TEAR FILM STRUCTURE AND ITS DYNAMICS BY DR. GUDIYA RANA (1).pptx

Tear film

1. STRUCTURE OF TEAR FILM AND ITS DYNAMICS BY GUDIYA RANA (JR 1 Ophthalmology)

2. 1. Lipid layer 0.1 μ m thick.   Produced by Meibomian, Zeiss and Moll glands Made up of low polarity wax and cholesterol esters. High polarity lipids-TG, FFA ,phospholipids. Prevents overflow of tears. Retards evaporation of tears. Acts as a barrier for preventing contamination of tear film.

3. 2. Aqueous Layer Formed by secretions from the main & accessory lacrimal glands of Krause & Wolfring . Constitutes the main bulk of tear film. Thickness over the cornea 8um. 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 .

4. 3. Mucin layer Secreted by conjunctival goblet cells. Spreads tears over corneal surface. Thickness of 0.2 -0.8 μ m Protects the cornea against foreign substances. Makes cornea smooth by filling in surface irregularities. Maintains stability of the tear film. Mucin converts hydrophobic corneal surface to a hydrophilic surface by adhering to the glycocalyx on the corneal microvilli.

5. NEW TEAR FILM MODEL: Recent observation— Mucins exist as a network distributed in the aqueous body of the tear film. Glycocalyx spread as transmembrane molecules into the aqueous & are anchored at the cell membrane. 

6. PHYSICAL PROPERTIES OF TEAR FILM Thickness 7-8 μ m Volume 4-13 μ l Rate of secretion 1.2 μ l 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 ֯ c at limbus Oxygen tension(PO2) 40-160 mmHg

7. TEAR FLUID 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 proteins , highest in conc. among all body fluids. Electrolytes sodium , potassium and chloride occur in higher concentrations in tears than in blood.

8. NEURAL ASPECTS OF TEAR PRODUCTION The trigeminal V1 (fifth cranial) nerve bears the sensory pathway(afferent) of the tear reflexes. The secretomotor or efferent pathway by parasympathetic supply of lacrimal gland. Applied: A newborn infant has insufficient development of nervous control, so she/he "cries without weeping”. After infant is 4 month old abnormal tearing start.

9. General functions of tear film Smooth optical surface Lubrication Keeps eyeball moist Defense like lysozyme Wash out debris Nutrition to cornea Pathway for WBCs in case of injuries

10. Tear film dynamics Secretion of tears Formation of preocular tear film Retention and redistribution of tear film Displacement phenomenon Evaporation from tear film Drying and breakup of tear film Dynamic events during blinking Elimination of tears

11. SECRETION OF TEARS Two types : Basal secretion Reflex secretion Tears are continuously secreted throughout the day by Accessory lacrimal gland basal secretion. The reflex tears is caused due to irritation of a foreign particle or an external stimulus. They are secreted by main lacrimal glands. The reflex tears attempt to wash out irritants that may have come in contact with the eye.

12. FORMATION OF PREOCULAR FILM Lids surfacing cornea with thin layer of mucus On this new surface, aqueous component of tear now spread spontaneously Then the superficial lipid layer spreads over aqueous film, probably contributing to it’s stability and retarding evaporation between blink.

13. RETENTION & REDISTRIBUTION OF TEAR FILM Retained at a uniform thickness over the corneal surface against gravitational force – positioned vertically. Redistribution occurs in the form of bringing of new tear fluid by way of marginal strip where there is constant tear flow.

14. DISPLACEMNT PHENOMENON If with a finger the lower lid is carefully displaced upwards over eyeball, the particles in the film are seen to move up the cornea Based on this, it has been concluded that cornea is covered by a film of certain stability, compressibility and elasticity.

15. EVAPORATION FROM TEAR FILM All lipid films including wax esters retard evaporation of water. Lipid layer is important in low humidity & turbulent air flow near cornea like windy & arid climate. Evaporation estimated to be about 10% of production rate 0.12 μ l/min (1.2 μ l/min production)

16. STABILITY, DRYING & RUPTURE OF TEAR FILM In the normal human eye the precorneal tear film has a short lived stability. When blinking is prevented after a brief time interval of 15-40 second ,tear film rupture & dry spot appear on various part of cornea. Drying of corneal surface – not only because of evaporation but also due to break up tear film.

17. MACHANISM OF TEAR FILM BREAKUP Holly has described a mechanism of tear film rupture – Holly & Lemp’s mechanism. Dry spots occur twice more in temporal side than nasal side – nasal areas are more protected against air currents & have comparatively higher temperature.

18. STEPS Tear film thins uniformly by evaporation. When thinned out to some critical thickness, significant number of lipid molecules begin to be attracted by mucin layer & migrate down. This migration enhanced if there is any spontaneous local thinning. After contamination of mucin layer by lipid migration from top surface of tear film mucin becomes hydrophobic & tear film rupture. Blinking repair rupture by removing lipid contaminant from mucin layer & restoring thick layer.

19. MECHANISM OF TEAR FILM BREAK UP

20. DYNAMIC EVENTS DURING BLINKING As upper lid moves downwards, the superficial layer is compressed. The compressed lipid layer has a thickness of 0.1 μ m. The lipid contaminated mucus is rolled up in a thread like shape and dragged into lower fornix. When eye opens, lipids spread as a monolayer against the upper eyelid.

21. BLINKING Dynamic events during blinking :

22. ELIMINATION OF TEARS

23. LACRIMAL PUMP MECHANISM

24. LACRIMAL PUMP MECHANISM on eyelid closing movement

25. Along with the increased tension on the lacrimal fascia ( which opens the lacrimal sac ) Inferior portion closes more tightly thereby preventing aspiration of air from nose

26. EYELID OPENING MOVEMENT

27. DRAINAGE INTO NASAL CAVITY

28. DRY EYE 1.Aqueous deficiency ( keratoconjunctivitis sicca ) causes: Sjogren’s syndrome ( primary keratoconjunctivitis sicca ) Non-Sjogren’s – Primary age related hyposecretion Lacrimal gland deficiency –congenital alacrimia , sarcoidosis, tumors, post radiation fibrosis of lacrimal gland and surgical removal Lacrimal gland duct obstruction -old trachoma , chemical burn , SJS Reflex hyposecretion ( neurogenic)-familial dysautonomia ( Riley –Day syndrome), Parkinson disease , reflex sensory block , 7 th cranial N. damage

29. 2.Evaporative dry eye Meibomian gland dysfunction Lagophthalmos Defective blinking Vit A deficiency

30. WATERING EYE HYPERLACRIMATION – Primary – direct stimulation of lacrimal gland Ex. Early stages of lacrimal gland tumors and cyst 2. Reflex - stimulation of sensory branches of 5 th nerve d/t irritation of cornea or conjunctiva 3. Central ( psychical lacrimation) – emotional states , voluntary lacrimation

31. Epiphora- 1.Physiological – lacrimal pump failure 2.Mechanical obstruction – Punctal- eversion of lower punctum , punctal obstruction Canalicular – canalicular obstruction Lacrimal sac – congenital mucous membrane fold , dacrocystitis , tumor NLD- noncanalization , partial canalization or imperforated membranous valve