Sensations evoked by selective mechanical, chemical, and thermal stimulation of the conjunctiva and cornea

To study the sensations evoked by selective mechanical, chemical, and thermal stimulation of the conjunctiva and compare them with those elicited by similar stimulation of the cornea.

Six young subjects participated in the study. Using a gas esthesiometer, selective mechanical (air puffs at flows from 0 to 264 ml/min), chemical (0--80% CO(2) in air), and thermal (air at temperatures from -10 degrees C to +80 degrees C) stimulation was performed on the center of the cornea and on the temporal conjunctiva. The intensity, degree of irritation, stinging and burning pain components, and thermal characteristics of the evoked sensation were evaluated after each stimulus in separate, 10-cm continuous visual analogue scales (VASs). The ability of the subjects to identify the quality of the stimulus applied to the cornea and the conjunctiva was also studied.

The subjective intensity and thermal components (cooling or warming) of the sensation reported after mechanical, chemical, and heat stimulation were similar in the conjunctiva and cornea, although lower VAS scores were always reported in the conjunctiva for the irritation and the stinging and burning pain components. In the cornea, stimulation with low temperatures was perceived as a cooling sensation with an irritative component. In the conjunctiva, cooling was perceived as a purely cold sensation. Subjects showed similar discrimination capability in the cornea and the conjunctiva for the various types of stimuli.

Sensations evoked in the cornea by selective mechanical, chemical, and heat and cold stimulation always presented an irritation component. In the conjunctiva, stimuli of the same intensity are always perceived as less irritating than in the cornea. Cold and other non-noxious subqualities of sensation can be evoked in the conjunctiva.