Human tears are distributed among the marginal tear strip (or tear meniscus) (Fig 7-1), the preocular film covering the exposed bulbar conjunctiva and cornea (precorneal tear film, also called tear film), and the conjunctival sac (between the eyelids and bulbar conjunctiva).
The primary functions of the precorneal tear film are to
provide a smooth optical surface at the air–cornea interface
allow diffusion of oxygen and other nutrients
serve as a medium for removal of debris and protect the ocular surface
The tear film protects the cornea and ultimately the entire eye by carrying tear constituents and debris to the lacrimal puncta, providing a medium for antimicrobial agents (lysozyme and lactoferrin, among others) and immunoglobulins, and preventing desiccation of the ocular surface barrier.
Historically, the precorneal tear film was viewed as a 3-layer “sandwich” composed of distinct lipid, aqueous, and mucin layers. Evidence continues to support a 2-phase model of the tear film, in which a lipid layer overlies a mucoaqueous phase (Fig 7-2). Components of the tear film (lipids, mucins, proteins, and salts) may interact to prevent tear film evaporation and collapse; however, additional studies are needed to confirm this concept.
Figure 7-1 Clinical photograph of the tear lake at the lower eyelid margin, stained with fluorescein. A normal tear lake is approximately 1 mm above the eyelid margin.
(Courtesy of Vikram S. Brar, MD.)
Figure 7-2 Two-phase model of the tear film. Schematic drawing of the structure of the tear film showing the outer lipid layer, the mucoaqueous layer, and the glycocalyx resting on the apical microvilli of the ocular surface epithelium.
(Reproduced with permission from Willcox MDP, Argüeso P, Georgiev GA, et al. TFOS DEWS II tear film report. Ocul Surf. 2017;15(3):366–403.)
Measurements of tear film thickness have differed widely, but more recent studies using optical coherence tomography (OCT) and reflectometry have found the tear film to be approximately 3.4 μm thick. The steady-state volume of tears is 7.4 μL for the unanesthetized eye and 2.6 μL for the anesthetized eye; this volume decreases with age. Some properties of the normal human tear film are given in Table 7-1.
Craig JP, Nelson JD, Azar DT, et al. TFOS DEWS II report executive summary. Ocul Surf. 2017;15(4):802–812.
Table 7-1 Approximate Properties of Normal Human Tear Film
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.