UPDATE Dry eye: A Focus on the Pathophysiology of Meibomian Gland Dysfunction

Meibomian glands help to form the superficial layer of fat on the tear film. This reduces the evaporation of the tear fluid, improves stability, protects the surface of the eye and is essential for good visual function. Meibomian Gland Dysfunction (MGD) can lead to dry eye due to increased evaporation. MGD is thus responsible for approximately 60 % of all cases of dry eye alone and for a further 20 % of cases associated with a deficiency in the aqueous tear film phase.

Meibomian Gland Dysfunction (MGD) is a chronic, diffuse impairment of the meibomian glands which is usually characterised by an obstruction/blockage in the excretory duct and/or qualitative or quantitative changes in gland secretion. This can impair the tear film or trigger symptoms of ocular irritation, clinically visible inflammation and superficial eye disease. 

Definition and classification of meibomian gland dysfunction: Report of the International Workshop on Meibomian Gland Dysfunction 1.
      

 

Meibomian Gland Dysfunction (MGD) can therefore be considered as the most common cause of dry eye.

Inflammation of the eyelid, increased bacterial growth on the eyelid accompanied by skin changes and severe corneal changes make MGD a multi-faceted disease. In a current publication, Baudoin et al. (2016) define MGD as a heterogeneous condition, which can be triggered by a combination of 5 separate pathophysiological conditions:

1. Inflammation of the eyelid
2. Conjunctival inflammation
3. Corneal inflammation
4. Microbiological changes
5. Dry eye (due to tear film instability)

The International Workshop on Meibomian Gland Dysfunction presented the pathophysiology of MGD as shown in the following diagram with the various interconnecting pathways. 1

 

In their publication, Baudouin et al. presented a new pathological diagram of MGD in an attempt to give a clearer picture of the basic mechanisms involved in the development and interaction of dry eye. It should facilitate a more effective treatment of MGD and dry eye in clinical practice.
The pathophysiology of MGD and dry eye can clearly be portrayed as a "vicious circle". The underlying pathophysiological mechanisms of dry eye and MGD interact and lead to a dual vicious circle.

The vicious circle in the pathophysiology of dry eye / MGD:


Various starting points / treatment options to break the MDG / dry eye vicious circle:

 

 

Conclusion for routine clinical practice
MGD should always be considered for all patients who present with dry eye as this condition is involved in dry eye in up to 80% of all cases. The lid margin should also be examined more closely in routine clinical practice for the characteristic signs of MGD. If MGD is diagnosed, specific treatment depending on the severity of the condition should be introduced: Basic therapy involves detailed instructions on lid margin hygiene, warming and massage. Lipid-containing substances should preferably be used as a tear substitute.

References:
1. Nichols KK, Foulks GN, Bron AJ et al. The international workshop on meibomian gland dysfunction: executive summary. Invest Ophthalmol Vis Sci 2011; 52 (04): 1922–1929
2. Baudouin C, Messmer EM, Aragona P, Geerling G, Akova YA, Benítez-Del-Castillo J, Boboridis KG, Merayo-Lloves J, Rolando M, Labetoulle M. Revisiting the vicious circle of dry eye disease: a focus on the pathophysiology of meibomian gland dysfunction. Br J Ophthalmol. 2016 May;100(3):300-6