Why the eye becomes dry: a cornea and lacrimal gland feedback model.

PURPOSE: Many explanations have been offered for why a large segment of the population develops dry eye. The purpose of this paper is to describe a new unifying theory of dry eye that incorporates all of these causes.
METHODS: Data from the Department of Ophthalmology, University of Iowa Hospitals and Clinics was analyzed from 520 patients with dry eye, blepharitis and other conditions to assess the relationship between dry eye and blepharitis. This data was reviewed in terms of the relationship between dry eye, menopause, and aging. Also examined in detail were many of the proposed causes for dry eye in the literature.
RESULTS: A close relationship between corneal damage and lacrimal gland function is hypothesized. Not only does decreased lacrimal gland output damage the ocular surface, but also damage to the corneal surface creates a negative feedback loop and damage to the lacrimal gland. There are probably several mechanisms by which this feedback occurs. One mechanism results from interruption or damage to the sensory corneal nerves. Damage to the nerves within the lacrimal gland may be another mechanism. Alteration of growth factor levels in the lacrimal gland, which occurs following corneal damage, represents another possible mechanism. Contact lenses and corneal refractive surgery are additional elements that may create negative feedback to the lacrimal gland.
CONCLUSION: The ocular surface and the lacrimal gland functions as a tightly integrated unit. Dry eye conditions damage the ocular surface and this in turn leads to further damage to the lacrimal gland.