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

Abstract

PURPOSE. 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. METHODS. 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 Ϫ10°C to ϩ80°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. RESULTS. 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. CONCLUSIONS. 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. (Invest Ophthalmol Vis Sci. 2001;42:2063-2067 T he quality of sensations arising from the application of different types of stimulating energy to the anterior surface of the eye has been a matter of discussion since von Frey's pioneering studies on ocular sensitivity. 3 However, in most of these studies the stimulation procedures precluded selective application of a given form of energy and a precise control of the stimulation parameters, thus making it difficult to define the site of origin of the sensation and its psychophysical characteristics. A more accurate procedure for stimulating the ocular surface was recently made available by the gas esthesiometer, 7 an instrument that allows the delivery of gas pulses of controlled mechanical force, temperature, and varying CO 2 concentration to the ocular surface. In humans, application with the gas esthesiometer of mechanical force, noxious heat, and low pH to the cornea evokes unpleasant sensations, with the degree of irritation, quality of pain, and thermal characteristics varying with the modality of stimulus. In contrast, moderate cold stimulation of the cornea elicits an innocuous sensation of cooling that becomes irritating when low temperatures are achieved. 9 MATERIALS AND METHODS Subjects Three women and three men were included in the study (mean age, 23.8 Ϯ 0.6 years). They received financial compensation for their participation. The research followed the tenets of the Declaration of Helsinki. The subjects signed an informed consent to a protocol approved by our institute and were free to interrupt the session at any time. None of them had a history of corneal or ocular disease. Three wore eyeglasses to correct myopia (less than Ϫ2 diopters [D]). Selective mechanical, chemical, and thermal stimulations of the cornea and the conjunctiva were performed in both eyes. The protocol was completed on four separate days. Esthesiometry A gas esthesiometer previously described 7 was used to apply to the corneal or conjunctival surface 3-sec gas jets of adjustable flow, composition, and temperature, separated by 2-minute pauses. Selective mechanical stimulation consisted of a series of nine pulses of air at variable flow (0 -264 ml/min) heated to 50°C at the tip of the probe to prevent changes in the temperature of the ocular surface during the air puff. A series of nine pulses of air and CO 2 mixture of different concentrations (0 -80% CO 2 ) at 50°C and with a flow 6.25 ml/min below mechanical threshold was used for chemical stimulation. Thermal (heat and cold) stimulation was performed by applying to the cornea and conjunctiva 10 pulses of air at different temperatures (Ϫ10°C to ϩ80°C), which produced variations of the basal corneal temperature (34.4°C) of Ϫ5°C to ϩ3°C (see Refs. 7 and 8) at a flow rate From th