Measurement of corneal sensitivity to mechanical and chemical stimulation with a CO2 esthesiometer

To develop an instrument to measure corneal sensitivity. Mechanical stimulation was performed with increasing air flow. Chemical stimulation consisted of local pH decreases induced by a mixture of air and CO2 at different concentrations.

Air and 98.5% CO2 were mixed with an electronic, proportional-direction control valve to obtain gas mixtures from 0% to 80% CO2. The regulated outflow of gas was carried to a probe mounted on a slit lamp holder, where it was warmed and its CO2 concentration monitored. An electronic valve directed gas pulses of controlled duration to the cornea. Corneal stimulation was performed in 17 young human subjects. The intensity of the experienced sensation was recorded in a continuous visual analog scale (VAS). To obtain threshold values and intensity-response curves, 3-second pulses were applied. For mechanical stimulation, air pulses of increasing flow were used. For chemical stimulation, gas mixtures of increasing CO2 concentration at subthreshold flow and CO2 in stepped increases of 5% was applied.

Mechanical and chemical stimulation of the cornea evoked a brief sensation of irritation. Mechanical threshold (flow values) varied among subjects but were reproducible within each subject and were higher with warmed air. The magnitude of the sensation increased proportionally to the flow of air. The mean chemical stimulation threshold (CO2 concentration) was 25% +/- 3%. Increases in CO2 concentration from 10% to 80% augmented proportionally the intensity of the evoked sensation.

The gas esthesiometer, which combines variable air flow and CO2 concentrations, permits application to the cornea of mechanical stimuli of controlled force and pH reductions of increasing magnitude. This instrument may be useful in a separate exploration of mechanical and chemical sensitivity of the cornea in human subjects.