Effect of intraocular pressure on the electroretinogram

Severity of middle cerebral artery occlusion determines retinal deficits in rats

Middle cerebral artery occlusion (MCAO) using the intraluminal suture technique is a common model used to study cerebral ischemia in rodents. Due to the proximity of the ophthalmic artery to the middle cerebral artery, MCAO blocks both arteries, causing both cerebral ischemia and retinal ischemia. While previous studies have shown retinal dysfunction at 48h post-MCAO, we investigated whether these retinal function deficits persist until 9days and whether they correlate with central neurological deficits. Rats received 90min of transient MCAO followed by electroretinography at 2 and 9days to assess retinal function. Retinal damage was assessed with cresyl violet staining, immunohistochemistry for glial fibrillary acidic protein (GFAP) and glutamine synthetase, and TUNEL staining. Rats showed behavioral deficits as assessed with neuroscore that correlated with cerebral infarct size and retinal function at 2days. Two days after surgery, rats with moderate MCAO (neuroscore <5) exhibited delays in electroretinogram implicit time, while rats with severe MCAO (neuroscore ≥5) exhibited reductions in amplitude. Glutamine synthetase was upregulated in Müller cells 3days after MCAO in both severe and moderate animals; however, retinal ganglion cell death was only observed in MCAO retinas from severe animals. By 9days after MCAO, both glutamine synthetase labeling and electroretinograms had returned to normal levels in moderate animals. Early retinal function deficits correlated with behavioral deficits. However, retinal function decreases were transient, and selective retinal cell loss was observed only with severe ischemia, suggesting that the retina is less susceptible to MCAO than the brain. Temporary retinal deficits caused by MCAO are likely due to ischemia-induced increases in extracellular glutamate that impair signal conduction, but resolve by 9days after MCAO.

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.

Retinal ischemia: mechanisms of damage and potential therapeutic strategies

Retinal ischemia is a common cause of visual impairment and blindness. At the cellular level, ischemic retinal injury consists of a self-reinforcing destructive cascade involving neuronal depolarisation, calcium influx and oxidative stress initiated by energy failure and increased glutamatergic stimulation. There is a cell-specific sensitivity to ischemic injury which may reflect variability in the balance of excitatory and inhibitory neurotransmitter receptors on a given cell. A number of animal models and analytical techniques have been used to study retinal ischemia, and an increasing number of treatments have been shown to interrupt the "ischemic cascade" and attenuate the detrimental effects of retinal ischemia. Thus far, however, success in the laboratory has not been translated to the clinic. Difficulties with the route of administration, dosage, and adverse effects may render certain experimental treatments clinically unusable. Furthermore, neuroprotection-based treatment strategies for stroke have so far been disappointing. However, compared to the brain, the retina exhibits a remarkable natural resistance to ischemic injury, which may reflect its peculiar metabolism and unique environment. Given the increasing understanding of the events involved in ischemic neuronal injury it is hoped that clinically effective treatments for retinal ischemia will soon be available.

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.

Intraocular pressure and blood flow of the optic disk: a fluorescent blood cell angiography study

The damage to disk microcirculation in glaucoma and ocular hypertension is correlated with the damage to the neuronal tissue. However, it is not clear whether the primary damage is vascular or neurogenic. We have developed a new method which allows in vivo observation of the retinal capillary blood cell flow and the plasma flow, separately. The method was used to examine the blood cell flow in the optic disks of cats' eyes. The intraocular pressure was controlled by extra-fine intraocular canula at the anterior chamber. It was found that blood cell flow became stagnant in a few capillaries at the optic disk with an intraocular pressure of 30-45% of the systemic mean blood pressure. When an induced IOP increase to 45% of systemic mean blood pressure was sustained for 30 minutes, microcirculatory blood flow recovery was complete two hours later. When this pressure was sustained for 120 minutes, the recovery was incomplete two hours later. This work represents a new tool in the research of retinal and disk hemodynamics and microvascular pathologies. The results presented in this paper support the hypothesis that early ischemic microdamage might be the basis of the pathogenesis of the chronic glaucomas.

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.

Non-competitive NMDA-receptor antagonists and anoxic degeneration of the ERG B-wave in vitro

Studies have been undertaken to see if the non-competitive NMDA antagonists, ketamine, MK-801 and dextromethorphan would preserve the b-wave of the electroretinogram (ERG) in vitro. The drugs had no effect on the ERG b-wave, nor prolonged its survival postmortem. The present results support previous evidence suggesting that NMDA-receptors are not involved directly in synaptic transmission between photoreceptors and ON-bipolar cells. Further, loss of the b-wave in post-mortem anoxia does not appear to be mediated via NMDA-receptors.

Electrophysiological consequences of retinal hypoxia

Experiments on cats show that electrical activity of the inner (proximal) retina is unaffected during systemic hypoxia as long as arterial oxygen tension (PaO2) is above 40 mm Hg. This is due to effective regulation of inner retinal tissue PO2 by the retinal circulation. In contrast, some electrical signals generated in the outer (distal) retina begin to change when PaO2 falls below 70-80 mmHg. The outer retinal responses are generated by the retinal pigment epithelium, but their susceptibility to hypoxia results primarily from their dependence on photoreceptors. Photoreceptor metabolism is sensitive to hypoxia because of the high oxygen consumption of photoreceptors and their reliance on the choroidal circulation, which cannot regulate PO2 in the outer retina. Retinal electrophysiology and oxygen distribution are altered by acutely elevated intraocular pressure just as by hypoxia. These results raise the question as to how inner retinal function can be preserved when outer retinal function is altered. The explanations proposed relate to (1) differences in conditions of light adaptation in different studies, (2) the possible inappropriateness of the previous measurements in the inner retina for revealing photoreceptor dysfunction, and (3) a possible preservation of photoreceptor electrical responses when their metabolism is altered. Comparison of cat and human studies suggests that the human retina is affected in much the same way during hypoxia as the cat retina, but further experiments are required for an understanding of the role of hypoxia in human disease.

Urethane as a sole general anaesthetic in cats used for electroretinogram studies

Cats are very sensitive to induction of anaesthesia by urethane. To anaesthetize cats with urethane, 1.0-1.3 g/kg, of freshly prepared urethane is administered intravenously at a rate of 1.92 g/h, while anaesthesia is maintained with 0.5% halothane in a 66%/33% nitrous oxide/carbogen gas mixture. Cats can then be maintained for up to 3 days by intravenous infusion at a rate of 4 ml/h of a 100 ml solution containing 50 IU heparin, 2.4 mg atropine, 4.7 g anhydrous D-glucose, and 240 mg urethane/kg. Using this anaesthetic, excellent electroretinograms can be recorded with no interfering eye movements.

Pneumatic retinopexy

Pneumatic retinopexy, a term introduced by Hilton in 1985, describes a recently revised and modified operation used for primary rhegmatogenous retinal reattachment. A gas bubble is injected into the vitreous cavity and the patient positioned so that the bubble closes the retinal break, permitting resorption of subretinal fluid. A chorioretinal adhesion formed around the break secures the retina in place. The history of pneumatic retinopexy is presented as well as recent studies on the subject. Characteristics of intraocular gases are reviewed. A protocol is described for pneumatic repair of complex retinal detachments. Patient selection and surgical techniques are recommended. Pneumatic retinopexy is compared to other retinal reattachment techniques and controversial issues are discussed.

Pneumatic retinopexy. A two-step outpatient operation without conjunctival incision

1,000 consecutive patients operated for retinal detachment were studied prospectively to determine the incidence of complications. Two parts of the operation were responsible for most of the vision-threatening complications: subretinal fluid drainage and the scleral buckle. In an attempt to reduce the complications of retinal detachment surgery, we have utilized a simplified procedure of transconjunctival cryotherapy and intravitreal gas injection with postoperative positioning. This study was limited to detachments with one or more breaks within one clock hour located within the superior eight clock hours of the fundus without signs of proliferative vitreoretinopathy. In a series of 20 consecutive patients, retinal reattachment was initially achieved in all cases. There were two recurrences that were reattached with scleral buckling. The final cure rate for the single pneumatic procedure, with six months follow-up, was 90%. No major complications were observed. This preliminary report suggests that pneumatic retinopexy has the advantages of reduced tissue trauma, no hospitalization, minimal complications, and reduced expense. The major disadvantage is the need for postoperative positioning for five days.

The pattern evoked electroretinogram associated with elevated intraocular pressure

Electroretinographic responses to pattern-reversal stimuli (P-ERG) were recorded in eight patients with protracted elevation of intraocular pressure. Pressures of bigger than 30 mm Hg result in marked amplitude reductions in the P-ERG. The observed changes probably reflect the impaired function of retinal ganglion cells caused by decreased oxygen supply.

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.

Experimental occlusion of the central artery of the retina. I. Ophthalmoscopic and fluorescein fundus angiographic studies

Transient experimental occlusion of the central artery of the retina (OCAR), lasting from 15 to 270 minutes, was produced by clamping the artery in the orbit in 63 eyes of rhesus monkeys. Ophthalmoscopic and fluorescein angiographic studies were performed before and during clamping of the artery, as well as periodically after unclamping, for periods of up to 22 weeks. The effects of transient retinal ischaemia on the retina, optic disc, and retinal vascular bed were studied. 89% of the eyes showed a variable amount of residual retinal circulation on angiography during CAR clamping, but this did not exercise any protective action against ischaemic damage. Duration of the ischaemia was the principal factor determining severity of damage. OCAR for up to 98 minutes produced no significant permanent neural damage, but OCAR for 105 minutes or longer produced irreversible permanent neural damage. There was no significant permanent damage to the retinal vascular bed, though a transient fluorescein leakage was seen after OCAR for 2 1/2-3 hours or longer. The findings revealed that the normal red colour of the optic disc represents retinal vascular filling in the surface layer of the disc and not deeper vascular filling. The various factors influencing the retinal circulation and neural damage in OCAR are discussed.

Experimental occlusion of the central artery of the retina. IV: Retinal tolerance time to acute ischaemia

Ophthalmoscopic, fluorescein angiographic, electrophysiological, and morphological studies on 63 eyes of rhesus monkeys with acute transient experimental occlusion of the central artery of the retina (OCAR) showed that the retina suffered irreparable damage after ischaemia of 105 minutes but recovered well after ischaemia of 97-98 minutes. The tolerance time of the brain to acute transient ischaemia is many times shorter than that of the retina. The metabolism of ischaemic neurones (in the retina and brain) is discussed with a view to explaining this difference, and also the various factors possibly responsible for the retina's longer tolerance to ischaemia, as compared to the brain.

[The electroretinogram and the visual evoked potential in normal and glaucomatous eyes (author's transl)]

The functional changes of the intraocular nerve structures caused by glaucoma were examined electro-ophthalmologically. The OPs, the photopic and scotopic ERG to examine the receptor and bipolar layers, as well as the EPs, elicited by luminance and pattern-reversal stimuli, for evaluation of the signal conduction in the optic nerve, were recorded. The problem was approached by way of three investigations: first was the question of which nerve structures are affected by glaucoma and exactly how the loss of visual field due to glaucoma can be determined. For this reason, 55 glaucomatous eyes with regulated intraocular pressure and different visual field losses were examined. The results show a functional diminution of all intraocular nerve structures in which the prelaminary part of the optic nerve is most affected. Differences in the visual field loss of both eyes can be well determined by the EPs. Second, the electro-ophthalmologic behavior in seven normal and eight pressure-regulated glaucomatous eyes was studied by gradually elevated intraocular pressure in order to obtain better insight into the functional pathology of glaucoma. The elevation of intraocular pressure was performed with a Müller spring dynamometer in five steps, depending on the ophthalmic blood pressure. The pressure behavior of the ERG components and the EPs is different. The amplitudes of the ERG components show a gradual decrease in normal as well as in glaucomatous eyes when intraocular pressure is increased, and are maintained when intraocular pressure reaches ophthalmic blood pressure. On the other hand, the EPs show a strong decrease in amplitude when intraocular pressure exceeds the mean ophthalmic blood pressure, particularly in the case of glaucomatous eyes. This behavior can be explained by a high pressure sensitivity of the preliminary part of the optic nerve, even greater in glaucomatous eyes. Third, the influence of pressure decrease on the electrical response was examined in glaucomatous eyes with chronic and acute pressure increase before and after pressure regulation. A mean pressure decrease of 37-13.6 mm Hg in ten eyes with chronic pressure increase led to no change in electrical responses other than a phase shift on the pattern-reversal EPs. In five cases with acute pressure increase, an amplitude increase on the luminance EPs was noticed after pressure regulation, with unchanged systemic blood pressure and almost unchanged ERG components. However, in one case an amplitude decrease on luminance EPs and ERG components was found with simultaneous blood pressure decrease. The increase of the amplitudes of the luminance EPs and the phase shifts of the pattern-reversal EPs can be explained as the functional improvement of the prelaminary part of the optic nerve caused by pressure decrease due to improved blood circulation in the prelaminary part of the optic nerve...

Post-mortem changes in the rabbit retina. A study by light microscopy

The course of post-mortem changes in rabbit retina has been followed. Short post-mortem periods are accompanied by degenerative changes limited mainly to the visual cells and retinal pigment epithelium. Long post-mortem periods are associated with degenerative changes throughout the retina. Retinal tissue maintained at room temperature was less affected than that kept at body temperature (37degreesC). Post-mortem changes are similar to those observed following periods of pressure induced ischaemia and it is thought that the mechanical effects of pressure on retinal tissue are minimal at the level of resolution afforded by light microscopy.