TEAR FILM AND DYNAMICS

SIVA TEJA CHALLA

THE TEAR FILM

The main role of lacrimal system is to establish & maintain a continuous tear film over the ocular surface.

The presence of the pre-corneal tear film was 1st demonstrated: by Fischer in 1928.

Rollet described it as the most superficial 6th layer of cornea.

STRUCTURE OF THE TEAR FILM

Wolff was the 1st to describe in detail the structure of the tear film.

Coined the term ‘PRE-CORNEAL FILM’.

Tear film consists of 3 layers.

1)Outer Lipid layer

2)Intermediate Aqueous layer &

3)Inner mucin layer

LIPID LAYER

Outer most layer.

0.1um thick

Formed from the secretions of Meibomian, Zeiss & Moll glands.

Contents:-1)low polarity lipids-wax & cholesterol esters 2)high polarity lipids-TG, FFA ,phospholipids.

Functions:-1)prevents the overflow of tears.

2)prevents evaporation.

AQUEOUS LAYER

Middle layer.

Formed by secretions from the main & accessory lacrimal glands of Krause & Wolfring.

Constitutes the main bulk of tear film.

thickness over the cornea 10um.

Film covering the cornea is thinner than over the conjunctiva.

Contents:- inorganic salts, glucose, urea, enzymes, proteins & glycoproteins.

Buffering capacity of the tear film is d/t HCO3 ions & protein

Functions: 1)provides O2 to corneal epithelium.

2)washes away debris & irritants.

3)contains antibacterial sub- lysozyme & betalysin.

MUCOUS LAYER

Innermost layer.

Secreted mainly by the conjunctival goblet cells

30um thick.can be demo in living eye by alician blue drops

Functions:-

1)plays a vital role in the stability of the tear film.

2)converts the hydrophobic corneal epithelium to a hydrophilic one.

3)lubricates the ocular & palpebral surfaces.

4)provides a slippery coating over the foreign bodies; thereby protecting the cornea & conjunctiva against the abrasive effects of such particles as they move about with blinking.

5)absorps various organic compounds in tears

NEW TEAR FILM MODEL Recent observation--

mucins exist as a network distributed in the aqueous body of the tear film.

Glycocalyx emanate as transmembrane molecules into the aqueous & are anchored at the cell membrane.

Membrane associated proteins-MUC1,4&16 as well as secretory mucins-MUC5AC &MUC7 have been identified at the ocular surface.

PHYSICAL PROPERTIES OF TEAR FILM

property

Thickness 4-8um

Volume 4-13ul

Rate of secretion 1.2ul per min

Turn over rate* 18% per min

Refractive index 1.357

Ph of tears* 7.3-7.7

Osmotic pressure* 0.90-0.95%

Temperature 30’c at cornea and 35’ at limbus

Oxygen tension 40-160 mm hg

TEAR FULID COMPOSITION

Mainly composed of three protein factors-albumin,globulin and lysozyme

IgA is the most prominent IG in tear film,IgE levels increase in patients with allergic conj., and IgM increases in patients with acute infections

Tear lysozyme constitutes 20% all tear protiens,highest in conc among all body fluids

Electrolytes sodium,potassium and chloride occur in higher concetrations in tears than in blood

FUNCTIONS OF TEAR FILM

Makes corea a smooth optical surface

Helps to wet cornea and conjunctiva and prevent them from drying

Flushes out debris and organisms from corneal surface

Has bactericidal properties due to presence of lusozyme,lactoferrin and betalysin

Ig’s and specific antibodies in tears defend the eye against externl infections

Frictional trauma between tarsal and bulbar conj and cornea is minimised by lubricating action

Enables anti infl., cells to reach injured cornea and conjunctiva

Provides epithelial cells with O2 glucose and growth factors

NEURAL ASPECTS OF TEAR PRODUCTION

The trigeminal v1 (fifth cranial) nerve bears the sensory pathway(afferent) of the tear reflexes.

The motor pathway is autonomic (involuntary), &, in general, uses the pathway of the facial (seventh) nerve in the parasympathetic division via pterygopalatine palatine ganglion, as efferent pathway.

Applied: A newborn infant has insufficient development of nervous control, so she/he "cries without weeping”.

APPLIED ASPECTS Crocodile tears syndrome/ Bogorad's

syndrome" is an uncommon consequence of nerve regeneration subsequent to Bell's palsy or other damage to the facial nerve in which efferent fibers from the superior salivary nucleus become improperly connected to nerve axons projecting to the lacrimal glands (tear ducts), causing one to shed tears (lacrimate) during salivation while smelling foods or eating.

It is presumed that one would also salivate while crying due to the inverse improper connection of the lacrimal nucleus to the salivary glands, but this would be less noticeable.

TEAR FILM DYNAMICS

Secretion of tears

Formation of tear film

Retention & redistribution of tear film

Displacement phenomenon

Evaporation from the tear film

Drying & break up of tear film

Dynamic events during blinking

Elimination of tears

1.SECRETION OF TEARS

BASAL SECRETION:-

• In the human eyes the cornea is continually kept moist & nourished by basal tears.

• They lubricate the eye & help to keep it clear of dust.

• Secreted by accessory lacrimal glands

REFLEX SECRETION:-

• Results from irritation of the eye by foreign particles.• Can also occur with bright light & hot & peppery stimuli to the tongue & mouth.• These reflex tears attempt to wash out irritants that may have come into contact with the eye.• Secreted by main lacrimal gland

Applied : If lacrimal gland malfunctions or is damaged in surgery or other failure of lacrimal function occur, it is not a serious matter, for the accessory glands are enough for general secretion

2.FORMATION OF PRE OCULAR TEAR FILM

Conjunctival mucus spreads on to the cornea by the action of the lids.

On this new surface- aqueous layer is spread spontaneously.

Over this the superficial lipid layer spreads; probably contributing to its stability & retarding evaporation b/w blinks.

3.RETENTION & REDISTRIBUTION

The outer most layer of corneal epithelium+ mucopolysaccharides leads to retention.

Precorneal film is stagnant.Redistribution occurs in the form of bringing of new tear fluid by way of marginal strip where there is constant flow of tears

4.DISPLACEMENT PHENOMENON

Demonstrate that cornea is covered by a film which has stability, compressibility, elasticity & unaffected by gravity

Demonstrated by upward movement of particles in the film on displacing lower eyelid upwards over eyeball

This phenomenon is possible due to presence of thin monomolecular layer on the surface of cornea

5.EVAPORATION FROM THE TEAR FILM

All lipid films including wax esters & cholesterol esters retard evaporation of water

Important in low humidity & turbulent air flow near cornea, such as exists in a windy & arid climate

Evaporation from tear film = 10% of production rate, so, evaporation = 0.12ul/min (as tear production = 1.2ul/min)

6.STABILITY, DRYING & RUPTURE OF TEAR FILM

Tears has to cover entire preocular surface to function properly

It is re-established completely after a blink , but has short lived stability

It takes 15-40 secs for tear film to rupture & dry spots to appear, when blinking is prevented

Drying of corneal surface cannot be a result of evaporation of water alone, as it takes at least 10 mins to eliminate whole tear film by drying alone.

7.ELIMINATION OF TEAR FILM HOLLY & LEMP’S MECHANISM 1st described by Holly in 1973.

Initially all the tear film thins uniformly by evaporation.

When thinned out to some critical thickness, some lipid molecules begin to be attracted by the mucin layer & migrate down to this layer.

When the mucin layer is sufficiently contaminated by lipid from the top, the mucin becomes hydrophobic & the tear film ruptures

Blinking can repair this and restore aqueous layer

8.DYNAMIC EVENTS DURING BLINKING

Was 1st described by Holly in 1980.

As the upper lid moves downwards, the superficial lipid layer is compressed b/w the lid edges

This will contaminate the mucus & this lipid contaminated mucus is rolled up in a thread like shape & dragged into lower fornix

When the eye opens, at 1st the lipid spreads in the form of a monolayer against the upper eye lid

Then spreading of the excess lipid follows & in about 1 sec multimolecular lipid layer is formed

The spreading lipid drags some aqueous tears with it thereby thickening the tear film.

9.ELIMINATION OF TEARS

Lacrimal fluid over the preocular surfacemarginal tear stripLacus lacrimalisinner canthus lacrimal passages nasal cavity

Lacrimal pump mechanism:- fibres of the pretarsal & preseptal portion of the Orbicularis which arise from the lacrimal fascia & posterior lacrimal crest.

This LPM operates with the blinking movements of the eyelids as follows:-

DRAINAGE OF LACRIMAL FLUID FROM NLD INTO NASAL CAVITY

Gravity helps downward flow.

Air currents in nose induce negative pressure within NLD draw the fluid down the potential lumen of the duct into the nose.

Hasner’s valve present at lower end of NLD, remains open as long as the pressure within nose is less than the NLD, allows the tears to flow from NLD to nose

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