ReviewTear film stability: A review
Introduction
The ocular surface is a complex unit comprising various epithelial and glandular tissues (cornea, bulbar and palpebral conjunctiva, and lacrimal and accessory eyelid glands). These tissues secrete the tear film that coats and protects the ocular surface and allows clear vision (Holly, 1973, Stern et al., 1998, Tutt et al., 2000, Gipson, 2007). Its complexity is highlighted by observations that the composition of tears varies between open eye and closed eye (Sack et al., 2000), stimulated and non-stimulated (Fullard and Snyder, 1990), and in diseased versus normal (Li et al., 2005, Tomosugi et al., 2005, Green-Church et al., 2008, Lema et al., 2010, Versura et al., 2010, Acera et al., 2011, Zhou et al., 2012) states. Therefore, deciphering the components of the tear film that are irregular and inadequate when it is unstable is a challenge for both scientists and clinicians, and the effects of a stable tear film on the ocular surface health is paramount. The purpose of this review is to bring to the forefront the current technologies and methods used to assess tear film stability, both in the clinical and laboratory setting, as well as critically revisit the literature on some of the concepts regarding tear film stability and what factors can influence tear film stability.
Section snippets
Measurement of tear film stability
In general, tear film stability is measured by its lack of stability, which is important clinically because it can be used for both diagnosis and assessment of treatments for dry eye states (Nichols et al., 2000, Bron, 2001). Its measurement is of importance to both clinicians and researchers. Clinicians are looking for evidence to support diagnosis of conditions that affect ocular health and patient comfort and quality of life, and to assess and monitor the effectiveness of treatments and
Factors controlling tear film stability
Instability is often regarded to be due to an excessive depletion of the aqueous component of tears and therefore, evaporation rates have often been a centrepoint of many studies into tear film stability. Mathers and Lane (1998) contended “... tear film stability actually refers primarily to evaporation rate; a stable tear film is one in which a minimum amount of tears evaporates.”
Fundamental to this idea is proving that the tear film thins due to evaporation rather than due to other mechanisms
Age, gender, race
Infants have highly stable tear films (Isenberg et al., 2003). As we age, the blink rate increases by up to 20 fold (Lawrenson et al., 2003) and tear film stability reduces (Patel and Farrell, 1989, Tonge et al., 1991, Cho and Yap, 1993, Ozdemir and Temizdemir, 2010, Sullivan et al., 2006).
Patel et al. (2000) used TTT to evaluate tear film stability with age in 55 males and females (110 subjects in total) ranging in age from 18 to 89 years. They reported tear film stability is lower in the aged
Conclusion and future directions
A number of procedures exist to measure tear film stability both in the clinical situation and for research purposes. Recent developments have focussed on technology that can examine the tear film dynamically and explain its kinetics. Further research directed to understanding what controls tear film stabilization is important. There are many individuals, some over the age of 50, with very stable tear films. These populations need to be identified and their tear film investigated along with
Disclosures
- a.
The author has no funding to disclose but was a former member of the Vision CRC which has been sponsored by Ciba Vision.
- b.
The author is a member of the Vision CRC which is sponsored by Ciba Vision and also receives funding from Allergan.
- c.
The author is a member of the Vision CRC which is sponsored by Ciba Vision.
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