Pattern discrimination perimetry in patients with glaucoma and ocular hypertension

Open angle glaucoma effects on preattentive visual search efficiency for flicker, motion displacement and orientation pop-out tasks

BACKGROUND/AIM

Preattentive visual search (PAVS) describes rapid and efficient retinal and neural processing capable of immediate target detection in the visual field. Damage to the nerve fibre layer or visual pathway might reduce the efficiency with which the visual system performs such analysis. The purpose of this study was to test the hypothesis that patients with glaucoma are impaired on parallel search tasks, and that this would serve to distinguish glaucoma in early cases.

METHODS

Three groups of observers (glaucoma patients, suspect and normal individuals) were examined, using computer-generated flicker, orientation, and vertical motion displacement targets to assess PAVS efficiency. The task required rapid and accurate localisation of a singularity embedded in a field of 119 homogeneous distractors on either the left or right-hand side of a computer monitor. All subjects also completed a choice reaction time (CRT) task.

RESULTS

Independent sample T tests revealed PAVS efficiency to be significantly impaired in the glaucoma group compared with both normal and suspect individuals. Performance was impaired in all types of glaucoma tested. Analysis between normal and suspect individuals revealed a significant difference only for motion displacement response times. Similar analysis using a PAVS/CRT index confirmed the glaucoma findings but also showed statistically significant differences between suspect and normal individuals across all target types.

CONCLUSIONS

A test of PAVS efficiency appears capable of differentiating early glaucoma from both normal and suspect cases. Analysis incorporating a PAVS/CRT index enhances the diagnostic capacity to differentiate normal from suspect cases.

Glaucoma: squaring the psychophysics and neurobiology

Advances in our understanding of the pathophysiology of retinal ganglion cell death in glaucoma are providing important insights into the functional changes occurring in retinal ganglion cells in the early stages of the disease. These exciting new findings may help us develop psychophysical tests to monitor early retinal ganglion cell damage, possibly before neurons are committed to the process of cell death.

Quantitative morphologic and functional evaluation of the optic nerve head in chronic open-angle glaucoma

Glaucoma leads to morphologic changes of the optic nerve head and to functional defects. Morphologic changes in the three-dimensional surface structure of the optic nerve head at its entrance site into the globe can be examined by laser scanning tomography. The standard technique for evaluating functional defects in glaucoma is static computerized perimetry. We compared these two techniques to determine which is more sensitive for follow-up of glaucomatous damage of the optic nerve head. If decreased function is presumed to precede imminent cell death, visual field analysis should be the more sensitive method, as cell death results in absolute defects of the visual field. However, the neuronal networks do not necessarily function in this way. In the case of loss of individual elements in the neuronal network, the complex linkages, even at the retinal level, are able to maintain functions and compensate for loss of function, which means that visual field defects would not be prominent. If the damage increases with time and is accompanied by a progressive loss of ganglion cells, however, compensation is eventually no longer possible, and the functional defects then become measurable by visual field analysis. Thus, morphologic absolute changes may be more prominent than visual field defects in the early stages of glaucoma. To evaluate the quantitative relationship between morphometrically measurable defects of the optic nerve head and measurable functional defects, we first examined the visual field with static computerized perimetry and then evaluated the surface structure with a laser scanning tomograph in 90 patients with chronic open-angle glaucoma, 10 patients with ocular hypertension, and 10 patients without any eye disease. Based on the 95th percentile of the standardized rim/disk area ratio, we calculated the relative rim area loss and correlated this with the mean defect in visual field analysis. The scatterplot shows an exponential curve. In the early stages of glaucoma, visual field defects were less prominent than morphologic absolute changes; 40% of the neuroretinal rim area is lost by glaucomatous optic nerve damage before first defects in visual field analysis appear. In late stages of glaucoma, changes in perimetry are more prominent than those observed with biomorphometry. These results show that in the follow-up of patients with early stages of glaucomatous damage, special attention should be given to morphologic absolute changes. In patients with advanced glaucoma, progress of the damage should be observed by repeated functional, rather than morphologic, examinations. It is important to keep in mind, however, that the sensitivity of any method is dependent on technology. One reason why functional tests may not be as sensitive as morphologic examination in observing patients with early stages of glaucoma may simply be that functional tests are not yet sensitive enough to detect early damage.

Newer visual function tests in the evaluation of glaucoma

Conventional visual field testing, with a uniform white-on-white stimulus, is used routinely to diagnose and follow patients with chronic open-angle glaucoma. Many investigators, however, believe that conventional perimetry may not detect the earliest visual dysfunction in patients with chronic open-angle glaucoma. Consequently, much research has been performed over the past decade to develop a visual function test which might diagnose chronic open-angle glaucoma earlier than conventional perimetry. This review discusses the mechanisms, clinical studies and the current usefulness of the most common new visual function techniques. These tests attempt to detect early glaucomatous visual loss, generally by placing the visual system under stress and by minimizing the influence of extensive functional redundancy in the retinal ganglion cell network due to widely overlapping receptive fields. Success of new visual function tests depends on the specificity and the sensitivity of the instrument, excellent patient acceptance, short test duration, standardization of techniques, and limited expense. In addition to searching for newer visual function tests to evaluate glaucoma, we should continue efforts to improve the diagnostic capability and shorten the test duration of conventional perimetry.

Anatomic features of the optic disc and risk of progression in ocular hypertension

We evaluated characteristics of the optic disc in ocular hypertensive patients which might portend future glaucomatous damage. We included in this study 12 patients with an intraocular pressure > 21 mmHg who on follow-up showed signs of optic nerve head damage. Each of these patients were matched to an ocular hypertensive patient who remained stable for > 5 years. This study found that neither optic disc, neural rim or peripapillary halo areas, vein or artery diameter over non-atrophied or atrophied retinal areas, differed significantly between groups (p > 0.05, Wilcoxon signed rank test). However, peripapillary atrophy was greater in patients who showed signs of glaucomatous disc damage (1.757 +/- 0.36 mm2, p = 0.02, Wilcoxon signed rank test) compared to stable patients (1.064 +/- 0.79 mm2), although this finding was not significant after the Bonferroni correction. Patients with < 0.6 mm2 area of peripapillary atrophy (n = 6), however, did not suffer damage. This study suggests that a relationship between the extent of peripapillary atrophy and the chance of developing optic disc damage in patients with ocular hypertension deserves further study.

Correlation of pattern discrimination perimetry to the optic disc and visual field in ocular hypertensive and chronic open-angle glaucoma patients

We evaluated 19 ocular hypertensive and 32 chronic open-angle glaucoma patients to determine the correlation of pattern discrimination field loss to known markers of glaucomatous damage on the visual field and the optic nerve head. This study found no statistical association of the findings on the pattern discrimination perimeter to the areas of the optic disc, peripapillary halo, peripapillary atrophy, neural rim, or cup/pallor discrepancy (P > 0.05). In addition, no statistical relationship was observed to the diameters of the largest vein or artery in the inferior- or superior-temporal quadrant adjacent to the optic disc (P > 0.05). Between pattern discrimination and automated perimetry no agreement was observed in any visual field cluster (Glaucoma Hemifield Test) greater than that expected by chance alone (P > 0.05). When both visual function tests disagreed, the proportion of abnormal diagnoses in any cluster did not differ statistically between tests in chronic open-angle glaucoma patients (P > 0.05). However, in ocular hypertensive patients a greater proportion of abnormal diagnoses was observed with pattern discrimination perimetry (P < 0.03). This study suggests that pattern discrimination perimetry appears to measure a different physiologic property of the retina than does automated perimetry.

Long-term and short-term fluctuation in pattern discrimination perimetry

We studied threshold fluctuation with the pattern discrimination perimeter in 24 healthy subjects at 56 locations within the central 30 degrees. This perimeter evaluates a subject's ability to discriminate a patterned stimulus measured by a percentage scale. We found an intraindividual fluctuation of 10.52% and an interindividual fluctuation of 8.78%. A statistically increased intraindividual, but not interindividual fluctuation was noted with increasing eccentricity from fixation (P less than .05, Bartlett's test). However, no correlation in fluctuation was noted with advancing age or increasing false-positive errors (P greater than .05, correlation coefficient). Also, no difference in fluctuation between superior or inferior hemifields was observed (P greater than .05, Student's t-test). The average threshold across all subjects was 54.3%, which provided an upper limit of normal, two standard deviations from the mean, of less than 80% for most locations. This study indicates that fluctuation should be considered when interpreting pattern discrimination fields, but that the extent of fluctuation generally allows for an adequate separation between normal and abnormal measurements.