The influence of forward light scatter on the visual field indices in glaucoma

Light scatter was induced in 15 glaucoma patients, exhibiting clear media and moderate field loss, using cells containing varying concentrations of 0.5 microns diameter latex beads in distilled water. The right eye was examined with program G1 on the Octopus 201, and with a 45 degrees threshold profile on the Dicon AP3000, with and without a given cell. Forward light scatter due to the cell was assessed by measuring the depression of contrast sensitivity, with the Nicolet CS2000, under glare conditions. Perimetric mean sensitivity decreased linearly and loss variance decreased curvilinearly with increase in forward scatter. Threshold values for 26 glaucoma patients, determined in the absence of induced scatter, were then corrected for the effects of naturally occurring intraocular light scatter. The recalculated mean defect decreased linearly while loss variance remained essentially unchanged. Forward light scatter therefore predominantly exaggerates diffuse loss; the apparent underestimation of focal defects caused by induced scatter is partly a computational artefact resulting from inapplicable age-matched normal reference data.