“Tear hyperosmolarity is a ubiquitous feature of dry eye disease” according to the Aug. 25, 2014 Recently Accepted Papers Alert of Investigative Ophthalmology & Visual Science (IOVS). Harumitsu et al provide extremely interesting evidence in the manuscript “Hyperosmolar tears enhance sensitivity of the special corneal nerves to ocular surface cooling: possible neural basis for cold-induced dry eye pain”, where they evaluate high threshold cold sensitive plus dry-sensitive (HT-CT + DS) neurons in rats is normally excited by strong (>4°C) cooling of the cornea which, when applied to humans, evokes the sensation of discomfort in humans. The researchers sought to determine if these nociceptors of the cornea could be sensitized by hyperosmolar tears such that they are now activated by small cooing of the cornea. They looked at responses of single corneal neurons to cooling stimuli presented in the presence of hyperosmolar (350-800mOms NaCl) tears which we know causes evaporation/breakdown of the tear film leading to significant cooling of the ocular surface.
This is exciting as we are now getting down to the cellular response to what hyperosmolar tears do the cells and possibly why some patients have significant discomfort when their tear film breaks down. It may also help us understand why while diabetics have such a high incidence of dry eye disease (DED), but often have very low symptoms and neurotrophic damage is likely the culprit. This is another strong reason why patients having diabetes need to have tear osmolarity as well as a very thorough evaluation to determine if they do have DED.
Read their conclusions here: Hyperosmolar Tears Enhance Sensitivity of the Special Corneal Nerves