Topographical spatial summation in glaucoma

Stimulus luminance (L) and area (A) are related by the equation L x A(k)=constant. The authors evaluated the k value at 66 positions of the central visual field in patients with glaucoma, to modify L and A simultaneously in order to examine advanced glaucomas with a bigger dynamic range.

The luminance limitation of a computer screen with automatic photometric control was compensated for by increasing the stimulus area in the range between 0 and 17 dB, using the k topographic values previously calculated on normal subjects. Four initial series of 21, 12, 10, and 10 glaucomas were sequentially examined with the Octopus 311 in which the stimulus size cannot be freely changed during the examination, and with the experimental method (Pulsar-SAP) modifying stimulus sizes to equal the results. k Final estimation was verified in 60 new cases.

k values increase progressively with defect deepness. Values higher than those of the normal population with equivalent topographic differences were obtained. Correlation between indices was as follows: MD: r=0.94 (p<0.0001); square root of the loss of variance (sLV): r=0.93 (p<0.0001). Frequency of local defects was similar in both procedures. Average topographic differences between thresholds were usually less than 1 dB. The average threshold difference favored Pulsar-SAP by 0.45 dB at those points where the average threshold of both examinations was less than 18 dB and 0.37 dB where such average was higher than or equal to 18 dB.

k value is higher in patients with glaucoma than in normal subjects, although the topographic features are similar. It is feasible to design a scale combining stimulus luminance and sizes to use screens with relative low brightness as surfaces for visual field examination.