Tear Film – Physiology and Disturbances in Various Diseases and Disorders

*Dorota Kopacz, Łucja Niezgoda, Ewa Fudalej, Anna Nowak and Piotr Maciejewicz*

## **Abstract**

The tear film is a thin fluid layer covering the ocular surface. It is responsible for ocular surface comfort, mechanical, environmental and immune protection, epithelial health and it forms smooth refractive surface for vision. The traditional description of the tear film divides it into three layers: lipid, aqueous and mucin. The role of each layer depends on the composition of it. Tear production, evaporation, absorption and drainage concur to dynamic balance of the tear film and leads to its integrity and stability. Nonetheless, this stability can be disturb in tear film layers deficiencies, defective spreading of the tear film, in some general diseases and during application of some general and/or topical medications. Dry eye disease is the result of it. In this review not only physiology of the tear film is presented. Moreover, we would like to discuss the influence of various diseases and conditions on the tear film and contrarily, spotlight tear film disorders as a manifestation of those diseases.

**Keywords:** tear film, dry eye, mucins, lipid layer, aqueous layer, ocular surface

#### **1. Introduction**

The tear film is a thin fluid layer covering the ocular surface; it is the interface of the ocular surface with the environment. It is responsible for ocular surface comfort, mechanical, environmental and immune protection, epithelial (both corneal and conjunctival) health and it forms smooth, refracting surface for vision [1, 2]. Tear production (about 1,2 microliters per minute, total volume 6 microliters, 16% turnover per minute), evaporation, absorption and drainage are responsible for dynamic balance of the preocular tear film [1, 3–5]. Homeostatic balance leads to stability of the tear film, that makes possible to realize its functions as lubrication, nutrition and protection of ocular surface [3, 6]. Nonetheless, this stability can be disturb in tear film layers deficiencies, defective spreading of the tear film, in some general diseases and during application of some systemic and/or topical medications and dry eye disease evolves as a consequence of it. These review focused on physiology of the tear film, it's meaning for the ocular surface stability and analyzed influence of various diseases and conditions on it.
