The outermost layer of the tear film, or lipid layer, has the following functions:
retards evaporation of the tear film
contributes to the optical properties of the tear film because of its position at the air–tear film interface
maintains a hydrophobic barrier (lipid strip) that prevents tear overflow by decreasing surface tension
prevents damage to eyelid margin skin by tears
The lipid layer is approximately 43 nm thick and contains polar and nonpolar lipids in multilayers with a complex lipid composition. Polar amphiphilic phospholipids interact with the mucoaqueous layer, and a thick layer of nonpolar hydrophobic lipids occupies the outermost layer at the air–eye interface (Fig 7-3). These phospholipids are secreted primarily by the meibomian (tarsal) glands, which are located in the tarsal plate of the upper and lower eyelids and are supplied by parasympathetic nerves that are cholinesterase-positive and contain vasoactive intestinal polypeptide (VIP). Sympathetic and sensory nerves are present but sparsely distributed. Neuropeptide Y (NPY)–positive nerves are abundant. There are approximately 30–40 meibomian glands in the upper eyelid and 20–30 in the lower eyelid. Each gland orifice opens onto the skin of the eyelid margin, between the tarsal gray line and the mucocutaneous junction (see Chapter 1, Fig 1-27). The sebaceous glands of Zeis, located at the eyelid margin close to the eyelash roots, also secrete lipid, which is incorporated into the tear film. Clinically, tear film evaporation can be evaluated by assessing the tear breakup time (see BCSC Section 8, External Disease and Cornea).
Figure 7-3 Schematic of the tear film demonstrating the lipid layer and distribution of nonpolar and polar lipids. Also shown is a proposed incorporation of proteins (pink) into the lipid layer.
(Modified with permission from Levin LA, Nilsson SFE, Ver Hoeve J, Wu SM. Adler’s Physiology of the Eye. 11th ed. Philadelphia: Elsevier/Saunders; 2011:361.)
The melting point of meibomian gland secretion ranges from 32°C to 40°C. With meibomian gland inspissation in chronic marginal blepharitis, the melting point is elevated, and the secretions become stagnant. In a study to determine whether tear film lipid layer thickness was altered after therapy with warm, moist compresses, samples of meibomian secretions from subjects without meibomian gland dysfunction (MGD) started to melt at 32°C, whereas secretion samples from subjects with MGD were found to begin melting at 35°C. Five minutes after initiation of compress therapy, the tear film lipid layer was shown to increase in thickness more than 80%.
Oral supplementation with omega-3 essential fatty acids (eg, fish oil) has been demonstrated to decrease symptoms associated with dry eye syndrome (DES) in women, presumably because of its direct effects on tear film fatty acids. However, research suggests that carotenoids and tocopherols in the oil or eicosanoids produced from the fatty acids of the oil may have a positive effect on inflammation (see the section Tear Dysfunction) and on differentiation of the meibomian gland cells.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.