Purpose: Measurement of sensitivity provides important clues about sensation on the ocular surface. This study aimed to evaluate whether measurements of threshold of sensation to an air stimulus are affected by corneal/conjunctival location, gender, age, time of day, ambient temperature or humidity.
Methods: A retrospective analysis is reported of ocular surface threshold measurements made by one examiner using the CRCERT-Belmonte esthesiometer. Multiple corneal measurements for 49 normal subjects (24M:25F) and conjunctival measurements for 33 subjects (16M:17F) were included in the analysis. Threshold was measured at the corneal apex and at the inferior conjunctiva 2 mm from the limbus. Measurements were made between 9 AM and 6 PM, at ambient temperature 20 to 26 degrees C and humidity 52 to 87%. Mixed model analysis of variance, paired-t-test and Pearson's correlation were used to examine effects of various factors on threshold.
Results: Mean group corneal threshold was 76.2+/-26.8 mL/min and conjunctival threshold 123.7+/-49.1 mL/min (n=33, p<0.001). Corneal and conjunctival threshold were well correlated (r=0.66, p<0.001). Thresholds were significantly higher for male than female subjects at both the cornea (M 82.2+/-23.5 mL/min, F 67.6+/-24.1 mL/min, p=0.04) and conjunctiva (M 144.1+/-40.7 mL/min, F 105.8+/-50.2 mL/min, p=0.02). A significant reduction in corneal threshold with age was apparent for females (n=25, r= -0.49 p=0.01) but not males. A similar effect on conjunctival sensitivity was not shown. No effect of time of day, ambient humidity or temperature was found on threshold at either site.
Conclusions: Corneal and conjunctival sensitivity were found to be associated. Corneal and conjunctival sensitivity is higher in female subjects, who also show an age-related increase in corneal sensitivity. No change in sensitivity of either tissue is apparent within normal levels of ambient temperature or humidity or over the course of a working day.