Meibomian Glands I: Role in Dry Eye Disease

Meibomian Glands I: Role in Dry Eye Disease

Meibomian Glands I: Role in Dry Eye Disease

By Gary N. Foulks, MD, FACS

The meibomian glands play a key role in maintaining a healthy ocular surface. By secreting lipids that help to stabilize the tear film, the meibomian glands minimize the evaporative loss of tear fluid. Maintaining aqueous tear volume keeps the ocular surface protected throughout the blink cycle and reduces the risk of hyperosmolarity. Lipids in the tear film also allow tear fluid to maintain the viscosity needed to spread quickly over the eye’s surface after each blink—without migrating out onto the skin of the eyelid.
   
Because healthy meibomian glands are integral to tear film integrity, meibomian gland abnormalities may create significant ocular surface problems. When patients do not have enough of the meibomian gland-secreted lipids in their tear film to prevent evaporation, they can develop dry eye, even if aqueous production remains normal. As a result, patients may have a shortened tear film breakup time, and in many cases, dry eye-related ocular surface damage can result. Therefore, we need to consider the possible involvement of the meibomian glands in any dry eye patient who appears to have an inadequate tear film.


Abnormalities in Meibomian Gland Function

Abnormalities in meibomian gland function can occur for a variety of  reasons, ranging from congenital absence of the meibomian glands, which is very rare, to loss of the gland from scarring due to inflammation. Changes in meibomian gland function usually result from obstruction of the glands, due either to the formation of an epithelial plug or an alternation in the gland’s secretions. When the glands are obstructed, the condition is referred to as meibomian gland dysfunction.
   
Normally, meibomian gland secretions are a thin, clear liquid, but meibomian gland dysfunction can involve changes in both the physical properties of these secretions and their chemical composition. In terms of viscosity, meibomian gland dysfunction often involves a thickening of the meibum, which impairs its ability to help in spreading the tear film evenly over the eye. This uneven spreading of tears can result in dry spots on the ocular surface.
   
The meibomian gland secretions can also be broken down by enzymes present in the tissue or from bacteria growing on the eyelid margin.  Enzymatic breakdown of the secretion results in the formation of diglyceride and fatty acid molecules which can become very irritating to the eye. Similarly, changes in the chemical composition of meibomian gland secretions can make the secretions supportive of bacterial growth, which results in accumulation of debris, crusting on the lashes, and general irritation to the eyelid.
   
Because it appears to be associated with changes in androgen levels, meibomian gland dysfunction is more commonly seen in older patients, particularly men. Medications such as retinoic acid (Accutane®) can obliterate meibomian glands and also contribute to meibomian gland dysfunction.

Conclusion

Although not all cases of dry eye disease involve abnormalities of the meibomian glands, meibomian gland dysfunction can be the underlying cause of some types of dry eye disease—particularly evaporative dry eye. Diagnosis and treatment of meibomian gland dysfunction can therefore aid in dry eye treatment.