TEAR FILM

Dr Md Ferdous Islam Department of Ophthalmology CMH, Dhaka

THE TEAR FILM

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

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

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 FILMProperty

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 3 protein factors-

albumin,globulin and lysozyme

IgA is the most prominent Ig. IgE levels increase in patients with allergic

conjunctivities. IgM increases in patients with acute infections.

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

Electrolytes Na,K and Cl higher concetrations in tears than in blood

FUNCTIONS OF TEAR FILM Makes corea a smooth optical surface Wets cornea and conjunctiva and prevent from

drying Flushes out debris and organisms Bactericidal properties (presence of

lusozyme,lactoferrin and betalysin) Ig’s and specific Ab in tears defend the eye against

infections Lubricating action Enables anti inflammatory cells to reach injured

cornea and conjunctiva Provides epithelial cells with O2 ,Glucose and

Growth Factors

NEURAL ASPECTS 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" 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.

Presumed that one would also salivate while crying due to the inverse improper connection of the lacrimal nucleus to the salivary glands, but less noticeable.

TEAR FILM DYNAMICSSecretion of tears Formation of tear filmRetention & redistribution of tear

filmDisplacement phenomenonEvaporation from the tear filmDrying & break up of tear filmDynamic events during blinkingElimination 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,bright light,hot & peppery stimuli to the tongue.

•These reflex tears attempt wash out irritants that may have come into contact with the eye.

•Secreted by main lacrimal gland

Applied If lacrimal gland 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 Demo that cornea is covered by a film

which has stability, compressibility, elasticity & unaffected by gravity

Demo 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 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

Initially all the tear film thins uniformly by evaporation.

When thinned out to critical thickness, some lipid molecules 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 repair this and restore aqueous layer

8.DYNAMIC EVENTS DURING BLINKING 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

CHIEF C/O THE PATIENTS WITH TEAR FILM DYSFUNCTION  Burning or Itching  Fluctuating Vision

 Foreign Body Sensation

 Grittiness or irritation

 Watering or excessive tearing

 Sore or tired eyes  History of Styes  Ocular Discharge  Light sensitivity

 Contact Lens Discomfort

HISTORY FOR A DRY EYE(DE) PATIENT Duration of reading or computer use

Using contact lens

Living in air conditioned environment

Frequent air traveling

Cigarettes smoking

Exposure to allergans or systemic allergies

Hormonal change

Autoimmune diseases

DISEASES RELATED TO DYSFUNCTION IN TEAR FILM

Evaporative Dry Eye

Oil deficiency- secondary to obstructive meibomian gland dysfunction

Defective resurfacing of the eye by the tear film (result of poor blinking or abnormal lid-globe congruity)

DRY EYE: MULTIFACTORIAL NATUREElderly woman

Contact lens user

Postmenopausal

Taking glaucoma

medications

Working for long hours in

front of computer

Air-conditioned environment

DISEASES RELATED TO DYSFUNCTION IN TEAR FILM

•Hyper secretion of tear film

C/O Wet eyes

Lacrimation from excess tearing

Obstructive epiphoria as a result of failure of tear drainage( Schirmer’s value)

TEAR FILM BREAK-UP TIME(BUT) Difference b/w the last blink

and the first randomly appearing dry spots

Assessed with fluorescein and cobalt blue filter in broad beam

Avg of three reading is taken

Suspect Dry Eye when BUT<10secs

CAUSES OF TEAR FILM DESTABILITY

Tear Film rupture occurs when hydrophobic lipid diffuses from the superficial layer and contaminates the underlying hydrophilic mucin layer

Epithelial change

SCHIRMER’S TEST Rate of tear formation is

estimated

Whatman filter paper no 41 is used

Dimension 5mm X 35mm

5mm tab is folded at one end

The bent end is placed at the junction of the lateral 1/3rd and medial 2/3rd of the lower conjunctival sac

Performed in dim light with fans and ACs switched off

SCHIRMER TEST Without Anesthesia

Measures Reflex Tear Secretion (dry eye = < 6mm wetting)

With Anesthesia Measures Basal Tear

Secretion (dry eye =< 3mm wetting)

JONES-I (PRIMARY) TEST Differentiates excessive watering due to blockage

in lacrimal passage with primary hypersecretion of tears

1 drop of 2% fluorescein in instilled in the conjunctival sac

After 5mins a cotton tipped bud (moistened with 4%proparacaine) is inserted under the inferior turbanate

Fluorescein if recovered from the nose then the excretory system is patent

Otherwise should go for Jones-II test

JONES-II (SECONDARY) TEST Helps to identify the probable site of partial obstruction

4% xylocaine instilled in the conjunctival sac

residual fuorescein washed out

NLD irrigated with NS

Patient is positioned his/her down by 45deg

+ve –fluorescein stained saline recovered from the nose showing functional patency of upper lac passage

-ve- unstained saline recovered from the nose shows block in the upper lac passage or defective lacrimal pump mechanism

Jones Dye-I

Jones Dye-II

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