A deficit in the quantity and/or quality of the tear film (e.g. a decrease in the tear flow, volume, thickness or lubricity) may have a number of damaging effects.
- Evaporation from the reduced volume of tears leads to an increase in the osmolarity of the tear fluid. This is damaging to the corneal epithelium and can give rise to many of the symptoms experienced in dry eye. Damaged cells shed some of their epithelial components (desquamation), leaving erosion on the cornea. The desquamated cells appear as increased cellular debris in the tear film. The corneal epithelial cells are also smaller in dry eye, while conjunctival cells appear larger. In addition, there is a change in the appearance of part of the nucleus of conjunctival epithelial cells (the chromatin).
- Even in mild dry eye, damage to the corneal surface can lead to redness of the eye and inflammation of the eyelids and conjunctiva (blepharitis and Meibomian disease). In addition, lysozyme (and probably lactoferrin) levels are reduced in dry eye, reducing the ability of the eye to resist infection. This can also lead to inflammation, eventually causing changes in the aperture of the eyelids.
- As the quality of the tear film decreases, it becomes unstable and begins to break up earlier than normal (indicated by a reduced TBUT). In combination with a reduction in the aqueous layer, this can result in dry spots on the corneal surface or, in more advanced cases, drying to the whole of the cornea.
- In between blinks, the tear film thins and begins to break up. If this continues without another complete blink, a dry spot or area of discontinuity develops within the tear film. This is thought to be due to contamination of the mucin by the outer lipid layer. The lipid-contaminated mucin creates a hydrophobic spot from which the aqueous tears retract. If another blink does not occur, the hydrophobic area enlarges and spreads, creating large areas of discontinuity in the tear film.
Altered tear film with dry spot
- As a result, the cornea and conjunctiva are not as well protected, leaving them more exposed to external damaging factors such as wind, dust and cigarette smoke. Damage to the conjunctiva causes a decrease in the density of goblet cells, leading to mucin deficiency. As the tear film breaks up, the behaviour of the mucin is also altered. The number of mucus threads increases and they become stringy and sticky. In addition, the cornea is deprived of the oxygen and nutrients that are present in the aqueous layer, and so rapidly undergoes destructive changes.
These changes lead to unpleasant sensations such as dryness, grittiness, pain, itching,
sensitivity to light and blurred vision. If not corrected, the changes can result in brown
pigmentation, scar tissue growth, ulcer development and blood vessel growth, leading
to partial vision loss.
Consequences of dry eye
The symptoms of dry eye, which are very subjective and can vary in severity, include a combination of the following:
- Itching, burning, irritation, redness, dryness
- Foreign body sensation, grittiness
- Stringy mucus in and around the eye
- Blurred vision that improves with blinking
- Excessive tearing
- Increased discomfort after periods of reading, watching television or working on a computer, discomfort when wearing contact lenses
In general, there is a gradual onset of symptoms, with a period of exacerbation that may be heightened by decreases in ambient humidity. The itchiness and irritation experienced by the patient often causes them to rub their eyes. They also often attempt to manually remove the mucin threads that are present. However, instead of alleviating the discomfort, such actions usually lead to further irritation and tearing.
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