PURPOSE. To compare 6 different scales for rating the ocular discomfort induced by CO2 delivered to the corneal surface using a Belmonte Esthesiometer.
METHOD. On 2 occasions (separated by at least one day), forty subjects rated ocular discomfort using simple horizontal and vertical analogue scales (only with verbal anchors at each end), magnitude estimation, a Likert scale and 2 detailed vertical analogue scales, one with numerical and verbal descriptors and the other only with numerical details. Discomfort was induced by 5 randomly presented levels of CO2 (from 10% to 50% CO2 in equal steps) using a Belmonte pneumatic esthesiometer. Flow rates were set at 10ml/min below previously determined mechanical thresholds (with 0% CO2).
RESULTS. Regardless of scale, there were significant linear associations between log rating and log %CO2 (average r=0.98). Repeated measures ANOVA showed no differences between scales or between visits and a significant difference between the ratings at each CO2 level (which were different from each other according to post hoc tests). Repeatability of the ratings was typically poor (highest intraclass correlation 0.38) but based on the standard deviations of the test-retest differences, the 2 detailed scales were the most repeatable.
CONCLUSIONS. Regardless of scale type, ocular discomfort induced by CO2 blown onto the cornea was monotonically related to the CO2 concentration suggesting that these scales accurately represent the physiological stimulus in some way. The most repeatable appeared to be the scales with the most detail.