Determining mechanisms of visual loss in glaucoma using Rarebit perimetry.

PURPOSE

Evidence for ganglion cell visual dysfunction in human glaucoma is often indirect, being either measured at the cellular level in animal models or being inferred from the pooled responses of a large number of ganglion cells in human observers. Rarebit perimetry (RBP) uses repeated, intense (150 cd/m2) stimuli-whose size is close to the spatial scale of a ganglion cell-to search for small retinal areas with zero sensitivity. Decreasing the stimulus luminance to 64 cd/m2 in normal observers does not alter the percentage of RBP stimuli detected [the mean hit rate (MHR)], and so we hypothesized that a similar response robustness should occur in glaucoma if the elements detecting the RBP target show no signs of visual dysfunction.

METHODS

Nineteen glaucoma subjects and 19 age-matched controls were tested with a customized RBP test at 13 stimulus luminances (10 to 150 cd/m2, 0.14 log unit intervals). A four-parameter (threshold, spread, false positive proportion, and miss rate) cumulative Gaussian psychometric function was fitted to the response rate data from a glaucoma-affected region (glaucoma subjects; MHR >50% and <80%) and from the corresponding region in an age-matched normal control. Our hypothesis would predict that only the miss rates should differ between groups.

RESULTS

Glaucoma subjects showed significantly higher miss rates (0.18 vs. 0.04, p < 0.001), lower false positive proportions (0.009 vs. 0.025, p = 0.004), greater spreads (0.30 vs. 0.19, p = 0.002), and elevated thresholds [1.57 log(cd/m2) vs. 1.13 log(cd/m2), p < 0.001].

CONCLUSIONS

Responses to RBP stimuli are not robust to decreasing luminances in glaucoma. Our results more directly imply the presence of ganglion cell visual dysfunction in human glaucoma than studies using larger targets where contrast sensitivity losses could result through ganglion cell death alone. Such dysfunction may not be detected by Rarebit's MHR given that dysfunctional elements may still respond to the very intense RBP stimulus.