The obstruction of meibomian glands is the most common cause for hyposecretion which results from the hypertrophy of ductal epithelium and keratinization due to aging, or decreased expression of androgen receptors, or medications. Hyposecretion is due to decreased Meibomian gland function due to gland atrophy or medications. Low delivery status is further classified into hyposecretory and obstructive conditions. MGD is classified based on the final consequence of Dry Eye Diseases (DED) into low delivery and high delivery categories based on the secretion status. The new classification system proposed by the 2011 International Workshop on MGD is shown in Figure 3. The international workshop on meibomian gland dysfunction: report of the definition and classification subcommittee. Classification of MGD.Adapted from Nelson JD, Shimazaki J, Benitez-del-Castillo JM, et al. Use of antibiotics, Isotretinoin for Acne, antihistamines, antidepressants, and hormone replacement therapy are found to be associated with MGD.ДЇigure 3. In addition, ophthalmic factors namely aniridia, chronic blepharitis, contact lens wear, eyelid tattooing, trachoma, Demodex folliculorum infection have been shown to impact Meibomian gland function. Risk factors of MGD include aging, deficiency of sex hormones notably androgens, other systemic conditions such as SjogrenвЂ™s syndrome (SS), Stevens-Johnson Syndrome (SJS), psoriasis, atopy, polycystic ovary syndrome (PCOS) and hypertension. Furthermore, the prevalence increases with age and appears to be higher in males when compared to females. For example, the prevalence ranges from 61-69.3% based on Telaniectasia among Asians while it ranges from 3.5-19.9% among Caucasians. The prevalence is less among Caucasians when compared to Asians and it ranges from 3.5-70% depending on the parameter looked at. It is estimated that 70% of Americans over the age of 60 have MGD. MGD is an underdiagnosed and undertreated disease and asymptomatic MGD is more common than the symptomatic MGD. Images of normal and progressive gland loss in patients with MGD are shown in Figure 2. It may result in alteration of the tear film, symptoms of eye irritation, clinically apparent inflammation, and ocular surface diseaseвЂќ. The International Workshop on MGD defined the disease as вЂњa chronic, diffuse abnormality of the meibomian glands, commonly characterized by terminal duct obstruction and/or qualitative/ quantitative changes in the glandular secretion. Korb and Henriquez coined the term Meibomian Gland Dysfunction (MGD) in a study addressing contact lens intolerance and the obstruction of the Meibomian gland orifices by desquamated epithelial cells. Reprinted with permission from American Academy of Ophthalmology from the article by Arita, Reiko et al (2008), Noncontact Infrared Meibography to Document Age-Related Changes of the Meibomian Glands in a Normal Population Ophthalmology,

Changes in meibomian glands were scored using the following grades in each eyelid (meiboscore): grade 0, no loss of meibomian glands grade 1, area loss was less than one third of the total meibomian gland area grade 2, area loss was between one third and two thirds grade 3, area loss was more than two thirds. The upper and lower eyelids were turned over and meibomian glands were observed using an infrared transmitting filter and an infrared charge-coupled device video camera. Images demonstrating representative cases of each grade of the meibomian gland changes. Aging, diet, sex hormones, usage of antibiotics and the dysfunction of Meibomian glands alters the composition of lipids and proteins in meibum resulting in altered tear film stability and function.ДЇigure 2.

Meibum contains over 100 major individual complex mixture of lipids, over 90 proteins, electrolytes that contribute to the stability of tear film in health and disease. The contraction of the orbicularis muscle during blinking and the contraction of Riolan muscle at the terminal part of the ductal system generates mechanical force that facilitates the secretion of meibum. Each gland opens into the lid margin secreting lipids to form tear film.

Each gland consists of clusters of about 10-15 secretory acini in the upper eyelid which drain into a central duct through smaller ductules. There are about 25-40 glands in the upper eyelid and about 20-30 in the lower eyelid with lengths of about 5.5 mm in upper eyelid versus 2 mm in lower eyelid. These glands are first described in detail by German physician Heinrich Meibom (1638-1700) and are named after him (Fig 1).

Meibomian glands or glandulae tarsales are large sebaceous glands present in eyelids which secrete lipids that form the superficial layer of tear film to protect evaporation of the aqueous phase. Heinrich Meibom.Reprinted with permission of the Herzog August Bibliothek, WolfenbuВЁttel, Germany, Signatur B 100

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