LACRIMAL SYSTEM

Precorneal tear film is consists of 3 layers

a) Outer oily layer

b) Middle aqueous layer

c) Inner mucoid layer

Fig.1: Precorneal tear film. A, Superficial lipid layer; B, aqueous layer; C, inner mucoid layer (David et al.,2008).

Functions of The precorneal tear film

1-   Provide antibacterial activity (lysozyme and leucocytes).

2-   Maintain the optical activity of the cornea. The film has a refractive index comparable to the cornea.

3-   Provide metabolic needs; oxygen and other metabolites are absorbed by the epithelium from the tear film.

4-   Remove waste products and debris.

5-   Provide lubrication for the eyelids.

6-   Mucous thread (Fine mucous threads lie in the superior and inferior conjunctival fornices in the normal animal).

Effects of Precorneal Tear Film Dysfunction

1-     Hypertonicity of the remaining tear film.

2-     Dehydration of the conjunctival and corneal epithelium.

3-     Hypoxia of the corneal epithelium and subepithelial stroma.

4-     lack of lubrication with frictional irritation of the ocular surface by the eyelids and third eyelid.

5-     Increase in numbers of microorganisms and mucus on the ocular surface and on the eyelids.

6-     Secondary inflammation of the conjunctiva and cornea with stromal vascularization and later pigmentation.

7-     Corneal erosions or ulcers.