Ocular Surface Cooling Corresponds to Tear Film Thinning and Breakup

Ocular thermography and clinical measurements in symptomatic and asymptomatic soft contact lens wearers

SIGNIFICANCE

Symptoms of dryness and discomfort are the main reasons for contact lens dropout. Clinical tests for this purpose are invasive or subjective. Ocular thermography may help to assess the ocular discomfort and dryness in noninvasive and objective manner.

PURPOSE

This study aimed to investigate the relationship of ocular thermography with clinical measurements in habitual symptomatic and asymptomatic soft contact lens wearers.

METHODS

Forty habitual contact lens wearers were evaluated in two age- and gender-matched asymptomatic and symptomatic groups (Contact Lens Discomfort Questionnaire scores ≤8 and ≥14, respectively). Clinical measurements took place during visit 1 (with contact lens in situ and after contact lens removal) and at baseline after a 2-week washout period (visit 2). The Ocular Surface Disease Index (OSDI) questionnaire, noninvasive tear breakup time, bulbar conjunctival hyperemia, and corneal staining were assessed. Thermal cooling rate was computed in the central and lower cornea during natural blinking (30 s) and sustained eye opening (10 s).

RESULTS

Dry eye symptoms (OSDI score) were significantly higher in the symptomatic group during contact lens wear (p<0.001) and at baseline (p = 0.001). Thermal cooling rate was significantly higher in the symptomatic group in the lower cornea (10 s, p=0.013) with the contact lens in situ and in the central cornea (30 s, p=0.045) after contact lens removal. At baseline, dry eye symptoms (OSDI score) significantly correlated with cooling rate in the central cornea region for the symptomatic group (30 s: r = -0.5, p=0.03; 10 s: r = -0.63, p=0.005). Noninvasive tear breakup time correlated with cooling rate in the central cornea region at baseline in the symptomatic group (30 s, r = 0.6, p=0.005; 10 s, r = 0.55, p=0.018). Cooling rate in the central cornea region (10-s duration, p<0.0001) and noninvasive tear breakup time (p<0.0001) were identified as significant predictor variables for dry eye symptoms at baseline.

CONCLUSIONS

Noninvasive tear breakup time and thermal cooling rate were identified as significant predictor variables for contact lens-induced dry eye. These findings may suggest the potential for the additional application of ocular thermography in the evaluation of contact lens discomfort.

High-Resolution Ocular Surface Imaging: Real-Time Visualization of Tear Film Dysfunction

PURPOSE

Recent advancements in infrared sensing technology have made it possible to visualize tear film dynamics in real time, enabling evaluation of tear film quality during blinking. A retrospective clinical evaluation was conducted to explore this by grading videos of the tear film and comparing grading data with dry eye diagnostic results using the OCULUS keratograph (K5M).

METHODS

Videos were used to grade patients' tear film perturbations as compared with healthy control subjects. The grading was then correlated with the ocular surface disease index (OSDI) scores, tear film breakup time (TFBUT), tear meniscus height (TMH), corneal staining, redness, and meibography data.

RESULTS

Infrared imaging of the ocular surface revealed instantaneous and recurring dynamic characteristics of the tear film, allowing for the differentiation between normal and abnormal tear films. Abnormal features included a complete absence of a spreading tear film, hindered spreading of the tear film after blinking, areas of tear film instability, or a combination of the latter 2. Some of these features show a resemblance to the tear film appearance after fluorescein staining. The grading of these features correlated with TFBUT and, to a lesser extent, with TMH but did not show significant correlation with any other diagnostic data from the K5M. Furthermore, the speed of tear film spreading after blinking showed a positive correlation with TMH.

CONCLUSIONS

Direct visualization of the tear film across the entire palpebral aperture using infrared sensing offers a noninvasive, reproducible, and rapid method for assessing the health and quality of the tear film.

Localized Evaporative Cooling Explains Observed Ocular Surface-Temperature Patterns

Purpose

We determined interblink corneal surface-temperature decline and tear-film evaporation rates of localized tear breakup cold regions (LCRs) and localized tear unbroken warm regions (LWRs) of the corneal surface, as well as that of the overall average corneal surface.

Methods

Each subject underwent 4 inter-day visits where the interblink corneal surface-temperature history of the right eye was measured using a FLIR A655sc infrared thermographer. Corneal surface temperature history was analyzed to determine the overall, LCR, and LWR temperature-decline rates. Evaporation rates of LCR and LWR regions were determined from the measured LCR and LWR temperature data using the physical model of Dursch et al.

Results

Twenty subjects completed the study. Mean (SD) difference of LCR temperature-decline rate was -0.08 (0.07)°C/s faster than LWR (P < 0.0001). Similarly, evaporation rates of LCR and LWR were statistically different (P < 0.0001). At ambient temperature, mean LCR and LWR evaporation rates were 76% and 27% of pure water evaporation flux, respectively. There was no statistically significant difference between the inter-day measured temperature-decline rates and the interblink starting temperature.

Conclusions

Significant differences in corneal temperature-decline rate and evaporation rate between LCR and LWR were quantified using infrared thermography. In agreement with literature, LCRs and LWRs correlate directly with fluorescein break-up areas and unbroken tear areas, respectively. Because lipid-evaporation protection is diminished in breakup areas, higher local evaporation rates and faster local cooling rates occur in LCRs relative to LWRs. Our results confirm this phenomenon clinically for the first time.

Ocular effects of exposure to low-humidity environment with contact lens wear: A pilot study

PURPOSE

To compare the ocular effects of exposure to a low-humidity environment with and without contact lens (CL) wear using various non-invasive tests.

METHODS

Fourteen habitual soft CL wearers were exposed to controlled low humidity (5% relative humidity [RH]) in an environmental chamber for 90 min on two separate occasions. First, when wearing their habitual spectacles and then, on a separate visit, when wearing silicone hydrogel CLs that were fitted specifically for this purpose. All participants had adapted to the new CL prior to data collection. Three non-invasive objective measurements were taken at each visit: blinking rate, objective ocular scatter (measured using the objective scatter index) and ocular surface cooling rate (measured using a long-wave infrared thermal camera). At each visit, measurements were taken before the exposure in comfortable environmental conditions (RH: 45%), and after exposure to environmental stress (low humidity, RH: 5%).

RESULTS

CL wearers showed increased blinking rate (p < 0.005) and ocular scatter (p = 0.03) but similar cooling rate of the ocular surface (p = 0.08) when compared with spectacle wear in comfortable environmental conditions. The exposure to low humidity increased the blinking rate significantly with both types of corrections (p = 0.01). Interestingly, ocular scatter (p = 0.96) and cooling rate (p = 0.73) were not significantly different before and after exposure to low humidity. There were no significant two-way interactions between correction and exposure in any of the measurements.

CONCLUSIONS

CLs significantly increased the blinking rate, which prevented a quick degradation of the tear film integrity as it was refreshed more regularly. It is hypothesised that the increased blinking rate in CL wearers aids in maintaining ocular scatter quality and cooling rate when exposed to a low-humidity environment. These results highlight the importance of blinking in maintaining tear film stability.

Relationship between ocular surface temperature and 0.1% cyclosporine a in dry eye syndrome with meibomian gland dysfunction

To determine the relationship between ocular surface temperature (OST) and 0.1% cyclosporine A in patients with dry eye syndrome and meibomian gland dysfunction (MGD). This study retrospectively analyzed 35 eyes from 18 patients with dry eye disease (DED) and MGD, who were divided into two groups. Group 1 was treated with artificial tears, and eyelid margin scrubs without anti-inflammatory eye drops, while group 2 received the same treatment as group 1 along with 0.1% cyclosporine A. The ocular surface disease index (OSDI), tear meniscus height (TMH), noninvasive tear breakup time (NIBUT), lipid layer thickness (LLT), meibum quality score (MQS), and OST were measured at baseline and 1 month later. Nineteen and 16 eyes were included in groups 1 and 2, respectively. Both groups showed a significant decrease in OSDI and OST; however, the decrease was more significant in group 2. No other significant differences in TMH, NIBUT, and LLT were observed; however, MQS significantly differed in group 2. This study found that 0.1% CsA administration can relieve symptoms in patients with DED and MGD although there were no definite keratitis clues, such as epithelial erosion. In addition, the conjunctival temperature showed a correlation with symptom improvement.

Tear-film evaporation flux and its relationship to tear properties in symptomatic and asymptomatic soft-contact-lens wearers

PURPOSE

With soft-contact-lens wear, evaporation of the pre-lens tear film affects the osmolarity of the post-lens tear film and this can introduce a hyperosmotic environment at the corneal epithelium, leading to discomfort. The purposes of the study are to ascertain whether there are differences in evaporation flux (i.e., the evaporation rate per unit area) between symptomatic and asymptomatic soft-contact-lens wearers, to assess the repeatability of a flow evaporimeter, and to assess the relationship between evaporation fluxes, tear properties, and environmental conditions.

METHODS

Closed-chamber evaporimeters commonly used in ocular-surface research do not control relative humidity and airflow, and, therefore, misestimate the actual tear-evaporation flux. A recently developed flow evaporimeter overcomes these limitations and was used to measure accurate in-vivo tear-evaporation fluxes with and without soft-contact-lens wear for symptomatic and asymptomatic habitual contact-lens wearers. Concomitantly, lipid-layer thickness, ocular-surface-temperature decline rate (i.e., °C/s), non-invasive tear break-up time, tear-meniscus height, Schirmer tear test, and environmental conditions were measured in a 5 visit study.

RESULTS

Twenty-one symptomatic and 21 asymptomatic soft-contact-lens wearers completed the study. A thicker lipid layer was associated with slower evaporation flux (p < 0.001); higher evaporation flux was associated with faster tear breakup irrespective of lens wear (p = 0.006). Higher evaporation flux was also associated with faster ocular-surface-temperature decline rate (p < 0.001). Symptomatic lens wearers exhibited higher evaporation flux than did asymptomatic lens wearers, however, the results did not reach statistical significance (p = 0.053). Evaporation flux with lens wear was higher than without lens wear but was also not statistically significant (p = 0.110).

CONCLUSIONS

The repeatability of the Berkeley flow evaporimeter, associations between tear characteristics and evaporation flux, sample-size estimates, and near statistical significance in tear-evaporation flux between symptomatic and asymptomatic lens wearers all suggest that with sufficient sample sizes, the flow evaporimeter is a viable research tool to understand soft-contact-lens wear comfort.

Features and influences on the normal tear evaporation rate

Tear evaporation is a normal physiological phenomenon that has an important role in regulating blink activity and tear production. An altered tear evaporation rate (TER) is a defining characteristic of evaporative dry eye disease (DED), and the measurement of tear evaporation is a useful clinical test for diagnosis. Reported values for a normal TER cover a broad range, which may be due to the influence of ocular, environmental, and systemic factors. For improved disease diagnosis, a fuller understanding of the normal TER range is essential. This paper reports on a literature review of the current knowledge of these normal influences on TER.

Prevention of localized corneal hyperosmolarity spikes by soft-contact-lens wear

PURPOSE

To determine whether localized hyperosmotic spikes on the pre-lens tear film (PrLTF) due to tear break up results in hyperosmotic spikes on the ocular surface during soft-contact-lens (SCL) wear and whether wear of SCLs can protect the cornea against PrLTF osmotic spikes.

METHODS

Two-dimensional transient diffusion of salt was incorporated into a computationally designed SCL, post-lens tear film (PoLTF), and ocular surface and solved numerically. Time-dependent localized hyperosmolarity spikes were introduced at the anterior surface of the SCL corresponding to those generated in the PrLTF. Salt spikes were followed in time until spikes penetrate through the lens into the PoLTF. Lens-salt diffusivities (D_s) were varied to assess their importance on salt migration from the PrLTF to the ocular surface. SCL and PoLTF initial conditions and the lens anterior-surface boundary condition were varied depending on the value of D_s and on dry-eye symptomatology. Determined corneal surface osmolarities were translated into clinical pain scores.

RESULTS

For D_s above about 10^-7cm²/s, it takes around 5-10 s for the PrLTF hyperosmotic break-up spikes to diffuse across the SCL and reach the corneal surface. Even if localized hyperosmotic spikes penetrate to the ocular surface, salt concentrations there are much lower than those in the progenitor PrLTF spikes. For D_s less than 10^-7cm²/s, the SCL protects the cornea from hyperosmotic spikes for both normal and dry eyes. When localized corneal hyperosmolarity is converted into transient pain scores, pain thresholds are significantly lower than those for no-lens wear.

CONCLUSIONS

A cornea can be protected from localized PrLTF hyperosmolarity spikes with SCL wear. With regular blinking (e.g., less than 10 s), SCL wear shields the cornea from significant hyperosmotic pain. Decreasing D_s increases that protection. Low-D_s soft contact lenses can protect against hyperosmotic spikes and discomfort even during infrequent blinking (e.g., > 10 s).

Ocular surface cooling rate associated with tear film characteristics and the maximum interblink period

The surface of the human eye is covered with a protective tear film that refreshes with each blink. Natural blinking occurs involuntarily, but one can also voluntarily blink or refrain from blinking. The maximum time one can refrain from blinking until the onset of discomfort is the maximum interblink period (MIBP). During the interblink period the tear film evaporates and thins from the ocular surface. Infrared thermography provides a non-invasive measure of the ocular surface temperature (OST). Due to evaporation, ocular surface cooling (OSC) generally occurs when the eyes are open and exposed to the environment. The purpose of our study was to investigate the effect of OSC rate on the MIBP, and to investigate the association of the MIBP with tear film characteristics in subjects who do and do not exhibit OSC. The MIBP was measured simultaneously with OST over time. Non-invasive tear breakup time, tear meniscus height, tear lipid layer thickness, and Schirmer I test strip wetted lengths were measured on a day prior to the thermography visit. Subjects were divided into cooling and non-cooling groups based on OSC rate, and demographic and tear film characteristics were tested for inter-group differences. A faster OSC rate was associated with an exponentially shorter duration of the MIBP overall and within the cooling group alone. Faster non-invasive tear breakup time was significantly associated with a shorter MIBP in both groups. These results suggest that tear film evaporation initiates a pathway that results in the onset of ocular discomfort and the stimulus to blinking. The presence of a subset of subjects with no or minimal OSC who nevertheless have a short MIBP indicates that evaporative cooling is not the only mechanism responsible for the onset of ocular discomfort.

CLEAR - Contact lens technologies of the future

Contact lenses in the future will likely have functions other than correction of refractive error. Lenses designed to control the development of myopia are already commercially available. Contact lenses as drug delivery devices and powered through advancements in nanotechnology will open up further opportunities for unique uses of contact lenses. This review examines the use, or potential use, of contact lenses aside from their role to correct refractive error. Contact lenses can be used to detect systemic and ocular surface diseases, treat and manage various ocular conditions and as devices that can correct presbyopia, control the development of myopia or be used for augmented vision. There is also discussion of new developments in contact lens packaging and storage cases. The use of contact lenses as devices to detect systemic disease has mostly focussed on detecting changes to glucose levels in tears for monitoring diabetic control. Glucose can be detected using changes in colour, fluorescence or generation of electric signals by embedded sensors such as boronic acid, concanavalin A or glucose oxidase. Contact lenses that have gained regulatory approval can measure changes in intraocular pressure to monitor glaucoma by measuring small changes in corneal shape. Challenges include integrating sensors into contact lenses and detecting the signals generated. Various techniques are used to optimise uptake and release of the drugs to the ocular surface to treat diseases such as dry eye, glaucoma, infection and allergy. Contact lenses that either mechanically or electronically change their shape are being investigated for the management of presbyopia. Contact lenses that slow the development of myopia are based upon incorporating concentric rings of plus power, peripheral optical zone(s) with add power or non-monotonic variations in power. Various forms of these lenses have shown a reduction in myopia in clinical trials and are available in various markets.

Impact of Ocular Surface Temperature on Tear Characteristics: Current Insights

Infrared (IR) thermographic assessment of ocular surface temperature (OST) is gaining interest as an adjuvant method to evaluate the ocular surface. It is a quick, non-invasive test that causes minimal, if any, discomfort to patients. The purpose of this article was to summarize research on how OST relates to tear film parameters and dry eye disease (DED). PubMed, Google Scholar, and Scopus searches for specific terms were carried out and eligible articles reviewed. OST of the central cornea is ~34-35°C when measured as a single time-point (typically right after a blink). Dynamically, OST values decrease over time at a rate of ~ -0.01 °C/s in healthy eyes. Single time-point OST values are impacted by temperature, with positive correlations noted with both ambient (1°C↓ results in ~0.16°C↓ in OST) and body (1°C↑ results in ~0.98°C↑ in OST) temperature. Single time-point OST values are also impacted by tear parameters, with negative correlations noted with tear break-up time (TBUT; r=-0.61) and positive correlations with lipid layer thickness (~r=0.50). Dynamically, the rate of OST cooling over the interblink period correlates with various tear parameters including Schirmer's test scores (r=-0.39), tear meniscus height (r=-0.52) and the rate of tear film break-up (r=-0.74). These data imply that OST decreases more rapidly in individuals with greater tear production, larger tear volumes, and shorter tear break-up times (faster rates of tear film break-up). There are discrepancies in relationships between OST and DED across studies, which is not surprising given that DED encompasses a number of different phenotypic presentations. However, most studies found that OST decreases at a more rapid rate in DED vs. control groups. As such, cooling rate may have utility as a screening tool in DED in combination with established point-of-care tests.

Novel Approaches for Imaging-Based Diagnosis of Ocular Surface Disease

Imaging has become indispensable in the diagnosis and management of diseases in the posterior part of the eye. In recent years, imaging techniques for the anterior segment are also gaining importance and are nowadays routinely used in clinical practice. Ocular surface disease is often synonymous with dry eye disease, but also refers to other conditions of the ocular surface, such as Meibomian gland dysfunction or keratitis and conjunctivitis with different underlying causes, i.e., allergies or infections. Therefore, correct differential diagnosis and treatment of ocular surface diseases is crucial, for which imaging can be a helpful tool. A variety of imaging techniques have been introduced to study the ocular surface, such as anterior segment optical coherence tomography, in vivo confocal microscopy, or non-contact meibography. The present review provides an overview on how these techniques can be used in the diagnosis and management of ocular surface disease and compares them to clinical standard methods such as slit lamp examination or staining of the cornea or conjunctiva. Although being more cost-intensive in the short term, in the long term, the use of ocular imaging can lead to more individualized diagnoses and treatment decisions, which in turn are beneficial for affected patients as well as for the healthcare system. In addition, imaging is more objective and provides good documentation, leading to an improvement in patient follow-up and education.

Normalized ocular surface temperature models for tear film characteristics and dry eye disease evaluation

PURPOSE

To construct normalized ocular surface temperature (NOST) models for different tear film characteristics and evaluate its potential in dry-eyes screening.

METHODS

We included 227 right eyes of 227 patients. Tear film characteristics were categorized into 4 types according to fluorescein tear film breakup time (FTBUT) and Schirmer test results, namely type 1: FTBUT >5 s and Schirmer >5 mm; type 2: FTBUT ≤5 s and Schirmer >5 mm; type 3: FTBUT ≤5 s and Schirmer ≤5 mm; and type4: FTBUT >5 s and Schirmer ≤5 mm. Ocular surface temperature was measured by a video-thermographer. Mean temperatures of the central cornea were calculated from the videos of each frame during the 4-s blink interval. We first constructed individual NOST model for every tear characteristic. Participants were included for further analysis when their OSDI was ≥23, FTBUT ≤5 s, and Schirmer >5 mm. They were subdivided into short-BUT and short BUT with SPK subgroups according to the absence or presence of corneal fluorescein-stain. The NOST models of the normal, short-BUT and short BUT with SPK groups were separately constructed and the potential of screening analyzed via ROC curves.

RESULTS

Each tear film type had a different NOST model. At 3 s after blinking, the order of NOST was type 4 >type 1 >type 3 >type 2. In dry-eye screening, the NOST was normal > short-BUT > short BUT with SPK. The NOST displayed a sensitivity 0.87, specificity 0.80, and AUC 0.88 for diagnosing short BUT with SPK.

CONCLUSION

NOST models are useful in differentiating tear film characteristics and screening dry-eyes. It alleviates the discomfort and inconvenience encountered during conventional dry-eye diagnosis.

Sex Disparity in How Pain Sensitivity Influences Dry Eye Symptoms

PURPOSE

Women have a higher dry eye disease prevalence compared with men, although only relatively minor differences in the ocular surface have been observed. Interestingly, a sex difference in pain sensitivity is known, and recent research suggests that pain sensitivity is associated with dry eye symptoms. This study attempts to discern whether the association between pain sensitivity and dry eye symptoms varies between women and men.

METHODS

In this prospective cross-sectional study, subjects were seen for one visit where they were asked to fill out a set of questionnaires consisting of the Pain Sensitivity Questionnaire, Ocular Surface Disease Index (OSDI), and other dry eye questionnaires. This was followed by an ocular surface assessment on both eyes.

RESULTS

Two hundred eighty-seven subjects (194 women, 93 men) completed the study. Intersex differences in the ocular surface were noted. Even after accounting for these differences, an interaction effect between sex and Pain Sensitivity Questionnaire-minor score on dry eye symptoms was observed, with only women noting increased symptoms on the OSDI (P < 0.005) and other dry eye questionnaires (P values ranging from 0.01 to <0.005) with greater pain sensitivity. After controlling for other variables, women with the highest pain sensitivity had a 17-point higher OSDI score and greater symptoms, as reported by all the other dry questionnaires compared with their male counterparts.

CONCLUSIONS

The role of pain sensitivity on dry eye symptoms appears to vary between women and men. This difference provides insight into why women have a significantly higher dry eye disease prevalence than men.

[Current methods of tear film stability assessment]

Dry eye syndrome (DES) is a multifactorial disease of the ocular surface characterized by a loss of homeostasis of the tear film, and accompanied by ocular symptoms, in which tear film instability and hyperosmolarity, ocular surface inflammation and damage, and neurosensory abnormalities play etiological roles. The clinical manifestations of DES can be highly variable, so its diagnosing is typically based on a combination of symptoms, signs, and clinical tests. Majority of the techniques involved in the diagnosis of this disease has a large degree of subjectivity. The present article reviews current dry eye diagnostic tests and presents a new tear film stability assessment technique that may potentially become the objective method for establishing DES diagnosis.

New approaches for diagnosis of dry eye disease

We reviewed the literature for different diagnostic approaches for dry eye disease (DED) including the most recent advances, contradictions and promising diagnostic tools and technique. We performed a broad literature search for articles discussing different methods for diagnosis of DED including assessment of tear osmolarity, tear film stability, ocular biomarkers and others. Articles indexed in PubMed and google scholar were included. With the growing cosmetic industry, environmental pollution, and booming of digital screens, DED is becoming more prevalent. Its multifactorial etiology renders the diagnosis challenging and invites the emergence of new diagnostic tools and tests. Diagnostic tools can be classified, based on the parameter they measure, into tear film osmolarity, functional visual acuity, tear volume, tear turnover, tear film stability, tear film composition, ocular biomarkers and others. Although numerous methods exist, the most accurate diagnosis can be reached through combining the results of more than one test. Many reported tests have shown potential as diagnostic/screening tools, however, require more research to prove their diagnostic power, alone or in combination. Future research should focus on identifying and measuring parameters that are the most specific to DED diagnosis.

Association among Blink Rate, Changes in Ocular Surface Temperature, Tear Film Stability, and Functional Visual Acuity in Patients after Cataract Surgery

Purpose

To investigate the association among the ocular surface temperature (OST), tear film stability, functional visual acuity (FVA), and blink rate in patients after cataract surgery.

Methods

We recruited 98 eyes of 69 patients (mean age, 73.7 ± 5.2 years) 1 month after phacoemulsification with implantation of acrylic intraocular lenses and assessed slit-lamp microscopy, corrected distance VA, FVA, noninvasive tear breakup time (NIBUT), and OST. We defined the changes in the OST from 0 to 10 seconds after eye opening as the ΔOST. We measured the FVA and blink rate using the FVA measurement system. We divided the patients into two groups based on tear film stability: stable tear film (NIBUT, >5.0 seconds) and unstable tear film (NIBUT, ≤5.0 seconds). We evaluated the differences between the two groups and the association between the blink rate and other clinical parameters.

Results

The unstable tear film group (56 eyes) had significantly (p < 0.0001, unpaired t-test) shorter NIBUTs than the stable tear film group (42 eyes). The ΔOSTs and blink rates were significantly (p < 0.0001) higher in the unstable tear film group than in the stable group. Linear single regression analysis showed that the ΔOST (r = −0.430, p < 0.0001), NIBUT (r = −0.392, p < 0.0001), and gender (r = −0.370, p=0.0002) were correlated significantly with the blink rate. Multiple regression analysis showed that the ΔOST independently contributed to the blink rate.

Conclusions

The frequency of blinks is associated with tear film stability in patients after cataract surgery. The blink rate may be useful for evaluating the tear film stability in clinical practice. The ΔOST should be an important contributing factor to the blink rate. [This trial is registered with UMIN000026970].

Characterization of the ocular surface temperature dynamics in glaucoma subjects using long-wave infrared thermal imaging

We evaluated the dynamics of ocular surface temperature using thermal imaging in 21 glaucoma subjects and 19 healthy subjects. On opening of the eye, subjects with glaucoma showed significantly cooler temperatures in the central cornea compared to the control group. The upper eyelid was also significantly cooler just before the eye opened. Immediately after opening the eye, the dynamic of temperature change was different in the two groups. In subjects with glaucoma, the eyes cooled significantly faster, with an average decrease of 0.49°C during the first second compared to 0.24°C in the control group. Our results support the hypothesis that both the stability of the tear film and changes in the ocular blood supply in subjects with glaucoma play an important role in thermal dynamics of the ocular surface.

Pre-contact lens and pre-corneal tear film kinetics

PURPOSE

Contact lenses (CL) insulate the corneal surface from the environment. It is possible that they influence the corneal sensory mechanism that contribute to spontaneous blinking. The study objective was to quantify the pre-CL and pre-corneal tear film kinetics (TFK) over blink period.

METHODS

The study population was 202 soft CL wearers, 133 non-lens wearers. TFK were quantified via post-hoc masked analysis of HD Tearscope videos. The parameters were: Non-Invasive Break Up Time (NIBUT), Exposed Area % at initial break (EA 1st Break) and at blink (EA Blink), Interblink period (IB), Protective Index (PI) and exposure speed of surface dehydration (ES mm2/s). The TFK of CL wearers were compared to non-lens wearers. The hypothesis was that pre-CL TFK was inferior to pre-corneal, specifically greater tear film anomalies presence at blink.

RESULTS

The pre-corneal NIBUT was longer than pre-CL NIBUT (9.1 vs. 5.1s, p < 0.001). The EA 1st Break was smaller for pre-corneal than pre-CL (0.003 vs. 0.43%, p < 0.001). The mean IB time was similar for pre-CL and pre-corneal (9.4 vs. 9.8s, p = 0.213). The EA Blink % was smaller for pre-corneal than pre-CL (0.03 vs. 6.66%, p < 0.001). The ES was faster for pre-CL than pre-corneal (0.339 vs. 0.004, p < 0.001). The PI was greater for pre-corneal than pre-CL (99.9 vs. 97.1%, p < 0.001).

CONCLUSIONS

Pre-CL TFK were significantly inferior than pre-corneal, confirmed the hypothesis. The NIBUT was shorter. Once the initial break occurred, ES was faster, and EA was much greater for pre-CL than pre-corneal. The differences identified may be an aetiological component of CL discomfort and the relationship between TFK and discomfort in contact lens wearers should be investigated.

Tear-Film Evaporation Rate from Simultaneous Ocular-Surface Temperature and Tear-Breakup Area

SIGNIFICANCE

A corneal heat-transfer model is presented to quantify simultaneous measurements of fluorescein tear-breakup area (TBA) and ocular-surface temperature (OST). By accounting for disruption of the tear-film lipid layer (TFLL), we report evaporation rates through lipid-covered tear. The modified heat-transfer model provides new insights into evaporative dry eye.

PURPOSE

A quantitative analysis is presented to assess human aqueous tear evaporation rate (TER) through intact TFLLs from simultaneous in vivo measurement of time-dependent infrared OST and fluorescein TBA.

METHODS

We interpret simultaneous OST and TBA measurements using an extended heat-transfer model. We hypothesize that TBAs are ineffectively insulated by the TFLL and therefore exhibit higher TER than does that for a well-insulting TFLL-covered tear. As time proceeds, TBAs increase in number and size, thereby increasing the cornea area-averaged TER and decreasing OST. Tear-breakup areas were assessed from image analysis of fluorescein tear-film-breakup video recordings and are included in the heat-transfer description of OST.

RESULTS

Model-predicted OSTs agree well with clinical experiments. Percent reductions in TER of lipid-covered tear range from 50 to 95% of that for pure water, in good agreement with literature. The physical picture of noninsulating or ruptured TFLL spots followed by enhanced evaporation from underlying cooler tear-film ruptures is consistent with the evaporative-driven mechanism for local tear rupture.

CONCLUSIONS

A quantitative analysis is presented of in vivo TER from simultaneous clinical measurement of transient OST and TBA. The new heat-transfer model accounts for increased TER through expanding TBAs. Tear evaporation rate varies strongly across the cornea because lipid is effectively missing over tear-rupture troughs. The result is local faster evaporation compared with nonruptured, thick lipid-covered tear. Evaporative-driven tear-film ruptures deepen to a thickness where fluorescein quenching commences and local salinity rises to uncomfortable levels. Mitigation of tear-film rupture may therefore reduce dry eye-related symptoms.

Mechanisms, imaging and structure of tear film breakup

Tear film breakup (BU) is an important aspect of dry eye disease, as a cause of ocular aberrations, irritation and ocular surface inflammation and disorder. Additionally, measurement of breakup time (BUT) is a common clinical test for dry eye. The current definition of BUT is subjective; here, a more objective concept of "touchdown" - the moment when the lipid layer touches down on the corneal surface - is proposed as an aid to understanding processes in early and late stages of BU development. Models of BU have generally been based on the assumption that a single mechanism is involved. In this review, it is emphasized that BU does not have a single explanation but it is the end result of multiple processes. A three-way classification of BU is proposed - "immediate," "lid-associated," and "evaporative." Five different types of imaging systems are described, which have been used to help elucidate the processes involved in BU and BUT; a new method, "high resolution chromaticity images," is presented. Three directions of tear flow - evaporation, osmotic flow out of the ocular surface, and "tangential flow" along the ocular surface - determine tear film thinning between blinks, leading to BU. Ten factors involved in BU and BUT, both before and after touchdown, are discussed. Future directions of research on BU are proposed.

TFOS DEWS II pain and sensation report

Pain associated with mechanical, chemical, and thermal heat stimulation of the ocular surface is mediated by trigeminal ganglion neurons, while cold thermoreceptors detect wetness and reflexly maintain basal tear production and blinking rate. These neurons project into two regions of the trigeminal brain stem nuclear complex: ViVc, activated by changes in the moisture of the ocular surface and VcC1, mediating sensory-discriminative aspects of ocular pain and reflex blinking. ViVc ocular neurons project to brain regions that control lacrimation and spontaneous blinking and to the sensory thalamus. Secretion of the main lacrimal gland is regulated dominantly by autonomic parasympathetic nerves, reflexly activated by eye surface sensory nerves. These also evoke goblet cell secretion through unidentified efferent fibers. Neural pathways involved in the regulation of meibomian gland secretion or mucin release have not been identified. In dry eye disease, reduced tear secretion leads to inflammation and peripheral nerve damage. Inflammation causes sensitization of polymodal and mechano-nociceptor nerve endings and an abnormal increase in cold thermoreceptor activity, altogether evoking dryness sensations and pain. Long-term inflammation and nerve injury alter gene expression of ion channels and receptors at terminals and cell bodies of trigeminal ganglion and brainstem neurons, changing their excitability, connectivity and impulse firing. Perpetuation of molecular, structural and functional disturbances in ocular sensory pathways ultimately leads to dysestesias and neuropathic pain referred to the eye surface. Pain can be assessed with a variety of questionaires while the status of corneal nerves is evaluated with esthesiometry and with in vivo confocal microscopy.

Evaporation and Hydrocarbon Chain Conformation of Surface Lipid Films

PURPOSE

The inhibition of the rate of evaporation (R_evap) by surface lipids is relevant to reservoirs and dry eye. Our aim was to test the idea that lipid surface films inhibit R_evap.

METHODS

R_evap were determined gravimetrically. Hydrocarbon chain conformation and structure were measured using a Raman microscope. Six 1-hydroxyl hydrocarbons (11-24 carbons in length) and human meibum were studied. Reflex tears were obtained from a 62-year-old male.

RESULTS

The Raman scattering intensity of the lipid film deviated by about 7 % for hydroxyl lipids and varied by 21 % for meibum films across the entire film at a resolution of 5 μm². All of the surface lipids were ordered. R_evap of the shorter chain hydroxyl lipids were slightly (7%) but significantly lower compared with the longer chain hydroxyl lipids. R_evap of both groups was essentially similar to that of buffer. A hydroxyl lipid film did not influence R_evap over an estimated average thickness range of 0.69 to >6.9 μm. R_evap of human tears and buffer with and without human meibum (34.4 μm thick) was not significantly different. R_evap of human tears was not significantly different from buffer.

CONCLUSIONS

Human meibum and hydroxyl lipids, regardless of their fluidity, chain length, or thickness did not inhibit R_evap of buffer or tears even though they completely covered the surface. It is unlikely that hydroxyl lipids can be used to inhibit R_evap of reservoirs. Our data do not support the widely accepted (yet unconfirmed) idea that the tear film lipid layer inhibits R_evap of tears.

Dynamic interfacial properties of human tear-lipid films and their interactions with model-tear proteins in vitro

This review summarizes the current state of knowledge regarding interfacial properties of very complex biological colloids, specifically, human meibum and tear lipids, and their interactions with proteins similar to the proteins found in aqueous part of human tears. Tear lipids spread as thin films over the surface of tear-film aqueous and play crucial roles in tear-film stability and overall ocular-surface health. The vast majority of papers published to date report interfacial properties of meibum-lipid monolayers spread on various aqueous sub-phases, often containing model proteins, in Langmuir trough. However, it is well established that natural human ocular tear lipids exist as multilayered films with a thickness between 30 and 100nm, that is very much disparate from 1 to 2nm thick meibum monolayers. We employed sessile-bubble tensiometry to study the dynamic interfacial and rheological properties of reconstituted multilayered human tear-lipid films. Small amounts (0.5-1μg) of human tear lipids were deposited on an air-bubble surface to produce tear-lipid films in thickness range 30-100nm corresponding to ocular lipid films. Thus, we were able to overcome major Langmuir-trough method limitations because ocular tear lipids can be safely harvested only in minute, sub-milligram quantities, insufficient for Langmuir through studies. Sessile-bubble method is demonstrated to be a versatile tool for assessing conventional synthetic surfactants adsorption/desorption dynamics at an air-aqueous solution interface. (Svitova T., Weatherbee M., Radke C.J. Dynamics of surfactant sorption at the air/water interface: continuous-flow tensiometry. J. Colloid Interf. Sci. 2003;261:1170-179). The augmented flow-sessile-bubble setup, with step-strain relaxation module for dynamic interfacial rheological properties and high-precision syringe pump to generate larger and slow interfacial area expansions-contractions, was developed and employed in our studies. We established that this method is uniquely suitable for examination of multilayered lipid-film interfacial properties. Recently it was compellingly proven that chemical composition of human tear lipids extracted from whole tears is substantially different from that of meibum lipids. To be exact, healthy human tear lipids contain 8-16% of polar lipids, similar to lung lipids, and they are mostly double-tailed phospholipids, with C16 and longer alkyl chains. Rationally, one would assume that the results obtained for meibum lipids, devoid of surface-active components such as phospholipids, and, above all, in a form of monolayers, are not pertinent or useful for elucidating behavior and stability of an averaged 60-nm thick ocular tear-lipid films in vivo. The advantage of sessile-bubble technique, specifically, using a small amount of lipids required to attain multilayered films, unlocks the prospect of evaluating and comparing the interfacial properties of human tear lipids collected from a single individual, typically 100-150μg. This is in sharp contrast with several milligrams of lipids that would be required to build equally thick films for Langmuir-trough experiments. The results of our studies provided in-depth understanding of the mechanisms responsible for properties and stability of human tear-lipid films in vivo. Here we summarize recent publications and our latest findings regarding human tear-lipid interfacial properties, their chemical composition, and their interaction with model proteins mimicking the proteins found in human tear-aqueous phase.

Static and Dynamic Measurement of Ocular Surface Temperature in Dry Eyes

Purpose. To study ocular surface temperature (OST) in dry eyes by static and dynamic measures. Methods. OST was recorded on 62 dry eyes and 63 age- and sex-matched controls. Static measures were study of absolute OST at t = 0, 5, and 10 s after eye opening. Dynamic measures were study of mean change and net change in OST over 10 s of sustained eye opening. Ten OST indices studied were temperatures of the geometric center of the cornea (GCC), extreme temporal (T1) and nasal conjunctiva (T4), midtemporal (CT) and nasal conjunctiva (CN), temporal (LT) and nasal (LN) limbus, and mean (MOST), maximum (Max T), and minimum (Min T) temperatures of the region of interest. Results. For static measures, dry eyes recorded significantly lower GCC, MOST, Min T, Max T, T4, CT, LT, LN, and CN. For dynamic measures, dry eyes had significantly steeper regression line of mean change (corresponding to greater net change) for Max T 5 s onward and T4 at 3 s onward. Conclusions. Both static and dynamic measures of the OST were valuable and can be used as clinical tool to assess dry eye.