Visual field defects and neural losses from experimental glaucoma

Modeling the sensitivity to variability relationship in perimetry

PURPOSE

Studies in glaucoma patients show that standard automated perimetry results increase in variability as sensitivity decreases. However, the reasons for this change are unclear. This study presents the principle of Divergent Dysfunction as a possible explanation for this change in variability, and incorporates it into a model that can be used to simulate perimetry.

METHODS

A computer program was written to simulate visual field test results based on the model, using a Full Threshold testing strategy. The validity of the simulation was tested by comparing it with normal sensitivity values, and with test-retest data from 63 participants evaluated five times each over the course of 1 month. The effect on the simulated data of varying parameters of the model was investigated, such as changing the magnitude of variability and the percentages of false positive and negative responses.

RESULTS

The correlation between subject and simulated test-retest data was 0.987. Several factors were found to affect the sensitivity-variability relationship for the simulated data, most notably the rate of sensitivity decline, the percentage of false positives, and the starting luminance of the test procedure.

CONCLUSIONS

The principle of Divergent Dysfunction presented here provides a plausible explanation for the sensitivity-variability relationship for standard automated perimetry in glaucomatous eyes. The model and resultant simulation program aim to provide an intuitive demonstration of the principle, which can also be used to examine the effectiveness of different testing strategies. These findings have great implications for future clinical research.

Association between localized visual field losses and thickness deviation of the nerve fiber layer in glaucoma

PURPOSE

To perform a correlation between morphology and function by studying local perimetric field losses and thickness reduction of the nerve layer in corresponding areas.

METHOD

Forty-seven patients with "preperimetric" open-angle glaucoma, 95 patients with "perimetric" open-angle glaucoma, and 75 control subjects had perimetric (Octopus) and polarimetric measurements. Criteria for exclusion: optic discs larger than 4 mm, media opacities, polarimetric hourglass pattern in the macular image. Thickness values of the retinal nerve fiber layer were determined in 14 upper and 14 lower sectors (10 degrees each) as well as nasally and temporally (40 degrees each) as provided from the GDx (software version 4000). The healthy control subjects served to determine age-corrected thickness deviation in all sectors. Analyses were calculated in eight nerve fiber bundle related areas. Due to non-linear relationships between perimetric defects and corresponding thickness deviation non-parametric tests were used.

RESULTS

Localized visual field defects in the present perimetric patients were highest in upper and lower visual field areas abutting the nasal meridian. Thickness loss of nerve fiber layer however was highest in more circumferential upper and lower nerve fiber bundles. Correlations between local mean defects and deviation of the retinal nerve fiber layer thickness from normals showed a clear association for corresponding areas. The correlation coefficients were significant (P < 0.01) for all arcuate superior and inferior visual field zones except horizontally and not for the area of the lower nasal step.

CONCLUSION

The present correspondence map indicates that focal perimetric defects can be identified best polarimetrically if they occur in the arcuate bundles of the visual field. A lack of correspondence was observed in the area of the papillo-macular bundle.

Glaucoma: thinking in new ways-a rôle for autonomous axonal self-destruction and other compartmentalised processes?

Glaucoma is a common neurodegenerative disease that affects retinal ganglion cells (RGCs). Substantial effort is being expended to determine how RGCs die in glaucoma. As in other neurodegenerative diseases, the majority of effort focuses on characterising apoptotic self-destruct pathways. However, apoptosis is not the only self-destruct mechanism that may be activated in neurons. It is now known that neurons have distinct classes of self-destruct programme that are spatially compartmentalised. In addition to the well-described intracellular suicide machinery in the neuronal soma, responsible for apoptosis, there is another, molecularly distinct, self-destruct programme localised in the axon. Evidence also supports the existence of compartmentalised degeneration programmes in synapses and dendrites. RGCs are no exception to this. Recent data, from in vitro studies and from an inherited mouse model of glaucoma, suggest that molecularly distinct degenerative pathways underlie the destruction of RGC somata and RGC axons. In various neurodegenerative diseases, axons, dendrites and synapses often degenerate well before the cells die, and there is increasing evidence that this is important for the production of clinical symptoms and signs. We hypothesise that such compartmentalised and autonomous programmes are of critical importance in the pathophysiology of glaucoma, and we suggest that studies of these processes are essential for a complete understanding of this complex disease.

Retinal ganglion cell activity from the multifocal electroretinogram in pig: optic nerve section, anaesthesia and intravitreal tetrodotoxin

Non-invasive recordings of the retinal activity have an important role to play in the diagnosis of retinal pathologies. The detection of diseases that involve retinal ganglion cells (RGCs), such as optic atrophy and glaucoma, may be improved by isolating the RGC contribution from the multifocal electroretinogram (mfERG). In this study, mfERGs were performed on 20 pigs, 1-6 weeks following unilateral retrobulbar optic nerve section (ONS). The stimuli were 103 non-scaled high-contrast hexagons from which summed and individual mfERG responses were obtained in experimental and control fellow eyes under conditions of ketamine (n = 11) or isoflurane anaesthesia (n = 9). The effect of intravitreal injection of tetrodotoxin (TTX; n = 6) was also investigated. The summed mfERG responses showed a first positive peak (P1) with a short latency (21 ms) followed by two smaller peaks (P2 and P3) of longer latency (46 and 65 ms, respectively). While P2 and P3 amplitude were highly correlated with the time post-optic nerve section (ONS) (P2: r(2) = 0.669; P = 0.007; P3: r(2) = 0.651; P = 0.005), P1 was not (r(2) = 0.193; P = 0.38). P1 and P2 showed no implicit time variation as a function of retinal location, while P3 implicit time varied along the axis of the visual streak, generating a naso-temporal asymmetry. However, the P3 implicit time did not vary consistently with distance away from the optic nerve head. Intravitreal injections of TTX reduced P2 and P3 in the control eyes, consistent with the effect of ONS, and also induced a series of regular oscillations lasting up to 200 ms post stimulus. Under isoflurane anaesthesia, all components of the mfERG ifn experimental and control eyes were, at all time points post-ONS, of similar amplitude and without naso-temporal asymmetry, suggesting a reduced participation of RGCs under these anaesthesic conditions. These data clearly demonstrate that it is possible to isolate the RGC contribution from non-invasive multifocal electroretinography.

Scaling the structure--function relationship for clinical perimetry

PURPOSE

The full ranges of glaucomatous visual field defects and retinal ganglion cell losses extend over several orders of magnitude and therefore an interpretation of the structure-function relationship for clinical perimetry requires scaling of both variables. However, the most appropriate scale has not been determined. The present study was undertaken to compare linear and logarithmic transformations, which have been proposed for correlating the perimetric defects and neural losses of glaucoma.

METHODS

Perimetry, by behavioural testing, and retinal histology data were obtained from rhesus monkeys with significant visual field defects caused by experimental glaucoma. Ganglion cell densities were measured in histologic sections of retina that corresponded to specific perimetry test locations for the treated and control eyes. The linear (percentage) and logarithmic (decibel) relationships for sensitivity loss as a function of ganglion cell loss were analysed.

RESULTS

With decibel scaling, visual sensitivity losses and ganglion cell densities were linearly correlated with high coefficients of determination (r(2)), although the parameters of the functions varied with eccentricity. The structure-function relationships expressed as linear percentage-loss functions were less systematic in two respects. Firstly, the relationship exhibited considerable scatter in the data for small losses in visual sensitivity and, secondly, visual sensitivity losses became saturated with larger losses in ganglion cell density. The parameters of the percentage-loss functions also varied with eccentricity, but the variation was less than for the decibel-loss functions.

CONCLUSIONS

Linear scaling of perimetric defects and ganglion cell losses might potentially improve the structure-function relationship for visual defects associated with small amounts of cell loss, but the usefulness of the relationship is limited because of the high variability in that range. With log--log co-ordinates, the structure--function relationship for clinical perimetry is relatively more accurate and precise for cell losses greater than about 3 dB. The comparatively greater accuracy and precision of decibel loss functions are a likely consequence of the logarithmic scale of stimulus intensities for perimetry measurements and because the relationship between visual sensitivity and the number of neural detectors is a form of probability summation.

Primate glaucoma models

The monkey model of ocular hypertension (OHT) with its resultant optic neuropathy closely reflects the optic neurodegeneration associated with human glaucoma. Utilization of the experimental glaucoma model (ExpG) in non-human primates (NHP) has led to advances in the understanding of aqueous humor dynamics, glaucomatous changes in the visual pathways from photoreceptors to the visual cortex, and anterior and posterior ocular segment pharmacological effects.

The psychophysics of glaucoma: improving the structure/function relationship

Perimetry of some kind remains an important tool in the detection, diagnosis and monitoring of glaucomatous damage to the visual pathway. However, recent studies have served to reinforce the suspicion that conventional perimetry does not possess the sensitivity to detect the earliest signs of functional loss resulting from glaucoma. The relationship between differential light threshold and ganglion cell loss is extremely weak and, in the early stages of glaucoma, non-existent. Alternative, more novel perimetric techniques seem to offer promise of better detectability for early loss by claiming to tap in to one or other of the separate parallel pathways of the visual system. While some of these tests show potential for better detection and monitoring of glaucoma, the reasons why this might be so are not always clearly formulated or represented. This leads to misunderstanding of what the test actually measures and of the glaucomatous disease process itself. This paper seeks to revisit and review the theory underlying psychophysical testing of visual function related to glaucoma and stresses the importance of developing tests that are based on a firm theoretical understanding of visual function and processing in order to both detect glaucoma at an earlier stage and better understand the mechanisms of loss from the disease process.

Comparison between monkey and human visual fields using a personal computer system

To use monkeys as models for eye diseases that may lead to blindness, we need to develop a method to precisely measure its visual field and to understand similarities and differences in visual field properties between monkeys and humans. The visual field of monkey was not measured precisely although the necessity. We established a new system with personal computers for precise measurement of the monkey visual field. Four monkeys and three humans served as subjects. The luminance-contrast sensitivity of the central 24 degrees field was measured while the subject was fixating a small spot. During the measurement, we continuously recorded the eye position, and discarded the data when fixation was broken. Reliability indices demonstrated high and stable behavioral performance by both monkeys and humans. The luminance-contrast sensitivity was highest around the fovea, and declined as eccentricity increased. The blind spot was clearly detected 15 degrees temporally. The overall sensitivity was higher in humans than in monkeys and the sensitivity dropped more sharply in the periphery in monkeys than in humans. We recommend this system as a convenient and reliable way to measure visual functions in monkeys in basic ophthalmologic research or in assessment of the drug effects on the visual field.

Longitudinal changes in the visual field and optic disc in glaucoma

The nature and mode of functional and structural progression in open-angle glaucoma is a subject of considerable debate in the literature. While there is a traditionally held viewpoint that optic disc and/or nerve fibre layer changes precede visual field changes, there is surprisingly little published evidence from well-controlled prospective studies in this area, specifically with modern perimetric and imaging techniques. In this paper, we report on clinical data from both glaucoma patients and normal controls collected prospectively over several years, to address the relationship between visual field and optic disc changes in glaucoma using standard automated perimetry (SAP), high-pass resolution perimetry (HRP) and confocal scanning laser tomography (CSLT). We use several methods of analysis of longitudinal data and describe a new technique called "evidence of change" analysis which facilitates comparison between different tests. We demonstrate that current clinical indicators of visual function (SAP and HRP) and measures of optic disc structure (CSLT) provide largely independent measures of progression. We discuss the reasons for these findings as well as several methodological issues that pose challenges to elucidating the true structure-function relationship in glaucoma.

Probing glaucoma visual damage by rarebit perimetry

AIM

To compare rarebit perimetry (RBP) with standard achromatic perimetry (SAP) in detecting early glaucomatous functional damage.

METHODS

43 patients with ocular hypertension (OH), 39 with early primary open angle glaucoma (POAG), and 41 controls were considered. Visual fields were assessed using the Humphrey field analyser (HFA) 30-2 and RBP tests. Differences among the groups were evaluated using Student-Newman-Keuls and chi(2) tests. Correlation between HFA and RBP parameters was assessed using the Pearson's correlation coefficients and regression analysis. Sensitivity and specificity of RBP in detecting early glaucomatous visual damage were calculated with different algorithms.

RESULTS

RBP-mean hit rate (MHR) was respectively 88.6% (SD 4.8%) in controls; 79.1% (10.9%) in the OH group; 64.3% (13.8%) in the POAG group (differences statistically significant). Good correlation in the POAG group was found between HFA-mean deviation and RBP-MHR. Largest AROC (0.95) and optimal sensitivity (97.4%) were obtained when an abnormal RBP test was defined as having (at least 1): MHR <80%; >15 areas with a non-hit rate of >10%; > or =2 areas with a non-hit rate of >50%; at least one area with a non-hit rate of > or =70%.

CONCLUSIONS

The RBP appeared to be a rapid, comfortable, and easily available perimetric test (requiring only a PC device), showing a high sensitivity and specificity in detecting early glaucomatous visual field defects.

Understanding mechanisms of pressure-induced optic nerve damage

Patients with glaucoma can suffer progressive vision loss, even in the face of what appears to be excellent intraocular pressure (IOP) control. Some of this may be secondary to non-pressure-related (pressure-independent) factors. However, it is likely that chronically elevated IOP produces progressive changes in the optic nerve head, the retina, or both that alter susceptibility of remaining optic nerve fibers to IOP. In order to understand the nature of these progressive changes, relevant, cost-effective animal models are necessary. Several rat models are now used to produce chronic, elevated IOP, and methods exist for measuring the resulting IOP and determining the extent of the damage this causes to the retina and optic nerve. A comparison of damage, pressure and duration shows that these models are not necessarily equivalent. These tools are beginning to uncover clear evidence that elevated IOP produces progressive changes in the optic nerve head and retina. In the optic nerve head, these include axonal and non-axonal effects, the latter pointing to involvement of extracellular matrix and astrocyte responses. In the retina, retinal ganglion cells appear to undergo changes in neurotrophin response as well as morphologic changes prior to actual cell death. These, and other, as yet uncovered, abnormalities in the optic nerve head and retina may influence relative susceptibility to IOP and explain progressive optic nerve damage and visual field loss, in spite of apparent, clinically adequate IOP control. Ultimately, this knowledge may lead to the development of new treatments designed to preserve vision in these difficult patients.

Restoration of retinal ganglion cell function in early glaucoma after intraocular pressure reduction: a pilot study

PURPOSE

To investigate the changes of pattern electroretinogram (PERG) after intraocular pressure lowering in glaucoma patients and normal controls.

DESIGN

Interventional retrospective cross-sectional study.

PARTICIPANTS

Twenty-five patients (49 eyes) with ocular hypertension or glaucoma undergoing topical treatment to lower IOP served as a study group; 22 patients (44 eyes) with ocular hypertension or glaucoma observed without treatment served as a control group for treated glaucoma patients; 9 normal subjects (18 eyes) receiving a 250-mg acetazolamide tablet served as a second study group; and 17 normal subjects (34 eyes) from a previous study served as a second control group for treated normal subjects.

METHOD

Pattern electroretinograms were recorded simultaneously from both eyes using skin electrodes and automated analysis. Visual field (VF) analyses were performed with white-on-white standard automated perimetry (SAP). Intraocular pressure was measured with Goldmann applanation tonometry; central corneal thickness was measured with pachymetry.

MAIN OUTCOME MEASURES

Pattern electroretinogram amplitude (microvolts), phase (pi rads), and test-retest variability (test 2-to-test 1 ratio, in decibels), SAP mean deviation (decibels), and IOP (millimeters of mercury).

RESULTS

In 56% of right eyes and 21% of left eyes of the treated glaucoma subgroup, the PERG amplitude and/or phase improved beyond the 95% confidence intervals of the test-retest variability of the untreated glaucoma control group. Pattern electroretinogram improvement with IOP lowering occurred in both high- and low-tension glaucoma eyes. Eyes with severely impaired VFs showed little improvement in PERG; however, eyes of normal subjects treated with acetazolamide did not show significant PERG changes relative to the test-retest variability of normal controls.

CONCLUSIONS

Retinal ganglion cell function can be at least partially restored after IOP reduction in glaucomatous eyes with early VF impairment.

S-cone, L + M-cone, and pattern, electroretinograms in ocular hypertension and glaucoma

Silent substitution and selective adaptation techniques were used to obtain full field S-cone and L + M-cone electroretinograms from 18 patients with ocular hypertension (OHT), 9 with normotensive glaucoma (NTG), 18 with early primary open angle glaucoma (POAG) and 19 normal controls. Pattern electroretinograms were also recorded, using a reduced check size to increase the contribution of retinal ganglion cells. In the OHT and POAG groups, statistically significant reductions (P = 0.05-0.001) were observed in the amplitudes, most notably in the late negative waves of all three types of ERG compared to the controls. These are thought to reflect ganglion cell activity. The results imply a diffusely distributed loss of activity (20-35%) affecting many retinal pathways to a similar extent in OHT and early POAG, with an additional amount (<5%) in POAG corresponding approximately to the loss associated with local field defects. The electrophysiology indicated that virtually all cases of untreated OHT have greater retinal dysfunction than the least affected cases of POAG. The NTG group showed a different pattern of loss in that the PERG was markedly affected but the S-cone ERG was not significantly reduced.

Responses of Müller glia to retinal injury in adult zebrafish

In an effort to identify the cellular events that enable neuronal regeneration in the vertebrate retina, the identity and characteristics of mitotic and apoptotic cells were examined in lesioned retinas of adult zebrafish. Following lesion a complex spatiotemporal pattern of mitosis was observed, including a delayed entry of Müller glia into the cell cycle. Characteristics of these proliferative Müller glia indicated they might serve as a stem/precursor cell of regenerated retina. The results suggested a model of retinal regeneration in which lesions are filled, in part, by a localized en place cytogenesis within intact retina surrounding the lesion site.

Vigabatrin-associated loss of vision: rarebit perimetry illuminates the dose-damage relationship

PURPOSE

The utility of vigabatrin in the treatment of epilepsy is partially offset by its retinal toxicity. The relationship between dosage and damage is obscure. This may be due to perimetric shortcomings. The new technique of rarebit ('microdot') perimetry might be more informative.

METHODS

Twelve patients who had been treated with vigabatrin for various durations were examined by manual, kinetic perimetry and by rarebit perimetry.

RESULTS

Rarebit results differed significantly between patients and normal controls and rarebit deficits were directly proportional to cumulated vigabatrin doses (correlation coefficients were - 0.92 in the nasal field and - 0.82 in the temporal field). Manual perimetry results were less clearly related to dosage (r = - 0.54 and r = - 0.73, respectively).

CONCLUSION

Rarebit perimetry indicates that each treated subject will develop visual loss and that visual loss will be proportional to the accumulated dose. Conventional perimetry is less well suited to detecting and quantifying vigabatrin-associated visual loss.

Inter-ocular and inter-session reliability of the electroretinogram photopic negative response (PhNR) in non-human primates

PURPOSE

To assess the inter-ocular and inter-session reliability for a range of parameters derived from the photopic electroretinogram (ERG) in a group of normal non-human primates.

METHODS

Inter-ocular differences for photopic ERGs were assessed in a group of normal anesthetized adult rhesus monkeys (Macaca mulatta, n=29); inter-session reliability was assessed for 23 eyes of 23 animals tested 3 months later. Signals were acquired using Burian-Allen contact lens electrodes, whereby the contralateral cornea served as a reference. Photopic ERGs were elicited using red Ganzfeld flashes (-0.5-0.67 log photopic cd.sm(-2)) on a rod suppressing blue-background (30 scotopic cdm(-2)). Measurement reliability was established for a-wave, b-wave, photopic negative response (PhNR) and oscillatory potential (OP) amplitudes, as well as for their implicit times, by calculation of the 95% limits-of-agreement (LOA) and the coefficient-of-variation (COV) for each parameter.

RESULTS

OP and a-wave amplitudes increased with intensity up to 0.67 log photopic cd.sm(-2), following a typical saturating function, whereas b-wave and PhNR amplitudes both declined above 0.42 log photopic cd.sm(-2). Inter-session variability was greater than inter-ocular variability. The inter-session COVs for PhNR amplitude (10-20%) were similar to the other photopic ERG components (a-wave: 12-17%, b-wave: 12-17%, OPs: 13-19%). Inter-session LOAs were also similar across components, but on average, were smallest for responses to moderate intensities (0.0-0.42 log photopic cd.sm(-2)).

CONCLUSION

In non-human primates, the 95% LOA for inter-session measurements of the photopic ERG a-wave, b-wave, OPs and PhNR are all similar. Inner-retinal damage may best be measured using the PhNR amplitude for moderately bright stimulus intensities. B-wave and PhNR amplitudes for brighter flashes are smaller and more variable. The ratio of PhNR:b-wave amplitudes manifests smaller variability and may therefore be useful for detection of selective PhNR loss.

Concordance of high-pass resolution perimetry and frequency-doubling technology perimetry results in glaucoma: no support for selective ganglion cell damage

PURPOSE

To compare the results of frequency-doubling technology perimetry (FDT), assumed to test the magnocellular visual pathway, and high-pass resolution perimetry (HRP), assumed to test the parvocellular visual pathway, in patients with ocular hypertension or glaucoma.

PATIENTS AND METHODS

Forty-eight consecutive patients with glaucoma or ocular hypertension, covering the entire range of optic nerve function from normal to severely damaged, were examined on the same day using FDT and HRP.

RESULTS

There was a linear correlation between both global and local indices in FDT and HRP (r = -0.84 P<0.0001 and r = 0.8 P<0.001, respectively). The HRP and FDT classifications agreed in 32 of the 48 (67%) eyes (Cohen kappa = 0.5). There was no significant difference between the ability of the different techniques to detect abnormality.

CONCLUSION

The observations in the present study indicate either that both cell populations are similarly affected by glaucomatous damage or that both methods measure activity in the same cell populations.

Application of Discriminant, Classification Tree and Neural Network Analysis to Differentiate between Potential Glaucoma Suspects with and without Visual Field Defects

Purpose: This study has two objectives. The first one is to investigate the question whether it is possible to discriminate between eyes with and without a glaucomateous visual field defect based on standard ophthalmologic examinations as well as optic nerve head topographic parameters. The second objective raises the question of the ability of several suggested statistical models to generalize their results to new, previously unseen patients.

Methods: To investigate the above addressed question: (a) independent, two-sidedt-tests, (b) a linear discriminant analysis with a forward stepwise variable selection algorithm, (c) four classification tree analyses and (d) three different neural network models with a forward, backward and a genetic variable selection algorithm were applied to 1020 subjects with a normal visual field and 110 subjects with a glaucomateous visual field defect. The Humphrey Visual Field Analyzer was used to test the visual fields and the TopSS®Scanning Laser Tomograph measured the optic nerve topography. A 10-fold cross-validation method was used for the models (b), (c) and (d) to compute approximative 95% confidence intervals for the specificity and sensitivity rates.

A literature study of 18 studies dealt with the question whether/how the generalization error was controlled (control of sample bias, cross-validation procedures, training net size for valid generalization). It was also looked up whether point estimators or 95% confidence intervals were used to report specificity and sensitivity rates.

Results: (a) Only few differences of the means could be found between both groups, including age, existing eye diseases, best corrected visual acuity and visual field parameters. The linear discriminant analysis (b), the classification tree analyses (c) and the neural networks (d) ended up with high specificity rates, but low sensitivity rates.

The literature study showed that three models did not apply a cross-validation procedure to report their results. Two models used a test sample cross-validation and thirteen used a v-fold cross-validation method. Although most authors reported confidence intervals for the area under the ROC, no author reported confidence intervals for the true, but unknown sensitivity and specificity rates.

Conclusions: (a) The results of this study suggest that the combination of standard ophthalmologic eye parameters and optic nerve head topographic parameters of the TopSS®instrument are not sufficient to discriminate properly among normal eyes and eyes with a glaucomateous visual field defect. (b) We follow important suggestions given in statistical learning theory for proper generalization and suggest to apply these methods to the given models or to models in future. At least three conditions should be met: (1) confidence intervals instead of point estimators to assess the classification performance of a model (control of test sample bias); (2) sensitivity and specificity rates should be estimated instead of reporting a point estimator for the area under the ROC and (3) the generalization error should be reported both in a training and a test sample and methods should be applied to select an appropriate training sample size for valid generalization.