Tear Film

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

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