## Production and composition
-  Protective covering of cornea (which is hydrophobic and non-wettable)-  Tears are distributed to three areas
-  Marginal tear strip/meniscus-  Precorneal tear film (covering bulbar conjunctiva and cornea)
-  About 3.5 microns thick
-  Conjunctival sac (between the lids and the bulbar conj)
-  Tear mucins are produced by conjunctival goblet cells and conjunctival squamous epithelium
-  Goblet cells secrete glycoproteins, proteins, lipoproteins
-  MUC5AC is the main tear mucin.
-  Stability is ensured by low surface tension (due to aqueous and lipid components) and spreading of tears during blinking
-  Tear film break-up time approximates to ‘stability’
-  Typically 10-30 seconds.
-  Typical tear volume is approximately 6-7 microlitres 
-  Tears are secreted at approx 4 microL/minute

<ClinicalCorrelate> <p> Schirmer’s test: Normal when there is 15-25mm of wetting over a period of 5 minutes </p> </ClinicalCorrelate>

### Biochemistry
-  pH = 7.4-  Note: tears have much higher potassium and chloride compared to plasma. They have lower glucose than plasma since they supply the cornea with glucose.-  Protein content: 7-10 mg/ml (less with high flow tears)
-  Mostly albumin

## Three layers of tear film
### Surface oily/lipid layer
-  Function: prevents evaporation and spillover (through surface tension) and ensures optical clarity
-  0.1 microns-  Produced by 30-40 **meibomian glands** in the upper lid and 20-30 in the lower lid
-  These are holocrine glands supplied by parasympathetic nerves
-  Production is stimulated by blinking (not neural control)-  Note: glands of Zeis also secrete some lipid

### Aqueous layer
-  Composed of proteins (IgA, lactoferrin, lipocalin, lysozyme, betalysin), water and electrolytes.
-  Functions: Oxygenates corneal epithelium, maintains electrolyte supply over ocular surface, antimicrobial defense, smooths irregularities for optics, and washes away debris
-  Produced by **lacrimal apparatus**
-  Main lacrimal gland (reflex secretion): rich parasympathetic/cholinergic innervation-  (Accessory) glands of Krause: located in lateral upper fornix-  (Accessory) glands of Wolfring: scattered along superior tarsal margin
-  Electrolytes
-  Osmolarity is similar to plasma
-  Sodium concentration is similar to blood
-  Potassium concentration is 5-7 times greater than blood
-  Chloride concentration higher than blood
-  Bicarbonate buffers tear pH
-  Proteins
-  IgA secretion: antibacterial 
-  Production increases with neural stimulation
-  Concentration decreases with age.

### Deep mucous layer
-  Makes cornea wettable and provides viscosity-  Coats the glycocalyx of corneal epithelium
-  Contains: mucins, proteins, electrolytes, water 
-  Functions: stabilises tear film, trap debris, lubrication

## Secretion
-  Secretion combines: accessory glands (baseline secretion) and the main lacrimal gland (reflex tearing) 
-  Physical irritation of cornea/conjunctiva (Note: there is parasympathetic communication between the pterygopalatine ganglion to the conjunctiva)
-  Psychogenic tearing
-  Bright light (via the optic nerve)

## Dysfunction
-  Change in tear constituents
-  Change in composition
-  Aqueous deficiency: keratoconjunctivitis sicca (possibly related to ocular surface inflammation)
-  Mucin deficiency/excess
-  Lipid abnormality (meibomian gland dysfunction)
-  Uneven dispersion due to surface irregularities
-  Contact lens use
-  Poor distribution due to eyelid-globe interface irregularity
-  Congenital, traumatic or neurogenic eyelid dysfunction
-  Dysfunctional blink mechanism

## Tests of tear film dysfunction
-  Tear breakup time
-  Lissamine green staining
-  Rose bengal staining
-  Osmolality tests
-  Schirmer's test

Production and composition Protective covering of cornea (which is hydrophobic and non-wettable)- Tears are distributed to three areas Marginal tear…

Orbit and ocular adnexae

Tear Film

Intro

FRCOphth Part 2 Study Notes

Tumours, masses and neoplasia

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Uveitis

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Corneal Structure And Development