tear film dr ferdous
TRANSCRIPT
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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