Meibomian Gland Dysfunction
Three Layers Protect the Eye Surface
The moisture-laden surface of the eye contains three interrelated layers known as the tear film. Stable continuity of that surface and the production of tears rely on the function of these three layers, which need to be produced in proper balanced amounts by the body to avoid dry eye syndrome:
- The innermost layer of the surface of the eye is a mucous layer that forms the bulk of the tears and contains electrolytes, a variety of proteins, and water. It also has some anti-microbial properties.
- On top, on the outside of the mucous layer, is a mildly alkaline aqueous layer (watery) comprising up to 90% of the thickness of the tear film.
- Outside the watery layer is an oily lipid layer that slows evaporation of the tear film. This thin layer is made up of meibum, produced by the meibomian gland.
Meibomian glands, also known as tarsal glands, are located along the edge of the eyelids. They are an essential part of the eye, keeping it healthy and lubricated. They secrete fatty and oily meibum, and with every blink (with help from a tiny muscle called the Riolan's muscle), they spread a thin layer over the tear film to slow the tear film's evaporation. Meibum also lowers tear-film surface tension so that the tear film remains contoured to the surface of the eye and tears don't spill down to the cheeks. It makes the eyelid airtight when closed.
Meibomian glands are located on both the upper (about 50 glands) and lower (about 25 glands) eyelids. Meibum is also produced by the Zeis and Moll glands. Meibum is fluid at body temperature. It slows tear film evaporation and Glands of Zeis are also sebaceous (oil) glands that protect the surface of the eyelid, adding a protective layer to the top of the tear film. These glands secrete sebum to the middle section of the eyelash follicle, keeping it lubricated, and keeping the eyelashes from becoming brittle.
Glands of Moll are located at the base of the eyelashes. They are modified sweat glands, secreting sebum. However, unlike Zeis glands, these secretions contain immune system ingredients: the enzyme lysozyme, mucin 1, and immunoglobulin A, which suggest that they are part of the localized eyelid immune system. Their function is not precisely known, but they may be protecting against pathogens on the surface of the eye. Further research supports this hypothesis with the discovery of additional immune-system components.
Symptoms and Diagnosis
Symptoms include red and/or itchy eyes, dry eyes, feeling grittiness, sometimes blurred vision. Your eyes may feel heavy. Your doctor can determine whether you have this condition. One test she may do is to press the eyelids to release some meibum. A diagnostic tool standardizes the amount of force needed to express meibum from the meibomian glands. The tool makes it easier for your doctor to assess not only whether there is a problem, but the severity.
Because meibomian dysfunction impacts the quality of your tear film, your doctor will also need to assess the quality, stability, and quantity of your tears. A common test is the tear breakup time (TBUT) test. Your doctor will add a slight amount of dye to the surface of your eyes and then examine your eyes with a light that makes your tears glow (cobalt blue light) to see how quickly the dye dissipates indicating tear film stability.
Retinoids used in cosmetics promote meibomian dysfunction.
Microscopic demodex mite infection. Demodex brevis lives in the meibomian glands. Demodex folliculorum specializes in the area around the eyelids and eyelashes where it feeds on skin cells and increases the number of skin cells on the surface of the eyelash. To further the self-reinforcing cycle of dysfunction, these mites carry bacteria that stimulate enzymes that degrade cellular proteins.
Contact lenses. Researchers have noted that meibomian gland alterations are linked to contact lens use and that discontinuing such usage doesn't eliminate the changes. Wearing soft contact lenses appears to cause a thickening of meibomian glands with consequent dysfunction.
Ocular microbiome. One study evaluated 157 subjects, who had mild (41) or moderate-to-severe (50) meibomian gland dysfunction. Bacteria were collected from their eyelids. Anterior blepharatis was not an independent indicator of dysfunction, but the presence of staphylococcus aureus was higher than anticipated, and coagulase-negative staphylococcus, corynebacterium and streptococci were lower than expected. The researchers concluded that similar ocular surface microbiome profiles suggested some common treatment possibilities.
Ductal hyperkeratinization is the inability of the cells lining the inside of a hair follicle to periodically slough off from the lining. It is mechanically involved in meibomian gland problems but does not appear to be a cause (at least in a mouse model).
Thyroid disease. Researchers have noted that patients with thyroid disease experience incomplete blinking (not completely closing the eyelid) and loss of meibomian gland structure.
Meibomian Gland Dysfunction and Other Eye Conditions
Most dry eye symptoms have to do with meibomian gland dysfunction (MGD). This occurs when natural oil produced by the lacrimal gland is either (a) blocked by the ducts along the upper and lower lids (25–30 in each lid), or (b) blocked along the openings at the lid margin. These oils produce the top layer of the tear film called the oil layer. When not secreted properly, tears evaporate more quickly causing dry eyes.
Meibomian gland dysfunction is the most common cause of posterior blepharitis; although it may also contribute to anterior blepharitis.
- Anterior blepharitis is found on the front and outside of the eyelid along the line of eyelashes. Bacteria and scalp dandruff are the most common causes of the inflammation there. Increased meibomian gland activity may also occur.
- Posterior blepharitis is found on the innermost part of the eyelid that touches the eyeball. It occurs when the meibomian glands are not functioning properly.
Two skin disorders can also cause posterior blepharitis: scalp dandruff and acne rosacea.
Chalazia occur when an oil gland becomes blocked; and can arise from meibomian gland dysfunction. A chalazion, or meibomian cyst, is a bump (similar to a stye) that occurs due to a blocked and inflamed meibomian or Zeis gland on the eyelid. Chalazia are more common in adults than children and most frequently occur in people aged 30-50, presumably because of hormone changes.
Chalazia are not caused by infection. Rather they are caused by foreign body cell reactions with the oily sebum secreted by the meibomian glands. Chalazia usually, but not always, do not involve redness, soreness, and swelling. Twenty-five percent of chalazia show no symptoms other than the visible bump, and they usually disappear without treatment. But they can grow to a bothersome size and even blur vision, because they distort the shape of the eye. Chalazia tend to take longer than styes to resolve, sometimes up to several months.
Styes are the result of a bacterial infection of the meibomian glands, or the glands of Zeis and Moll (glands supplying oil to the eyelashes). Staph infections are usually the cause. This means that styes almost always involve redness, soreness, and localized swelling in the eye, which is not typically the case with chalazia. Styes are tender and painful to the touch. Ordinarily, they swell for about three days, and then they break open and drain, with healing usually occurring within a week.
Ocular Rosacea. In the healthy eye, oily meibum covers the surface of the cornea, helping to slow evaporation of tears. But increased toxic and irritating biochemicals (proinflammatory cytokine and matrix metalloproteinase) cause the meibum to thicken that in turn reduces its ability to protect the tear film. In this way, the process toward chronic inflammation reinforces itself. Chronic inflammation in turn, suppresses meibomian gland function that further reinforces development of ocular rosacea, not to mention dry eye syndrome.