The classical description of the tear film describes it as a trilaminar structure: lipid layer on the surface, aqueous layer in the middle, and mucous layer adjacent to the eye surface . However, recent research has shown that this clear-cut demarcation between layers does not seem to exist. Rather, it is postulated that soluble mucins are dispersed in the aqueous layer, being more concentrated near the ocular surface .
Figure 1 – Structure of the tear film (courtesy of Alcon)
Functions of different elements of the tear film
1. Lipid layer The lipid layer is the thinnest of all, and is uppermost. It is mostly produced by the meibomian glands in the upper and lower tarsus in the eyelids, and secreted on the lid margins. It consists of lipids that are not identical to skin sebum.
Its main functions are preventing aqueous tear layer evaporation, and providing a smooth,
regular optical surface. In the case of an absent lipid layer, aqueous tear evaporation increases four-fold.  In addition, it prevents tear overflow over the lid margins. 
2. Aqueous layer The aqueous-mucin layer is the thickest of all the tear layers. The aqueous component is secre- ted by the lacrimal gland, as well as by the accessory lacrimal glands of Krause and Wolfring. In it are dispersed electrolytes, proteins, metabolites and desquamated epithelial cells. Among the various proteins present, immunoglobulins, lysozyme and lactoferrin are very important, as they have an antibacterial role in the defence of the tear surface. Aqueous tear osmolality is an expression of the total concentration of dissolved particles in a solution without regard to their
size, density, configuration or electrical charge. Osmolality is one of the tear film parameters that virtually always increases in all types of dry eye.  The main function of the aqueous tear layer is wetting of the ocular surface, thus providing its optimal optical clarity. Besides that, it forms the first line of antibacterial defence and washes away foreign bodies, dust etc.
3. Mucous component The mucous component consists of proteins, mainly glycoproteins, secreted by the goblet cells of the conjunctiva. Glycoproteins form the glycocalyx, a protein meshwork, that helps bind proteins to the epithelial surface, and promotes ocular surface wettability.  Its viscoelasti- city also enables it to quickly repair and fill any surface defect or irregularity. 
CLASSIFICATION OF DRY EYE
The most comprehensive classification of dry eye is The Madrid triple classification of dry eye [1, 2]. It includes the whole spectrum of causes of dry eye, and uses three criteria: ethiopatho- genesis, affected glands and severity. Although a general practitioner or general paediatric p r a c t i t i o n e r ( G P / G P P ) s h o u l d b e i n f o r m e d a b o u t a l l t h e c a u s e s o f d r y e y e , i t i s h i g h l y r e c o m -
mended that he/she should endeavour to treat only those listed below in italics – other causes of dry eye should be referred to a subspecialist experienced in treating dry eye.
According to ethiopathogenesis:
others may be immunopathic, hyponutritional, inflammatory, traumatic, neurologic and tantalic.
According to affected glands:
other types are mucodeficiency, epitheliopathy and non-lacrimal affected exocrine glands.
According to severity:
Grade 1 – symptoms without significant clinical signs
Grade 2 – reversible clinical signs, such as corneal staining and hyperaemia
Grade 3 – irreversible sequelae of dry eye, such as scarring and leucoma.
Rheinstrom (1999) made a more precise differentiation of tear deficient and evaporative dry eye, which is relevant for this guideline.  McCulley et al. proposed using terms hyposecre- tory and hyperevaporative dry eye (2003). 
Dry eye (keratoconjunctivitis sicca) Tear deficient (hyposecretive)
Non-Sjoegren tear deficiency