Probing visual function with psychophysics and photochemistry

Use of high spatial resolution perimetry to identify scotomata not apparent with conventional perimetry in the nasal field of glaucomatous subjects

AIM

To examine whether high spatial resolution perimetry (HSRP) could identify fine scale scotomata which may not be apparent with conventional perimetry. The HSRP was performed in the nasal field, as this location is a recognised site for the early occurrence of glaucomatous defects.

METHOD

16 early glaucoma eyes, 17 glaucoma suspect eyes, and 20 age matched healthy control eyes underwent conventional automated perimetry using the 24-2 program of the Humphrey field analyser (HFA) and HSRP. The HSRP was performed in the nasal field by testing 9 x 9 degrees of 100 tested points separated by 1 degree and the results compared with the HFA 24-2 program.

RESULTS

Mean HSRP thresholds were significantly abnormal in the suspect and glaucoma eyes, with elevated levels of asymmetry between the superior and inferior nasal field. Overall, 7/17 (41%) suspect eyes (95% confidence interval 5/17 (29%) to 7/17 (41%)) had nasal scotomata on HSRP, although their HFA 24-2 fields failed to identify any defects. In glaucomatous eyes, 15/16 (94%) eyes had HSRP scotomata (95% CI 14/16 (88%) to 15/16 (94%)). In 12 these coexisted with HFA 24-2 defects at the same location, while in three eyes only HSRP identified scotomata in the nasal field.

CONCLUSION

HSRP can identify scotoma in glaucomatous eyes in the nasal field which may be missed with the lower spatial resolution of conventional perimetry.

Abnormal motion displacement thresholds are associated with fine scale luminance sensitivity loss in glaucoma

This study tests the hypothesis that abnormal motion displacement thresholds coexist with scotomas on a finer spatial scale than is measurable by conventional Humphrey perimetry. Eighteen patients with primary open angle glaucoma in one eye, and 18 age matched normal controls underwent motion displacement threshold testing and high spatial resolution perimetry. The motion displacement thresholds were significantly elevated in the glaucoma eyes, in 73% this exceeded normal limits. Ten glaucoma eyes had normal Humphrey 24-2 field nearest the motion test site: of these seven had abnormally elevated motion displacement thresholds and six had fine scale threshold depressions detected with high spatial resolution perimetry. This result suggests that glaucomatous elevations of motion displacement threshold may be present in areas of normal Humphrey 24-2 field, and this may coexist with measurable scotomas beyond the resolution of conventional Humphrey perimetry in some, but not all patients.

High spatial resolution automated perimetry in glaucoma

BACKGROUND

Automated perimetry is of fundamental importance in assessing visual function in glaucoma. A technique was evaluated to perform high spatial resolution automated perimetry to allow a more detailed assessment of the luminance sensitivity in selected regions of the visual field than is possible with conventional perimetry.

METHOD

High spatial resolution perimetry was performed using a Humphrey automated perimeter by measuring luminance sensitivity across a 9 by 9 degree custom grid of 100 test locations with a separation between adjacent locations of 1 degree. Quantitative analysis of the raw and Gaussian filtered thresholds was performed to assess the repeatability of the technique in normals, glaucoma suspects, and glaucoma patients.

RESULTS

The testing protocol was well tolerated by all subjects. High spatial resolution perimetry in glaucomatous eyes demonstrated fine luminance sensitivity loss not suspected with conventional perimetry. High spatial resolution perimetry also demonstrated reproducible areas of sensitivity loss in some glaucomatous eyes in areas of the visual field which appear normal with conventional programmes. The repeatability of the technique correlated with mean threshold sensitivity and was substantially improved to clinically acceptable levels by Gaussian filtering the thresholds.

CONCLUSION

This technique of high spatial resolution perimetry allows the practical assessment of selected regions of the visual field at higher resolution than conventional perimetry, and may be clinically useful in glaucoma.

Image processing of computerised visual field data

BACKGROUND

Computerised perimetry is of fundamental importance in assessing visual function. However, visual fields are subject to patient response variability which limits the detection of true visual loss.

METHODS

A method of improving the repeatability of visual field data was demonstrated by applying techniques used in image processing. An illustrative sample of nine normals and nine patients with field loss was used. Two successive Humphrey fields were selected for each subject. Repeatability was defined as the standard deviation of the pointwise differences between sensitivity values of the reference field and repeat field. The field data were then separately subjected to Gaussian and median image processing filters and the repeatability was compared with the unprocessed field results.

RESULTS

Improvement in repeatability, by a factor of approximately 2, was demonstrated by both processes.

CONCLUSION

These techniques may improve the reliable detection of loss of visual function using computerised perimetry.