Changes in optic disk characteristics and number of nerve fibers in experimental glaucoma

Assessing Surface Shapes of the Optic Nerve Head and Peripapillary Retinal Nerve Fiber Layer in Glaucoma with Artificial Intelligence

Purpose

To assess 3-dimensional surface shape patterns of the optic nerve head (ONH) and peripapillary retinal nerve fiber layer (RNFL) in glaucoma with unsupervised artificial intelligence (AI).

Design

Retrospective study.

Participants

Patients with OCT scans obtained between 2016 and 2020 from Massachusetts Eye and Ear.

Methods

The first reliable Cirrus (Carl Zeiss Meditec, Inc) ONH OCT scans from each eye were selected. The ONH and RNFL surface shape was represented by the vertical positions of the inner limiting membrane (ILM) relative to the lowest ILM vertical position in each eye. Nonnegative matrix factorization was applied to determine the ONH and RNFL surface shape patterns, which then were correlated with OCT and visual field (VF) loss parameters and subsequent VF loss rate. We tested whether using ONH and RNFL surface shape patterns improved the prediction accuracy for associated VF loss and subsequent VF loss rates measured by adjusted r² and Bayesian information criterion (BIC) difference compared with using established OCT parameters alone.

Main Outcome Measures

Optic nerve head and RNFL surface shape patterns and prediction of the associated VF loss and subsequent VF loss rates.

Results

We determined 14 ONH and RNFL surface shape patterns using 9854 OCT scans from 5912 participants. Worse mean deviation (MD) was most correlated (r = 0.29 and r = 0.24, Pearson correlation; each P < 0.001) with lower coefficients of patterns 10 and 12 representing inferior and superior para-ONH nerve thinning, respectively. Worse MD was associated most with higher coefficients of patterns 5, 4, and 9 (r = –0.16, r = –0.13, and r = –0.13, respectively), representing higher peripheral ONH and RNFL surfaces. In addition to established ONH summary parameters and 12–clock-hour RNFL thickness, using ONH and RNFL surface patterns improved (BIC decrease: 182, 144, and 101, respectively; BIC decrease ≥ 6; strong model improvement) the prediction of accompanied MD (r² from 0.32 to 0.37), superior (r² from 0.27 to 0.31), and inferior (r² from 0.17 to 0.21) paracentral loss and improved (BIC decrease: 8 and 8, respectively) the prediction of subsequent VF MD loss rates (r² from 0 to 0.13) and inferior paracentral loss rates (r² from 0 to 0.16).

Conclusions

The ONH and RNFL surface shape patterns quantified by unsupervised AI techniques improved the structure–function relationship and subsequent VF loss rate prediction.

Clinical Utility of Optical Coherence Tomography in Glaucoma

Optical coherence tomography (OCT) has established itself as the dominant imaging modality in the management of glaucoma and retinal diseases, providing high-resolution visualization of ocular microstructures and objective quantification of tissue thickness and change. This article reviews the history of OCT imaging with a specific focus on glaucoma. We examine the clinical utility of OCT with respect to diagnosis and progression monitoring, with additional emphasis on advances in OCT technology that continue to facilitate glaucoma research and inform clinical management strategies.

[Imaging of the lamina cribrosa for early detection of glaucoma : Latest trends from the annual ARVO meeting 2016]

Studying the lamina cribrosa (LC) is relevant to understand the mechanisms of retinal ganglion cell degeneration in glaucoma and develop new diagnostic and therapeutic strategies. We would like to present some of the emerging trends and hot topics in imaging of the lamina cribrosa in glaucoma from the 2016 ARVO (The Association of Research in Vision and Ophthalmology) annual meeting, which was held in Seattle, WA, USA. Presentation of the path of ganglion cells through the pores of the lamina cribrosa as well as changes to the shape of the lamina cribrosa with increase of the intraocular and intracerebral pressure have been of great international interest.

The non-human primate experimental glaucoma model

The purpose of this report is to summarize the current strengths and weaknesses of the non-human primate (NHP) experimental glaucoma (EG) model through sections devoted to its history, methods, important findings, alternative optic neuropathy models and future directions. NHP EG has become well established for studying human glaucoma in part because the NHP optic nerve head (ONH) shares a close anatomic association with the human ONH and because it provides the only means of systematically studying the very earliest visual system responses to chronic intraocular pressure (IOP) elevation, i.e. the conversion from ocular hypertension to glaucomatous damage. However, NHPs are impractical for studies that require large animal numbers, demonstrate spontaneous glaucoma only rarely, do not currently provide a model of the neuropathy at normal levels of IOP, and cannot easily be genetically manipulated, except through tissue-specific, viral vectors. The goal of this summary is to direct NHP EG and non-NHP EG investigators to the previous, current and future accomplishment of clinically relevant knowledge in this model.

A comparison between amblyopic and fellow eyes in unilateral amblyopia using spectral-domain optical coherence tomography

Purpose

To compare the macular retinal thickness and characteristics of optic nerve head (ONH) parameters in amblyopic and fellow eyes in patients with unilateral amblyopia.

Patients and methods

A total of 21 patients with unilateral amblyopia (14 patients with anisometropic amblyopia, four patients with strabismic amblyopia, and three patients with both) were examined using spectral-domain optical coherence tomography. The mean age of the patients was 8.5±3.5 years. The examined parameters included the mean macular (full, inner, and outer), ganglion cell complex and circumpapillary retinal nerve fiber layer (cpRNFL) thicknesses, and ONH parameters (rim volume, nerve head volume, cup volume, rim area, optic disc area, cup area, and cup-to-disc area ratio).

Results

The amblyopic eyes were significantly more hyperopic than the fellow eyes (P<0.001). Among the macular retinal thickness parameters, the cpRNFL thickness (P<0.01), macular full retinal thickness (3 mm region) (P<0.01), and macular outer retinal thickness (1 and 3 mm regions) (P<0.05) were significantly thicker in the amblyopic eyes than in the fellow eyes, while the ganglion cell complex thickness, macular full retinal thickness (1 mm region), and macular inner retinal thickness (1 and 3 mm regions) were not significantly different. Among the ONH parameters, the rim area was significantly larger and the cup-to-disc area ratio was smaller in the amblyopic eyes than in the fellow eyes (P<0.05). None of the other ONH parameters were significantly different between the investigated eyes. The differences in the cpRNFL thickness and macular outer retinal thickness in the 1 mm region were significantly correlated with the difference in axial length (P<0.05, r=−0.48; P<0.01, r=−0.59, respectively) and refractive error (P<0.05, r=0.50; P<0.01, r=0.60, respectively). The other parameters were not significantly related to the difference in axial length, refractive error, or best corrected visual acuity.

Conclusion

We found significant differences in some of the morphological measurements between amblyopic and fellow eyes that appear to be independent of abnormalities in the visual cortex.

Imaging of the Lamina Cribrosa using Swept-Source Optical Coherence Tomography

The lamina cribrosa (LC) is the presumed site of axonal injury in glaucoma. Its deformation has been suggested to contribute to optic neuropathy by impeding axoplasmic flow within the optic nerve fibers, leading to apoptosis of retinal ganglion cells. To visualize the LC in vivo, optical coherence tomography (OCT) has been applied. Spectral domain (SD)-OCT, used in conjunction with recently introduced enhanced depth imaging (EDI)-OCT, has improved visualization of deeper ocular layers, but in many individuals it is still limited by inadequate resolution, poor image contrast and insufficient depth penetrance. The posterior laminar surface especially is not viewed clearly using these methods. New generation high-penetration (HP)-OCTs, also known as swept-source (SS)-OCT, are capable to evaluate the choroid in vivo to a remarkable level of detail. SS-OCTs use a longer wavelength (1,050 nm instead of 840 nm) compared to the conventional techniques. We review current knowledge of the LC, findings from trials that use SD-OCT and EDI-OCT, and our experience with a prototype SS-OCT to visualize the LC in its entirety.

Key Points

What is known?

•     The LC is the presumed site of axonal injury in glaucoma

•     Compared to spectral domain-OCT, enhanced depth imaging-OCT improves imaging of the LC

•     Even so, currently used SD-OCT techniques are restricted by poor wavelength penetrance of the deeper ocular layers

What our findings add?

•    SS-OCT may be a superior imaging modality for deep ocular structures

•    Prior studies used SS-OCT to evaluate choroidal thickness in both healthy and ‘normal tension glaucoma’ eyes

•    SS-OCT enables good evaluation of three-dimension (3D) lamina cribrosa morphology.

How to cite this article: Nuyen B, Mansouri K, Weinreb RN. Imaging of the Lamina Cribrosa using Swept-Source Optical Coherence Tomography. J Current Glau Prac 2012;6(3): 113-119.

Comparison of neuroretinal rim area measurements made by the Heidelberg Retina Tomograph I and the Heidelberg Retina Tomograph II

PURPOSE

To investigate the agreement between neuroretinal rim area (RA) measurements using the Heidelberg Retina Tomograph I (HRT Classic) and Heidelberg Retina Tomograph II (HRT II). To compare apparent RA changes in follow-up series of HRT II topographies when using either an HRT Classic or HRT II mean topography as baseline.

DESIGN

Cross-sectional study and "no-change," short time series study.

PARTICIPANTS

Forty-three ocular hypertensive and 31 primary open angle glaucoma subjects.

METHODS

Five HRT Classic and 5 HRT II examinations were acquired from 1 eye of each subject, across 2 visits within 6 weeks. For the cross-sectional study, follow-up RA measurements from HRT Classic and HRT II were compared, using the same HRT Classic mean topography as the baseline. The linear rates of RA change were compared in 2 short time series with either an HRT Classic or an HRT II mean topography as baseline, and 4 follow-up HRT II mean topographies. Intervals between topographies were arbitrarily set at 1 year for meaningful comparisons of rates. Rates of RA change over time were calculated by linear regression. Separate analyses were performed using 3 available reference planes (RP).

MAIN OUTCOME MEASURES

Global and sectoral RA measurements in HRT Classic and HRT II mean topographies; linear rates of RA change.

RESULTS

HRT Classic minus HRT II mean differences (95% limits of agreement) were 0.09 (-0.17, 0.35) mm, 0.09 (-0.13, 0.32) mm, and 0.11 (-0.24, 0.46) mm for the Moorfields, 320 µm, and standard RPs, respectively (P<0.001 for all RPs, Wilcoxon rank sum test). In the time series, the mean differences (95% limits of agreement) of RA rates of change (HRT Classic baseline minus HRT II baseline) were -0.01 (-0.06, 0.03) mm/y, -0.01 (-0.06, 0.04) mm/y, and -0.0 (-0.09, 0.05) mm/y using the Moorfields, 320 µm, and standard RPs, respectively.

CONCLUSION

Although HRT software is backward-compatible, follow-up RA measurements made in the same eye using HRT Classic and HRT II devices display statistically and clinically meaningful systematic differences when HRT Classic topographies are used as a baseline.

'Structure-function relationship' in glaucoma: past thinking and current concepts

An understanding of the relationship between functional and structural measures in primary open-angle glaucoma is necessary for both grading the severity of disease and for understanding the natural history of the condition. This article outlines the current evidence for the nature of this relationship and highlights the current mathematical models linking structure and function. Large clinical trials demonstrate that both structural and functional change are apparent in advanced stages of disease, and at an individual level, detectable structural abnormality may precede functional abnormality in some patients, whereas the converse is true in other patients. Although the exact nature of the 'structure-function' relationship in primary open-angle glaucoma is still the topic of scientific debate and the subject of continuing research, this article aims to provide the clinician with an understanding of the past concepts and contemporary thinking in relation to the structure-function relationship in primary open-angle glaucoma.

Visual advantage in deaf adults linked to retinal changes

The altered sensory experience of profound early onset deafness provokes sometimes large scale neural reorganisations. In particular, auditory-visual cross-modal plasticity occurs, wherein redundant auditory cortex becomes recruited to vision. However, the effect of human deafness on neural structures involved in visual processing prior to the visual cortex has never been investigated, either in humans or animals. We investigated neural changes at the retina and optic nerve head in profoundly deaf (N = 14) and hearing (N = 15) adults using Optical Coherence Tomography (OCT), an in-vivo light interference method of quantifying retinal micro-structure. We compared retinal changes with behavioural results from the same deaf and hearing adults, measuring sensitivity in the peripheral visual field using Goldmann perimetry. Deaf adults had significantly larger neural rim areas, within the optic nerve head in comparison to hearing controls suggesting greater retinal ganglion cell number. Deaf adults also demonstrated significantly larger visual field areas (indicating greater peripheral sensitivity) than controls. Furthermore, neural rim area was significantly correlated with visual field area in both deaf and hearing adults. Deaf adults also showed a significantly different pattern of retinal nerve fibre layer (RNFL) distribution compared to controls. Significant correlations between the depth of the RNFL at the inferior-nasal peripapillary retina and the corresponding far temporal and superior temporal visual field areas (sensitivity) were found. Our results show that cross-modal plasticity after early onset deafness may not be limited to the sensory cortices, noting specific retinal adaptations in early onset deaf adults which are significantly correlated with peripheral vision sensitivity.

Self-reported glaucoma in Canada: findings from population-based surveys, 1994-2003

BACKGROUND

To provide population estimates and 10-year trends in the self-reported prevalence of glaucoma in Canada, including age and sex profiles, frequency of contact with eye specialists, and associated chronic medical conditions.

METHODS

Analyses were based on cross-sectional, self-reported data for ages >or=20 years from 5 national surveys: the 1994-95, 1996-97, and 1998-99 National Population Health Survey (minimum sample size [n] = 33 153) and the 2000-01 and 2002-03 Canadian Community Health Survey (min n = 113 212). Prevalence estimates were derived from descriptive analyses; multivariate logistic regression analyses examined the association of income, education, and chronic medical conditions with the likelihood of reporting glaucoma.

RESULTS

In 2002-03, an estimated 409,000 Canadians had glaucoma, including 2.7% of those >or=40 years and 11% of those >or=80 years. Overall, the prevalence of self-reported glaucoma increased from 1.1% in 1994-95 to 1.8% in 2002-03 (p < 0.01). Significant increases in age-specific prevalences over time were also observed in 4 of 6 age groups. In total, 14% of respondents with glaucoma reported no recent contact with an eye specialist. Arthritis (and other rheumatic conditions), high blood pressure, migraines, and diabetes were associated with glaucoma.

INTERPRETATION

Our review suggests glaucoma is at least as prevalent in Canada as in other Western nations. It is most common in the over-80 age group and appears to be associated with other medical conditions.

Surgical lowering of elevated intraocular pressure in monkeys prevents progression of glaucomatous disease

Recent reports from large clinical trials have clearly demonstrated that lowering intraocular pressure (IOP) in persons with ocular hypertension has a beneficial effect on reducing the progression of glaucomatous disease. Few studies of this effect have been conducted in controlled laboratory settings, however, none have been conducted using non-human primates, the model of experimental glaucoma considered most similar to the human disease. Using data collected retrospectively from a trabeculectomy study using 16 cynomolgous monkeys with experimental ocular hypertension, we evaluated both the threshold of elevated IOP required to cause clinically observable damage to the optic nerve head and also if lowering IOP below this threshold prevents further damage. An index of the level of elevated IOP experienced by experimental eyes (the Pressure Insult) was calculated as the slope of the difference in cumulative IOP between experimental and control eyes during four intervals of time over the course of the experiment, while damage to the optic nerve head was evaluated by measuring the Cup:Disc ratio for each eye from stereoscopic photographs taken at the end of each interval. An increase in the Cup:Disc ratio was significantly associated with both the maximum IOP obtained in the experimental eye during each interval (r=0.573, P<0.001) and the Pressure Insult (r=0.496, P<0.001). Pressure Insult values less than 11 mm Hg Days/Day were not associated with glaucomatous damage in monkey eyes, whereas values greater than 11 showed a significant correlation with increasing Cup:Disc ratios (P<0.001). Trabeculectomy to reduce the Pressure Insult below 11 was correlated with an attenuation of the rate of progression of the Cup:Disc ratio in eyes that had exhibited damage before surgery. These results contribute further to our understanding of this model of experimental glaucoma by demonstrating a threshold at which IOP needs to be elevated to stimulate damage, while also providing corroborating evidence that lowering IOP in ocular hypertensive monkeys can attenuate the progression of glaucomatous disease.

Nerve fiber layer measurement using scanning laser polarimetry with fixed corneal compensator in normal cynomolgus monkey eyes

PURPOSE

The purpose of this study was to examine retinal nerve fiber layer thickness in normal cynomolgus monkeys using a scanning laser polarimeter with a fixed corneal compensator (GDx FCC), and to clarify the reproducibility and symmetries (right-left differences) between both eyes for the GDx parameters.

METHODS

GDx parameters were measured in 36 normal eyes of 18 cynomolgus monkeys aged 4.0-5.5 years. The retardation values (thickness parameters) at peripapillary and macular areas obtained from the GDx FCC were measured and calculated thickness, ratio, and modulation parameters in the superior and inferior quadrants. Mean and standard deviation (SD), coefficient of variation (CV), and binocular differences were obtained for each parameter from three independent measurements made during a 1-week period. Correlation between both eyes in macular retardation and baseline values, which indicated the combined minimum retardation values for the nasal and temporal quadrants, and between macular retardation and baseline values were analyzed.

RESULTS

The intraocular pressure values (mean +/- SD, n = 18) obtained for the right and left eyes were 20.7 +/- 3.8 and 20.0 +/- 3.2 mm Hg, respectively (no significant differences in both eyes). No significant differences between right and left eyes were detected for any GDx parameters. All parameters showed small right-left differences. The CVs (SD/mean x 100) for all parameters were less than 10%. Highly significant correlations were seen between bilateral eyes for macular retardation (r = 0.936, p < 0.0001) or baseline values (r = 0.946, p < 0.0001). A significant correlation (r = 0.883, p < 0.0001) was also seen between macular retardation and baseline values.

CONCLUSIONS

Considering individual differences in corneal birefringence, GDx parameters obtained from a GDx FCC may be useful for the objective evaluation of time-related changes in individual eyes or for binocular comparisons in cynomolgus monkeys.

Optic disc topographic parameters measured in the normal cynomolgus monkey by confocal scanning laser tomography

AIM

To study optic disc topographic parameters in normal cynomolgus monkeys by Heidelberg retina tomograph (HRT).

METHODS

12 optic disc topographic parameters were investigated in 36 normal eyes in 18 male monkeys. Mean (SD) and interocular differences were obtained for each parameter from three independent measurements made during a 1 week period. Correlations among the topographic parameters were analysed, too.

RESULTS

No significant differences between right and left eyes were detected for any topographic parameters. Disc area, rim area, and height variation contour showed smaller right-left differences than other parameters. The coefficients of variation for rim area, height variation contour, rim volume, mean cup depth, maximum cup depth, mean retinal nerve fibre layer (RNFL) thickness, and RNFL cross section area were less than 10% (for rim area, less than 5%). Rim area and height variation contour showed relatively weak interrelations and neither showed a correlation with disc area.

CONCLUSION

For evaluating time related changes in the optic disc by HRT in monkeys, rim area and height variation contour might be useful parameters because coefficients of variation and right-left differences were lower than for other parameters and because these parameters showed weak interrelations and no correlation with disc area.

Blood flow per unit retinal nerve fibre tissue volume is lower in the human inferior retina

AIM

To determine if perfusion per unit tissue volume of retinal nerve fibre layer and optic nerve head in the inferior sector is lower than in the superior sector.

METHODS

Heidelberg retinal tomogram (HRT) for topographic measurement of optic nerve head and retinal nerve fibre layer and Heidelberg retinal flowmeter (HRF) for retinal blood flow were performed on 19 normal healthy subjects. Measurements from the superior and inferior sectors were compared. The perfusion/nerve fibre ratio (PNR); the blood flow per unit retinal nerve fibre tissue volume, was calculated in each sector with a formula; HRF flow measurements divided by HRT measurements.

RESULTS

Retinal nerve fibre layer thickness in the inferior retina was significantly higher than in the superior retina (p<0.05). There were, however, no differences in retinal blood flow between the superior and inferior retinal sectors. The PNR in the inferior sector were significantly lower than in the superior sector (p=0.047 for HRF mean flow/rim volume and p = 0.0282 for HRF 75th percentile flow/rim volume).

CONCLUSIONS

The inferior sector of retinal nerve fibre layer and optic nerve head may have lower blood flow per unit nerve tissue volume compared to the superior sector. This result suggests that the inferior sector is more vulnerable to elevated intraocular pressure (IOP) and ischaemic insults in glaucomatous optic neuropathy.

Mapping the visual field to the optic disc in normal tension glaucoma eyes

PURPOSE

To establish the anatomical relationship between visual field test points in the Humphrey 24-2 test pattern and regions of the optic nerve head (ONH) DESIGN: Cross-sectional study.

PARTICIPANTS

Glaucoma patients and suspects from the Normal Tension Glaucoma Clinic at Moorfields Eye Hospital.

METHODS

Sixty-nine retinal nerve fiber layer (RNFL) photographs with well-defined RNFL defects and/or prominent bundles were digitized. An appropriately scaled Humphrey 24-2 visual field grid and an ONH reference circle, divided into 30 degrees sectors, were generated digitally. These were superimposed onto the RNFL images. The relationship of visual field test points to the circumference of the ONH was estimated by noting the proximity of test points to RNFL defects and/or prominent bundles. The position of the ONH in relation to the fovea was also noted.

MAIN OUTCOME MEASURES

The sector at the ONH corresponding to each visual field test point, the position of the ONH in relation to the fovea, and the effect of the latter on the former.

RESULTS

A median 22 (range, 4-58), of a possible 69, ONH positions were assigned to each visual field test point. The standard deviation of estimations was 7.2 degrees. The position of the ONH was 15.5 degrees (standard deviation 0.9 degrees ) nasal and 1.9 degrees (standard deviation 1.0 degrees ) above the fovea. The location of the ONH had a significant effect on the corresponding position at the ONH for 28 of 52 visual field test points.

CONCLUSIONS

A clinically useful map that relates visual field test points to regions of the ONH has been produced. The map will aid clinical evaluation of glaucoma patients and suspects, as well as form the basis for investigations of the relationship between retinal light sensitivity and ONH structure.

Relation of optic disc topography and age to thickness of retinal nerve fibre layer as measured using scanning laser polarimetry, in normal subjects

AIMS

To evaluate the relation of the optic nerve head topographic measurements and age with the thickness of the retinal nerve fibre layer (RNFL) in normal Caucasoid subjects by means of scanning laser polarimetry and tomography.

METHODS

Topographic optic disc measurements and RNFL thickness values of 38 normal Caucasoid subjects of both sexes aged 20 to 78 were measured using a confocal scanning laser ophthalmoscope and a confocal scanning laser polarimeter. One eye was randomly selected for statistical analysis. The effects of optic disc size, age, and optic disc head topographic measurements of total and regional RNFL thickness were evaluated.

RESULTS

Age showed a significant correlation with the integral of the total RNFL thickness (R=-0.341, p<0.05). The optic disc size showed a significant correlation with the integral of the total, superior, and inferior quadrant RNFL thickness (R=0.425, p<0.01), (R=0.361, p<0.05), (R=0.468, p<0.05). Neuroretinal rim area (NRA) had a correlation with the superior and inferior quadrant RNFL thickness values (R=0.339, p<0.05) (R=0.393, p<0.05). There was no significant correlation between the other optic disc topographic measurements and RNFL thickness values (p>0.05).

CONCLUSION

The thickness of total as well as superior and inferior quadrant peripapillary RNFL as measured by scanning laser polarimetry increased significantly with an increase in optic disc size. The cross sectional area occupied by superior and inferior polar RNFL increased significantly with an increase in NRA. The total cross sectional area occupied by RNFL decreased significantly with an increase in age. The effects of optic disc size, age, and NRA should be considered when the peripapillary RNFL thickness is evaluated.

Inter- and intraobserver variation in the analysis of optic disc images: comparison of the Heidelberg retina tomograph and computer assisted planimetry

AIMS

The development of imaging and measurement techniques has brought the prospect of greater objectivity in the measurement of optic disc features, and therefore better agreement between observers. The purpose of this study was to quantify and compare the variation between observers using two measurement devices.

METHODS

Optic disc photographs and images from the Heidelberg retina tomograph (HRT) of 30 eyes of 30 subjects were presented to six observers for analysis, and to one observer on five separate occasions. Agreement between observers was studied by comparing the analysis of each observer with the median result of the other five, and expressed as the mean difference and standard deviation of differences between the observer and the median. Inter- and intraobserver variation was calculated as a coefficient of variation (mean SD/mean x 100).

RESULTS

For planimetry, agreement between observers was dependent on observer experience, for the HRT it was independent. Agreement between observers (SD of differences as a percentage of the median) for optic disc area was 4.0% to 7.2% (planimetry) and 3.3% to 6.0% (HRT), for neuroretinal rim area it was 10.8% to 21.0% (planimetry) and 5.2% to 9.6% (HRT). The mean interobserver coefficient of variation for optic disc area was 8.1% (planimetry) and 4.4% (HRT), for neuroretinal rim area it was 16.3% (planimetry) and 8.1% (HRT), and (HRT only) for rim volume was 16.3%, and reference height 9.1%. HRT variability was greater for the software version 1.11 reference plane than for version 1.10. The intraobserver coefficient of variation for optic disc area was 1.5% (planimetry) and 2.4% (HRT), for neuroretinal rim area it was 4.0% (planimetry) and 4.5% (HRT).

CONCLUSIONS

Variation between observers is greatly reduced by the HRT when compared with planimetry. However, levels of variation, which may be clinically significant, remain for variables that depend on the subjective drawing of the disc margin.

Measurement of optic disc size: equivalence of methods to correct for ocular magnification

AIMS

To compare methods available to correct the magnification of images that result from the optics of the eye and identify errors, and source of error, of the methods.

METHODS

11 methods were applied to ocular biometry data from three independent cohorts. Each method was compared with the method of Bennett, which uses most biometric data. The difference between each method and Bennett's is the "error" of the method. The relation between the error and axial length, ametropia, and keratometry was explored by linear regression analysis.

RESULTS

Methods using axial length had the lowest mean (+0.5 to +2.6%) and standard deviation (0.6 to 1.2%) of errors. Of methods using keratometry and ametropia only, the lowest mean (-1.4% to +4.4%) and standard deviation (2.9 to 4.3%) of errors was found for a new method described in this paper, and that used by the Heidelberg retina tomograph (HRT). The highest mean error (+2.2 to +7.1%) was found for Littmann's method. Littmann's correction was larger than the HRT's by 3.5 to 3.7%. The mean difference between the new and HRT methods and the "abbreviated axial length" method of Bennett is -1.3 to +2.0%. The error of the "keratometry and ametropia" methods is related to axial length.

CONCLUSIONS

Methods using axial length are most accurate. The abbreviated axial length method of Bennett differs little from more detailed calculations and is appreciably more accurate than methods using keratometry and ametropia alone. If axial length is unknown, the new and the HRT methods give results closest to the abbreviated axial length method.

Aging changes of the optic nerve head in relation to open angle glaucoma

AIMS

To determine the age related changes in optic nerve head structure in a group of normal subjects and assess the significance of any changes in relation to those found in open angle glaucoma.

METHODS

A group of 88 white volunteers and friends and spouses of patients with a normal visual field and normal intraocular pressure was studied. Two different imaging and measurement devices were used (computer assisted planimetry and scanning laser ophthalmoscopy), and the results from each were compared. Measurements were made of the optic disc, optic cup, and neuroretinal rim areas, and the vertical optic disc diameter and cup/disc diameter ratio.

RESULTS

Neuroretinal rim area declined at the rate of between 0.28% and 0.39% per year. Vertical optic cup diameter and optic cup area increased with age. The mean cup/disc diameter ratio increased by about 0.1 between the ages of 30 and 70 years.

CONCLUSIONS

Age related changes are significant and measurable, and should be taken into account when assessing the glaucoma suspect, and when estimating the rate of progression of glaucomatous optic neuropathy in patients with established disease.

Comparison of methods to evaluate the optic nerve head and nerve fiber layer for glaucomatous change

PURPOSE

To compare the rates of optic nerve damage in early human glaucoma as measured by four methods to evaluate change in the optic nerve and nerve fiber layer.

METHODS

Four techniques were used to detect progressive glaucomatous damage in a prospective, longitudinal study: (1) qualitative evaluation of stereoscopic color optic disk photographs, (2) qualitative evaluation of monochromatic nerve fiber layer photographs, (3) manual stereoplanimetric measurements of disk rim area, and (4) computerized measurement of peripapillary nerve fiber layer height. One eye of each patient with glaucoma or ocular hypertension was evaluated at the beginning and end of a follow-up period of not less than one year. The rates of structural change measured by these techniques and the rate of visual field change measured with threshold automated perimetry were determined.

RESULTS

We followed up 193 patients for a mean (+/- S.D.) of 3.3 +/- 1.0 years (range, one to six years). Twenty-nine (15%) of 193 eyes progressed by qualitative optic disk evaluation, 14 (7.2%) of 193 eyes progressed by qualitative nerve fiber layer evaluation, seven (3.6%) of 193 eyes progressed by stereoplanimetry, and 24 (13.2%) of 182 eyes progressed by measurement of nerve fiber layer height. Visual field deterioration was detected in 12 (5.2%) of 193 patients and correlated best with qualitative optic disk and nerve fiber layer evaluations. Evaluation by stereoplanimetry and nerve fiber layer height measurement detected change in eyes with primarily diffuse structural damage, a pattern not well detected by qualitative methods.

CONCLUSION

Both qualitative and quantitative methods of optic disk and nerve fiber layer evaluation contribute to the identification of progressive damage, depending on the stage of disease and the characteristics of optic nerve cupping.

Comparison of clinician judgment with digitized image analysis in the detection of induced optic disk change in monkey eyes

PURPOSE

To compare the ability of clinicians to detect change in the photographic appearance of the optic disk with the performance of a system for digitized image analysis.

METHODS

In 11 monkey eyes, a Topcon Imagenet System was used to acquire eight digitized image pairs and four stereoscopic photographs at an intraocular pressure of 10 mm Hg, and then, again, 45 minutes after intraocular pressure was increased to 45 mm Hg. We recently reported detection of global (ten of 11 eyes) and regional (11 of 11 eyes) change in the digitized images of these eyes by using two new statistical strategies for optic disk analysis. For the current study, we evaluated the ability of three clinicians (the authors) to detect a change within the stereoscopic photographs of these 11 optic disks. For each eye, the eight stereoscopic photographs (four at intraocular pressure of 10 mm Hg and four at intraocular pressure of 45 mm Hg) were developed as stereoscopic slides and arranged into four pairs (10/10, 45/45, 10/45, and 45/10 mm Hg). Thus, two pairs represented no change in intraocular pressure (10/10 and 45/45 mm Hg) and the other two pairs represented either an increase or a decrease in intraocular pressure (10/45 and 45/10 mm Hg). The 44 pairs of stereoscopic slides (four pairs for each of 11 eyes) were masked then randomly mixed. On two separate occasions, each clinician evaluated each pair of stereoscopic slides for the presence of absence of optic disk change.

RESULTS

Reproducibility between the two readings of each clinician ranged from .50 to .64 (kappa statistic). Clinicians correctly detected change (as detected by image analysis) within 45% to 64% of the 10/45 and 45/10 pairs of stereoscopic slides. Clinicians correctly indentified no change within 86% to 100% of the 10/10- and 45/45-mm Hg pairs of stereoscopic slides. Clinicians correctly identified no change significantly more often than change (P < .01, chi 2 test). Change was not detected consistently by all three clinicians in any of the 11 eyes.

CONCLUSION

In a controlled experimental setting, digitized image acquisition with extensive secondary statistical analysis more sensitively detected small short-term changes in the surface of the optic disks of monkeys than did three masked clinicians.

Ganglion cell death in glaucoma: pathology recapitulates ontogeny

I review some improvements in our knowledge about the death of retinal ganglion cells in glaucoma during the last 20 years. These include the realisation that glaucoma damage precedes its detection by perimetry, the fact that the lamina cribrosa is a major site of axonal injury to ganglion cells, and the association between regional structure of optic nerve head connective tissue and the pattern of glaucoma damage. The selective susceptibility of larger retinal ganglion cells and its functional significance are described. Apoptosis is the mode of cell death in at least some ganglion cells in experimental glaucoma. This supports a theory that retrograde axonal transport failure leads to loss of trophic factor influence on ganglion cells, causing them to initiate their own suicide. As a consequence of this theory, two therapeutic avenues are suggested for prevention of glaucoma injury and cell death: delivery of trophic factors and manipulation of ganglion cell genetic expression of controlling influences over programmed cell death.

Histomorphometry of the optic nerves of normal dogs and dogs with hereditary glaucoma

The beagle dog with hereditary primary open-angle glaucoma, unlike other animal models of human glaucoma, possesses a slowly progressive, sustained elevation of intraocular pressure. The effects of this insidious elevation in intraocular pressure on the axons of the optic nerves of three beagles at early stages of glaucoma and two beagles with advanced signs of glaucoma were compared to the optic nerves of four age-matched normal dogs. Plastic embedded optic nerve cross-sections (1 micron) 1 mm posterior to the lamina cribrosa were osmicated and stained with Toluidine Blue. Axons from 0.2 to > 2.0 microns in diameter were counted and measured in 16 cross-sectional regions of equal size within the whole optic nerve using a computerized image analysis system. The mean optic nerve axon diameters in the normal, early glaucomatous, and advanced glaucomatous dogs were 1.53, 1.25 and 1.13 microns respectively. The average total optic nerve axon count in the normal dogs was 148,303. Approximately 16% of the total axonal fibers were counted in each nerve. The counts of optic nerve axons 2.0 microns or greater in diameter were reduced by up to 60% in the central regions of the optic nerves of affected beagles. The large diameter axons of the peripheral optic nerve of the beagle dogs with glaucoma were more resistant to the elevated intraocular pressure. The counts of axons > 0.6 to 0.8 micron in diameter were significantly increased in glaucomatous beagles.

Treatment and prognosis of secondary glaucoma in Fuchs' heterochromic iridocyclitis

After reviewing the records of 111 patients with Fuchs' heterochromic iridocyclitis, we studied the therapy and prognosis of secondary glaucoma in 30 of these 111 patients (27%) who had glaucoma or could be considered glaucoma suspects. Maximal medical therapy was unsuccessful in 22 of the 30 patients (73%). Surgical intervention (mostly trabeculectomies, half with 5-fluorouracil) successfully controlled intraocular pressure (< or = 21 mm Hg with or without medication) in 13 of the 18 operated-on patients (72%) after a mean follow-up of 26 months. All successfully operated-on patients retained a visual acuity of 20/80 or better. We had favorable results, possibly because of modern surgical techniques (use of 5-fluorouracil, sodium hyaluronate) or earlier surgical intervention, or both.