Tear Film

• Tear Structure

  • Lipid layer - Originates from Meibomian glands, Glands of Zeiss, Glands of Moll
  • Aqueous layer - Originates from Lacrimal gland, Gland of Krause, Gland of Wolfring
  • Mucus layer - Origin: Goblet cells (glands of Manz and crypts of Henle), Non-goblet epithelial cells, Lacrimal gland
  • Microvilli
  • Corneal epithelium
• Aqueous layer


  • Composition - Electrolytes, hydrogen ions (Na+, K+, Ca2+ and others), Proteins/Enzymes (Lysozyme, Lactoferrin, Lipocalin, IgA, 60+ others), Metabolites - Glucose
• Functions of Tear Film

  • Optical - Fills in corneal epithelial irregularities, provides a perfect smooth optical surface
  • Mechanical - Lubrication, Flushes debris, removes water from the cornea
  • Nutrition - O2 and nutrients
  • Defence - Antibacterial agents
• Tear film has a refractive index of roughly 1.33
• What can we assess in the tear film? Physical parameters - Tear volume, Tear turnover/production, Tear evaporation rate, Tear film stability, Lipid layer thickness
• Basal vs Reflex tears - Reflex tearing as a result of Peripheral sensory stimulation of cornea or cilia, Central sensory stimulation of the retina (light), Psychogenic (emotional) factors
• Tear production/volume - Total volume = 4-10 microlitres, Mean volume = 7.0 microlitres, Max capacity = 25 microlitres
• The tear film is more than 10 times thinner than a sheet of paper
• Mucus layer

  • Composition - High molecular weight mucins (protein with many carbohydrate chains attached)
  • Properties - Allows the aq to spread over the non-wetable corneal epithelium
  • Function - Essential in making the tear film spread over cornea, Important in maintaining tear stability
• Lipid layer


  • Composition - polar and non-polar lipids
  • Properties - Spreads well over the tear film, does not spread on saline, aqueous layer contains special proteins
  • Function - Retards evaporation of aqueous, prevents skin lipid contamination, prevents over-spilling onto eyelids
• Clinical tests used to assess tear production and volume: Schirmer test - Insert filter paper onto tear film for around 5 minutes, Phenol red thread - Insert thin phenol red thread onto tear volume for 15 seconds, Tear meniscus assessment - Observe the shape and size of the tear meniscus
• Dynamics and drainage

  1. Excretory System - Gravity (Tears fall by gravity into lower meniscus), Capillary attraction (Conducts tears to the puncta), Lid movements

  2. Evaporation

  3. Conjunctival absorption
• FBUT - Fluorescein impregnated strip wetted with saline and a drop put into the eye, observed with slit-lamp and cobalt blue filter, look for breakup (dark spots). This is widely used however is not the optimal test, a large volume is added to the tears (30+ microlitres) and is known to destabilise tears
• To assess the lipid layer appearance

  Use a TEARSCOPE-PLUS, this can examine the tear film quality non-invasively and can estimate the quality, quantity, and stability of the tear film
• 10-40% of tears are lost by evaporation. Results indicate lipid layer abnormalities. Evaporimeter is not clinically available due to the cost and number of evaporimeters
• Clinical tests used to assess tear production and volume

  1. Schirmer test - Insert filter paper onto tear film for around 5 minutes

  2. Phenol red thread - Insert thin phenol red thread onto tear volume for 15 seconds

  3. Tear meniscus assessment - Observe the shape and size of the tear meniscus. Best with a graticule in slit-lamp. Dry eye is irregular and scalloped
• Patient history is very useful when observing the tear film, McMonnie's questionnaire is the most widely used, Ocular Surface Disease Index (OSDI) is also widely used
• Dry eye is an important health issue with 9 million sufferers in the US alone (~3%). It is even more common in the older population with 15% in 55-72 years and 35% in 63-92 years. It is also economically important with markets for dry eye lubricants, contact lenses, and true dry eye treatment amounting to billions of US dollars
• Break up time

  1. Fluorescein break up time (FBUT)

  2. Non-invasive tear break up time (NITBUT) - HIRCAL grid, TearScope, Loveridge grid
• NITBUT - Dry spots or loss of transparency in the tear film can be observed using diffuse, low luminance, heat filtered white light and moderate magnification. A grid or concentric ring pattern is projected onto the cornea and the time that elapses to distortion of the pattern is measured
• Research tests used to assess tear production and volume

  1. Ocular Fluorophotometry - Best way to determine tear volume and turnover. 1 microlitre of fluorescein is put into tear film, the decay of fluorescein is monitored by optical methods (Fluorotron) for 25 minutes. The decay curve is calculated. Back extrapolation to time zero gives the tear volume. This is unfortunately not clinically viable as it costs $40,000 and takes 30 minutes per patient

  2. Gamma Scintigraphy - Good method but need gamma emitting isotope, does show passage down puncta into nasolacrimal duct
• When the tear film does not function properly

  It causes Dry Eye
• To examine Ocular Surface Quality

  1. Fluorescein staining

  2. Rose Bengal staining

  3. Lissamine Green staining