction of oxygen by an enduring "polar penumbra" that permeates or largely replaces the infarct core. On retinal reperfusion some days later, the retinal venous oxygen saturation reverts to normal and vision improves. Thus, penumbral inner retina, marginally oxygenated by the choroid or by residual circulation, isn't at risk of delayed apoptotic infarction (unlike hypoxic cerebral cortex). Emergency fibrinolytic intervention is inappropriate, therefore, once the duration of CRAO exceeds 2 h.

Coupling blood flow and neural function in the retina: a model for homeostatic responses to ocular perfusion pressure challenge

Retinal function is known to be more resistant than blood flow to acute reduction of ocular perfusion pressure (OPP). To understand the mechanisms underlying the disconnect between blood flow and neural function, a mathematical model is developed in this study, which proposes that increased oxygen extraction ratio compensates for relative ischemia to sustain retinal function. In addition, the model incorporates a term to account for a pressure-related mechanical stress on neurons when OPP reduction is achieved by intraocular pressure (IOP) elevation. We show that this model, combining ocular blood flow, oxygen extraction ratio, and IOP mechanical stress on neurons, accounts for retinal function over a wide range of OPP manipulations. The robustness of the model is tested against experimental data where ocular blood flow, oxygen tension, and retinal function were simultaneously measured during acute OPP manipulation. The model provides a basis for understanding the retinal hemodynamic responses to short-term OPP challenge.

In vivo identification of alteration of inner neurosensory layers in branch retinal artery occlusion

Background/aims

To characterise the extension and progression of alteration of neurosensory layers following acute and chronic branch retinal artery occlusion (BRAO) in vivo using spectral-domain optical coherence tomography.

Methods

In this observational case series, eight eyes with acute BRAO and nine eyes with chronic BRAO were analysed using a Spectralis Heidelberg Retina Angiograph (HRA)+optical coherence tomography system including eye tracking. Patients with acute BRAO were examined within 36±5 h after primary event and at weekly/monthly intervals thereafter. Segmentation measurements of all individual neurosensory layers were performed on single A-scans at six locations in affected and corresponding non-affected areas. The thickness values of the retinal nerve fibre layer together with the ganglion cell layer (NFL/GCL), inner plexiform layer (IPL), inner nuclear layer together with outer plexiform layer (INL/OPL), outer nuclear layer (ONL), and photoreceptor layers together with the retinal pigment epithelium (PR/RPE) were measured and analysed.

Results

Segmentation evaluation revealed a distinct increase in thickness of inner neurosensory layers including the NFL/GCL (35%), IPL (80%), INL/OPL (48%) and mildly the ONL by 21% in acute ischaemia compared with corresponding layers in non-ischaemic areas. Regression of intraretinal oedema was followed by persistent retinal atrophy with loss of differentiation between IPL and INL/OPL at month 2. In contrast, the ONL and subjacent PR/RPE retained their physiological thickness in patients with chronic BRAO.

Conclusion

In vivo assessment of retinal layer morphology allows a precise identification of the pathophysiology in retinal ischaemia.

Studies on acute and late stages of experimental central retinal artery occlusion in the Cynomolgus monkey. I. Intensity-amplitude relations of the D.C. recorded ERG with special reference to the c-wave

The main positive component of the c-wave of the ERG is generated by the pigment epithelium-receptor complex, which is supplied from the choroidal circulation. Occlusion of the central retinal artery (OCRA) causes serious morphological damage only to the inner retina. An effect on the c-wave is therefore not primarily to be expected. In this study on five Cynomolgus monkeys with unilateral, laser-induced OCRA the c-wave was decreased, slightly in the early stage and markedly in the late stage. A hypothetical explanation of the reduced c-wave amplitude, involving changes in potassium ion concentration, is discussed. The conflicting results in other reports may be explained by uncontrolled influence of slow c-wave amplitude oscillations, by differences between acute and late stages, and by possible surgical damage to the choroidal circulation in some cases.

Studies on acute and late stages of experimental central retinal artery occlusion in the Cynomolgus monkey. II. Influence on the cyclic changes in the amplitude of the c-wave of the ERG and in the standing potential of the eye

The slow (2-3/h) oscillations of the c-wave amplitude and of the standing potential of the eye (SP) were studied in the Cynomolgus monkey during the acute and late stages after experimentally induced (laser photocoagulation) occlusion of the central retinal artery (OCRA). Whereas the healthy control eyes showed large cyclic variations in both the c-wave amplitude and the SP, no oscillations in the c-wave amplitude were observed in the OCRA eyes at any stage, and the SP oscillations were barely detectable in these eyes. OCRA causes morphological damage to the inner retina but not to the pigment epithelium-photoreceptor complex, which generates the positive component of the c-wave, and where the SP is also believed to originate. The findings of the present study strongly indicate that the SP and c-wave oscillations are related, and that they are either dependent on an intact inner retina or that the pigment epithelium-photoreceptor complex is functionally affected by OCRA. If not taken into consideration, the marked difference in oscillations between the OCRA eye and the healthy eye may be a major source of error when comparing c-waves from the two eyes, and it seems that some of the conflicting results reported by others can be explained in this way.

Retinal Detachment Neuropathology and Potential Strategies for Neuroprotection

Understanding the neuropathology of retinal detachment from postmortem and animal models allows identification of cellular targets, receptors and mediators for pharmacological manipulation. In this review, concepts of retinal detachment and neuropathology are examined at cellular and structural anatomical levels using postmortem and animal model data. Possible neuroprotective strategies are reviewed in the setting of the new environment created by successful retinal reattachment surgery.

Correlation of retinal function with retinal histopathology following ischemia-reperfusion in rat eyes

PURPOSE

To investigate the relationship between retinal function and histopathology following retinal ischemia-reperfusion in rats.

METHODS

Retinal ischemia was induced in 27 Sprague-Dawley rats by raising the intraocular pressure for 60- or 90-minutes. Two weeks after the retinal ischemia-reperfusion, ganzfeld stimuli of different luminances (maximum 0.51 log cd-s/m2) were used to elicit full-field ERGs from the right (experimental) and left (control) eyes. The amplitudes of the scotopic b-waves were measured, and intensity-response curves were plotted. The b-wave amplitudes at the initial peak (VbIP), the dip (Vb(dip)), and the maximum stimulus intensity (Vb(max)) were measured. The animals were sacrificed after the ERG recordings to determine the surviving retinal cells in the inner nuclear layer (INL).

RESULTS

The log values of the VbIP, Vb(dip) and Vb(max) were linearly correlated with the number of surviving cells in the INL (r = 0.928, 0.905, and 0.913 for VbIP, Vb(dip) and Vb(max), respectively; p < 0.0001 for each). The slope of the regression line for the Vb(dip) was significantly steeper than that for the VbIP and Vb(max) (p < 0.005). The regression line for the Vb(dip) reached undetectable level of the b-wave (<1 microV) even though approximately 25% of the cells still remained in the INL.

CONCLUSIONS

The amplitude of the scotopic b-wave is a sensitive index of the surviving INL cells. However, if one selects the stimulus intensity at the dip, the functional loss would be exaggerated in spite of the presence of cells in the INL.

Correlating retinal function and amino acid immunocytochemistry following post-mortem ischemia

We wanted to determine the characteristics associated with electrophysiological and neurochemical changes secondary to ischemic insult as well as correlate these electrophysiological and neurochemical changes. A Ganzfeld source was used to elicit electroretinograms in anesthetized adult Sprague-Dawley rats. Following baseline recordings, one eye was removed for control quantitative amino acid immunocytochemistry, and ischemic insult was induced by cervical dislocation. Following the induction of ischemia, a single electroretinogram signal was collected at 1, 2, 4, 6, 8, 16, 32 or 64 min, after which the eye was removed for immunocytochemistry. The post-receptoral b-wave was undetectable after 1 min post-ischemia, whereas phototransduction declined more gradually and persisted for up to 16 min post-mortem. Both phototransduction saturated amplitude and sensitivity decayed with a similar time course (tc=3.06 (2.73, 3.48) versus 3.29 (2.61, 4.62)min). Significant elevation of amino acid neurotransmitter levels was not observed until 6 min post-mortem. Between 8 and 16 min post-ischemia, glutamate and GABA were significantly accumulated in neurons and Müller cells (p<0.05). Beyond 16 min, the neurotransmitter elevation in neurons and Müller cells was relatively attenuated. Aspartate immunoreactivity was significantly elevated at 4 and 6 min post-ischemia in neurons, prior to a change in any other amino acid. Moreover, of the amino acids assessed the post-ischemic change in aspartate immunoreactivity showed the best correlation with phototransduction decay (r2=0.68). Our findings show that complete impairment of phototransduction coincides with the accumulation of amino acid neurotransmitter. The correlation of aspartate immunoreactivity and phototransduction provides evidence of heightened glutamate oxidation during ischemic insult.

Effect of ocular perfusion pressure on retinal function in the rabbit

The electroretinogram (ERG) vs perfusion pressure relation was studied in anesthetized rabbits during mechanically induced changes in mean arterial pressure (MAP). After 1 hr of dark adaptation, ERGs were obtained at perfusion pressures from 10 to 95 mmHg with each pressure held for 30-60 sec, and at three levels from 15 to 75 mmHg with each pressure held for 5 min. The slopes of the b-wave amplitude vs perfusion pressure data were not significantly different from zero during either the brief or longer manipulations of perfusion pressure. However, the b-wave was nearly extinguished 5 min after death. The results indicate that the rabbit ERG is maintained over a wide range of perfusion pressure.

A paired comparison of two models of experimental retinal ischemia

Increased intraocular pressure and vascular ligation models are often used in studies of global ocular ischemia. The purpose of this study is to perform a paired comparison of retinal recovery in these paradigms. Our data indicate that ERG b-wave recovery profiles, following identical periods of ischemia, differ significantly between models. We propose that increased intraocular pressure models induce greater retinal injury than vascular ligation models. We suggest that pressure or another aspect of the increased intraocular pressure model induces injury beyond that caused by ischemia alone and caution against direct comparison of results obtained using these two models.

Quantitation of ischemic damage in the rat retina

In order to determine thresholds for irreversible cellular injury in the rat retina, timed acute no-flow ischemic episodes of 30-180 min duration were produced by elevation of intraocular pressure (IOP) above systolic pressure. Quantitation of irreversible degeneration and cell loss following a 2-week post-ischemic interval was performed by computer-assisted measurements from histologic sections. Alterations of thickness of retinal layers and linear cell density were determined for ischemia of selected durations (30, 60, 80, 90, 120 and 180 min). Different thresholds were evident for inner and outer retinal damage. Neurons of the inner nuclear layers showed extensive loss with episodes at 60 min. Decrease in the thickness of the inner plexiform layer provided the best index of this inner nuclear damage. The outer retina was more resistant, with photoreceptors showing extensive damage only after 90 min in conjunction with pigment epithelial metaplasia and degeneration. Two-hour episodes produced full-thickness degeneration with loss of pigment epithelium and sparing of the peripheral retina. Greater sensitivity of the inner retina suggested problems with restoration of the retinal circulation. Horseradish peroxidase infusions did reveal central microcirculatory defects in retinal wholemounts of some specimens with episodes longer than 60 min. Refinements of the methods resulted in outcomes sufficiently reproducible for quantitative assessment of acute ischemic injury. The rat retina provides an economical basic tissue model of acute ischemic injury affecting neurons, glia, and microvasculature. Quantitation of this injury promises great utility in testing agents with potentially protective effects on acute ischemic injury.

Dysregulation of neuroendocrine crossroads: depression, circadian rhythms and the retina--a hypothesis

The pathophysiology of depression and the mechanism of action of lithium and other antidepressant drugs involve alterations in circadian rhythms. These include changes in both the intrinsic rhythm of circadian oscillators and in the sensitivity of the retina to LIGHT. The retina in humans is the only photoreceptor for circadian entrainment. The retinal-hypothalamic-pineal axis is the essential pathway for neuronal entrainment of rhythms which use light as a phase cue. A common substance throughout this axis in many species is MELATONIN. Retinal melatonin has been implicated in regulation of the sensitivity of the retina to light. The hypothalamus, at THE NEUROENDOCRINE CROSSROADS, has a central role in the integration of neurotransmitters and hormones in circadian rhythms. DYSREGULATION of the hypothalamic-pituitary-adrenal, as well as -gonadal, axes has been documented in depressed patients. Abnormalities in circulating melatonin have also been found in patients with affective disorders. It is speculated that the availability of melatonin along the retinal-hypothalamic-pineal axis may have important implications in the genesis of affective disorders. More specifically--is there a latent biochemical defect which causes a phase shift and change in circadian rhythms of melatonin and/or other neurotransmitters in the retina which then alters the sensitivity of the retina to light (for the visible spectrum) which in turn desynchronizes all other biological rhythms thus disrupting mental well-being? We suggest that variations of retinal photosensitivity in humans can be measured by using a visual testing system, and that depressed patients might show changes in photosensitivity which could be corrected when treated with lithium and/or antidepressants. It is our working hypothesis that the primary defect in depression may be a change in retinal function, and that behavioural and neuroendocrine concomitants of this disorder are secondary events.

The hypoxic retinopathy syndrome

We examined electrophysiologically six children who had had cardiac or respiratory arrests with resultant neurologic and visual damage. Their electroretinograms were initially subnormal, but with time and return of visual acuity the electroretinograms became normal. The subnormal electroretinograms may have reflected transient hypoxic retinal damage.

Examination of the fast oscillation of the corneoretinal potential under clinical conditions

The "fast oscillation" of the corneoretinal potential can be precisely examined by means of cyclic light stimuli and an automatic measuring device. This new procedure is carried out during the period of adaptation as part of the usual EOG-test. Normal values and some clinical findings are presented. The fast oscillation may provide additional information when compared with EOG and ERG results.

The c-wave of the human electroretinogram in central retinal artery occlusion

The pigment epithelium-receptor complex, which is supplied from the choroidal circulation, is the main source of the c-wave of the ERG. Occlusion of the central retinal artery (OCRA), which causes serious damage to the inner retinal layers, should therefore not primarily be expected to affect the c-wave amplitude. Nevertheless, conflicting reports of increased, decreased and unchanged c-waves have appeared in the literature. In the present study on four patients with OCRA the affected eyes showed diminished b- and c-waves. On the other hand, both the a-wave and the trough between the b- and c-waves were accentuated in the diseased eyes. Experiments in progress on Cynomolgus monkeys with experimentally induced unilateral OCRA show similar results.

Electroretinographic changes and choroidal defects in a case of central retinal artery occlusion

A 62-year-old white woman with hypertensive diabetes had central retinal artery occlusion of her right eye. Serial electroretinographic (ERG) studies demonstrated reversible changes in the scotopic components but permanent reduction of the photopic b-wave. Serial simultaneous fluorescein and indocyanine green angiographic studies demonstrated a perfusion defect in the choroid involving part of the macula which subsided after reopening the retinal circulation. Differential hypoxic susceptibility of the ERG photopic and scotopic b-waves was demonstrated, and changes in the patient's retinal tissue oxygenation were postulated on the basis of retinal and choroidal angiographic studies performed during and after central retinal artery occlusion to account for the observed changes in serial ERGS.

Occlusion of posterior ciliary artery. IV. Electroretinographic studies

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