Influence of the "fatigue effect" on the mean deviation measurement in perimetry

To quantify the influence of the reduction of the threshold during perimetric study on the mean deviation (MD).

One hundred-57 eyes of 94 patients with ocular hypertension and glaucoma and 123 eyes of 62 normal patients were explored using the Humphrey perimeter with a Delphi program specially modified to make 12 consecutive measurements of the MD.

Hypertension and glaucoma population: during the test, which lasted 13.88 +/- 1.25 minutes, the estimated MD showed a tendency to decrease by 2.9 dB on average from the beginning to the end of the exploration. This diminution was more important in patients with deeper defects, in whom the average was near 6 dB (Normal population: 13.26 +/- 2.91 minutes; mean decrease 2.22 dB). It was found that, in fact, this reduction is related to age (p < 0.01) and not to the level of the glaucomatous defect.

Age, through a "fatigue effect", influences the measured depth of the glaucomatous defect. Since in a conventional exploration the thresholds of the points are obtained from the whole test, we can estimate that the "fatigue effect" influences the MD to the extent of approximately half (1.45-1.11 dB), with respect to the numbers indicated above.