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

Dynamics and function of the tear film in relation to the blink cycle

Great strides have recently been made in quantitative measurements of tear film thickness and thinning, mathematical modeling thereof and linking these to sensory perception. This paper summarizes recent progress in these areas and reports on new results. The complete blink cycle is used as a framework that attempts to unify the results that are currently available. Understanding of tear film dynamics is aided by combining information from different imaging methods, including fluorescence, retroillumination and a new high-speed stroboscopic imaging system developed for studying the tear film during the blink cycle. During the downstroke of the blink, lipid is compressed as a thick layer just under the upper lid which is often released as a narrow thick band of lipid at the beginning of the upstroke. "Rippling" of the tear film/air interface due to motion of the tear film over the corneal surface, somewhat like the flow of water in a shallow stream over a rocky streambed, was observed during lid motion and treated theoretically here. New mathematical predictions of tear film osmolarity over the exposed ocular surface and in tear breakup are presented; the latter is closely linked to new in vivo observations. Models include the effects of evaporation, osmotic flow through the cornea and conjunctiva, quenching of fluorescence, tangential flow of aqueous tears and diffusion of tear solutes and fluorescein. These and other combinations of experiment and theory increase our understanding of the fluid dynamics of the tear film and its potential impact on the ocular surface.

Prevalence of dry eye syndrome after a three-year exposure to a clean room

OBJECTIVE

To measure the prevalence of dry eye syndrome (DES) among clean room (relative humidity ≤1%) workers from 2011 to 2013.

METHODS

Three annual DES examinations were performed completely in 352 clean room workers aged 20-40 years who were working at a secondary battery factory. Each examination comprised the tear-film break-up test (TFBUT), Schirmer's test I, slit-lamp microscopic examination, and McMonnies questionnaire. DES grades were measured using the Delphi approach. The annual examination results were analyzed using a general linear model and post-hoc analysis with repeated-ANOVA (Tukey). Multiple logistic regression was performed using the examination results from 2013 (dependent variable) to analyze the effect of years spent working in the clean room (independent variable).

RESULTS

The prevalence of DES among these workers was 14.8% in 2011, 27.1% in 2012, and 32.8% in 2013. The TFBUT and McMonnies questionnaire showed that DES grades worsened over time. Multiple logistic regression analysis indicated that the odds ratio for having dry eyes was 1.130 (95% CI 1.012-1.262) according to the findings of the McMonnies questionnaire.

CONCLUSIONS

This 3-year trend suggests that the increased prevalence of DES was associated with longer working hours. To decrease the prevalence of DES, employees should be assigned reasonable working hours with shift assignments that include appropriate break times. Workers should also wear protective eyewear, subdivide their working process to minimize exposure, and utilize preservative-free eye drops.

Hyperosmolar tears enhance cooling sensitivity of the corneal nerves in rats: possible neural basis for cold-induced dry eye pain

PURPOSE

Tear hyperosmolarity is a ubiquitous feature of dry-eye disease. Although dry-eye patients' sensitivity to cooling is well known, the effects of tear hyperosmolarity on a small amount of cooling in the corneal nerves have not been quantitatively examined. Recently reported corneal afferents, high-threshold cold sensitive plus dry-sensitive (HT-CS + DS) neurons, in rats is normally excited by strong (>4°C) cooling of the cornea, which, when applied to healthy humans, evokes the sensation of discomfort. However, corneal cooling measured between blinks does not exceed 2°C normally. Thus, we sought to determine if these nociceptors could be sensitized by hyperosmolar tears such that they are now activated by small cooling of the ocular surface.

METHODS

Trigeminal ganglion neurons innervating the cornea were extracellularly recorded in isoflurane-anesthetized rats. The responses of single corneal neurons to cooling stimuli presented in the presence of hyperosmolar (350-800 mOsm NaCl) tears were examined.

RESULTS

The HT-CS + DS neurons with thresholds averaging 4°C cooling responded to cooling stimuli presented after 15 minutes of hyperosmolar tears with thresholds of less than 1°C. The response magnitudes also were enhanced so that the responses to small (2°C) cooling emerged, where none was observed before.

CONCLUSIONS

These results demonstrate that after exposure to hyperosmolar tears, these nociceptive corneal neurons now begin to respond to the slight cooling normally encountered between blinks, enabling the painful information to be carried to the brain, which could explain the cooling-evoked discomfort in dry eye patients.

Is there a relationship between pathologic myopia and dry eye syndrome?

PURPOSE

The aim of this study was to investigate the relationship between pathologic myopia and the dry eye syndrome.

METHODS

Forty-five patients with a spherical equivalence (SE) greater than -6.0 diopters (D) and an axial length (AL) >26.5 mm were assigned to the pathological myopia group (group 1). Forty-four healthy individuals were selected from subjects with emmetropia whose SE values ranged from -1.0 to +1.0 D (group 2). Ocular surface disease index (OSDI) scores of all the patients were determined. All the participants underwent the following: Schirmer 1 test without anesthesia, corneal staining, tear break-up time (TBUT), Schirmer 1 test with anesthesia, and AL measurement.

RESULTS

The mean age of group 1 and group 2 patients was 40.2 ± 12.3 and 38.8 ± 9.3 years. The mean values of SE, keratometry, and AL were -9.6 ± 3.8 D, 43.9 ± 1.1 D, and 27.4 ± 0.6 mm in group 1 and -0.1 ± 0.5 D, 42.3 ± 1.4 D, and 23.0 ± 0.2 mm in group 2. The mean values of the Schirmer 1 test without and with anesthesia were 14.4 ± 6.1 and 9.5 ± 4.5 mm in group 1 and 16.7 ± 6.2 and 11.4 ± 6.0 mm in group 2. The mean TBUT in group 1 was 7.2 ± 3.4 seconds, whereas that in group 2 was 13.6 ± 3.7 seconds. There was a significant difference between the groups in SE, keratometry, AL, TBUT, and OSDI scores (P < 0.001).

CONCLUSIONS

Patients with pathologic myopia have lower TBUT scores and higher OSDI scores when compared with healthy individuals.

Tear film dynamics with evaporation, wetting, and time-dependent flux boundary condition on an eye-shaped domain

We study tear film dynamics with evaporation on a wettable eye-shaped ocular surface using a lubrication model. The mathematical model has a time-dependent flux boundary condition that models the cycles of tear fluid supply and drainage; it mimics blinks on a stationary eye-shaped domain. We generate computational grids and solve the nonlinear governing equations using the OVERTURE computational framework. In vivo experimental results using fluorescent imaging are used to visualize the influx and redistribution of tears for an open eye. Results from the numerical simulations are compared with the experiment. The model captures the flow around the meniscus and other dynamic features of human tear film observed in vivo.

Evaporation-driven instability of the precorneal tear film

Tear-film instability is widely believed to be a signature of eye health. When an interblink is prolonged, randomly distributed ruptures occur in the tear film. "Black spots" and/or "black streaks" appear in 15 to 40 s for normal individuals. For people who suffer from dry eye, tear-film breakup time (BUT) is typically less than a few seconds. To date, however, there is no satisfactory quantitative explanation for the origin of tear rupture. Recently, it was proposed that tear-film breakup is related to locally high evaporative thinning. A spatial variation in the thickness of the tear-film lipid layer (TFLL) may lead to locally elevated evaporation and subsequent tear-film breakup. We examine the local-evaporation-driven tear-film-rupture hypothesis in a one-dimensional (1-D) model for the evolution of a thin aqueous tear film overriding the cornea subject to locally elevated evaporation at its anterior surface and osmotic water influx at its posterior surface. Evaporation rate depends on mass transfer both through the coating lipid layer and through ambient air. We establish that evaporation-driven tear-film breakup can occur under normal conditions but only for higher aqueous evaporation rates. Predicted roles of environmental conditions, such as wind speed and relative humidity, on tear-film stability agree with clinical observations. More importantly, locally elevated evaporation leads to hyperosmolar spots in the tear film and, hence, vulnerability to epithelial irritation. In addition to evaporation rate, tear-film instability depends on the strength of healing flow from the neighboring region outside the breakup region, which is determined by the surface tension at the tear-film surface and by the repulsive thin-film disjoining pressure. This study provides a physically consistent and quantitative explanation for the formation of black streaks and spots in the human tear film during an interblink.

Organization of lipids in the tear film: a molecular-level view

Biophysical properties of the tear film lipid layer are studied at the molecular level employing coarse grain molecular dynamics (MD) simulations with a realistic model of the human tear film. In this model, polar lipids are chosen to reflect the current knowledge on the lipidome of the tear film whereas typical Meibomian-origin lipids are included in the thick non-polar lipids subphase. Simulation conditions mimic those experienced by the real human tear film during blinks. Namely, thermodynamic equilibrium simulations at different lateral compressions are performed to model varying surface pressure, and the dynamics of the system during a blink is studied by non-equilibrium MD simulations. Polar lipids separate their non-polar counterparts from water by forming a monomolecular layer whereas the non-polar molecules establish a thick outermost lipid layer. Under lateral compression, the polar layer undulates and a sorting of polar lipids occurs. Moreover, formation of three-dimensional aggregates of polar lipids in both non-polar and water subphases is observed. We suggest that these three-dimensional structures are abundant under dynamic conditions caused by the action of eye lids and that they act as reservoirs of polar lipids, thus increasing stability of the tear film.

A model for tear film thinning with osmolarity and fluorescein

PURPOSE

We developed a mathematical model predicting dynamic changes in fluorescent intensity during tear film thinning in either dilute or quenching regimes and we model concomitant changes in tear film osmolarity.

METHODS

We solved a mathematical model for the thickness, osmolarity, fluorescein concentration, and fluorescent intensity as a function of time, assuming a flat and spatially uniform tear film.

RESULTS

The tear film thins to a steady-state value that depends on the relative importance of the rates of evaporation and osmotic supply, and the resulting increase of osmolarity and fluorescein concentrations are calculated. Depending on the initial thickness, the rate of osmotic supply and the tear film thinning rate, the osmolarity increase may be modest or it may increase by as much as a factor of eight or more from isosmotic levels. Regarding fluorescent intensity, the quenching regime occurs for initial concentrations at or above the critical fluorescein concentration where efficiency dominates, while lower concentrations show little change in fluorescence with tear film thinning.

CONCLUSIONS

Our model underscores the importance of using fluorescein concentrations at or near the critical concentration clinically so that quenching reflects tear film thinning and breakup. In addition, the model predicts that, depending on tear film and osmotic factors, the osmolarity within the corneal compartment of the tear film may increase markedly during tear film thinning, well above levels that cause marked discomfort.

Tear film images and breakup analyzed using fluorescent quenching

PURPOSE

Tear evaporation should increase fluorescein concentration, causing fluorescence dimming from self-quenching for high but not low fluorescein concentration. This prediction was tested and compared to the predicted effect of "tangential flow" that fluorescence dimming should be similar for high and low concentrations.

METHODS

A custom optical system was used for video recording of tear film fluorescence in 30 subjects. The subjects were asked to blink at the start of the recording and try to keep their eyes open for the rest of the 60-second recording. An initial recording was made after instillation of 1 μL 0.1% fluorescein followed by further recordings at 5-minute intervals using 0.5% and 5% fluorescein.

RESULTS

Decay of fluorescence was considerably greater for the high (5%) concentration condition than for the low (0.1%) concentration. This is shown by "ratio images" (ratio of the intensity of a fluorescence image at a later time divided by that of an earlier image), fluorescence decay curves, fluorescence decay rates, and histograms of estimated tear thickness decrease. For example, for the high concentration condition, decay rates were higher than for the low concentration for all 30 subjects (P < 0.0001, binomial test). Additionally, breakup time was significantly reduced for the high compared to the low concentration condition.

CONCLUSIONS

The greater fluorescence decay and more rapid breakup for the high concentration condition are the results expected if thinning and breakup are mainly due to evaporation, hence causing self-quenching. Fluorescence decay rate for the low concentration condition was not significantly greater than zero.

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.

Four characteristics and a model of an effective tear film lipid layer (TFLL)

It is proposed that a normal, effective tear film lipid layer (TFLL) should have the following four characteristics: 1) high evaporation resistance to prevent water loss and consequent hyperosmolarity; 2) respreadability, so it will return to its original state after the compression-expansion cycle of the blink; 3) fluidity sufficient to avoid blocking secretion from meibomian glands; 4) gel-like and incompressible structure that can resist forces that may tend to disrupt it. These characteristics tend to be incompatible; for example, lipids that form good evaporation barriers tend to be disrupted by compression-expansion cycles. It is noted that clues about the function and organization of the TFLL can be obtained by comparison with other biological lipid layers, such as lung surfactant and the lipid evaporation barrier of the skin. In an attempt to satisfy the conflicting characteristics, a "multilamellar sandwich model" of the TFLL is proposed, having features in common with the skin evaporation barrier.

Dry eye as a mucosal autoimmune disease

Dry eye is a common ocular surface inflammatory disease that significantly affects quality of life. Dysfunction of the lacrimal function unit (LFU) alters tear composition and breaks ocular surface homeostasis, facilitating chronic inflammation and tissue damage. Accordingly, the most effective treatments to date are geared towards reducing inflammation and restoring normal tear film. The pathogenic role of CD4+ T cells is well known, and the field is rapidly realizing the complexity of other innate and adaptive immune factors involved in the development and progression of disease. The data support the hypothesis that dry eye is a localized autoimmune disease originating from an imbalance in the protective immunoregulatory and proinflammatory pathways of the ocular surface.

Quantitative analysis of tear film fluorescence and discomfort during tear film instability and thinning

PURPOSE

The purpose of this study was to test the association between tear film fluorescence changes during tear break-up (TBU) or thinning and the concurrent ocular sensory response.

METHODS

Sixteen subjects kept one eye open as long as possible (MBI), indicated their discomfort level continuously, and rated ocular sensations of irritation, stinging, burning, pricking, and cooling using visual analog scales (VAS). Fluorescence of the tear film was quantified by a pixel-based analysis of the median pixel intensity (PI), TBU, and percentage of dark pixels (DarkPix) over time. A cutoff of 5% TBU was used to divide subjects into either break-up (BU) or minimal break-up (BUmin) groups.

RESULTS

Tear film fluorescence decreased (median PI) and the percentage of TBU and DarkPix increased in all trials, with the rate significantly greater in the BU than the BUmin group (Mann-Whitney U test, P < 0.05). The rate of increasing discomfort during trials was highly correlated with the rate of decrease in median PI and developing TBU (Spearman's, r ≥ 0.70). Significant correlations were found between corneal fluorescence, MBI, and sensory measures.

CONCLUSIONS

Concentration quenching of fluorescein dye with tear film thinning best explains decreasing tear film fluorescence during trials. This was highly correlated with increasing ocular discomfort, suggesting that both tear film thinning and TBU stimulate underlying corneal nerves, although TBU produced more rapid stimulation. Slow increases in tear film hyperosmolarity may cause the gradual increase in discomfort during slow tear film thinning, whereas the sharp increases in discomfort during TBU suggest a more complex stimulus.

Estimating tear film spread and stability through tear hydrodynamics

PURPOSE

The stability and ease of spread of the tear-film over the contact lens surface may be an indicator of contact lens surface dewetting. The present in vivo methods of determining lens dewetting are complex. This study introduces a novel and objective way of determining the upward spread and stability of the tear-film through measurement of tear-film particle dynamics.

METHODS

Ten adapted contact lens wearers wore the same type of contact lens. Using a video camera mounted to a slit-lamp, the tear-film spread over the lens surface was recorded after a blink, at 8× magnification and capture rate of 30 frames per second, at morning after lens insertion, and after 8 h of lens wear. Images from 20 videos, without blinks and without an observable change in fixation were analyzed without any further postprocessing of the images. Using a customized calibrated ImageJ macro for particle tracking, the velocity of naturally occurring reflective particles was determined. The results were analyzed using the R program and ProFit.

RESULTS

The results established that the upward particle velocity was highest immediately after a blink and declined with time. The spread of the tear film measured through upward particle velocity was different on lens insertion than after 8 h of lens wear (p = 0.001). The exponential time constants ± SE were 346.02 ± 29.0 for lens insertion at morning and 1413.13 ± 419.6 after 8 h of lens wear.

CONCLUSIONS

A novel and non-invasive way to measure in vivo spread and stability of the prelens tear-film has been developed. Additional studies are needed to understand whether this simple measure is able to differentiate the performance of different soft contact lenses and how this method may help in the understanding other aspects of lens performance such as non-invasive tear breakup time, surface deposition, and lens comfort.

Eye disorders in old people

Background:

Visual impairment is one of a major public health problem among elderly people.

Object:

Aim of this study was determining the prevalence of visual impairment in median and old peoples in Qazvin (Iran).

Method:

In this cross sectional study, with a simple random sampling, 446 patients older than 5o years who were referred to outpatient ophthalmology clinics at Avecina hospital of Qazvin (a province of Iran) in 2010 were enrolled. Participants first complete a questionnaire with 25 questions toward demographic and past medical history and then were examined by ophthalmologist. These examinations includes direct and indirect ophthalmoscopy, slit lamp examination, measurement of uncorrected visual acuity and visual acuity with current glasses, lensometery of the previous glasses, refraction with and without the use of cycloplegic and determining the best corrected visual acuity. All slit lamp examinations were performed by the same ophthalmologist. Data were analyzed with SPSS16 with use of Chi – Square test with Pvalue <0.05.

Results:

In this study 446 patients were examined that 54.7% were male. Mean age of study population was 62+-9.3 years old. 96.4% of participants had refractory disorder. Prevalence of myopia, hyperopia and astigmatism were 33.6%, 45.9% and 16.8% respectively. Of patients 17.4% had diabetes. Of participants 28.9% had temporal headache, 37% red eye, 41.2% flashing, 27.3% and 28% had dryness and discharge of eye respectively. 31.1% of participants had eyelide problem, 4.7% Color Vision Deficiency (CVD) and 3.8% had family history of CVD. Of total 4.5% had glaucoma, 3.3% macular degeneration and 21.7% had hypertension. 0.6% of population had macular degeneration, 0.4% of population had glaucoma Of 892 eyes (446 individuals), 36.2% had visual acuity less than 7/10, 1.7% light perception (LP) and 0.22% no light perception (NLP) and 2.7% finger count.

Conclusion:

Refractory errors, cataract and amblyopia were most important eye disorders in older people in Qazvin.

Menthol activation of corneal cool cells induces TRPM8-mediated lacrimation but not nociceptive responses in rodents

PURPOSE

Stimulation to the cornea via noxious chemical and mechanical means evokes tearing, blinking, and pain. In contrast, mild cooling of the ocular surface has been reported to increase lacrimation via activation of corneal cool primary afferent neurons. The purpose of our study was to determine whether menthol induces corneal cool cell activity and lacrimation via the transient receptor potential melastatin-8 (TRPM8) channel without evoking nociceptive responses.

METHODS

Tear measurements were made using a cotton thread in TRPM8 wild type and knockout mice after application of menthol (0.05-50 mM) to the cornea. In additional studies, nocifensive responses (eye swiping and lid closure) were quantified following cornea menthol application. Trigeminal ganglion electrophysiologic single unit recordings were performed in rats to determine the effect of low and high concentrations of menthol on corneal cool cells.

RESULTS

At low concentrations, menthol increased tear production in TRPM8 wild type and heterozygous animals, but had no effect in TRPM8 knockout mice, while nocifensive responses remained unaffected. At the highest concentration, menthol (50 mM) increased tearing and nocifensive responses in TRPM8 wild type and knockout animals. A low concentration of menthol (0.1 mM) increased cool cell activity, yet a high concentration of menthol (50 mM) had no effect.

CONCLUSIONS

These studies indicated that low concentrations of menthol can increase lacrimation via TRPM8 channels without evoking nocifensive behaviors. At high concentrations, menthol can induce lacrimation and nocifensive behaviors in a TRPM8 independent mechanism. The increase in lacrimation is likely due to an increase in cool cell activity.

Evaluation of dry eye

Dry eye is a common yet complex condition. Intrinsic and extrinsic factors can cause dysfunction of the lids, lacrimal glands, meibomian glands, ocular surface cells, or neural network. These problems would ultimately be expressed at the tear film-ocular surface interface. The manifestations of these problems are experienced as symptoms such as grittiness, discomfort, burning sensation, hyperemia, and secondary epiphora in some cases. Accurate investigation of dry eye is crucial to correct management of the condition. Techniques can be classed according to their investigation of tear production, tear stability, and surface damage (including histological tests). The application, validity, reliability, compatibility, protocols, and indications for these are important. The use of a diagnostic algorithm may lead to more accurate diagnosis and management. The lack of correlation between signs and symptoms seems to favor tear film osmolarity, an objective biomarker, as the best current clue to correct diagnosis.

Quantitative characterization reveals three types of dry-sensitive corneal afferents: pattern of discharge, receptive field, and thermal and chemical sensitivity

This study reveals that the cold-sensitive (CS) + dry-sensitive (DS) corneal afferents reported in a previous study consist of two types: 1) low threshold (LT)-CS + DS neurons with <1°C cooling sensitivity, and 2) high threshold (HT)-CS + DS neurons with a wide range of cooling sensitivities (~1-10°C cooling). We also found DS neurons with no cooling sensitivity down to 19°C [cold-insensitive (CI) + DS neurons]. LT-CS + DS neurons showed highly irregular discharge patterns during the dry cornea characterized by numerous spiking bursts, reflecting small temperature changes in the cornea. Their receptive fields (RFs) were mainly located in the cornea's center, the first place for tears to ebb from the surface and be susceptible to external temperature fluctuations. HT-CS and CI + DS neurons showed a gradual rise in firing rate to a stable level over ~60 s after the dry stimulus onset. Their RFs were located mostly in the cornea's periphery, the last place for tears to evaporate. The exquisite sensitivity to cooling in LT-CS + DS neurons was highly correlated with heat sensitivity (~45°C). There was a perfect correlation between noxious heat sensitivity and capsaicin responsiveness in each neuron type. The high sensitivity to noxious osmotic stress was a defining property of the HT-CS and CI + DS neurons, while high sensitivity to menthol was a major characteristic of the LT-CS + DS neurons. These observations suggest that three types of DS neurons serve different innocuous and nociceptive functions related to corneal dryness.

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.

Wax-tear and meibum protein, wax-β-carotene interactions in vitro using infrared spectroscopy

Protein-meibum and terpenoids-meibum lipid interactions could be important in the etiology of meibomian gland dysfunction (MGD) and dry eye symptoms. In the current model studies, attenuated total reflectance (ATR) infrared (IR) spectroscopy was used to determine if the terpenoid β-carotene and the major proteins in tears and meibum affect the hydrocarbon chain conformation and carbonyl environment of wax, an abundant component of meibum. The main finding of these studies is that mucin binding to wax disordered slightly the conformation of the hydrocarbon chains of wax and caused the wax carbonyls to become hydrogen bonded or experience a more hydrophilic environment. Lysozyme and lactoglobulin, two proteins shown to bind to monolayers of meibum, did not have such an effect. Keratin and β-carotene did not affect the fluidity (viscosity) or environment of the carbonyl moieties of wax. Based on these results, tetraterpenoids are not likely to influence the structure of meibum in the meibomian glands. In addition, these findings suggest that it is unlikely that keratin blocks meibomian glands by causing the meibum to become more viscous. Among the tear fluid proteins studied, mucin is the most likely to influence the conformation and carbonyl environment of meibum at the tear film surface.

Inputs and outputs of the lacrimal system: review of production and evaporative loss

Meta-analyses were carried out of studies of tear production (by fluorophotometry, tear turnover rate[TTR]) and evaporation (from capture of fluid loss from the eye). TTR was reduced in dry eye relative to normal at 9.26 +/- 5.08%/min (0.54 +/- 0.28 μl/min) vs 16.19 +/- 5.1%/min (1.03 +/- 0.39 μl/min); with values of 7.71 +/- 1.02 %/min (0.4 +/- 0.10 μl/min) in aqueous deficiency dry eye (ADDE) and 11.95 +/- 4.25%/min (0.71 +/- 0.25 l/min) in evaporative dry eye (EDE). Evaporation was increased in dry eye at 21.05 +/- 13.96 x 10(-7)g/cm(2)/s (0.21 +/- 0.13 μl/min) vs 13.57 +/- 6.52 x 10(-7)g/cm(2)/s (0.14 +/- 0.07 μl/min) in normals; with values of 17.91 +/- 10.49 x 10(-7)g/cm(2)/s (0.17 +/- 0.1 μl/min) in ADDE and 25.34 +/- 13.08 x 10(-7)g/cm(2)/s (0.26 +/- 0.16 μl/min) in EDE. Evaporation rate from tear film thinning was also considered, and possible reasons and consequences for the much higher rates thereby reported are discussed. A new statistical approach determined diagnostic efficacy of cut-offs for dry eye derived from the meta-analyses; sensitivities and specifications ranging from 69.5 to 98.6% and 58.7 to 96.8% (TTR) and 45.5 to 61.2% and 79.8 to 90.6% (evaporation). Indices of tear dynamics were reconsidered, and ratios of evaporation and TTR suggest that an increase of between 2 and 3 times may be associated with dry eye.

Tear dysfunction and the cornea: LXVIII Edward Jackson Memorial Lecture

PURPOSE

To describe the cause and consequence of tear dysfunction-related corneal disease.

DESIGN

Perspective on effects of tear dysfunction on the cornea.

METHODS

Evidence is presented on the effects of tear dysfunction on corneal morphology, function, and health, as well as efficacy of therapies for tear dysfunction-related corneal disease.

RESULTS

Tear dysfunction is a prevalent eye disease and the most frequent cause for superficial corneal epithelial disease that results in corneal barrier disruption, an irregular optical surface, light scattering, optical aberrations, and exposure and sensitization of pain-sensing nerve endings (nociceptors). Tear dysfunction-related corneal disease causes irritation and visual symptoms such as photophobia and blurred and fluctuating vision that may decrease quality of life. Dysfunction of 1 or more components of the lacrimal functional unit results in changes in tear composition, including elevated osmolarity and increased concentrations of matrix metalloproteinases, inflammatory cytokines, and chemokines. These tear compositional changes promote disruption of tight junctions, alter differentiation, and accelerate death of corneal epithelial cells.

CONCLUSIONS

Corneal epithelial disease resulting from tear dysfunction causes eye irritation and decreases visual function. Clinical and basic research has improved understanding of the pathogenesis of tear dysfunction-related corneal epithelial disease, as well as treatment outcomes.

Ocular dryness excites two classes of corneal afferent neurons implicated in basal tearing in rats: involvement of transient receptor potential channels

This study reports the findings of two classes of corneal afferents excited by drying of the cornea (dry responses) in isoflurane-anesthetized rats: cold-sensitive (CS; 87%) and cold-insensitive (CI; 13%) neurons. Compared with CI neurons, CS neurons showed significantly higher firing rates over warmer corneal temperatures (~31-15°C) and greater responses to menthol, drying, and wetting of the cornea but lower responses when hyperosmolar solutions were applied to the ocular surface. We proposed that the dry responses of these corneal afferents derive from cooling and an increased osmolarity of the ocular surface, leading to the production of basal tears. An ocular application of the transient receptor potential channel TRPM8 antagonist BCTC (20 μM) decreased the dry responses by ~45-80% but failed to completely block them, whereas the TRPA1 antagonist HC030031 did not influence the responses to drying of the cornea or hyperosmolar tears. Furthermore, the responses produced by cold stimulation of the cornea accounted for only 28% of the dry responses. These results support the view that the stimulus for basal tearing (corneal dryness) derives partly from cooling of the cornea that activates TRPM8 channels but that non-TRPM8 channels also contribute significantly to the dry responses and to basal tearing. Finally, we hypothesized that activation of TRPM8 by cooling in CS corneal afferents not only gives rise to the sensation of ocular coolness but also to the "wetness" perception (Thunberg's illusion), whereas a precise role of the CI afferents in basal tearing and other ocular dryness-related functions such as eye blink and the "dryness" sensation remain to be elucidated.

A solute gradient in the tear meniscus. I. A hypothesis to explain Marx's line

Marx's line is a line of mucosal staining behind the mucocutaneous junction. It can be demonstrated throughout life in all normal lids by staining with lissamine green and related dyes. Of all the body orifices, only the mucosae of the eye and mouth are directly exposed to the atmosphere. In this paper, we suggest that for the eye, this exposure leads to the formation of Marx's line. The tear meniscus thins progressively toward its apex, where it is pinned at the mucocutaneous junction of the lid. It also thins toward the black line, which segregates the meniscus from the tear film after the blink. We predict that, because of the geometry of the tear meniscus, evaporation generates a solute gradient across the meniscus profile in the anteroposterior plane, which peaks at the meniscus apices at the end of the interblink. One outcome would be to amplify the level of tear molarity at these sites so that they reach hyperosmolar proportions. Preliminary mathematical modeling suggests that dilution of this effect by advection and diffusion of solute away from the meniscus apex at the mucocutaneous junction will be restricted by spatial constraints, the presence of tear and surface mucins at this site, and limited fluid flow. We conclude that evaporative water loss from the tear meniscus may result in a physiological zone of hyperosmolar and related stresses to the occlusal conjunctiva, directly behind the mucocutaneous junction. We hypothesize that this stimulates a high epithelial cell turnover at this site, incomplete epithelial maturation, and a failure to express key molecules such as MUC 16 and galectin-3, which, with the tight junctions between surface epithelial cells, are necessary to seal the ocular surface and prevent penetration of dyes and other molecules into the epithelium. This is proposed as the basis for Marx's line. In Part II of this paper (also published in this issue of The Ocular Surface), we address additional pathophysiological consequences of this mechanism, affecting lid margins.

Spontaneous eyeblink activity

Spontaneous blinking is essential for maintaining a healthy ocular surface and clarity of vision. The spontaneous blink rate (SBR) is believed to reflect a complex interaction between peripheral influences mediated by the eye surface and the central dopaminergic activity. The SBR is thus extremely variable and dependent on a variety of psychological and medical conditions. Many different methods have been employed to measure the SBR and the upper eyelid kinematics during a blink movement. Each has its own merits and drawbacks, and the choice of a specific method should be tailored to the specific needs of the investigation. Although the sequence of muscle events that leads to a blink has been fully described, knowledge about the neural control of spontaneous blinking activity is not complete. The tear film is dynamically modified between blinks, and abnormalities of the blink rate have an obvious influence on the ocular surface.

Forward light scatter analysis of the eye in a spatially-resolved double-pass optical system

An optical analysis is developed to separate forward light scatter of the human eye from the conventional wavefront aberrations in a double pass optical system. To quantify the separate contributions made by these micro- and macro-aberrations, respectively, to the spot image blur in the Shark-Hartmann aberrometer, we develop a metric called radial variance for spot blur. We prove an additivity property for radial variance that allows us to distinguish between spot blurs from macro-aberrations and micro-aberrations. When the method is applied to tear break-up in the human eye, we find that micro-aberrations in the second pass accounts for about 87% of the double pass image blur in the Shack-Hartmann wavefront aberrometer under our experimental conditions.

Lateral shearing interferometry for analysis of tear film surface kinetics

PURPOSE

To date, there have been no measuring techniques available that could clearly identify all phases of tear film surface kinetics in one interblink interval.

METHODS

Using a series of cases, we show that lateral shearing interferometry equipped with a set of robust parameter estimation techniques is able to characterize up to five different phases of tear film surface kinetics that include: (i) initial fast tear film build-up phase, (ii) further slower tear film build-up phase, (iii) tear film stability, (iv) tear film thinning, and (v), after a detected break-up, subsequent tear film deterioration.

RESULTS

Several representative examples are given for estimating tear film surface kinetics in measurements in which the subjects were asked to blink and keep their eyes open as long as they could.

CONCLUSIONS

Lateral shearing interferometry is a noninvasive technique that provides means for temporal characterization of tear film surface kinetics and the opportunity for the analysis of the two-step tear film build-up process.

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.

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).