King-Smith et al.
1 recently published an enlightening piece of work showing that tear film evaporation does not necessarily correlate with thickness of the tear film lipid layer. This has important implications in the field of dry eye research, where the consensus opinion for many years has been that a thicker lipid layer serves to prevent evaporation of the aqueous component of the tear film, helping to prevent the development of dry eye symptoms.
2,3 We were especially interested in the work of King-Smith et al.
1 because it corroborates our own work on the relationship between lipid layer thicknesses and tear film breakup.
A prospective study at our center involving 86 undifferentiated patients (mean age 56.1 ± 13.7 years) found that noninvasive tear breakup time (NIBUT) did not significantly correlate with lipid layer thickness (r2 = 0.001, slope = −0.126, P = 0.750). In our study, a LipiView interferometer was employed to noninvasively assess lipid layer thickness among the 86 patients (mean thickness 64.9 ± 23.5 nm). Optical interference patterns produced by light reflected from the tear film in situ are used to derive lipid layer thickness in this method. An Oculus Keratograph model 5M was used to measure NIBUT among the same patients (mean NIBUT 9.21 ± 6.44 seconds) and the data were analyzed using an F test for regression.
In light of these findings, we are inclined to agree with the suggestion by King-Smith and coworkers
1 that tear film breakup may be impacted by more than simply lipid layer thickness, and may also be a function of lipid layer composition. Other workers have recently shown that tear film evaporation is dictated by only certain components of the lipid layer.
4 Moreover, some dry eye patients appear to have abnormal lipid compositions that may impact the normal functioning of the tear film.
5 Thus, it remains to be seen how best to interpret tear breakup times in dry eye patients, but it almost certainly requires more information than simply lipid layer thickness data. At the same time, the role and significance of lipid layer thickness in dry eye, previously thought to be functionally important, requires reexamination.