A Review of Current Technology Used in Evaluating Visual Function in Glaucoma

Visual function tests for glaucoma practice - What is relevant?

Glaucoma represents one of the most important ocular diseases causing irreversible ganglion cell death. It is one of the most common causes of visual impairment and morbidity in the elderly population. There are various tests for measuring visual function in glaucoma. While visual field remains the undisputed method for screening, diagnosis, and monitoring disease progression, other tests have been studied for their utility in glaucoma practice. This review discusses some of the commonly used tests of visual function that can be routinely used in clinics for glaucoma management. Among the various modalities of testing visual function in glaucoma, this review highlights the tests that are most clinically relevant.

Trends in Utilization of Visual Field Tests for Glaucoma Patients: A Nationwide Study Using the Korean Health Insurance Review and Assessment Database

Purpose

To analyze 10-year trends in utilization of visual field tests for adult glaucoma or glaucoma-suspect patients using the Korean Health Insurance Review and Assessment data.

Methods

Health claims for the years 2010 to 2019, as recorded via Korea’s Health Insurance Review and Assessment service, were accessed. We identified glaucoma patients using the glaucoma diagnostic codes H40 (glaucoma) and H42 (glaucoma in other diseases classified elsewhere). For verification of the glaucoma diagnosis, information on any antiglaucoma medication prescriptions and ocular surgery history also was obtained. Visual field testing data was isolated using procedural codes E6690 (kinetic perimetry) and E6691 (standard automated perimetry [SAP]) performed in tertiary hospitals. Any changes in visual field test utilization were identified using regression trend analysis.

Results

From 2010 to 2019, the total number of SAP procedures performed in tertiary hospitals for either glaucoma or glaucoma-suspect patients increased gradually from 93,459 to 216,433. With regard to kinetic perimetry examinations, the total number decreased gradually from 6,364 to 3,792. The yearly average SAP number per patient showed a slight increase, from 1.168 to 1.248 (β = 0.008, R² = 0.669, p = 0.004). Meanwhile, the yearly average number of kinetic perimeter examinations per patient showed a significant decrease, from 1.093 to 0.940 (β = −0.013, R² = 0.580, p = 0.010).

Conclusions

Between 2010 and 2019, the yearly average number of SAP procedures performed per glaucoma or glaucoma-suspect patient increased in Korea. Meanwhile, the yearly average number of kinetic perimetry examinations per patient significantly decreased.

Three-dimensional Neuroretinal Rim Thickness and Visual Fields in Glaucoma: A Broken-stick Model

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Precis:

In open-angle glaucoma, when neuroretinal rim tissue measured by volumetric optical coherence tomography (OCT) scans is below a third of the normal value, visual field (VF) damage becomes detectable.

Purpose:

To determine the amount of neuroretinal rim tissue thickness below which VF damage becomes detectable.

Methods:

In a retrospective cross-sectional study, 1 eye per subject (of 57 healthy and 100 open-angle glaucoma patients) at an academic institution had eye examinations, VF testing, spectral-domain OCT retinal nerve fiber layer (RNFL) thickness measurements, and optic nerve volumetric scans. Using custom algorithms, the minimum distance band (MDB) neuroretinal rim thickness was calculated from optic nerve scans. “Broken-stick” regression was performed for estimating both the MDB and RNFL thickness tipping-point thresholds, below which were associated with initial VF defects in the decibel scale. The slopes for the structure-function relationship above and below the thresholds were computed. Smoothing curves of the MDB and RNFL thickness covariates were evaluated to examine the consistency of the independently identified tipping-point pairs.

Results:

Plots of VF total deviation against MDB thickness revealed plateaus of VF total deviation unrelated to MDB thickness. Below the thresholds, VF total deviation decreased with MDB thickness, with the associated slopes significantly greater than those above the thresholds (P<0.014). Below 31% of global MDB thickness, and 36.8% and 43.6% of superior and inferior MDB thickness, VF damage becomes detectable. The MDB and RNFL tipping points were in good accordance with the correlation of the MDB and RNFL thickness covariates.

Conclusions:

When neuroretinal rim tissue, characterized by MDB thickness in OCT, is below a third of the normal value, VF damage in the decibel scale becomes detectable.

Analysis of Neuroretinal Rim by Age, Race, and Sex Using High-Density 3-Dimensional Spectral-Domain Optical Coherence Tomography

Précis:

Neuroretinal rim minimum distance band (MDB) thickness is significantly lower in older subjects and African Americans compared with whites. It is similar in both sexes.

Purpose:

To evaluate the relationship between age, race, and sex with the neuroretinal rim using high-density spectral-domain optical coherence tomography optic nerve volume scans of normal eyes.

Methods:

A total of 256 normal subjects underwent Spectralis spectral-domain optical coherence tomography optic nerve head volume scans. One eye was randomly selected and analyzed for each subject. Using custom-designed software, the neuroretinal rim MDB thickness was calculated from volume scans, and global and quadrant neuroretinal rim thickness values were determined. The MDB is a 3-dimensional neuroretinal rim band comprised of the shortest distance between the internal limiting membrane and the termination of the retinal pigment epithelium/Bruch’s membrane complex. Multiple linear regression analysis was performed to determine the associations of age, race, and sex with neuroretinal rim MDB measurements.

Results:

The population was 57% female and 69% white with a mean age of 58.4±15.3 years. The mean MDB thickness in the normal population was 278.4±47.5 µm. For this normal population, MDB thickness decreased by 0.84 µm annually (P<0.001). African Americans had thinner MDBs compared with whites (P=0.003). Males and females had similar MDB thickness values (P=0.349).

Conclusion:

Neuroretinal rim MDB thickness measurements decreased significantly with age. African Americans had thinner MDB neuroretinal rims than whites.

The Effect of Simulated Visual Field Loss on Optokinetic Nystagmus

Purpose

Assessment of functional vision across the visual field is hampered by a reliance on patients' subjective judgement of the presence of a stimulus, and the accompanying demands (time and attention) this places on them. As a first step toward determining whether an objective measure of an involuntary eye movement (optokinetic nystagmus [OKN]) could provide an objective measure of field loss, we determined how various measures of OKN depend on the extent of simulated visual field loss (SVFL).

Methods

We used infrared eye-tracking to measure the eye movements of 16 healthy participants viewing horizontally translating 2-dimensional noise patterns over trials of varying contrasts and different levels of SVFL. We quantified the strength of OKN by estimating the velocity of tracking eye movements compared to the stimulus (OKN gain). These measurements were made using an open-loop SVFL paradigm, where a varying amount of gaze-contingent peripheral stimuli was occluded.

Results

Full-field stimulation led to an average OKN gain of 0.92 ± 0.15. This value fell steadily with increasing SVFL to a value of 0.38 ± 0.20 when the periphery was not stimulated at all (i.e., the stimulus was a 5-deg. diameter foveal patch). We note considerable individual variation in OKN gain in all conditions.

Conclusions

Measuring the extent of visual field loss using an objective measure of OKN gain is feasible.

Translational Relevance

Simulated visual field loss reduces optokinetic nystagmus, but further refinement of this technique would be required to overcome individual differences and to pick up clinically relevant field defects.

Chromatic Multifocal Pupillometer for Objective Perimetry and Diagnosis of Patients with Retinitis Pigmentosa

PURPOSE

To assess visual field (VF) defects and retinal function objectively in healthy participants and patients with retinitis pigmentosa (RP) using a chromatic multifocal pupillometer.

DESIGN

Cross-sectional study.

PARTICIPANTS

The right eyes of 16 healthy participants and 13 RP patients.

METHODS

Pupil responses to red and blue light (peak, 485 and 625 nm, respectively) presented by 76 light-emitting diodes, 1.8-mm spot size at different locations of a 16.2° VF were recorded. Subjective VFs of RP patients were determined using chromatic dark-adapted Goldmann VFs (CDA-GVFs). Six healthy participants underwent 2 pupillometer examinations to determine test-retest reliability.

MAIN OUTCOME MEASURES

Three parameters of pupil contraction were determined automatically: percentage of change of pupil size (PPC), maximum contraction velocity (MCV; in pixels per second), and latency of MCV (LMCV; in seconds). The fraction of functional VF was determined by CDA-GVF.

RESULTS

In healthy participants, higher PPC and MCV were measured in response to blue compared with red light. The LMCV in response to blue light was relatively constant throughout the VF. Healthy participants demonstrated higher PPC and MCV and shorter LMCV in central compared with peripheral test points in response to red light. Test-retest correlation coefficients were 0.7 for PPC and 0.5 for MCV. In RP patients, test point in which the PPC and MCV were lower than 4 standard errors from the mean of healthy participants correlated with areas that were indicated as nonseeing by CDA-GVF. The mean absolute deviation in LMCV parameter in response to the red light between different test point was significantly higher in RP patients (range, 0.16-0.47) than in healthy participants (range, 0.02-0.16; P < 0.0001) and indicated its usefulness as a diagnostic tool with high sensitivity and specificity (area under the receiver operating characteristic curve (AUC), 0.97, Mann-Whitney-Wilcoxon analysis). Randomly reducing the number of test points to a total of 15 points did not significantly reduce the AUC in RP diagnosis based on this parameter.

CONCLUSIONS

This study demonstrates the feasibility of using a chromatic multifocal pupillometer for objective diagnosis of RP and assessment of VF defects.

Correlation of retinal nerve fiber layer thickness and visual fields in glaucoma: a broken stick model

PURPOSE

To determine the retinal nerve fiber layer (RNFL) thickness at which visual field (VF) damage becomes detectable and associated with structural loss.

DESIGN

Retrospective cross-sectional study.

METHODS

Eighty-seven healthy and 108 glaucoma subjects (1 eye per subject) were recruited from an academic institution. All patients had VF examinations (Swedish Interactive Threshold Algorithm 24-2 test of the Humphrey Visual Field Analyzer 750i) and spectral-domain optical coherence tomography RNFL scans. Comparison of RNFL thickness values with VF threshold values showed a plateau of VF threshold values at high RNFL thickness values and then a sharp decrease at lower RNFL thickness values. A broken stick statistical analysis was used to estimate the tipping point at which RNFL thickness values are associated with VF defects. The slope for the association between structure and function was computed for data above and below the tipping point.

RESULTS

The mean RNFL thickness value that was associated with initial VF loss was 89 μm. The superior RNFL thickness value that was associated with initial corresponding inferior VF loss was 100 μm. The inferior RNFL thickness value that was associated with initial corresponding superior VF loss was 73 μm. The differences between all the slopes above and below the aforementioned tipping points were statistically significant (P < .001).

CONCLUSIONS

In open-angle glaucoma, substantial RNFL thinning or structural loss appears to be necessary before functional visual field defects become detectable.

Analysis of normal retinal nerve fiber layer thickness by age, sex, and race using spectral domain optical coherence tomography

PURPOSE

To determine the effects of age, sex, and race on the retinal nerve fiber layer (RNFL) in the normal human eye as measured by the spectral domain optical coherence tomography (SD-OCT) Spectralis machine (Heidelberg Engineering).

METHODS

Peripapillary SD-OCT RNFL thickness measurements were determined in normal subjects seen at a university-based clinic. One randomly selected eye per subject was used for analysis in this cross-sectional study. Multiple regression analysis was applied to assess the effects of age, sex, ethnicity, and mean refractive error on peripapillary RNFL thickness. Results are expressed as means±SD wherever applicable.

RESULTS

The study population consisted of 190 healthy participants from 9 to 86 years of age. Of the 190 participants, 62 (33%) were men, 125 (66%) Caucasians, 26 (14%) African Americans, 14 (7%) Hispanics, 16 (8%) Asians, and 9 (5%) other races. The mean RNFL thickness for the normal population studied was 97.3 ± 9.6 µm. Normal RNFL thickness values follow the ISNT rule with decreasing RNFL thickness values starting from the thickest quadrant inferiorly to the thinnest quadrant temporally: inferior quadrant (126 ± 15.8), superior quadrant (117.2±16.13), nasal quadrant (75 ± 13.9), and temporal quadrant (70.6 ± 10.8 µm). Thinner RNFL measurements were associated with older age (P<0.001); being Caucasian, versus being either Hispanic or Asian (P=0.02 and 0.009, respectively); or being more myopic (P<0.001). For every decade of increased age, mean RNFL thickness measured thinner by approximately 1.5 µm (95% confidence interval, 0.24-0.07). Comparisons between ethnic groups revealed that Caucasians had mean RNFL values (96 ± 9.2 µm) slightly thinner than those of Hispanics (102.9 ± 11 µm; P=0.02) or Asians (100.7 ± 8.5 µm; P=0.009). African Americans RNFL values (99.2 ± 10.2 µm) were not significantly different when compared with Caucasians. There was no relationship between RNFL thickness and sex.

CONCLUSIONS

The thickest RNFL measurements were found in the inferior quadrant, followed by the superior, nasal, and temporal quadrants (ISNT rule applied to the RNFL). Thinner RNFL measurements were associated with older age and increasing myopia. Caucasians tend to have thinner RNFL values when compared with Hispanics and Asians. SD-OCT analysis of the normal RNFL showed results similar to time domain OCT studies.

The East London glaucoma prediction score: web-based validation of glaucoma risk screening tool

AIM

It is difficult for Optometrists and General Practitioners to know which patients are at risk. The East London glaucoma prediction score (ELGPS) is a web based risk calculator that has been developed to determine Glaucoma risk at the time of screening. Multiple risk factors that are available in a low tech environment are assessed to provide a risk assessment. This is extremely useful in settings where access to specialist care is difficult. Use of the calculator is educational. It is a free web based service. Data capture is user specific.

METHOD

The scoring system is a web based questionnaire that captures and subsequently calculates the relative risk for the presence of Glaucoma at the time of screening. Three categories of patient are described: Unlikely to have Glaucoma; Glaucoma Suspect and Glaucoma. A case review methodology of patients with known diagnosis is employed to validate the calculator risk assessment.

RESULTS

Data from the patient records of 400 patients with an established diagnosis has been captured and used to validate the screening tool. The website reports that the calculated diagnosis correlates with the actual diagnosis 82% of the time. Biostatistics analysis showed: Sensitivity = 88%; Positive predictive value = 97%; Specificity = 75%.

CONCLUSION

Analysis of the first 400 patients validates the web based screening tool as being a good method of screening for the at risk population. The validation is ongoing. The web based format will allow a more widespread recruitment for different geographic, population and personnel variables.

'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.