The Anatomy of Evaporative Dry Eye
“Overall, Meibomian gland dysfunction (MGD) is an important, under-estimated condition, and is very likely the most frequent cause of dry eye disease.” – The Tear Film and Ocular Surface Society (TFOS)
The tear film is composed of three layers. The innermost layer, in contact with the cornea, is the mucus layer. The middle layer, comprising the bulk of the tear film, is the aqueous layer. The outermost layer is a thin layer, measured in nanometers, comprised of lipids known as “meibum.”
Meibomitis, an inflammation of the meibomian glands that can lead to their dysfunction, is commonly believed to be accompanied by blepharitis (inflammation of the lids). Meibomian gland obstruction may accompany meibomitis and blepharitis, but often meibomian gland obstruction is present without meibomitis or any obvious inflammation of the lid or lid margin. While meibomitis can be identified with an inspection of the external lids, meibomian gland obstruction may not be immediately evident even when examined with the magnification of the slit-lamp biomicroscope, and external signs may be so minimal that they are overlooked.
The importance of this non-obvious obstruction is that meibomian gland dysfunction, resulting from obstruction, is now thought to be the leading cause of dry eye.
Meibomian gland obstruction results from the formation of keratotic plugs, which obstruct gland orifices and compromise secretory functions of individual glands. These plugs are made up of a combination of keratin, sebaceous ground substance, bacteria and dead, desquamated epithelial cells.
Risk factors
Hormonal changes in women during menopause, particularly changing levels of estrogen, can cause thickening of the oils secreted by the meibomian glands, which results in obstructed meibomian glands and dysfunction. Additionally, decreased estrogen levels may enhance conditions under which staphylococcal bacteria can proliferate, causing migration of the bacteria into the glands. This results in a decreased secretion rate. Additional factors that may cause or exacerbate meibomian gland disease include age, contact lens wear and hygiene, cosmetic use, and other illnesses, particularly diabetes.
Effective evaluation
Clinical evaluation of the meibomian glands includes the application of standardized, gentle expression to the external surface of the eyelids over the meibomian glands, in order to determine whether liquid secretion is obtained from the individual gland. If gentle pressure does not provide secretion, forceful expression may be utilized to determine if secretion can be obtained.
The state of an individual meibomian gland can vary from optimal (where clear meibomian fluid is expressed with gentle pressure), to mild or moderate meibomian gland dysfunction (where milky fluid, inspissated or creamy secretion often resembling pus may be obtained), to total blockage (where no secretion of any sort can be obtained even with extreme pressure). Significant chemical changes of secretions occur with meibomian gland dysfunction, altering the composition of the naturally occurring tear film and contributing to ocular disease generally known as “dry eye.”
The cause of lipid deficiency
Recently, it has been proposed by Blackie, et al.2 that lipid deficiency is primarily due to non-obvious blockage of the meibomian glands (without obvious infection or inflammation), and that this non-obvious blockage may be the primary initiator of a cascade of recognizable dry eye signs and symptoms. The Tear Film and Ocular Surface Society (TFOS), an international group of world leading physicians specializing in dry eye, began meeting in early 2009 to address and bring to the forefront the largely unrecognized role of lipid deficiency due to meibomian gland dysfunction and its overall importance in dry eye disease. Their report is was issued March 2011.