dry eye and tx
Meibomian Glands and Dry Eye: News From the Research Lab
BY WILLIAM TOWNSEND, OD, FAAO
The role of meibomian glands in maintaining a healthy ocular surface has become increasingly well recognized in the past decade. These structures produce lipids that reduce evaporation of the aqueous tear layer and reduce friction forces between the eyelids and ocular surface.
In two recent articles, Donald Korb, OD, FAAO, and Caroline Blackie, OD, PhD, FAAO, presented the results of their ongoing research with respect to meibomian gland function and production of tear lipids.
Meibomian Gland Secretion
Dr. Korb's interest in the meibomian glands extends back for decades. In 1980, Korb and Henriquez reported that meibomian glands in asymptomatic individuals secrete more lipids than do those in symptomatic individuals, and that forceful expression of meibomian glands yielded more material than gende expression of the same glands. They did not quantify “forceful” and “gentle.”
In the December 2008 issue of Cornea, Korb and Blackie published “Meibomian gland diagnostic expressibility: correlation with dry eye symptoms and gland location.” This study investigates the relationship between subjective symptoms as defined by the Standard Patient Evaluation of Eye Dryness (SPEED) questionnaire and by the number of secreting meibomian glands. The second goal was to determine the production of meibomian glands by region (nasal, temporal, or central) of the eyelid. In developing a protocol for this study, the authors designed a device specifically for expressing the glands in a consistent manner at a constant force of 1.25g/mm2.
The results of this study reveal several facts relevant to dry eye care. It shows that the number of meibomian glands yielding liquid secretion (MGYLS) is 0.27 ± 0.06 in the temporal lid, 2.14 ± 0.13 in the central lid, and 3.10 ± 0.14 in the nasal lid.
Korb and Blackie also found a positive correlation between the SPEED score and the number of MGYLS in the nasal and medial lower lids. Patients who had reduced functional glands in these areas were more likely to suffer from symptoms of dryness. As you might expect, the temporal glands had minimal effect on the SPEED score. Also of interest is the finding that there was minimal correlation between the number of MGYLS and age or gender. Asymptomatic patients in this study had an average of six MGYLS, while severely symptomatic subjects had an average of four MGYLS.
Gland Recovery Time
In the April 2009 Cornea, these authors published the article, “Recovery time of an optimally secreting meibomian gland.” The study was designed to assess the time it takes for a meibomian gland that has been emptied via expression to refill. In the initial evaluation, meibomian glands that secreted freely with expression were marked with a waterproof marker. Then each gland was individually drained with pressure of 1g/mm2. Each gland was then reevaluated until expression yielded fluid. They found that a single meibomian gland could be drained in eight-to-20 seconds. The average recovery time was just over two hours.
These studies help us to better understand what we are seeing when we evaluate or express meibomian glands. We now understand that it is normal to see little if any secretions in the temporal third of the eyelid and that when we see less than six functional glands per eye, we should not be surprised to hear of ocular dryness symptoms. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #166.
Dr. Townsend practices in Canyon, Texas and is an adjunct faculty member at UHCO. E-mail him at firstname.lastname@example.org.