COMPARISON OF SPATIAL THRESHOLDS AND INTENSITY THRESHOLDS IN GLAUCOMA

Spatial resolution perimetry and conventional perimetry measure different qualities of the functional performance of the eye. Theoretically, spatial resolution is directly related to the density of intact sensory units, but the relation of intensity to the density of intact sensory units is unknown. Previous studies indicated an almost linear relationship of global indices of spatial thresholds and intensity thresholds. The purpose of our study is to look at any difference in the behaviour of local threshold values and their precise relation. We examined 41 eyes of 23 patients with open angle glaucoma. Perimetry was carried out using the Humphrey Field Analyzer, and program 30-S or program 30-2 and the ring perimeter and standard program. Because test point patterns of the two examinations were different, the best matching points were calculated. If the distance between next points in both examinations was more than 5 degrees, the location was excluded. The Humphrey program 30-2 provided 47 matching points, the Humphrey program 30-S furnished 49 matching points for 50 locations. For the range of 20..36 dB in conventional perimetry, linearity could be verified in relation to the local thresholds in ring perimetry. The spatial threshold units used in the ring perimeter are the logarithm of spatial extend. Vice versa, spatial resolution is a power function of spatial thresholds. Based on the linear relationship of both thresholds, spatial resolution is a power function of intensity thresholds. In other words, if the intensity threshold is reduced from 36 to 30 dB, the spatial resolution is nearly half. If intensity threshold is reduced by a further 6 dB, spatial resolution is diminished to one quarter, and so on. If we accept that two-dimensional spatial resolution is directly related to the density of functional units, most of these units are lost when only small changes in dB values of conventional perimetry occur. For revaluation of visual fields in early glaucoma, our results are important for the otherwise rather meaningless decibel numbers.