precorneal film

(redirected from tear layer)

pre·cor·ne·al film

a protective film, 7-9 nm thick, consisting of external oily, intermediate watery, and deep mucoprotein layers.
Synonym(s): tear film
Farlex Partner Medical Dictionary © Farlex 2012

film, precorneal

The field covering the anterior surface of the cornea which consists of lacrimal fluid and of the secretion of the meibomian and conjunctival glands. Its total thickness was thought to be about 9 μm but recent investigations have questioned that value and point to a much larger figure. It is composed of three layers: (1) The deepest and densest is the mucin layer (or mucous layer) which derives from the conjunctival goblet cells, as well as some secretion from the lacrimal gland. (2) The watery lacrimal fluid is the middle layer, called the lacrimal (or aqueous layer). It is secreted by the lacrimal gland and the accessory glands of Krause and Wolfring. It forms the bulk of the film and contains most of the bactericidal lysosyme and other proteins, inorganic salts, sugars, amino acids, urea, etc. (3) The oily layer (or lipid layer) is the most superficial and is derived principally from the meibomian glands in the lids as well as some secretion from the glands of Zeis. It greatly slows the evaporation of the watery layer and may provide a lubrication effect between lid and cornea (Fig. F6). Note: Some authors have suggested that the precorneal film is made up of only two layers; an innermost aqueous and mucin gel layer and an outer lipid layer. Syn. lacrimal layer; preocular tear film; tear film; tear layer. See hyperlacrimation; mucin; tear secretion; Tearscope; break-up time test.
Fig. F6 Diagram of the three layers of the precorneal film attached to the squamous epithelial cellsenlarge picture
Fig. F6 Diagram of the three layers of the precorneal film attached to the squamous epithelial cells
Millodot: Dictionary of Optometry and Visual Science, 7th edition. © 2009 Butterworth-Heinemann
References in periodicals archive ?
Jessen fitted myopic patients with conventional corneal piano polymethyl methacrylate (PMMA) lenses flatter than corneal curvature to correct their refractive error using the corrective abilities of the post-lens tear layer. Later, he found that the cornea flattened, allowing improved unaided vision after contact lens removal.
"Far-UVC light has a very limited range and cannot penetrate through the outer dead-cell layer of human skin or the tear layer in the eye, so it's not a human health hazard.
In addition, tear evaporation may accelerate when the preconjunctival tear layer temperature rises due to hyperemia.
The lens, combined with the tear layer, act as a shield against the environmental hazards caused by many outdoor activities, such as wind, dust, and debris.
"Tear film breakup time" is a term ophthalmologists use to indicate how rapidly the normally smooth tear layer disperses, leaving the eye exposed to dryness and potential injury.
This tear layer also cleanses the eye and washes away foreign particles or irritants that are wrapped up by the other major component--mucin.
Sensitivity change in cornea and tear layer due to incision difference on cataract surgery with either manual small incision cataract surgery or phacoemulsification.
The outermost layer consists of lipids produced by the tarsal glands (meibomian glands) which form a thin oily cover (0.05-0.1im) on the aqueous tear layer, thus reducing the effects of evaporation.
Instability of the precorneal tear layer causing corneal epithelial damage is related to factors produced by the lacrimal glands and conjunctival goblet cells.
If there are insufficient tears behind the lens it will need to be 'lifted' further away from the cornea by selecting a lens which is deeper/has a greater sag, with the opposite being true for a lens that has a tear layer thickness of greater than 300[micro]m.
(7,16) The studies which have investigated the effects of the tear layer on soft contact lens movement have done so in relation to hydrogel lens materials.
Observe the tear film with the slit lamp using the cobalt blue filter, instruct the patient to blink, and note the time for spots or streaks to appear in the tear layer. Typically this should be in excess of 15 seconds, anything under 10 seconds would be considered abnormal.