“Dry eye occurs when the eye does not produce tears properly, or when the tears are not of the correct consistency and evaporate too quickly” [1]. The tear film is composed of three layers; mucin, aqueous fluid, and lipid.
The mucin layer is the deepest layer and supplies the epithelial layer of the cornea with nutrients though secretions from goblet cells. The aqueous fluid is the middle layer of the tear film and functions to keep the eye hydrated. The most anterior layer of the tear film is the lipid which functions as the barrier between the tear film and the environment. An insufficient lipid layer results in increased exposure and tear film evaporation. This lipid layer is secreted by the Meibomian glands which are located in the upper and lower eye lids.
The Meibomian glands are tube-like structures that secrete lipid onto the tear film through the mechanical motion of the eye lids blinking. All three layers of the tear film function together to attain the proper amount of tears and consistency within the tear film.
Historically, aqueous fluid production has been thought to be the main contributor to dry eye syndrome however, recent studies have shown that the Meibomian gland dysfunction has a significant contributor. According to American Academy of Ophthalmology, “MGD is the most under recognized, underappreciated and undertreated disease in ophthalmic care. It is so common as to be taken as ‘normal’ in many clinical practices” [2]. The most common form of MGD is due to gland obstruction [3]. The orifice of the Meibomian gland becomes blocked from cell debris and external particulates. Once blocked, the gland begins to atrophy and reduce function. Factors that “increase Meibomian gland obstruction range from advancing age and hormonal disturbances, the toxic effects of medication and chemicals and the breakdown products of Meibomian lipids, or influences of external factors such as epinephrine eye drops, to [contact lens] wear” [4]. Without ample lipid in the tear film, the aqueous fluid evaporates at an accelerated rate and exposes the corneal surface. This can lead to common issues such as computer eye strain and eye fatigue. Once atrophied, even the best severe dry eye treatment cannot regain the function of lost glands. Research suggests that MGD is a main contributor to dry eye syndrome and should be treated early in order to maintain gland function.
1) National Eye Institute. “Dry Eye Facts”. National Institute of Health. https://nei.nih.gov/health/dryeye/dryeye. [Accessed July 13, 2016]
2) Linda Roach. “Rethinking Meibomian Gland Dysfunction: How to Spot It, Stage It and Treat It”. EyeNet Magazine. August 2011. http://www.aao.org/eyenet/article/rethinkingmeibomian-gland-dysfunction-how-to-spot?julyaugust-2011
3) Nichols, Kelly. The International Workshop on Meibomian Gland Dysfunction: Executive Summary. Investigative Ophthalmology Vision Science. 2011 Mar; 52(4): 1922–1929. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3072157/
4) Nichols, Kelly. The International Workshop on Meibomian Gland Dysfunction: Executive Summary. Investigative Ophthalmology Vision Science. 2011 Mar; 52(4): 1922–1929. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3072157/
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