Is meibomian gland dysfunction related to dry eye disease?
In this first blog post on meibomian gland disease, I am going to explain to you the pathology of meibomian gland dysfunction. Meibomian gland dysfunction is critical to understand because of the very prevalent condition of dry eyes in our society, which worsens
- as we get older,
- in female patients, particularly during and after the menopause, and
- worse in young people in particular whose jobs involve them in long hours of screen use.
What is the meibomian gland?
The meibomian gland is a type of sebaceous gland located in the eyelids in a part of the eyelid called the tarsal plate.
Although the whole of the tarsal plate contains the meibomian gland, there are meibomian glands throughout the tarsal plate; each one forms phospholipids, cholesterol, wax esters and cholesteryl esters.
These components are known as polar and non-polar lipids. To make it simpler, we just call the oil or the lipid that is produced meibum, as it comes from the Meibomian glands. Think ear wax, which is also a type of lipid, and can be soft or thick and hard. Similarly, the meibum can vary.
The meibum drains from about 30 individual secreting glands within each eyelid through a small opening on the lid margin where the meibum can then be delivered from close to the posterior edge of the lid margin over the eye surface. There, it forms a very thin oily coating over the aqueous layer of the tears. This oily layer helps to provide tear stability and also protects against microbial agents and organic matter.
What causes Meibomian Gland Dysfunction?
Meibomian gland dysfunction (MGD) is a term used to describe a group of disorders which can be congenital and acquired. Linking these disorders is one unifying functional abnormality of the meibomian glands which is an alteration in either the quality or the quantity of meibum. This condition then leads to an altered tear film composition and protection. That can lead to what is called ocular surface disease along with eye discomfort, eyelid discomfort, and dry eyes.
Meibomian gland dysfunction is classified by the rate of the gland secretion of meibum. There can be low production of meibum called a low delivery state where there is meibomian gland hyposecretion or actual obstruction of the openings of the gland or substance of the gland either from scarring or non-scarring causes. In contrast, there can be high delivery states where there is meibomian gland hypersecretion which can also cause symptoms.
For instance, there are scarring conditions at the front of the eye such as mucus membrane pemphigoid which can cause an obstructive cicatricial MGD. In contrast, acne rosacea and seborrhoeic dermatitis are secondary causes of obstructive non-cicatricial MGD and hypersecretory MGD. However, the most common MGD is low delivery state with gland obstruction.
The pathophysiology (cause) is epithelial hyperkeratinisation along the lid margin and in the glands, which leads to duct obstruction with meibum stasis, subsequent duct cystic dilation and a eventually a disuse duct acinar atrophy and eventual individual meibomian gland duct dropout.
Inside each meibomian gland there are meibocytes and their function can also be influenced by both extrinsic and intrinsic factors such as extrinsic environmental stress and intrinsic age. Both factors affect meibomian gland meibocyte cells differentiation and renewal.
It is not your fault if you have got meibomian gland disease or meibomian gland dysfunction and dry eyes. It could well be that there is an intrinsic problem such as
- you are getting older,
- there has been a hormonal change, or
- your eyelids have been subjected to an extrinsic environmental stress (such as air conditioning, central heating and a lot of screen use).
In the next blog post, we are going to look a little bit more at the risk factors for meibomian gland dysfunction.