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M.C. Acosta, M. Alfaro, C. Belmonte, J. Gallar; AGE DIFFERENCES IN CORNEAL AND CONJUNCTIVAL SENSITIVITY TO MECHANICAL AND CHEMICAL STIMULUS. . Invest. Ophthalmol. Vis. Sci. 2004;45(13):3946.
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Purpose: To measure differences in threshold sensitivity to mechanical and chemical stimulation of the cornea and conjunctiva between human subjects of different age. Methods: 54 eyes from 27 healthy subjects (thirteen women and fourteen men, ages from 21 to 62 years) were explored using a gas esthesiometer (Belmonte et al., 1999). Mechanical stimulation pulses ranged between 40 and 200 ml/min. chemical stimulation pulses applied at a subthreshold flow of 60 ml/min ranged between 5% and 80% CO2. In order to avoid thermal stimulation of the tissues, the gas was heated to 50ºC at the tip of the probe, reaching the corneal or conjunctival surface at a temperature of around 34ºC. Mechanical and chemical thresholds were determined using the method of levels Results: Both in the cornea and in the conjunctiva the threshold for mechanical and chemical stimulation increased with age. Differences in corneal and conjunctival sensitivity to mechanical stimulation were apparent only in individuals over 50. For chemical stimulation, the sensitivity decrease was already apparent in the age group of 30–40 years. Overall, mechanical and thermal threshold increases with age were more prominent in the conjunctiva than in the cornea. Conclusions: Corneal and conjunctival sensitivity decreased with age, but this reduction seems to affect in a greater extent polymodal than mechanosensory nerve fibers. The cornea is more richly innervated than the conjunctiva and experienced a lesser reduction in sensitivity with age. Thus, it can be speculated that sensitivity decreases with age are associated with the initial density of corneal innervation.
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