Application of a novel interferometric method to investigate the relation between lipid layer thickness and tear film thinning

Abstract

Purpose: The lipid layer of the tear film forms a barrier to evaporation. Evaporation is a major cause of tear thinning between blinks and tear breakup. The purpose of this study was to investigate the relation between tear film thinning and lipid layer thickness before and after instillation of an emulsion eye drop.

Methods: Fifty non-contact lens wearers were studied. Spectral interferometry was used to measure the thinning rate of the precorneal tear film for up to 19 seconds after a blink. Simultaneously, lipid layer thickness was measured based on an absolute reflectance spectrum. After a 2-minute recovery, the measurement was repeated. A drop of the lipid emulsion was then instilled; 15 minutes later, two interferometry measurements were performed similarly.

Results: A histogram of thinning rates was fitted by a bimodal distribution with narrow and broad peaks corresponding to slow and rapid thinning, respectively. The correlation between repeated thinning rate measurements was modest, but repeatability was considerably more significant when analyzed in terms of the slow/rapid dichotomy. Similarly, the correlation between thinning rate and lipid thickness was modest but was more evident when analyzed in terms of the slow/rapid dichotomy. Instillation of an emulsion eye drop significantly increased the thickness of the lipid layer but did not significantly alter the thinning rate.

Conclusions: The proposed slow/rapid dichotomy of thinning rates presumably relates to a good/poor barrier to evaporation of the lipid layer. The imperfect correlation between thinning rate and lipid thickness indicates that other factors, such as the composition and structure of the lipid layer, are important (e.g., sufficient polar lipids may be needed to form good interface between nonpolar lipids and the aqueous layer).

Figure 1.

(a) Absolute value of calculated reflectance from the front of the tear film as a function of wave number and lipid thickness. (b) Relative reflectance (curves scaled to match at a central wave number).

Figure 2.

Simplified diagram of the optical systems with the eye slightly misaligned. L₁, L₂, L₃, lenses; thick rays, illuminating beam; thin rays, reflected beam.

Figure 3.

Histogram of lipid thickness (uniform thickness assumption, n = 100). Error bars indicate 1 SE, derived from Poisson statistics.

Figure 4.

Histogram of thinning rates (n = 99). Error bars indicate 1 SE, derived from Poisson statistics. The histogram has been fit by a sum of two Gaussian distributions corresponding to slow (solid curve) and rapid (dashed curve) thinning. The intersection of the two Gaussian functions occurs at a rate of 4 μm/min and is marked by the vertical line. This rate is considered to be the boundary between slow and rapid thinning.

Figure 5.

Second measurement of lipid thickness as a function of the first measurement (n = 50). Filled circles: seven eyes with OSDI scores 21 (possible dry eye). Dashed line: equality of the two measurements.

Figure 6.

Second measurement of thinning rate as a function of the first measurement (n = 49). Filled circles: seven eyes with OSDI scores 21. Dashed line: equality of the two measurements. Horizontal and vertical lines: boundary between slow and rapid thinning. Numbers of subjects in the four resultant areas are indicated.

Figure 7.

Thinning rate as a function of lipid thickness; data have been averaged for each subject (n = 50). Filled circles: seven eyes with OSDI scores 21. Horizontal line: boundary between slow and rapid thinning. Inset: ROC curve indicating the ability of lipid thickness measurements to predict rapid thinning.

Figure 8.

Lipid thickness after instillation of lipid emulsion as a function of thickness before instillation (averages of two measurements per subject; n = 50). Filled circles: seven eyes with OSDI scores 21. Dashed line: equality of thickness before and after instillation.

Figure 9.

Thinning rates 15 minutes after instillation of lipid emulsion as a function of thinning rate before instillation (n = 50). Filled circles: seven eyes with OSDI scores 21. Dashed line: equality of thinning rates before and after instillation. Horizontal and vertical lines: boundary between slow and rapid thinning rates of 4 μm/min. Numbers of observations in the four resultant regions are indicated.