**2.3 The lipid layer**

The lipid layer is the most significant layer in terms of DES. Alterations of its thickness and composition are associated with DES. The lipid layer is the outermost layer of the precorneal tear film and is the thinnest at 0.04 μm. The lipids within this layer are secreted from mainly the meibomian glands, with minor contribution from the Moll and Zeiss glands located in the eyelids [1, 13, 15].

The lipid layer is composed of mostly cholesteryl esters and waxes, with the rest of its composition made up of diesters, fatty acids, cholesterol, and triacylglycerol. The main function of this layer is to prevent the rapid evaporation of tears, followed by the prevention of spillage of tears at the lid margin [16, 17]. This prevention is achieved by the formation of a water-tight seal with the closure of the lids. Furthermore, the lipid layer functions as a clear optical medium.

Deficiency of the lipid layer occurs with meibomian gland dysfunction (MGD). Of cases of dry eye, MGD makes up 60% of cases of DES [13]. With the dysfunction of the meibomian gland, the thickness of the lipid layer is decreased, leading to rapid evaporation of tears and spillage of tear film over the lid margin, ending in eye dryness. To individuals with DES, this spillage can give the false sensation

**Figure 4.** *Lacriminal pump mechanism corresponding with Figure 5.*

#### **Figure 5.**

*Illustration of Lacriminal pump. (A) Pump at resting state. (B) Closure of eyelid. (C) Opening of eyelid.*

of excessive watering as opposed to dryness. Other conditions effecting the meibomian glands are infections from *Staphylococcus aureus* and other bacteria which produce cholesterol esterase and fatty wax esterase capable of hydrolyzing the meibomian lipids and forming "froths" at the lid margin [18].

Non-invasive tests including interferometry, meibography, and meibometry are carried out to detect abnormalities in the lipid layer and meibomian gland. However, meibometry is the only test which measures the basal lipid production volume of the meibomian glands [17, 19, 20]. The test is done with the use of an 8-mm wide loop of translucent plastic tape and a "Laser Meibometer" which measures the optical density of the tape. Before beginning the test, the optical density *The Physiology of Tear Film DOI: http://dx.doi.org/10.5772/intechopen.98945*

of the plastic tape is measured with the laser meibometer as a control. While the patient is gazing upwards with their lower lid pulled downwards, the loop is then pushed against the lid margin with a pressure of 0 mmHg for 3 seconds and is set aside for 3 minutes to evaporate any tear fluid contaminants. Afterwards, the laser meibometer is used to measure the "casual" or basal lipid level. This measurement is calculated as (C-B) where C is the casual reading and B is the reading from the untouched tape [20].
