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Nociceptive Sensation and Sensitivity Evoked from Human Cornea and Conjunctiva Stimulated by CO₂

From the School of Optometry, University of Waterloo, Waterloo, Ontario, Canada.

PURPOSE. To compare sensation and sensitivity evoked from human cornea and conjunctiva stimulated by CO_2.

METHODS. Twenty healthy participants were recruited for the study. Central corneal and temporal conjunctival chemical sensation and sensitivity of only one eye of each subject were evaluated. Air mixed with different concentrations of CO₂ was delivered by a modified Belmonte pneumatic esthesiometer. The ascending method of limits was used to determine the sensitivity and subjects were required to characterize the sensation at threshold.

RESULTS. The sensations evoked by CO₂ in the cornea and conjunctiva were stinging or burning. The sensation evoked by mechanical stimulation was that of irritation. The corneal and conjunctival chemical thresholds were 31% ± 2% and 54% ± 5% CO₂ (mean ± SE), respectively. The corneal and conjunctival mechanical thresholds were 80 ± 6 and 140 ± 10 mL/min (mean ± SE), respectively. The corneal sensitivity was significantly higher for both mechanical and chemical stimuli (P < 0.05).

CONCLUSIONS. The results suggest that CO₂ stimulates similar corneal and conjunctival nociceptors in that the interpretations were the same (i.e., nociceptive). The central cornea had a higher sensitivity to CO₂ than the temporal conjunctiva, which may reflect a different peripheral innervation, such as different nerve density or different receptor characteristics. Sensations evoked by mechanical and chemical stimulation were different, which suggests that at the peripheral level, the two modalities stimulate two different kinds of molecular receptors or channels and that this information is somehow retained within the nociceptive system.

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