ReviewMechanisms, imaging and structure of tear film breakup
Section snippets
Aims and scope
Breakup (BU) is an important but poorly understood aspect of the tear film and dry eye disorders. The aim of this review is to describe the wide range of imaging methods for studying tear BU and to propose mechanisms and structure for BU that incorporate current understanding of the fluid dynamics involved. Much of the information presented in this review is based on the authors' experience and thus is descriptive rather than quantitative.
The following areas are covered in this review. First,
Definition of breakup
According to the 2007 Dry Eye Workshop [1], “the tear film breakup time is defined as the interval between the last complete blink and the first appearance of a dry spot or disruption in the tear film.” The “first appearance of a dry spot” may relate to the appearance of a dark area in FBUT, whereas the “disruption of the tear film” may correspond to distortions of the tear surface seen in NIBUT. Lemp and Hamill [18] added that, from trial to trial, the BU position should be randomly located,
Breakup time is an important clinical test of tear film function
Together with the Schirmer test, staining tests, and history, FBUT is one of the preferred diagnostic tests for dry eye used by eye care practitioners [67]; since that report, osmolarity testing has become another common test [68]. Abelson et al. [69] found a normal mean FBUT of 7.1 s, which was reduced to 2.2 s in dry eye; they recommended a cutoff for dry eye diagnosis of ≤5 s, while previous studies proposed a cut off of 10 s [70], [71]. It may be noted that a larger cutoff value increases
Surface physical chemistry models
An early model and much referenced paper by Holly [13] was based on ideas from surface physical chemistry; the model involved diffusion of lipids through the aqueous layer, causing the mucus layer to become hydrophobic and thus generating BU. A critical review of this and other models based largely on physical chemistry of the mucus layer and corneal surface has been published by Peng et al. [55] A limitation of such models has sometimes been imperfect in vivo experimental evidence; for
Classification of breakup
BU is a complicated and still poorly understood process, so a complete classification of BU is probably not possible at present. In the classification proposed here, we have tried to distinguish three types of BU, which depend mainly on different mechanisms and can be differentiated from each other quite readily.
BU has sometimes been classified by shape of the BU area. Bitton and Lovasik [107] described three distinct patterns – “dots” with a circular shape, “streaks” having a linear shape, and
Images of breakup obtained by different methods
In this review, the importance of different imaging methods for studying mechanisms of BU is emphasized. Five different types of methodology for imaging the tear film are illustrated in Fig. 6 and will be reviewed in this section. Note that both the tear and corneal surfaces are rough, so both can contribute to spatial variations in tear film thickness. This may be expressed by the equationh(x,y,t) = a(x,y,t)-c(x,y,t)where h is tear film thickness, x and y give position on the tear surface, t
Three directions of tear flow determine tear thinning and breakup
Tear film thickness changes within a small area of tear film are determined by three directions of tear flow [34]. First, water may flow outwards into the air by evaporation causing tear thinning. Second, water may flow across the corneal surface, typically by osmosis [142]; in the interblink interval, evaporation causes increased tear osmolarity and hence osmotic flow into the tear film, tending to oppose tear film thinning. Third, there may be “tangential flow” (flow along the corneal
Ten factors in tear film breakup and breakup time
Section 7 listed three directions of tear flow, which determine the thinning of the tear film, leading to BU. This section elaborates a number of factors that affect these three directions of flow. In addition, the initial thickness of the tear film deposited by a blink [111] is discussed because it is an important factor in BUT. Finally, factors involved in causing touchdown and after touchdown will also be considered.
Future directions
There is an important need for a more quantitative understanding of the processes involved in BU and BUT. This applies to the thinning of the tear film before touchdown (Fig. 1, Fig. 17), as well as the processes after touchdown. In Section 7, three directions of tear flow contributing to, or countering, tear film thinning were discussed, namely evaporation, flow across the corneal surface (osmotic flow), and tangential flow along the corneal surface. The relations between these three
Conclusions
Tear film BU is an essential characteristic of dry eye. BUT is reduced in dry eye, so it is an important clinical test. BU often causes high PCTF osmolarity and may sometimes cause mechanical shear of the cornea; hence, BU stresses the corneal surface causing irritation and inflammation.
The definition of BU suffers from uncertainty (does the tear film form a dry spot?) and lack of objectivity (e.g., how dim should fluorescence be at BU?). It is proposed that a more objective definition would be
Acknowledgement
The authors have no commercial or proprietary interest in any product or concept described in this article.
Financial support: RO1 EY017951 (King-Smith), RO1 EY021794 (Begley), NSF 1412085 (Braun).
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