Meibomian gland

Anatomy

Although the upper lid has a greater number and volume of meibomian glands than the lower lid, there is no consensus whether it contributes more to the tear film stability. The glands do not have direct contact with eyelash follicles. The process of blinking releases meibum into the lid margin.

Function

Meibum

Lipids

Lipids are the major components of meibum (also known as "meibomian gland secretions"). The term "meibum" was originally introduced by Nicolaides et al. in 1981.

The biochemical composition of meibum is extremely complex and very different from that of sebum. Lipids are universally recognized as major components of human and animal meibum. An update was published in 2009 on the composition of human meibum and on the structures of various positively identified meibomian lipids.

Currently, the most sensitive and informative approach to lipidomic analysis of meibum is mass spectrometry, either with direct infusion or in combination with liquid chromatography.

The lipids are the main component of the lipid layer of the tear film, preventing rapid evaporation and it is believed they lower the surface tension which helps to stabilize the tear film.

Proteins

In humans, more than 90 different proteins have been identified in meibomian gland secretions.

Clinical significance

Dysfunctional meibomian glands often cause dry eyes, one of the more common eye conditions. They may also contribute to blepharitis. Inflammation of the meibomian glands (also known as meibomitis, meibomian gland dysfunction, or posterior blepharitis) causes the glands to be obstructed by thick, cloudy-to-yellow, more opaque and viscous-like, oily and waxy secretions, a change from the glands' normal clear secretions. Besides leading to dry eyes, the obstructions can be degraded by bacterial lipases, resulting in the formation of free fatty acids, which irritate the eyes and sometimes cause punctate keratopathy.

Meibomian gland dysfunction is more often seen in women and is regarded as the main cause of dry eye disease. Factors that contribute to meibomian gland dysfunction can include things such as a person's age and/or hormones, or severe infestation of Demodex brevis mite.

Treatment can include warm compresses to thin the secretions and eyelid scrubs with a commercial eyelid cleanser or baby shampoo, or emptying ("expression") of the gland by a professional. Lifitegrast and ciclosporin are topical medication commonly used to control the inflammation and improve the oil quality. In some cases, topical steroids and topical (drops or ointment)/oral antibiotics (to reduce bacteria on the lid margin) are also prescribed to reduce inflammation. Intense pulsed light (IPL) treatments have also been shown to reduce inflammation and improve gland function. Meibomian gland probing is also used on patients who experience deep clogging of the glands.

Meibomian gland dysfunction may be caused by some prescription medications, notably isotretinoin. A blocked meibomian gland can cause a chalazion (or "meibomian cyst") to form in the eyelid.