Conjunctival Redness Response to Corneal Stimulation.

Significance: The conjunctiva is an integral component of the ocular surface, and its vasculature forms a terminal vascular bed of the human internal carotid artery. No research on the response of conjunctival vasculature to ocular surface stimulation exists; however, it is essential to understand the local physiological and pathological responses to such a stimulus.

Purpose: The purpose of this study was to characterize the ocular redness levels in healthy participants after the application of noxious corneal stimulation.

Methods: A computerized Belmonte pneumatic esthesiometer was used to determine detection thresholds (using ascending method of limits) and to randomly deliver mechanical and chemical stimuli from levels of detection threshold to twice the threshold in 50% steps to the central cornea of 15 healthy subjects. For each suprathreshold stimulus, a spectrophotometer was used to measure ipsilateral and contralateral conjunctival redness before and after delivery of the corneal stimulus. Redness between the stimulated and unstimulated eyes was analyzed using dependent t tests. The effects of stimulus intensity and modality on conjunctival redness were analyzed using repeated-measures analysis of variance (ANOVA). Tukey honestly significant difference tests were used for all post hoc analyses. P ≤ .05 was considered statistically significant.

Results: In mechanical and chemical stimulation experiments, the stimulated eye became redder than the unstimulated eye (all t tests, P > .05). On average, redness increased from baseline as the corneal stimulus intensity increased with corneal stimulation (ANOVA, P < .05). Chemical stimulation produced greater conjunctival redness than did mechanical stimulation at all stimulation levels (ANOVA, P < .05; all Tukey honestly significant difference tests, P < .05).

Conclusions: Stimulation of the central cornea by noxious mechanical and chemical stimuli evokes a dose-dependent autonomic conjunctival redness response. Chemical stimulation of the cornea seems to evoke a greater response compared with mechanical stimulation. This study serves as a basis for the characterization of the local stimulus-response neural circuitry relating nociceptive ocular surface stimuli to autonomic responses.