Tear Film And Blinking

PresenterJunu ShresthaB. Optom 3rd yearMMC, IOM

Moderator Sanjeev Bhattarai

Contents • Tear film introduction

– Composition– Properties– Dimensions

• Tear film dynamics– Secretion – Distribution – Stability – Tear flow– Drainage

• Tear film functions

• Blinking

• Clinical correlations

Tear Film• Layer of fluid on the surface of cornea

• First demonstrated by Fischer in 1928 by

using reflectography

• Also called precorneal film-Wolff(1946)

• Is a trilaminar structure

Development of tear film• Within 24 hours of birth with normal tear production rate

within 1 week

• Abnormal or reflex tearing starts only when the infant is 4 months old

Composition

• Tear film….

composed of three layers:

• (a) the outer or superficial lipid layer

• (b) the middle aqueous layer

• (c) the inner mucoid layer

1. Lipid layer• Derived from secretions of

Meibomian glands, Glands of Zeis &

Glands of Molls.

• Primary Constituents: hydrocarbons,

fatty acids, sterol esters, wax esters, triacylglycerol.

Functions of lipid layer

• Retards evaporation.

• Contribute to the optical properties of the film.

• Maintain a hydrophobic barrier that prevents tear overflow by increasing surface tension.

• Prevent damage to lid margin skin by tears.(skin lipid contamination)

2. Aqueous layer• Secreted by Lacrimal glands,

Accessory lacrimal glands (Krause & Wolfring)

• Primary constituents: water, inorganic electrolytes, organic substances of low and high molecular weights

• urea, glucose, lactate, citrate, ascorbate & amino acids.

• 6.5 -7.5 micro meter.

Functions of aqueous layer

• Supply oxygen to the avascular corneal epithelium.

• Antibacterial (lysozymes & lactoferrin), immunoglobulins (IgA, IgM, IgD, IgE, S- IgA) and antiviral (interferons) defense.

• Wash away debris (cleansing)

• Maintain a constant electrolyte composition over the ocular surface epithelium.

• Smoothing minute irregularities of anterior corneal surface (optical surface enhancement)

• Bicarbonates regulates pH

3. Mucus layer• Deepest structure of the film

• Covers the microplicae of the superficial corneal epithelial cells & forms fine network over the conjunctival surface.

• Contains mucins, proteins, electrolytes & water.

• Tear mucins are secreted principally by conjunctival goblet cells ,the stratified squamous cells of the conjunctival & corneal epithelia ,& minimally by lacrimal glands of Henle & Manz.

Functions of mucus layer• Converts the corneal epithelium from a hydrophobic to hydrophilic

layer .

• The mucin also interacts with tear lipids layer to lower surface tension thereby stabilizing the tear film.

• Provides lubrication for eyelid movements.

• Loose mucin network covering the bulbar conjunctiva traps exploited surface cells, foreign particles & bacteria.

Dimensions • Distribution of Tear Volumes

• Pre-corneal: 1 μL

• Tear meniscus: 3 μL

• Cul-de-sac: 4 μL

• Total thickness: 3 – 12 µm

• Lipid layer: 40 - 150 nm

• Aqueous layer: 7 µm

• Tear volume: 6.5 - 8.5 µL

• Tear turnover rate: 15-16%/min

• Tear meniscus volume: 0.51 – 3.15 µL

• Osmolarity: 295 - 318 mOsm/L

• Production rates: 0.5 - 1.78 µL/min

• Evaporation: 1.58 - 14.7 X10–7g/cm2/sec

• pH: 6.50 - 7.83

• Temperature (corneal): 34.2 - 34.8 °C

• Temperature (conjunctival): 34.9 - 35.4°C

• Surface tension: 35 - 43.6 mN/m2

• Lactoferrin levels: 1.5 - 1.68 mg/mL

• Refractive index (epithelium): 1.376 - 1.401

• Refractive index (tears): 1.3357 - 1.3370

Tear film

dynamics

Secretion

• Are continuously secreted throughout the day by – Accessory lacrimal glands (basal secretion)– Main lacrimal glands (reflex secretion)

• Reflex secretion occurs in response to sensations from the cornea and conjunctiva.

• Afferent pathway of this secretion is formed by 5th nerve and efferent by parasympathetic (secretomotor) supply of lacrimal gland.

• Tear turnover rate: 5 to 7 minutes

Distribution Distribution of tear is possible due to

• Conjunctival mucus that makes cornea hydrophilic(wetting) Holly and Lemp

• Interaction of eyelids and tears in corneal wetting (blinking) Doane 1980

Sequence of events in the formation of the precorneal continum:

1. Lids surfacing cornea with a thin layer of mucus

2. On this, the aqueous then spreads spontaneously

3. Then the superficial lipid layer spreads over it retarding its evaporation and enhancing stability.

• As the upper lid moves downwards, the superficial layer is compressed. As it thickens it begins to exhibit interference colors. The whole lipid layer together with the associated biopolymers is compressed between the edges.

• This lipid contaminated mucus is rolled up in a thread like shape and dragged into lower fornix.

• On opening the eye, at first the lipid spreads in the form of a monolayer against the upper eyelid. The spreading lipid drags some aqueous tears with it, thereby thickening the tear film.

Stability

• The compositional factors are responsible for producing 3 major tear components: aqueous fluid, lipids, and mucins.

• Hydrodynamic factors generate effective tear clearance (turnover) and spread to cover the ocular surface and intermittent closure to avoid unwanted exposure and desiccation.

The 2 factors are integrated by 2 neural complexes

The tear film is transient.

After a finite period of time the integrity of the tear’s structure is lost, leading to tear film rupture (and, thus, loss of confluent coverage of the ocular surface).

• 2 theories:

Tearflow • Tear flow is aided by:

• Capillary action.

• Gravity.

• Blinking.

Tear Flow: Lid Closure

• Contraction of the orbicularis oculi muscle causes lid closure with a scissor-like action towards the nose.

• Lid closure propels the tears towards the medial canthus.

Drainage • Tear is drained from lacus lacrimalis to the nasolacrimal

duct via active lacrimal pump mechanism.

• Operates during blinking

• On closure of eyelids contraction of pretarsal fibres of the orbicularis compresses the ampulla and shortens the canaliculi. This propels the tear present in the ampulla and horizontal part of the canaliculi toward the lacrimal sac.

• Contraction of preseptal fibres of orbicularis pulls the lacrimal fascia and lateral wall of a the lacrimal sac laterally, thereby opening the normally closed lacrimal sac.

• Produces a relative negative pressure and draws the tears from the canaliculi into the lacrimal sac.

• Along with the increased tension on the lacrimal fascia, the inferior portion closes more tightly, preventing aspiration of air from the nose.

• When the eyelids are open

Relaxation of pretarsal fibres of orbicularis allows the canaliculi to expand and reopen. It draws in the lacrimal fluid through the puncti from the lacrimal lake.

• Relaxation of portions of preseptal fibres allows the lacrimal sac to collapse. It then expels the fluid therein into the now open nasolacrimal duct

Functions of tear film

Optical functionsThe tear film is spread by the eyelids over the rough ocular surface to form a uniform surface for refraction of light that is to focus on the retina.

Protective functions

MechanicalDebris and metabolic wastes are flushed from the ocular surface by the tears. Foreign bodies result in increased tear secretion

The ocular surface is a complex biological continuum responsible for:

AntimicrobialThe tears contain antibacterial substances (lysozyme, lactoferrin, beta-lysin) antiviral immunoglobulins (secretory IgA), and complement.  

EnvironmentalThe tears protect the surface of the eye from the external environment by responding to constant, varying challenges, such as desiccation, bright light, cold, mechanical stimulation, and noxious chemicals.

There is exquisite control of tear volume, composition, and structure in response to these challenges.

Lubricative function• The conjunctival and corneal surfaces are lubricated by the tears so that they

slide over each other during blinking.

Osmotic function• Appropriate corneal hydration levels are maintained partly through the osmotic

activity of the tears.

Nutritional function• Oxygen passes through the tears and across the anterior surface of the eye.

Fat soluble nutrient (e.g., vitamin A) could be provided through the tears, although it is absent in tears.

LAMBS

The five major elements that causes tearfilm dysfunction are

• L = lipid abnormality

• A = aqueous abnormality

• M = mucin abnormality

• B = base abnormality (or epitheliopathy)

• S = surfacing abnormality (spreading of the tears by the lids) or causes of dry eye produced by exposure

Dry eye

‘Dry eye’ is a generic term for a group of conditions characterized

• symptomatically by irritated, gritty, burning eyes, and

• clinically by alterations in the tear film and anterior surface of the eye.

In a classic review, this syndrome was defined as

• ‘a disorder of the tear film due to tear deficiency or excess tear evaporation which causes damage to the interpalpebral ocular surface and is associated with symptoms of ocular discomfort’ (Lemp, 1995).

Dry eye

Tear deficient

Sjogren’s syndrome

Non Sjogren’ tear deficiency

Lacrimal disease

Lacrimal obstructio

nReflex

Evaporative

Oil deficient

Lid related

Contact lens

Surface change

Blinking

Blinking • Coordinated opening and closing movements of the

eyelids.

Blinking

CompleteMovement of both eyelids, which begins in the normal alert open

position, reaches a halfway point when the upper and lower

eyelids’ciliary margin appose each other along at least one half of their ciliary margins and ends when the

upper and lower eyelids return to the starting, alert open position

Incomplete Blink which is terminated at any

stage that occurs in complete blink phase

Blinking

Voluntary

Involuntary

Spontaneous

Reflex

Closure of Eyelids

• Brought about by the contraction of the orbicularis oculi muscle (OO), innervated by the facial nerve (N7).

• The lid action is ‘zipper-like’ or ‘scissor-like’ from the temporal to the nasal canthus. The lower lid moves relatively little during a normal blink.

• Blink rate: approx. 15 blinks/min.

• Duration: 0.3 - 0.4 seconds.

Closure of eyelids• The globe moves up and in towards the nose as well as

backwards, and then returns on eye opening.

• Forced closure involves the whole of the orbicularis oculi (especially the orbital portion) and Müller's muscle.

• Eye closure in sleep involves tonic stimulation of the orbicularis oculi and concurrent inhibition of the levator palpebrae superioris muscles.

Opening of Eyelids

• Brought about by contractions of the levator palpebrae superioris (LPS) muscle.

• Some assistance comes from Müller's muscle which is smooth and sympathetically innervated.

• Main innervation comes from the oculomotor nerve (N3). As it has no reciprocal innervation, the orbicularis oculi does not relax even when the levator palpebrae superioris elevates the upper lid.

Voluntary blinking• Blinking carried out as a willed act in both eyes

• When in one eye – winking

• Produced by simultaneous contraction of palpebral and orbital portions of orbicularis muscle

Involuntary blinking• Spontaneous blinking occurs without any obvious external

stimulus or voluntary willed efforts

• Since it does not require retinal stimulation, it is also present in visually challenged (blind) people

• Does not produce discontinuity in vision, despite vision is interrupted during the blink for about 130 msec

• Reflex blinking occurs reflexly in response to direct stimulus

• Depending on stimulus it can be

• Tactile reflex afferent via 5th nerve and efferent via 7th nerve excited by tactile stimulation of conjunctiva, cornea, eyelashes, eyelid and eyebrow

• Optic reflex afferent 2nd nerve and efferent via 7th nerve -bright light or menace reflex

• Auditory reflex afferent 8th nerve an efferent 7th nerve – loud noise

• Stretch reflex in orbicularis

• Blink Reflexes

• Facial nerve (N7) nucleus connects with– superior colliculus (optic impulses)– trigeminal nucleus (sensory impulses)– superior olive (acoustic impulses).

• Optic reflex– eye closure when threatened– eye closure when exposed to bright light/dazzle.

• Sensory reflex – eye closure when lid or cornea is touched.

• Auro-palpebral and cochleo-palpebral reflexes – eye closure caused by a loud sound stimulus.

• Stretching or striking reflex eye closure– resulting from stretching or striking anatomical features close

to the lids (protective).

• Psychogenic reaction (non-reflex)– eye closure caused by emotional stimulus (this reflex is

involuntary).

Tear film evaluation

TEAR QUANTITY ASSESSMENT

• Tear film fluorophotometry/fluorescein clearance

• Schirmer test

• Phenol red thread test

OTHER TESTS

• Marginal tear strip

• Fluorescein staining

• Rose Bengal staining

• Conjunctival impression cytology

• TBUT test

Tearfilm fluorophotometry/ fluorescein clearance

• Fluorescein sodium is applied topically whish is homogenously on the corneal surface and conjunctival sac after several blinks.

• Tears containing fluorescein are removed by flow and are replaced by fresh tears not containing fluorescein.

• Measurement of disappearance via fluorophotometry or other fluorescein clearance tests is used to determine tear turnover

• which is percentage decrease of fluorescein concentration in tears per unit of time (% minute-1).

• Basal tear turnover, defined as the tear turnover at the lowest level of reflex tear production possible under physiologic conditions, is taken as an indirect quantitative assessment of tear production.

Schirmer’s Tear test • Test for tear quantity

• Based on wetting length of the strip 5x35mm Whatman 41 filter paper

• Placed in the lower fornix at medial 1/3rd and lateral 1/3rd

• 2 variations

Schirmer’s test I• Measures total tear secretion (basic and reflex)

• Open eye technique

• Normal– 10-30 mm at the end of 5min

• If wetting >30mm before 5 min– Reflex tearing overactive/ insufficient tear drainage

• Value < 5mmHyposecretion

Basic Secretion Test• Variant of Schirmer test I measures basal tear secretion

• Topical anesthetic is applied

• Cul-de-sac is dried out before the strip is inserted

• Difference between Schirmer I and BST reading gives reflex secretion

• Normal value > 10mm

• Basic Secretion of 3mm or less Abnormal

Schirmer Tear test II• Measures reflex tear secretion

• Irritate nasal mucosa by rubbing it with cotton swab or smelling ammonia

• Measure wetting after 2min

• Wetting < 15mm Failure of reflex secretion

• Parasympathetic supply

Phenol Red Thread Test• Measures basal secretion

• Sensitive to change in pH

• 75mm long cotton thread impregnated with phenol red-temporal cul-de-sac for 15 secs

• Color changes from Yellow to red (pH range 6.6-8.2)

• Wetting of ≥16.7 mm in 15 secs (Normal)

• ≤6mm (Dry Eye)

• Phenolsulphophthalein

• Fluorophotometry offers the greatest potential for providing an accurate assessment of tear production. Although unconventional, the PRT appears to offer the next best accuracy and reproducibility followed by the Schirmer test.

• Quantitative assessment of tear production: A review of methods and utility in dry eye drug discovery Michelle Senchyna and Martin B Wax

• J Ocul Biol Dis Infor. 2008 March;

1.Tear Meniscus Height

• Tear lake (or prism) accumulates at the junction of the bulbar conjunctiva in the inferior lid margin

• is approx 0.3 to 0.4 mm in ht.

• Decrease in tear meniscus & with accumulation of debris, is suggestive of lacrimal deficiency, an unstable tear film.

• A measurement scale in the reticule or an adjustable slit beam height can be used to measure tear meniscus height.

2. Fluorescein Staining

• Fluorescein dye is used to detect epithelial defects on the anterior surface of the eye.

• Penetrates only the corneal epithelium at sites of interrupted continuity of the epithelial surface.

• Is enhanced with the use of cobalt blue filter that blocks extraneous light and highlights staining patterns.

3. Rose Bengal Staining

• Devitalized cells on the cornea and conjunctiva and mucus in the tear film

• detected using 1% rose bengal

• highlighted by red punctate staining

• Rose bengal staining patterns are classically graded on a 0 to 3 graded scale proposed by van Bijesterveld,

• Three regions of the interpalpebral ocular surface are assessed – the triangular wedge of the nasal interpalpebral conjunctiva,– the corneal surface– the wedge of the temporal conjunctiva

• The grade of each region is summed, and a score greater than 3.5 is considered indicative of dry eye

4. TBUT

• Tear film Break Up Time in cobalt blue filter

• Interval between the last blink & the first appearance of black spot in the fluorescein-stained tear film (Norn & Hamil 1973)

• A black island in the sea of green fluorescein

• Assessment of tear film stability

• >10 sec Normal

5. Conjunctival Impression Cytology

• Millipore filter paper of cellulose acetate (with 0.02-µm pores) can be employed to assess conjunctival goblet cell density.

• The filter paper is cut into strips approximately 5 x 5 x10 mm in size. After instillation into the inferior cul-de-sac of one drop of proparacaine or a similar anesthetic, with the aid of a forceps, the pieces of filter paper are pressed against the nasal, temporal, inferior, and superior bulbar conjunctiva.

• Pressure is applied to the paper for 2 to 3 seconds.

• The filter paper is then fixed for 10 minutes in a mixture of 70% ethyl alcohol, 37% formaldehyde, and glacial acetic acid in a proportion of 20:1:1.

• Each paper is stained, using periodic acid–Schiff (PAS), hematoxylin and eosin, and Papanicolaou.

• Under the light microscope, the epithelial cells are evaluated for morphology and density.

Nelson (1988) classified impression cytology of the conjunctiva in grades:

• Stage 0: normal cellular structure

• Stage 1: early loss of goblet cells without keratinisation

• Stage 2: total loss of goblet cells with slight enlargement of epithelial cells

• Stage 3: early and mild keratinisation

• Stage 4: moderate keratinisation

• Stage 5: advanced keratinisation

• Grades 1 and 2 are considered normal.

Clinical correlation

Tear film1. Deturgensence of cornea

2. Tear film in contact lens

3. Dry eye related contraindication of contact lens wear

4. Effect of blinking during contact lens wear

1. Deturgesence of cornea• Is maintained due to osmotic pressure of the precorneal

tear film and the aqueous humour

• The osmotic pressure of the precorneal tear film is greater than that of corneal epithelium due to constant evaporation of tears that causes water to be removed from the cornea.

• But blinking is necessary for the replenishment of oxygen to the cornea

2. Optics of the RGP lens• Depends largely on the tear layer between cornea and the

lens.

• Refraction that we think as taking place at the cornea as measured by keratometer actually takes place at the air tear interface.

• Contact lens alters the curvature of air tear interface if the back surface of the lens is not parallel with the interface.

• When the curvature of the tear lens is steepened, the eye is made more myopic(back surface of the contact lens is steeper than the tear layer).

• When the curvature is flattened, the eye is made more hyperopic (back surface flatter than tear layer)

3. Benefits of blinking on contact lens wear• Better oxygen and carbon dioxide exchange, providing

increased comfort, better tolerance of the lenses on eyes

• Removal of keratinized epithelial cells and other debris from underneath the lenses

• Cleaner lens surfaces, reducing the possibility of the occurrence of coating

• Reduced possibility of lens dehydration and the deleterious effect that may follow

• Extends lens life

4. Dry eye related contraindications of contact lens wear

Dryness

• Volume or quality of tears.

• Poor blinking.

• Dry environment.

• Drug-induced (e.g. antihistamine).

• Work-induced (e.g. VDUs).

From which component of the tear film do contact lens deposits originate ?

• originate in any of the layers of the tear film.

• The aqueous layer of the tear film is composed of 98% water; 2% is mineral content, including K, Cl, and Ca, ascorbate ions

• Among proteins, lysozyme has a special attraction to the materials from which hydrophilic contact lenses are made.

• Lipids produced by the meibomian glands also deposit on contact lenses, as do products of the mucin layer.

Short -term changes in the tear film induced by contact lens

• decrease in osmolarity due to the contact lens storage solutions and reflex tearing due to lens insertion.

• mild short-term fluctuations in the tear film pH.

• The pre–contact lens tear film differs from the post-lens tear film once the tear film has stabilized.

• The prelens tear film - postulated to be lipid and slightly aqueous, while the post-lens layer - mucin and aqueous.

3 & 9-O'Clock Staining/Peripheral Corneal Desiccation

• Epithelial punctate staining of the peripheral cornea in regions adjacent to the edge of a rigid contact lens. 

• Its location is limited to the 2 to 4-o'clock and 8 to 10-o'clock regions of the peripheral cornea. 

• The presence of a lens may exaggerate the effect by shifting the lid margin further from the cornea thereby increasing the size of the bridged zone. It is in this bridged zone that 3 & 9 o’clock staining occurs.

The corneo-scleral sulcus represents a zone of no contact between the globe and the upper eyelid margin, i.e. it bridges the midperipheral cornea and the perilimbal conjunctiva.

• The fundamental cause of 3 & 9 o'clock staining is the irritation produced by the lens which inhibits the normal blinking process leading to a disturbanceof the tear layer, and a failure of wetting of the corneal epithelium adjacent to the edge of the lens

• consists of isolated punctate stains. It can coalesce, with engorgement of the adjacent conjunctival blood vessels.

Blinking • Reflex blepharospasm

• Occurs due to reflex sensory stimulation via branches of 5th nerve in conditions such as phlyctenular keratitis, Interstitial Keratitis, corneal foreign body, corneal ulcers, iridocyclitis

• Benign essential blepharospasm (BEB)

• is a bilateral, involuntary, spasmodic forced eyelid closure with accompanying brow depression in the absence of any other ocular or adnexal cause.

• May also be secondary to ocular surface disease or neurodegenerative diseases such as Parkinson’s.

• BEB affects women more often than men, and typically presents during 5th to 7th decades of life.

Effect of incomplete blinking on tear film stability.M1, Uozato H, Kawamorita T, Shibata Y, Yamamoto S.

Optom Vis Sci 2013

Purpose : assess changes in tear film stability caused by incomplete blinking.

Methods : 11subjects (mean age, 21.3 years) participated in study. All subjects had a visual acuity of 20/20 or better and normal ocular health. The subjects were asked to play a game for 60 min on a personal computer as part of a visual display terminal (VDT) experiment. Each subject's blinking was observed by a Web camera that was attached to the top of the display. Every 15 min, the VDT experiment was interrupted for measurement.

• .

Results :

Although the total blink rate changed very little, the complete and incomplete blink rates fluctuated during the VDT experiment. Noninvasive breakup time at 30 min (4.33 ± 2.57 s) was significantly shorter (p < 0.01) than that at baseline before the VDT experiment (8.62 ± 1.54 s). After 30 min, the incomplete blink rate began decreasing (fewer incomplete blinks), whereas the complete blink rate began increasing. Ring breakup time increased (improved) after 45 min; however, the incomplete blink rate began to increase again after approximately 50 min.

Conclusions :

Even if the total blink rate decreases, the tear film remains stable so long as almost all blinks are complete. The incomplete blinking contributes to tear film instability and is variable with prolonged VDT exposure. The tear film stability was determined by blinking quality, and the predominance of blinking type relates to tear film stability

References 1. IACLE Module 1, 2 and 7

2. Anatomy and physiology of eye 2nd edition A.K. Khurana

3. Adler’s Physiology of the eye 11th edition

4. Dry eye a practical approach Butterworth

5. Quantitative assessment of tear production: A review of methods and utility in dry eye drug discovery

6. Impression cytology of the ocular surface Br J Ophthalmol. 2005 December

Thank you……