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B. J. Winterson, A. Pasha, K. Gassman, I. D. Meng; Tear Volume, Blink Rate, and Sensation Evoked by Humidified Air Stimulation of the Cornea in Normal Adult Humans. Invest. Ophthalmol. Vis. Sci. 2010;51(13):3368.
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Dry eye syndrome (DES) is a disorder of inadequate tears on the ocular surface often associated with symptoms of pain, discomfort and visual disturbance. Recently, we have characterized corneal primary afferent neurons in the rat that are activated by drying and innocuous cooling, and inhibited by warming of the cornea. These neurons may be important for activating the reflex arc responsible for basal tearing and blinking. The aim of the present study was to determine whether non-noxious temperature changes of the cornea in normal adult humans affects tear production, blink rate, and subjective sensations.
The cornea was stimulated with a modified gas esthesiometer. Air was bubbled through either hot or room temperature water and delivered to the cornea at a rate set slightly below mechanical threshold for 2 minutes. Warmed humidified air was approximately 32 o C at the point of measurement immediately in front of the cornea. Tear volume was measured with phenol red threads (Zone-Quick®). Blink rate and subjective reports were determined from video recordings. Subjects were 6 male and 14 female volunteers between the ages of 18 -46. Exclusion criteria were the following conditions: DES, diabetes, history of corneal trauma, cluster headache, contact lens use and history of eye irritation.
Tear volume was significantly (p < 0.01) reduced (X = 23.25 mm, sem= 1.87) by warm air when compared to baseline condition (no air stream: X = 26.7 mm, sem = 2.09) and room temperature air (X= 27.05 mm, sem = 2.03). Blink rate was not significantly different among the 3 experimental conditions. There was an increase in the number of subjects reporting sensations (Chi-square=11.56, df =1, p <0.001) between the baseline condition (4) and when stimulated (21). Subject reports did not correspond to the stimulus temperature.
Warming of the cornea reduces tearing. The present results support the hypothesis that corneal cold cells, which are inhibited by warming, are important for modulating basal tear production to non-noxious stimuli. Reduced activity of this class of corneal sensory neurons may contribute to DES in some individuals.
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