The effect of refractive blur on the visual field using the ring perimeter

Are high-pass resolution perimetry thresholds sampling limited or optically limited?

PURPOSE

It has been reported that high-pass resolution perimetry (HRP) provides a means of noninvasively determining retinal ganglion cell density. However, there is evidence to suggest that this may not be true. The purpose of the present study was to determine whether HRP thresholds are sampling limited, which is a necessary condition for being able to determine retinal ganglion cell density psychophysically.

METHODS

This study measured resolution and detection performance for a range of grating-based stimuli under the testing conditions that HRP uses and compared these with performance of the ring stimulus.

RESULTS

The results show that detection and resolution acuity under HRP test conditions were often equivalent, in accordance with previous investigations. However, the results also show that the thresholds underestimated the true level of resolution acuity in the periphery because increasing stimulus contrast increased performance.

CONCLUSION

These findings suggest that HRP thresholds cannot be regarded as sampling limited, but rather they are optically limited. We therefore conclude that HRP thresholds cannot be regarded as a direct measure of the underlying ganglion cell density.

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.

Properties of high-pass resolution perimetry targets

Relationships between high-pass resolution perimetry ring targets and conventional perimetry and acuity targets were explored by optical analytical techniques and by comparative measurements in normal humans. High-pass resolution and acuity targets produced closely proportional resolution measurements, showing that the critical ring detail is the width of the bright core. High-pass resolution and conventional perimetry thresholds were not equally well correlated. From a purely optical point of view, high-pass resolution targets appeared to lose somewhat more contrast on defocusing. On the other hand, high-pass resolution appears to show less variability in practical, clinical use.

High-pass resolution perimetry. A clinical review

Review of all available reports comparing high-pass resolution perimetry (HRP) and conventional perimetry in normals and in subjects with different visual disorders reveals closely comparable aspects of sensitivity, specificity, and reliability. HRP shows important advantages concerning variability, test duration, and subject preferences. Drawbacks seem largely limited to somewhat loose renditions of visual field defects of small area and large depth. Otherwise HRP's novel format of graphic result presentation may be better suited to visual evaluation than conventional gray-scale maps with their lumping of thresholds and extensive interpolations. Several examples are provided of visual field defects due to various lesions throughout the visual system.

The effect of lens opacity on the resolution visual field in normal subjects

The sensitivity of the visual field declines with age. The density of the crystalline lens increases with age and may be partly responsible for the decline in sensitivity. To determine whether lens opacity influences the mean threshold of the visual field, 82 normal subjects were enrolled. All had their lens opacity measured with the Interzeag Lens Opacity Meter 701 and the visual field assessed with the Ring Resolution Perimeter. Lens opacity was highly correlated with age, while mean visual field threshold was also correlated with age but showed much greater variation. Multiple regression failed to show any influence of lens opacity measurements on the mean field threshold after age had been accounted for.

Effect of intermittent versus continuous patient monitoring on reliability indices during automated perimetry

PURPOSE

This prospective, randomized study was undertaken to determine the effect of intermittent versus continuous patient monitoring on reliability indices (fixation losses, false-positive errors, and false-negative errors) during automated static perimetry.

METHODS

A practice Humphrey Program C30-2 visual field was administered to 169 subjects (mean age +/- standard deviation, 56 +/- 18 years) for 1.5 minutes, during which time trained technicians continuously monitored all subjects and assessed patient fixation as noted on the eye monitor. The computer-generated reliability indices during the 1.5 minute practice test were recorded. After completion of the practice test, the 169 eyes were randomized to either intermittent or continuous monitor for the permanent C30-2 test. During intermittent monitoring, the technician returned periodically to the examination room to assess the subject's performance. For continuous monitoring, the technician continually assessed the subject's performance while remaining in the examination room throughout the test duration.

RESULTS

The mean number of visits per visual field test during intermittent monitoring was 4.0, and mean test duration was 15.8 minutes. There was no difference in the mean deviation, pattern standard deviation, and short-term fluctuation (P = 0.85, 0.98, and 0.41, respectively) of the visual fields for intermittent and continuous monitoring, suggesting similar diffuse depression and localized defects in the visual fields for each group. The mean fixation losses (6.9%), false-positive errors (1.8%), and false-negative errors (5.8%) for the intermittently monitored group were not different from the mean fixation losses (7.8%), false-positive errors, (3.1%) and false-negative errors (5.0%) for the continuously monitored group (P = 0.40, 0.24, and 0.36, respectively).

CONCLUSION

Although recommended, continuous monitoring does not appear necessary for all patients undergoing automated perimetry. Error-free reliability indices combined with the technician's judgment of patient reliability during the first 1.5 minutes of C30-2 testing may be a guideline for choosing intermittent monitoring.

The unidirectionality of cerebral polyopia

Polyopia, visual perseveration in space, has been associated with seizure activity, afterimage formation, and the presence of visual field defects. It can be interpreted both as a positive and a negative visual phenomenon. A patient with polyopia associated with the acute onset of hemianopsia is presented. The phenomenon has been investigated objectively with a simple procedure. The polyopia was highly correlated to movement of the eyes into the hemianoptic visual field and to increased contrast but not duration of the stimulus. This type of polyopia could be the result of incomplete visual processing due to poor visuospatial localization in a hemianopic field.

High-pass resolution perimetry in eyes with ocular hypertension and primary open-angle glaucoma

We analyzed the results of high-pass resolution perimetry on normal eyes, eyes with ocular hypertension, and eyes with primary open-angle glaucoma. The subjects were matched for age and lens density. We controlled for refraction, visual acuity, pupil size, miotic medications, and learning effects. Under these conditions the glaucomatous eyes showed a significant reduction in overall resolution threshold compared to both normal (P less than or equal to .01) and hypertensive eyes (P less than .01). The hypertensive eyes were not significantly different from normal. Standard increment threshold fields and high-pass resolution visual fields had an agreement of abnormality of 67%. Resolution perimetry showed a 92% agreement with standard perimetry in the location of the defect, when the defect was apparent on both tests. These results indicated that resolution perimetry may be useful for diagnosis and management of primary open-angle glaucoma.