P. Ewen King-Smith

The thickness of the tear film

Measurements of the thickness of the pre-corneal tear film, pre-lens tear film, post-lens tear film, and the lipid layer on the surface of the tear film are summarized. Spatial and temporal variations in tear film thickness are described. Theoretical predictions of tear film thickness are discussed. Mechanisms involved in the upward drift of the tear film after a blink, and in the formation of dry spots, are considered.

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

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).

The Contribution of Lipid Layer Movement to Tear Film Thinning and Breakup

PURPOSE To investigate whether the tear film thinning between blinks is caused by evaporation or by tangential flow of the tear film along the surface of the cornea. Tangential flow was studied by measuring the movement of the lipid layer. METHODS Four video recordings of the lipid layer of the tear film were made from 16 normal subjects, with the subjects keeping their eyes open for up to 30 seconds after a blink. To assess vertical and horizontal stretching of the lipid layer and underlying aqueous layer, lipid movement was analyzed at five positions, a middle position 1 mm below the corneal center, and four positions respectively 1 mm above, below, nasal, and temporal to this middle position. In addition, in 13 subjects, the thinning of the tear film after a blink was measured. RESULTS The total upward movement could be fitted by the sum of an exponential decay plus a slow steady drift; this drift was upward in 14 of 16 subjects (P = 0.002). Areas of thick lipid were seen to expand causing upward or downward drift or horizontal movement. The velocity of the initial rapid upward movement and the time constant of upward movement were found to correlate significantly with tear film thickness but not with tear-thinning rate. CONCLUSIONS Analysis indicated that the observed movement of the lipid layer was too slow to explain the observed thinning rate of the tear film. In the Appendix, it is shown that flow under a stationary lipid layer cannot explain the observed thinning rate. It is concluded that most of the observed tear thinning between blinks is due to evaporation.

Contributions of evaporation and other mechanisms to tear film thinning and break-up.

Purpose. To evaluate the contribution of three mechanisms—evaporation of the tear film, inward flow of water into the corneal epithelium or contact lens, and “tangential flow” along the surface of epithelium or contact lens—to the thinning of the tear film between blinks and to tear film break-up. In addition to a discussion of relevant studies, some previously unpublished images are presented illustrating aspects of tear film break-up. Contributions of Three Mechanisms to Tear Film Break-Up. Inward flow of water into the epithelium or contact lens is probably unimportant, and a small flow in the opposite direction may actually occur. Tangential flow is probably important in certain special cases of tear film break-up—at the black line near the tear meniscus, over surface elevations, after partial blinks, and from small thick lipid spots in the tear film. In all these special cases it is argued that tangential flow is important initially, but evaporation may be needed for final thinning to break-up. It is argued that most of the observed tear film thinning between blinks is due to evaporation, rather than tangential flow, and that large “pool” break-up regions are the result of evaporation over an extended area. Conclusion. Evaporation in our “free-air” conditions may be four to five times faster than the average of the values reported in the literature when air currents are prevented by preocular chambers. However, recent evaporation measurements using “ventilated chambers” give higher values, which may correspond better to free-air conditions. Thus evaporation may be fast enough to explain many cases of tear film break-up, and to give rise to considerable increases in the local osmolarity of the tear film between blinks.

Evidence for the Major Contribution of Evaporation to Tear Film Thinning between Blinks

PURPOSE To determine the contribution of evaporation to the thinning of the precorneal tear film between blinks. METHODS The rate of tear film thinning after a blink was measured using spectral interferometry from the right eyes of 37 subjects. Data were obtained under two different conditions: free air and air-tight goggles. RESULTS The mean (±SD) tear film thinning rates for subjects was 3.22 ± 4.27 μm/min in free air and -0.16 ± 1.78 μm/min (i.e., a slight but not significant thickening) for the same subjects wearing air-tight goggles; this reduction in thinning rates was significant (P < 0.0001). CONCLUSIONS The large reduction in thinning rate caused by wearing goggles indicates that evaporation is the major cause of thinning between blinks. The mean thinning rate in free air is greater than reported evaporation rates; it is argued that the preocular chambers used in evaporimeters restrict movement of air over the tear film and reduce evaporation compared to our free air condition.

Interferometric measurement of tear film thickness by use of spectral oscillations

A method of measuring the tear film thickness is described in which interference causes oscillations in the reflectance spectrum from the tears. Strong oscillations were usually observed when a contact lens was worn. Measurement of modulation and phase of these oscillations confirmed that they were associated with the tear layer in front of the contact lens. Calculated thickness of this layer averaged 2.7 microns. In one out of five subjects, weak oscillations were sometimes observed without a contact lens. These oscillations probably arose from the aqueous layer of the tears with a thickness of approximately 3 microns. The relative merits of three interference methods of measuring the tear film are discussed.

The Use of Fluorescent Quenching in Studying the Contribution of Evaporation to Tear Thinning

PURPOSE The purpose of our study was to test the prediction that if the tear film thins due to evaporation, rather than tangential flow, a high concentration of fluorescein in the tear film would show a greater reduction in fluorescent intensity compared to a low concentration of fluorescein due to self-quenching at high concentrations. METHODS Tear film thickness, thinning rate, and fluorescent intensity were measured continuously and simultaneously with a modified spectral interferometer in 30 healthy subjects with two different concentrations (2% followed by 10%) of 1 μL of liquid fluorescein on the eye. Measurements of fluorescein self-quenching (fluorescent efficiency as a function of fluorescein concentration) are described in an Appendix and are reported in arbitrary units. RESULTS Under low and high fluorescein concentration conditions, there were no differences in tear film thickness (P = 0.09) or thinning rates (P = 0.76). While the mean initial fluorescent intensity was similar between groups (637.47 ± 381.47 vs. 672.09 ± 649.72, P = 0.55), the mean rate of fluorescent decay was 4-fold faster in the high (16.57 ± 29.34) than in the low (4.11 ± 6.78) concentration group (P < 0.01). CONCLUSIONS The large difference in the rate of fluorescent decay between groups can be explained by the effects of evaporation and self quenching of fluorescein; the latter is expected to be greater for high than for low fluorescein concentration. Fluorescence decay due to tangential flow would be expected to be similar at high and low fluorescein concentrations. This supports previous evidence that evaporation has the primary role in normal tear thinning between blinks.

High resolution microscopy of the lipid layer of the tear film.

Tear film evaporation is controlled by the lipid layer and is an important factor in dry eye conditions. Because the barrier to evaporation depends on the structure of the lipid layer, a high resolution microscope has been constructed to study the lipid layer in dry and in normal eyes. The microscope incorporates the following features. First, a long working distance microscope objective is used with a high numerical aperture and resolution. Second, because such a high resolution objective has limited depth of focus, 2000 images are recorded with a video camera over a 20-sec period, with the expectation that some images will be in focus. Third, illumination is from a stroboscopic light source having a brief flash duration, to avoid blurring from movement of the lipid layer. Fourth, the image is in focus when the edge of the image is sharp - this feature is used to select images in good focus. Fifth, an aid is included to help align the cornea at normal incidence to the axis of the objective so that the whole lipid image can be in focus. High resolution microscopy has the potential to elucidate several characteristics of the normal and abnormal lipid layer, including different objects and backgrounds, changes in the blink cycle, stability and fluidity, dewetting, gel-like properties and possible relation to lipid domains. It is expected that high resolution microscopy of the lipid layer will provide information about the mechanisms of dry eye disorders. Illustrative results are presented, derived from over 10,000 images from 375 subjects.

Tear film breakup and structure studied by simultaneous video recording of fluorescence and tear film lipid layer images.

PURPOSE The thinning of the precorneal tear film between blinks and tear film breakup can be logically analyzed into contributions from three components: evaporation, flow into the cornea, and tangential flow along the corneal surface. Whereas divergent tangential flow contributes to certain types of breakup, it has been argued that evaporation is the main cause of tear thinning and breakup. Because evaporation is controlled by the tear film lipid layer (TFLL) it should therefore be expected that patterns of breakup should match patterns in the TFLL, and this hypothesis is tested in this study. METHODS An optical system is described for simultaneous video imaging of fluorescein tear film breakup and the TFLL. Recordings were made from 85 subjects, including both with healthy and dry eyes. After instillation of 5 μL2% fluorescein, subjects were asked to blink 1 second after the start of the recording and try to maintain their eyes open for the recording length of 30 or 60 seconds. RESULTS Areas of tear film thinning and breakup usually matched corresponding features in the TFLL. Whereas thinning and breakup were often matched to thin lipid, surprisingly, the corresponding lipid region was not always thinner than the surrounding lipid. Occasionally, a thin lipid region caused a corresponding region of greater fluorescence (thicker aqueous layer), due to convergent tangential flow. CONCLUSIONS Areas of tear thinning and breakup can generally be matched to corresponding regions of the TFLL as would be expected if breakup is largely due to evaporation. Surprisingly, in some examples, the corresponding lipid area was not thinner and possibly thicker than the surrounding lipid. This indicates that the lipid was a poor barrier to evaporation, perhaps because of deficiency in composition and/or structure. For example, bacterial lipases may have broken down esters into component acids and alcohols, causing a defective TFLL structure with increased evaporation.

The effect of eye closure on the post-lens tear film thickness during silicone hydrogel contact lens wear.

Purpose. The purpose of this study was to investigate the effect of eye closure on the thickness on the post-lens tear film (POLTF) during silicone hydrogel contact lens wear. Methods. Ten subjects (mean age, 30.2 ± 8.6 years; seven males) wore balafilcon A silicone hydrogel contact lenses in both eyes during the experimental procedure (power = –2.00 D, base curve = 8.6 mm). Previously described interference techniques, based on oscillations in reflectance spectra, were used to measure the post-lens tear film. Reflectance spectra (562–1030 nm) from the front of the eye wearing a contact lens were measured at normal incidence where the thickness of the POLTF was derived from the “frequency” of the oscillations. Six baseline measures of POLTF thickness were taken, followed by eye closure in the supine position for 30 minutes. Post-lens tear film thickness measures were taken after 5 and 15 minutes of eye closure. After 30 minutes of eye closure, 1 measure of POLTF was taken every minute for an additional 15 minutes. Results. The average baseline POLTF thickness was 2.00 ± 0.30 &mgr;m (median, 2.00) for the 10 subjects. The average POLTF thickness decreased to 1.58 ± 0.33 and 1.20 ± 0.23 &mgr;m after 5 and 15 minutes of eye closure, respectively (repeated measures ANOVA, F = 42.18, P < 0.0001). Only 5 of 60 POLTF thickness estimates were obtained for the first 6 minutes following completion of 30 minutes of eye closure, indicating that the POLTF may be too thin (i.e., <1 &mgr;m) to measure using this method. However, there was a significant increase in POLTF thickness for 15 minutes after prolonged eye closure (mixed modeling regression, thickness = 0.85 + 0.07 × time after eye opening; P < 0.0001). Conclusions. The POLTF thickness is rapidly reduced by eye closure during contact lens wear. After 30 minutes of eye closure, the POLTF thickness may be reduced to values less than 1 &mgr;m for the first several minutes after opening the eyes.