March 2015
Volume 56, Issue 3
Free
Letters to the Editor  |   March 2015
More to Stable Tears Than Thickness of the Tear Film Lipid Layer
Author Affiliations & Notes
  • Beau J. Fenner
    Singapore General Hospital, Singapore;
  • Louis Tong
    Singapore National Eye Centre, Singapore;
Investigative Ophthalmology & Visual Science March 2015, Vol.56, 1601. doi:10.1167/iovs.15-16490
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      Beau J. Fenner, Louis Tong; More to Stable Tears Than Thickness of the Tear Film Lipid Layer. Invest. Ophthalmol. Vis. Sci. 2015;56(3):1601. doi: 10.1167/iovs.15-16490.

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      © ARVO (1962-2015); The Authors (2016-present)

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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 coworkers1 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. 
References
King-Smith PE Reuter KS Braun RJ Nichols JJ Nichols KK. Tear film breakup and structure studied by simultaneous video recording of fluorescence and tear film lipid layer images. Invest Ophthalmol Vis Sci. 2013; 54: 4900–4909. [CrossRef] [PubMed]
Blackie CA Solomon JD Scaffidi RC Greiner JV Lemp MA Korb DR. The relationship between dry eye symptoms and lipid layer thickness. Cornea. 2009; 28: 789–794. [CrossRef] [PubMed]
Ngo W Situ P Keir N Korb D Blackie C Simpson T. Psychometric properties and validation of the Standard Patient Evaluation of Eye Dryness questionnaire. Cornea. 2013; 32: 1204–1210. [CrossRef] [PubMed]
Rantamäki AH Wiedmer SK Holopainen JM. Melting points--the key to the anti-evaporative effect of the tear film wax esters. Invest Ophthalmol Vis Sci. 2013; 54: 5211–5217. [CrossRef] [PubMed]
Butovich IA Lu H McMahon A Biophysical and morphological evaluation of human normal and dry eye meibum using hot stage polarized light microscopy. Invest Ophthalmol Vis Sci. 2014; 55: 87–101. [CrossRef] [PubMed]
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