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

Correlation between meibomian gland dysfunction and sebaceous adenitis in dogs

BACKGROUND

Sebaceous adenitis (SA) is an immune-mediated disease targeting the sebaceous glands. Meibomian gland dysfunction is a disease affecting meibomian glands with inflammatory features. Sebaceous and meibomian glands share anatomical, physiological and embryological similarities. The involvement of meibomian glands in dogs with SA is currently unknown.

OBJECTIVES

To evaluate meibomian glands in dogs affected by SA and compare them with healthy dogs.

ANIMALS

Eighteen dogs were enrolled. Nine dogs with SA were retrospectively identified from clinical records and represented the case group. Nine healthy, breed- and age-matched dogs were prospectively enrolled in the control group.

MATERIALS AND METHODS

Both groups underwent dermatological examination, Schirmer tear test-1 (STT-1), tear meniscus height (TMH), slit-lamp biomicroscopy, interferometry (INT) and noncontact infrared meibography (NIM).

RESULTS

One third of SA dogs presented subepithelial crystalline opacities. No significant difference between groups was observed in TMH (p = 0.944) and STT-1 values (p = 0.066). INT (p = 0.016) and NIM grades (p = 0.010) were significantly higher and lower in the SA group compared to the control group, respectively. INT values decreased with age (η = 0.930), while NIM scores (η = 0.935) increased.

CONCLUSIONS

Clinical Relevance: Subepithelial crystalline opacities in SA dogs might reflect a reduced tear film quality. In the absence of standardised methods, INT and NIM proved to be noninvasive and useful methods to examine meibomian glands. Dogs with SA showed a thinner lacrimal lipid layer and more severe meibomian gland abnormalities than control dogs, which seemed to progress with age.

Aqueous-Deficient Dry Eye Increases Lipid Layer Thickness Measured by Dynamic Tear Interferometry

PURPOSE

To investigate the impact of aqueous deficiency on lipid layer thickness (LLT) measurement in dry eye disease before and after treatment of aqueous-deficient dry eye (ADDE) using dynamic tear interferometry.

METHODS

This prospective comparative study included 230 eyes from 230 patients with ADDE. The treatment group with punctal plug insertion was compared with the control group. The average LLT (LLTave) was measured with a LipiView II tear interferometer at baseline and after 1 month. Correlation analyses were performed with the indices of the dynamics of the tear film lipid layer (TFLL).

RESULTS

Baseline LLTave values were 116.1 ± 26.7 and 112.3 ± 27.6 nm in the treatment group (n = 121) and the control group (n = 109), respectively. At 1 month, LLTave significantly decreased in the treatment group (difference -41.8 [95% confidence interval (CI) -47.2 to -36.3], P = 0.003). The values of tear meniscus height (TMH) at baseline and 1-month follow-up were negatively correlated with LLTave (both P < 0.001). Spreading time of TFLL to equilibrium and the deviation of TFLL were positively correlated with LLTave (all P < 0.001) and negatively correlated with TMH (P < 0.001 and 0.009). In multivariate analysis (adjusted R2 = 0.411, P < 0.001), LLTave was associated with TMH (B = -1.068; P < 0.001), adjusting for age, sex, and meibomian gland expressibility.

CONCLUSIONS

In ADDE, the TFLL was measured to be thicker than the normal range of 60 to 99 nm and became thinner with treatment. LLT was affected by not only meibum secretion but also aqueous status due to changes in the dynamics of TFLL.

High Resolution Images of Human Meibum Spread on Saline

Purpose

Understanding of the role of the tear film lipid layer (TFLL) in evaporative dry eye requires knowledge of its structure. X-ray studies show 11.1-nm thick lamellae in meibum at tear film temperature (approximately 35°C), whereas below 30°C, 4.88-nm thick lamellae predominate. Here, high resolution microscopy of meibum spread on saline is studied as a function of temperature, to compare with x-ray results.

Methods

A purpose-built high resolution color microscope, previously used to study the TFLL, was used to study meibum from 10 subjects. It was spread on buffered saline at near 40°C, and allowed to cool to room temperature. Analytical methods from previous studies were applied to measure meibum and lamellar thickness.

Results

Initially, an irregular "island" was formed, surrounded by a "background layer" of 7.8 ± 0.3 nm thickness. Dewetting of the meibum layer always occurred, leading to the formation of lens-shaped droplets. Below 30°C, the lenses start to emit "tails" having a multilamellar structure containing up to about 49 lamellae superimposed on the background layer, each lamella being 4.82 ± 0.13 nm thick.

Conclusions

Below 30°C, meibum spread on saline shows a multilamellar structure like the 4.88 nm thickness in x-ray studies, demonstrating the ability to observe and measure tightly stacked lamellae. In contrast, above 30°C, the 11.1 nm lamellae were not observed as in x-ray studies, indicating that these lamellae were not tightly stacked but may be separated by disordered lipid. The role of these findings in evaporative dry eye is discussed.

A Novel Noninvasive Screening Tool for Dry Eye Disease

PURPOSE

To assess the feasibility and the diagnostic accuracy of the new tool, DEvice© (AI, Rome, Italy), for screening patients with dry eye disease (DED).

METHODS

This study was performed at the University Magna Græcia of Catanzaro. Enrolled patients were classified as affected by DED (group 1) or not (group 2) using an already validated tool (Keratograph 5M, Oculus, Germany), evaluating the noninvasive keratograph breakup time (NIKBUT), tear meniscus height (TMH), meibomian gland loss (MGL), and bulbar redness. All the patients were then examined by means of DEvice©, which allowed the measurement of the relative humidity (RH) and temperature of the ocular surface. Symptoms were scored using the Ocular Surface Disease Index (OSDI) questionnaire.

RESULTS

Overall, 40 patients (17 males and 23 females, mean age 38.0 ± 17.1 years) were included: of these, 20 belonged to group 1 and the remaining 20 to group 2. Using Keratograph 5M, significant differences between groups 1 and 2 were detected for NIKBUT-first (respectively, 4.97 ± 1.85 vs. 13.95 ± 4.8 s; p < 0.0001) and for NIKBUT-average (10.55 ± 4.39 vs. 15.96 ± 4.08 s; p = 0.0003). No statistically significant changes were detected for TMH (p = 0.565), MGL (p = 0.051), and bulbar redness (p = 0.687). Using Device©, a statistically significant higher value of RH was found in group 1 compared to group 2 (respectively, 85.93 ± 10.63 vs. 73.05 ± 12.84%; p = 0.0049). A statistically significant correlation was found between RH and OSDI (r = 0.406; p = 0.009). The value RH showed a discriminating power to detect DED with an AUC = 0.782 (standard error 0.07264; 95% CI 0.6401-0.9249; p = 0.0022).

CONCLUSIONS

The DEvice© can effectively discriminate DED patients from healthy subjects. The parameter RH showed good sensitivity, making this tool ideal for a fast and noninvasive DED screening.

Evaluation of tear film in patients with AIDS without opportunistic ocular infections in the era of COVID-19: A case-control study

PURPOSE

The assessment of tear film and ocular surface conditions in patients with acquired immunodeficiency syndrome (AIDS) has been poorly studied thus far. We aim to assess tear film parameters, ocular surface characteristics, and dry eye disease (DED) symptoms of patients with AIDS who did not undergo highly active antiretroviral treatment (HAART).

METHODS

This case-control study included 154 age-, sex-, and ethnicity-matched healthy controls and patients with AIDS. All participants underwent comprehensive ocular surface assessment and subjective DED symptomology evaluation. Data were collected between March 2022 and July 2022.

RESULTS

HAART-naïve patients with AIDS had a shorter noninvasive tear film breakup time (median 3.76 vs. 8.54 s), thinner tear film lipid layer thickness (median 73.00 vs. 91.00 nm), and lower Schirmer I test values (median 5.00 mm/5 min vs. 12.00 mm/5 min) (all P < 0.001). Moreover, higher corneal fluorescein staining scores (median 1.00 vs. 0.00) and higher upper, lower, and total meibomian gland grades were observed in AIDS patients (all P < 0.05). Negative correlations between the blood viral load and the Ocular Surface Disease Index score ( r = -3.50, P = 0.027) and the Schirmer I test score ( r = -0.374, P = 0.017) were detected in patients with AIDS.

CONCLUSION

Altered tear film status was observed in individuals with HAART-naïve AIDS, even when there were no other ocular symptoms present. Therefore, patients with AIDS should be encouraged to undergo comprehensive ocular surface examinations to detect any subclinical tear film alterations occurring.

One Soul and Several Faces of Evaporative Dry Eye Disease

The ocular surface system interacts with, reacts with, and adapts to the daily continuous insults, trauma, and stimuli caused by direct exposure to the atmosphere and environment. Several tissue and para-inflammatory mechanisms interact to guarantee such an ultimate function, hence maintaining its healthy homeostatic equilibrium. Evaporation seriously affects the homeostasis of the system, thereby becoming a critical trigger in the pathogenesis of the vicious cycle of dry eye disease (DED). Tear film lipid composition, distribution, spreading, and efficiency are crucial factors in controlling water evaporation, and are involved in the onset of the hyperosmolar and inflammatory cascades of DED. The structure of tear film lipids, and subsequently the tear film, have a considerable impact on tears' properties and main functions, leading to a peculiar clinical picture and specific management.

Immune Fingerprint in Diabetes: Ocular Surface and Retinal Inflammation

Diabetes is a prevalent global health issue associated with significant morbidity and mortality. Diabetic retinopathy (DR) is a well-known inflammatory, neurovascular complication of diabetes and a leading cause of preventable blindness in developed countries among working-age adults. However, the ocular surface components of diabetic eyes are also at risk of damage due to uncontrolled diabetes, which is often overlooked. Inflammatory changes in the corneas of diabetic patients indicate that inflammation plays a significant role in diabetic complications, much like in DR. The eye's immune privilege restricts immune and inflammatory responses, and the cornea and retina have a complex network of innate immune cells that maintain immune homeostasis. Nevertheless, low-grade inflammation in diabetes contributes to immune dysregulation. This article aims to provide an overview and discussion of how diabetes affects the ocular immune system's main components, immune-competent cells, and inflammatory mediators. By understanding these effects, potential interventions and treatments may be developed to improve the ocular health of diabetic patients.

Classification of Tear Film Lipid Layer En Face Maps Obtained Using Optical Coherence Tomography and Their Correlation With Clinical Parameters

PURPOSE

The purpose of this study was to investigate the correlation between the pattern of optical coherence tomography (OCT) en face maps of the tear film lipid layer (TFLL) and lipid layer thickness (LLT), fluorescein breakup time (FBUT), and Schirmer I test values in healthy subjects.

METHODS

Measurements from four clinical data sets were retrospectively analyzed, and TFLL patterns were classified into 3 categories: homogeneous (HOM), wavy (WAV), or dotted (DOT) appearance. Linear mixed model analyses were performed. Intraclass correlation coefficients and index of qualitative variation were computed to investigate interrater and intrasubject variabilities.

RESULTS

For the LLT, a significant difference between HOM and DOT ( P < 0.001, β HOMvsDOT = -6.42 nm) and WAV and DOT ( P = 0.002, β WAVvsDOT = -4.04 nm) was found. Furthermore, the difference between WAV and DOT regarding FBUT ( P < 0.001, β WAVvsDOT = -3.065 seconds) was significant, while no significant differences between any of the classes with respect to the Schirmer I test values were found. An intraclass correlation coefficient of 89.0% reveals a good interrater reliability, and an index of qualitative variation of 60.0% shows, on average, a considerable variability in TFLL pattern class for repeated measurements over 1 hour.

CONCLUSIONS

A new classification method for OCT en face maps of the TFLL is presented. Significant differences between patterns were found with respect to LLT and FBUT. A dotted pattern on dark background appears to be the most stable type of TFLL. The analysis of OCT en face maps of the TFLL provides complimentary information to conventional imaging methods and might give new insights into the characteristics of the TFLL.

Biophysical profiling of synthetic ultra-long tear film lipids

The tear film lipid layer (TFLL) is a unique biological membrane of importance to the maintenance of ocular surface health. The underlying factors at play, e.g. the ability to retard evaporation and offer protection from the environment, are all closely connected to the properties of individual lipid components and their interplay. The TFLL contains unique ultra-long polar lipid species such as O-acyl-ω-hydroxy fatty acids, type I-St diesters and type II diesters, which are considered important for its proper function. Herein, we have synthesized model compounds from these categories and studied their biophysical and surface rheological properties at the aqueous interface. Altogether, we provide insights on the distinct biophysical profiles of these lipid classes and discuss how their interplay may affect the structure and function of the TFLL.

Nano-based eye drop: Topical and noninvasive therapy for ocular diseases

Eye drops are the most accessible therapy for ocular diseases, while inevitably suffering from their lower bioavailability which highly restricts the treatment efficacy. The introduction of nanotechnology has attracted considerable interest as it has advantages over conventional ones such as prolonged ocular surface retention time and enhanced ocular barrier penetrating properties, and achieving higher bioavailability and improved treatment efficacy. This review describes various ocular diseases treated with eye drops as well as the physiological and anatomical ocular barriers faced with through drug administration. It also summarizes the recent advances regarding the utilization of nanotechnology in developing eye drops, and how to optimize the nanocarrier-based ocular drug delivery systems. The prospective future research directions for nano-based eye drops are also discussed here.

A biophysical study of tear film lipid layer model membranes

The tear film lipid layer (TFLL), the final layer of the human tear film is responsible for surface tension reduction while blinking, water evaporation retardation and maintaining the stability of the tear film. The study of the composition-structure-function relationship of TFLL is paramount, as a compromised structure of TFLL leads to the emergence of dry eye disease (DED) which is one the most prevalent ophthalmic surface diseases of the modern world, associated with chronic pain and reduced visual capability. In this model membrane study, a systematic approach is used to study the biophysical properties of TFLL model membranes as a function of composition. Three mixed-lipid model membranes are studied along with their individual components comprising cholesteryl oleate (CO), glyceryl trioleate (GT), L-α-phosphatidylcholine (egg PC) and a free fatty acid mixture. The models become progressively more complex from binary to quaternary mixtures, allowing the role of each individual lipid to be derived. Langmuir balance, Brewster Angle Microscopy (BAM) and Profile Analysis Tensiometer (PAT) are used to study the surface activity and compression-expansion cycles, morphology, and rheological behaviour of the model membranes, respectively. Evidence of multilayering is observed with inclusion of CO and a reversible collapse is associated with the GT phase transition. An initially more coherent film is observed due to the addition of polar PC. Notably, these individual behaviours are retained in the mixed films and suggest a possible role for each physiological component of TFLL.

Dry eye syndrome: comprehensive etiologies and recent clinical trials

Dry eye syndrome (DES) is multifactorial and likely to be a cause of concern more so than ever given the rapid pace of modernization, which is directly associated with many of the extrinsic causative factors. Additionally, recent studies have also postulated novel etiologies that may provide the basis for alternative treatment methods clinically. Such insights are especially important given that current approaches to tackle DES remains suboptimal. This review will primarily cover a comprehensive list of causes that lead to DES, summarize all the upcoming and ongoing clinical trials that focuses on treating this disease as well as discuss future potential treatments that can improve inclusivity.

In-vitro evaluation of the evaporation retardation by Meibomian lipids in homogeneous and non-homogeneous evaporation

HYPOTHESIS

An important function of the Tear Film Lipid Layer (TFLL) is the retardation of evaporation. We propose two micro-scaled systems to quantify the influence of the TFLL in evaporation for single patients, which may contribute as an improvement on the diagnosis of Meibomian Gland Dysfunctions (MGD).

EXPERIMENTS

Meibum was extracted from 10 patients with hypersecretory MGD and 9 healthy controls. The lipids were placed over water, and the evaporation was determined in the case of homogeneous evaporation over a surface (pendant drop), and the case where the evaporation depends on a pinned triple contact line (meniscus).

FINDINGS

For the homogeneous case, the presence of Meibum reduced evaporation in 30%, although there was no significant difference between controls and MGD patients. However, evaporation induced by menisci was 25 % higher in MGD patients. Our results contribute to the evidence of the inhibition of evaporation by Meibum. Our study also suggests that the evaporation induced by contact points may be a more relevant model to measure differences in evaporation due to the composition of Meibum. This model may also have connotations in the occurrence of internal stresses in the tear film, inducing its instability.

Management Strategies for Evaporative Dry Eye Disease and Future Perspective

Dry eye disease (DED) is a common disorder that remains challenging from a clinical perspective. Unstable or deficient tear film is a major factor contributing to DED and the inability to resolve the loss of tear film homeostasis that accompanies DED can result in a vicious circle of inflammation and treatment-refractory disease. Recently recognized as a multifactorial disease, the main etiological subtypes of DED are aqueous-deficient and evaporative which exist on a continuum, although evaporative dry eye (EDE) is the more frequent classification. Although attaining greater recognition in recent years, there is currently no consensus and no clear recommendation on how to manage EDE. Clarity on the early diagnosis and treatment of EDE may facilitate the avoidance of progression to chronic inflammation, permanent damage to the ocular surface, and treatment-refractory disease. The purpose of this review was to identify current best practice for management of EDE in order to help clinicians in providing accurate diagnosis and optimized treatment. We summarize recent literature considering the role of the lipid layer on tear film stability, the importance of its composition and of its dynamic behavior, and the link between its malfunction and the insurgence and maintenance of tear film-related diseases. We have provided an assessment of the best management of lipid-deficient EDE based upon an understanding of disease pathophysiology, while indicating the flow of current treatments and possible future evolution of treatment approaches. Lipid containing eye drops may be considered as a step closer to natural tears from artificial aqueous tears because they more closely mimic the aqueous and lipid layers and may be used in combination with other management approaches. As a next step, we recommend working with a wider expert group to develop full guidelines to enable patient-centered management of EDE. Key pointsDry eye is a multifactorial disease of variable presentation with the tendency to become a chronic disease for which it is essential to identify and treat the main pathogenic mechanisms involved and tailor the treatment to the individual patient.Early intervention is needed to prevent the vicious cycle of DED and may require a multi-faceted management approach.EDE is not just a problem of MGD but can be the result of anything affecting blinking, mucin spreading, aqueous layer volume and content.Lipid-containing eye drops may provide significant relief of symptoms by improving the lipid layer and its spreading ability and, as such, are an appropriate component of the overall management of lipid-deficient EDE; natural lipid-containing eye drops should be the preferred treatment.

Diurnal variations of tear film osmolarity on the ocular surface

The measurement of tear film osmolarity has been suggested as a gold standard in the diagnosis of dry eye. Many tear film physiological variables oscillate during the day. This review summarises current clinical knowledge regarding diurnal osmolarity variation in the tear film. A critical analysis is presented in respect of of sample size and characteristics, differences in the diurnal osmolarity variation on healthy versus altered tear film conditions or environment, and time of day and number of measurements undertaken. A comparison of 21 studies was made for studies in which one of the main objectives was to analyse the variance of tear film osmolarity at different time-points in a day on human cohorts. Tear film osmolarity appeard to be somewhat influenced by the time of day in healthy subjects and patients with ocular surface disease, or altered by environmental conditions. Both healthy and non-physiological tear film cohorts showed variations in results depending on the study: no variations during the day or statistically different values at some point in the day. These differences could be in the middle of the day or between the beginning and the end of the day, with higher values in the morning than in the afternoon, or even the opposite situation. The possibility of diurnal variations in tear film osmolarity should be considered by the clinician since the time of day when the tear film measurements are made can be critical in making the right diagnosis. Future studies in the diurnal variation field may have to use a well-established range of measurement time-points and a larger group of healthy subjects and and subjects who have a tear film altered by pathological or environmental conditions.

Characterization of the thickness of the Tear Film Lipid Layer in Meibomian Gland Dysfunction using high resolution optical microscopy

PURPOSE

To evaluate the thickness of the tear film lipid layer (TFLL) in meibomian gland dysfunction (MGD) using a high-resolution optical microscope.

METHODS

The Ocular Surface Disease Index (OSDI) and meibum grade score (MGS) were used to classify 190 subjects into four groups: normal (OSDI<13 and MGS<10), mixed (OSDI≥13 and MGS<10), asymptomatic MGD (OSDI<13 and MGS≥10), and MGD (OSDI≥13 and MGS≥10). The high-resolution optical microscope was used to capture TFLL images in vivo. The histograms of TFLL thickness were analyzed and curve-fitted using probability density functions (PDFs).

RESULTS

There were three obvious peaks in the distributions of TFLL across the groups. From the curve-fitting process, the main outcomes are displayed according to each Gaussian function with the position of peak (μ) and the summed percentage within the range of standard deviation (σ). The normal group had distribution as follows: 33.3 ± 0.005 nm, 26%; 53.9 ± 0.019 nm, 40%; 79.4 ± 0.064 nm, 12%. The mixed group had a distribution as follows: 33.8 ± 0.004 nm, 32%; 53.1 ± 0.115 nm, 21%; 71.7 ± 0.232 nm, 27%. The asymptomatic MGD group had a distribution as follows: 33.5 ± 0.004 nm, 20%; 49.2 ± 0.041 nm, 25%; 62.9 ± 0.063 nm, 47%. The MGD group had a distribution as follows: 34.3 ± 0.004 nm, 34%; 53.7 ± 0.022 nm, 28%; 74.9 ± 0.060 nm, 16%.

CONCLUSIONS

The MGD and mixed groups had the largest percentages of TFLL thicknesses fall within the thinnest modes (peak 34.3 and 33.8 nm, respectively). These data show that measures of central tendency (e.g., averages, medians) do not fully appreciate the variable distributions of TFLL across disease spectra.

Orthokeratology Lens Wear for 2 Years in Children Did Not Alter Tear Film Lipid Thickness by Non-Invasive Interferometry

Purpose

Previous studies suggest that overnight orthokeratology (OOK) may be detrimental to tear function. We aimed to investigate the effect of OOK on lipid layer thickness (LLT), blink pattern, and meibomian gland and elucidate the relationship of these variables.

Methods

Thirty-seven participants who wore OOK lenses every night for at least 2 years and twenty-four healthy non-contact lens wearers (controls) were enrolled in this retrospective study. LipiView interferometry, blink pattern analysis, the ocular surface status, and morphology and function of the meibomian gland were determined.

Results

The OOK group and healthy controls had similar LLT, blink patterns, ocular surface status, and the function of the meibomian gland. OOK participants demonstrated higher meiboscore in the lower eyelids (p < 0.05) but not in the upper eyelids. Within the OOK group, LLT was significantly impacted by the partial blink rate (p < 0.05) and the total score of the upper meibomian gland (p = 0.10).

Conclusions

Wearing OOK for 2 or more years increased lower eyelid meibomian gland dropout but did not have a reduction of LLT.

Human meibum and tear film derived cholesteryl and wax esters in meibomian gland dysfunction and tear film structure

PURPOSE

This study evaluated the presence and roles of cholesteryl esters (CEs) and wax esters (WEs) from human tear film and meibum in meibomian gland dysfunction (MGD).

METHODS

Out of 195 enrolled subjects, 164 and 179 subjects provided tear and meibum samples, respectively. Subjects were classified into normal, asymptomatic MGD, MGD, and mixed (MGD & aqueous deficient). The precorneal tear film (PCTF) thinning rate (evaporation) was measured using optical coherence tomography. Lipids extracted from tear and meibum samples were infused into a SCIEX 5600 TripleTOF mass spectrometer. CE and WE intensities quantified with Analyst 1.7 TF and LipidView 1.3 were compared across disease groups in MetaboAnalyst 5.0 and correlated with PCTF thinning rates.

RESULTS

The numbers of unique CEs and WEs identified in the samples were 125 and 86, respectively. Unsupervised Principal Component (PC) analysis and supervised Partial Least Square Discriminant analysis exhibited little separation among groups for both CEs and WEs in tears and meibum. Spearman's correlation analyses showed no association between either the first or second PC scores with PCTF thinning rates.

CONCLUSION

The abundances of human PCTF and meibum-derived CEs and WEs were independent of MGD disease status and PCTF thinning (evaporation). CEs and WEs alterations do not contribute to alterations in tear film dynamics in MGD, such as has been demonstrated by the (O-acyl) ω-hydroxy fatty acids (OAHFAs).

Reference values for selected dry eye tests in normal Beagle dogs: a pilot study

BACKGROUND

Dry eye disease (DED) cannot be diagnosed by a single test because it is a multifactorial disorder of the ocular surface. Although studies on various dry eye tests (DETs) in dogs have been reported, standard criteria have not been established except for the Schirmer tear test 1 (STT-1).

OBJECTIVES

To establish reference values for DETs in dogs with normal STT-1 values (≥ 15 mm/min) and to analyze the correlation between DETs.

METHODS

The STT-1 was performed in 10 healthy Beagle dogs (20 eyes). After 20 min, interferometry (IF) for estimating the lipid layer thickness (LLT) of the tear film, tear meniscus height (TMH), non-invasive tear breakup time (NIBUT), and meibomian gland loss rate of the upper eyelid (MGLRU) and lower eyelid (MGLRL), were measured using an ocular surface analyzer.

RESULTS

Mean values for STT-1, TMH, and NIBUT were as follows: 21.7 ± 3.4 mm/min, 0.41 ± 0.21 mm, and 19.1 ± 9.5 sec, respectively. The most frequent LLT range, measured by IF, was 30-80 nm (11 eyes), followed by 80 nm (5 eyes) and 80-140 nm (4 eyes). MGLRU ≤ 25% was measured in 11 eyes and 26%-50% in 9 eyes; MGLRL ≤ 25% in 8 eyes and 26%-50% in 12 eyes. Besides positive correlation between TMH and NIBUT (P = 0.038), there were no significant associations between DETs.

CONCLUSIONS

Data obtained in this study provided normative references that could be useful for diagnosing DED and for further research into correlation between DETs in dogs with DED.

Protection against corneal hyperosmolarity with soft-contact-lens wear

Hyperosmotic tear stimulates human corneal nerve endings, activates ocular immune response, and elicits dry-eye symptoms. A soft contact lens (SCL) covers the cornea preventing it from experiencing direct tear evaporation and the resulting blink-periodic salinity increases. For the cornea to experience hyperosmolarity due to tear evaporation, salt must transport across the SCL to the post-lens tear film (PoLTF) bathing the cornea. Consequently, limited salt transport across a SCL potentially protects the ocular surface from hyperosmotic tear. In addition, despite lens-wear discomfort sharing common sensations to dry eye, no correlation is available between measured tear hyperosmolarity and SCL-wear discomfort. Lack of documentation is likely because clinical measurements of tear osmolarity during lens wear do not interrogate the tear osmolarity of the PoLTF that actually overlays the cornea. Rather, tear osmolarity is clinically measured in the tear meniscus. For the first time, we mathematically quantify tear osmolarity in the PoLTF and show that it differs significantly from the clinically measured tear-meniscus osmolarity. We show further that aqueous-deficient dry eye and evaporative dry eye both exacerbate the hyperosmolarity of the PoLTF. Nevertheless, depending on lens salt-transport properties (i.e., diffusivity, partition coefficient, and thickness), a SCL can indeed protect against corneal hyperosmolarity by reducing PoLTF salinity to below that of the ocular surface during no-lens wear. Importantly, PoLTF osmolarity for dry-eye patients can be reduced to that of normal eyes with no-lens wear provided that the lens exhibits a low lens-salt diffusivity. Infrequent blinking increases PoLTF osmolarity consistent with lens-wear discomfort. Judicious design of SCL material salt-transport properties can ameliorate corneal hyperosmolarity. Our results confirm the importance of PoLTF osmolarity during SCL wear and indicate a possible relation between PoLTF osmolarity and contact-lens discomfort.

Tear Film Lipid Layer Structure: Self-Assembly of O-Acyl-ω-hydroxy Fatty Acids and Wax Esters into Evaporation-Resistant Monolayers

In healthy eyes, the tear film lipid layer (TFLL) is considered to act as an evaporation resistant barrier, which prevents eyes from drying. Seeking to understand the mechanisms behind the evaporation resistance of the TFLL, we studied mixtures of lipid layer wax esters and O-acyl-ω-hydroxy fatty acids. Analyzing their self-assembly and biophysical properties led to new discoveries concerning the structure and function of the TFLL. We discovered how these lipids self-assemble at the air-water interface and form an efficient antievaporative barrier, demonstrating for the first time how the interaction of different tear film lipid species can improve the evaporation resistance compared with individual lipid classes on their own. These results provide a potential mechanism for the evaporation resistance of the lipid layer. In addition, the results serve as a base for the future development of improved dry eye treatments and other applications where the evaporation of water represents a significant challenge.

Physiological fluid interfaces: Functional microenvironments, drug delivery targets, and first line of defense

Fluid interfaces, i.e. the boundary layer of two liquids or a liquid and a gas, play a vital role in physiological processes as diverse as visual perception, oral health and taste, lipid metabolism, and pulmonary breathing. These fluid interfaces exhibit a complex composition, structure, and rheology tailored to their individual physiological functions. Advances in interfacial thin film techniques have facilitated the analysis of such complex interfaces under physiologically relevant conditions. This allowed new insights on the origin of their physiological functionality, how deviations may cause disease, and has revealed new therapy strategies. Furthermore, the interactions of physiological fluid interfaces with exogenous substances is crucial for understanding certain disorders and exploiting drug delivery routes to or across fluid interfaces. Here, we provide an overview on fluid interfaces with physiological relevance, namely tear films, interfacial aspects of saliva, lipid droplet digestion and storage in the cell, and the functioning of lung surfactant. We elucidate their structure-function relationship, discuss diseases associated with interfacial composition, and describe therapies and drug delivery approaches targeted at fluid interfaces. STATEMENT OF SIGNIFICANCE: Fluid interfaces are inherent to all living organisms and play a vital role in various physiological processes. Examples are the eye tear film, saliva, lipid digestion & storage in cells, and pulmonary breathing. These fluid interfaces exhibit complex interfacial compositions and structures to meet their specific physiological function. We provide an overview on physiological fluid interfaces with a focus on interfacial phenomena. We elucidate their structure-function relationship, discuss diseases associated with interfacial composition, and describe novel therapies and drug delivery approaches targeted at fluid interfaces. This sets the scene for ocular, oral, or pulmonary surface engineering and drug delivery approaches.

Human Meibum and Tear Film Derived (O-Acyl)-Omega-Hydroxy Fatty Acids as Biomarkers of Tear Film Dynamics in Meibomian Gland Dysfunction and Dry Eye Disease

Purpose

To investigate the association between precorneal tear film (PCTF)- and meibum-derived (O-Acyl)-omega-hydroxy fatty acids (OAHFAs) and PCTF thinning in meibomian gland health and dysfunction.

Methods

Of 195 eligible subjects (18-84 years, 62.6% female), 178 and 170 subjects provided both PCTF optical coherence tomography (OCT) imaging and mass spectrometry data for tears (n = 178) and meibum (n = 170). The PCTF thinning rate was measured in the right eye using an ultra-high-resolution, custom-built OCT. Tear and meibum samples from the right eye were infused into the SCIEX 5600 TripleTOF mass spectrometer in the negative ion mode. Intensities (m/z) of preidentified OAHFAs were measured with Analyst 1.7TF and LipidView 1.3 (SCIEX). Principal component (PC) analyses and Spearman's correlations (ρ) were performed to evaluate the association between OAHFAs and PCTF thinning rates.

Results

In meibum and tear samples, 76 and 78 unique OAHFAs were detected, respectively. The first PC scores of the meibum-derived OAHFAs had statistically significant correlations with PCTF thinning rates (ρ = 0.18, P = 0.016). Among 10 OAHFAs with the highest first PC loadings, six OAHFAs had negative correlations with PCTF thinning rate (18:2/16:2, ρ = -0.19, P = 0.01; 18:2/30:1, ρ = -0.21, P = 0.008; 18:1/28:1, ρ = -0.22, P = 0.004; 18:1/30:1, ρ = -0.22, P = 0.005; 18:1/25:0, ρ = 0.22, P = 0 .006; and 18:1/26:1, ρ = -0.22, P = 0.006), while one OAHFA had a positive correlation with PCTF thinning rate (18:2/18:1, ρ = 0.48, P = 0.006). Tear film-derived OAHFAs had no association with the PCTF thinning rate.

Conclusions

Several human meibum-derived OAHFAs showed significant associations with PCTF thinning, suggesting that these OAHFAs could be implicated in the mechanism underlying the stabilization and thinning of the PCTF. The tear-film derived OAHFAs were, however, independent of the rate of PCTF thinning.

Dynamics and mechanisms for tear breakup (TBU) on the ocular surface

The human tear film is rapidly established after each blink, and is essential for clear vision and eye health. This paper reviews mathematical models and theories for the human tear film on the ocular surface, with an emphasis on localized flows where the tear film may fail. The models attempt to identify the important physical processes, and their parameters, governing the tear film in health and disease.

Human meibum and tear film derived (O-acyl)-omega-hydroxy fatty acids in meibomian gland dysfunction

PURPOSE

The molecular basis of the tear film and lipid layer alterations in meibomian gland dysfunction (MGD) is unknown. This study aimed to identify and compare (O-acyl)-omega-hydroxy fatty acids (OAHFAs) derived from human meibum and tears in MGD.

METHODS

Of 195 eligible subjects (18-84 years, 62.6% female), 183 and 174 provided samples for tears and meibum, respectively. Subjects were classified into four groups: Normal, Asymptomatic MGD, MGD, and Mixed. Samples from the right eye of each subject were infused into the SCIEX 5600 TripleTOF mass spectrometer in negative ion mode. Lipid intensities identified with Analyst1.7 TF and SCIEX LipidView1.3 were normalized by an internal standard and total ion current, then statistically compared in MetaboAnalyst 4.0.

RESULTS

In meibum and tears, 76 and 78 unique OAHFAs were identified, respectively. The five most frequent and abundant OAHFAs were 18:2/16:2, 18:1/32:1, 18:1/30:1, 18:2/32:1, and 18:1/34:1. Two OAHFAs, 18:2/20:2 and 18:2/20:1, were identified only in tears. Initial univariate analysis revealed three differently regulated OAHFAs in meibum and eight in tears. Partial Least Square Discriminant Analysis showed 18:1/32:1, 18:2/16:2, 18:1/34:1 and 18:0/32:1 in tears, and 18:2/16:2, 18:1/32:1 and 18:2/32:2 in meibum, had variable importance in projection scores >1.5 and contributed the most to the separation of groups. In both meibum and tears, all OAHFAS except 18:2/16:2 were reduced in MGD compared to the normal group.

CONCLUSION

MGD is accompanied by differential expression of specific OAHFAs in meibum and tears. These results suggest OAHFAs play a role in the altered biochemical profile of the tear film lipid layer in humans with MGD.

Monolayers of Cholesterol and Cholesteryl Stearate at the Water/Vapor Interface: A Physico-Chemical Study of Components of the Meibum Layer

Langmuir monolayers containing different amounts of cholesterol and cholesteryl stearate were studied at two different temperatures (24 °C and 35 °C). The main goal was to contribute towards the understanding of how the variations in the chemical composition may affect the physico-chemical properties of these specific lipid monolayers. The model mixture was chosen considering that cholesteryl esters are present in cell membranes and some other biological systems, including human tear lipids. Therefore, an investigation into the effect of the lipid monolayer composition on their interfacial properties may elucidate some of the fundamental reasons for the deficiencies in cell membranes and tear film functioning in vivo. The experimental results have shown that the molar ratio of the mixture plays a crucial role in the modulation of the Langmuir film properties. The condensing effects of the cholesterol and the interactions between the lipids in the monolayer were the main factors altering the monolayer response to dilatational deformation. The modification of the mixture compositions leads to significant changes in the Langmuir films and the mechanical performance, altering the ability of the monolayer to reduce the surface tension and the viscoelastic properties of the monolayers. This suggests that subtle modifications of the biomembrane composition may significantly alter its physiological function.

Pathophysiology of Meibomian Glands - An Overview

Purpose: The meibomian glands are located in the tarsal plate of the upper and lower eyelid and are responsible for the production of a lipid-rich secretion, the meibum, which forms the outer component of the tear film. Meibomian gland dysfunction results in excessive evaporation of the tear film and is the leading cause of dry eye disease (DED). Despite the high prevalence of DED, the etiology of meibomian gland dysfunction is only basically understood. In addition, the molecular mechanisms of meibomian gland maturation and physiological function are currently the focus of research.Methods: A systematic literature search was performed using the main scientific databases, including all relevant published articles up to September 2020.Results: This article provides an overview of the current state of knowledge about meibomian gland stem cells, cell surface marker expression and PPARγ signaling, as well as the pathological causes of meibomian gland dysfunction.Conclusion: Androgen deficiency, hyperkeratinization, PPARγ signaling and inflammatory reactions including neutrophil extracellular traps (NETs) seem to be key factors within the pathological processes of the meibomian gland.

Ocular Surface Changes Associated with Ophthalmic Surgery

Dry eye disease causes ocular discomfort and visual disturbances. Older adults are at a higher risk of developing dry eye disease as well as needing for ophthalmic surgery. Anterior segment surgery may induce or worsen existing dry eye symptoms usually for a short-term period. Despite good visual outcomes, ocular surface dysfunction can significantly affect quality of life and, therefore, lower a patient’s satisfaction with ophthalmic surgery. Preoperative dry eye disease, factors during surgery and postoperative treatment may all contribute to ocular surface dysfunction and its severity. We reviewed relevant articles from 2010 through to 2021 using keywords “cataract surgery”, ”phacoemulsification”, ”refractive surgery”, ”trabeculectomy”, ”vitrectomy” in combination with ”ocular surface dysfunction”, “dry eye disease”, and analyzed studies on dry eye disease pathophysiology and the impact of anterior segment surgery on the ocular surface.

Parameter Estimation for Mixed-Mechanism Tear Film Thinning

Etiologies of tear breakup include evaporation-driven, divergent flow-driven, and a combination of these two. A mathematical model incorporating evaporation and lipid-driven tangential flow is fit to fluorescence imaging data. The lipid-driven motion is hypothesized to be caused by localized excess lipid, or "globs." Tear breakup quantities such as evaporation rates and tangential flow rates cannot currently be directly measured during breakup. We determine such variables by fitting mathematical models for tear breakup and the computed fluorescent intensity to experimental intensity data gathered in vivo. Parameter estimation is conducted via least squares minimization of the difference between experimental data and computed answers using either the trust-region-reflective or Levenberg-Marquardt algorithm. Best-fit determination of tear breakup parameters supports the notion that evaporation and divergent tangential flow can cooperate to drive breakup. The resulting tear breakup is typically faster than purely evaporative cases. Many instances of tear breakup may have similar causes, which suggests that interpretation of experimental results may benefit from considering multiple mechanisms.

CLEAR - Anatomy and physiology of the anterior eye

A key element of contact lens practice involves clinical evaluation of anterior eye health, including the cornea and limbus, conjunctiva and sclera, eyelids and eyelashes, lacrimal system and tear film. This report reviews the fundamental anatomy and physiology of these structures, including the vascular supply, venous drainage, lymphatic drainage, sensory innervation, physiology and function. This is the foundation for considering the potential interactions with, and effects of, contact lens wear on the anterior eye. This information is not consistently published as academic research and this report provides a synthesis from all available sources. With respect to terminology, the report aims to promote the consistent use of nomenclature in the field, and generally adopts anatomical terms recommended by the Federative Committee for Anatomical Terminology. Techniques for the examination of the ocular surface are also discussed.

Human precorneal tear film and lipid layer dynamics in meibomian gland dysfunction

PURPOSE

To evaluate the precorneal tear film (PCTF) and lipid layer (TFLL) thicknesses and thinning rates in meibomian gland dysfunction (MGD) using a combined ultra-high-resolution optical coherence tomography (OCT) and thickness dependent fringe (TDF) interferometry system.

METHODS

Based on the Tear Film and Ocular Surface Society (TFOS) International Workshop on Meibomian Gland Dysfunction diagnostic algorithm, the Ocular Surface Disease Index (OSDI) and meibum grade score (MGS) were used to classify subjects into four groups: Normal (OSDI<13 and MGS<10), MGD (OSDI≥13 and MGS≥10), Asymptomatic MGD (OSDI<13 and MGS≥10), and Mixed (OSDI≥13 and MGS<10). The OCT/TDF system was used to capture PCTF and TFLL thicknesses and thinning rates. Kruskal-Wallis was used to compare median PCTF and TFLL thicknesses and thinning rates.

RESULTS

There were 190 subjects categorized into four groups: Normal (n = 63), MGD (n = 51), Asymptomatic MGD (n = 29), and Mixed (n = 47). The PCTF was significantly thinner in the Mixed group (3.3 [1.2]) than in the Normal (p < 0.001), MGD (p < 0.001) and Asymptomatic MGD (p = 0.009) groups. Relative to the Normal (4.5 [4.5] μm/min) and Mixed (5.0 [2.0] μm/min) groups, the rate of PCTF thinning was faster in the MGD (8.1 [3.0] μm/min, both p < 0.001) and Asymptomatic MGD (6.9 [3.1] μm/min, p = 0.009 and p = 0.04, respectively) groups. The correlation between PCTF thinning rate and TFLL thickness was ρ = -0.46, p < 0.001.

CONCLUSIONS

Symptomatic and asymptomatic MGD shows rapid PCTF thinning rates (evaporation), while the PCTF thickness was reduced in mixed disease. Thicker lipid layers were associated with slower PCTF thinning.

The Properties and Role of O-Acyl-ω-hydroxy Fatty Acids and Type I-St and Type II Diesters in the Tear Film Lipid Layer Revealed by a Combined Chemistry and Biophysics Approach

The tear film lipid layer (TFLL) that covers the ocular surface contains several unique lipid classes, including O-acyl-ω-hydroxy fatty acids, type I-St diesters, and type II diesters. While the TFLL represents a unique biological barrier that plays a central role in stabilizing the entire tear film, little is known about the properties and roles of individual lipid species. This is because their isolation from tear samples in sufficient quantities is a tedious task. To provide access to these species in their pure form, and to shed light on their properties, we here report a general strategy for the synthesis and structural characterization of these lipid classes. In addition, we study the organization and behavior of the lipids at the air–tear interface. Through these studies, new insights on the relationship between structural features, such as number of double bonds and the chain length, and film properties, such as spreading and evaporation resistance, were uncovered.

Lipid conformational order and the etiology of cataract and dry eye

Lens and tear film lipids are as unique as the systems they reside in. The major lipid of the human lens is dihydrosphingomylein, found in quantity only in the lens. The lens contains a cholesterol to phospholipid molar ratio as high as 10:1, more than anywhere else in the body. Lens lipids contribute to maintaining lens clarity, and alterations in lens lipid composition due to age are likely to contribute to cataract. Lens lipid composition reflects adaptations to the unique characteristics of the lens: no turnover of lens lipids or proteins; the lowest amount of oxygen of any tissue; and contains almost no intracellular organelles. The tear film lipid layer (TFLL) is also unique. The TFLL is a thin (100 nm) layer of lipid on the surface of tears covering the cornea that contributes to tear film stability. The major lipids of the TFLL are wax esters and cholesterol esters that are not found in the lens. The hydrocarbon chains associated with the esters are longer than those found anywhere else in the body (as long as 32 carbons), and many are branched. Changes in the composition and structure of the 30,000 different moieties of TFLL contribute to the instability of tears. The focus of the current review is how spectroscopy has been used to elucidate the relationships between lipid composition, conformational order and function, and the etiology of cataract and dry eye.

Characteristics of dry eye patients with thick tear film lipid layers evaluated by a LipiView II interferometer

PURPOSE

To investigate the characteristics of eyes with dry eye disease (DED) whose lipid layer thickness (LLT) measured 100 nm on a LipiView II interferometer and compare the DED parameters of them to those with LLT below 100 nm.

METHODS

A total of 201 eyes of 102 enrolled DED patients (mean age 56.4 ± 11.8 years) were classified into 3 groups according to their average LLT; < 60 nm as thin-LLT (n = 49), 60-99 nm as normal-LLT (n = 77), and 100 nm as thick-LLT (n = 75). LLT, meiboscore, Schirmer I test, tear film break-up time (TBUT), ocular surface staining (OSS), and ocular surface disease index (OSDI) were assessed.

RESULTS

The OSS and TBUT were significantly worse in the thick-LLT group than in the normal-LLT group (p = 0.020, and p = 0.028, respectively). The OSDI was significantly higher in the thick-LLT group than in the thin-LLT group (p = 0.006). However, the meiboscore was not different among the three groups (p = 0.33). Age, OSS, and OSDI showed a positive correlation with LLT (r = 0.16, p = 0.023; r = 0.213, p = 0.003; and r = 0.338, p = 0.001, respectively). In sensitivity analyses, eyes with corneal erosions had a significantly higher average LLT (p = 0.015), higher OSDI (p = 0.009), shorter TBUT (p < 0.001), and shorter Schirmer I value (p = 0.024) than those with clear corneas.

CONCLUSION

The average LLT of eyes with corneal erosions was thicker than those without erosions, suggesting that the LLT of 100 nm in the eyes with corneal erosions should not be regarded as a stable physiologic condition. Cautious interpretation of LLT along with other dry eye parameters is required.

Comparison of nanoemulsion and non-emollient artificial tears on tear lipid layer thickness and symptoms

PURPOSE

Dry eye disease (DED) is often managed with over-the-counter eye drops. This study evaluated the diurnal effects of a single drop of two ocular lubricants (nanoemulsion vs. non-emollient) on tear film lipid layer thickness (LLT) and symptoms of ocular dryness. Subjects were also assessed after 1 month of nanoemulsion eye drop use.

METHODS

Part 1 was a cross-over comparison of a nanoemulsion and a non-emollient eye drop. LLT and dry eye symptoms were measured at baseline and at 15min, 1h, 2h, 4h and 6h after instillation of each drop. Part 2 was a 1-month observational study assessing LLT and symptoms after 30-day use of the nanoemulsion drop four times daily (qid).

RESULTS

Total of 20 subjects completed the study (mean age=45.6±7.9, 15 female). Part 1 found a significant increase in average LLT 15min after nanoemulsion drop instillation in the overall and inferior third of the tear film for subjects with baseline LLT values <50nm. Average LLT values did not increase after use of the non-emollient. Symptoms of dryness improved up to 6h following instillation of both drops. Part 2 results found that using the nanoemulsion eye drop for 1 month improved symptoms reported on symptom surveys.

CONCLUSION

Nanoemulsion eye drop use increased average LLT in subjects with low baseline levels. Statistically and clinically significant improvement in symptoms were found on symptom surveys after qid-use (four times a day) of the nanoemulsion drop. Results suggest that a nanoemulsion eye drop can benefit subjects with dry eye symptoms.

Changes in meibum composition following plaque bachytherapy for choroidal melanoma

Objectives

Dry eye is common when external beam radiation is used for the treatment of choroidal melanoma (CM). As meibum structure and composition have been related to dry eye, we determined if plaque bachytherapy for CM alters meibum composition.

Design

¹H-NMR spectroscopy was used to measure the lipid composition of meibum.

Setting

The University of Louisville, Kentucky, USA.

Participants

All 13 participants had CM and one participant had iris melanoma.

Main outcome measures

Cholesteryl ester (CE) to wax ester (WE) ratio, amount of meibum esters (ME) and meibum lipid saturation were measured.

Results

ME decreased by 80%±18% (±99% CI) in 11 eyes that were treated compared with the contralateral untreated eye. ME increased by 181% in two eyes that were treated compared with the contralateral untreated eye. The mole % CE/WE for meibum was significantly (p<0.0001) 67% lower in eyes that were irradiated compared with control eyes from donors without CM and were not treated. Plaque brachytherapy induced the de-esterification of CE. The intensity of the meibum cis double bond resonances did not change significantly (p>0.05).

Conclusion

Eyes that had plaque brachytherapy had a lower amount of expressible meibum and a lower CE/WE ratio compared with meibum from the contralateral eye that received no treatment and eyes that did not have uveal melanoma. Both the quality and quantity of meibum should be considered in designing a therapy for dry eye after plaque brachytherapy.

Fluorescent contact lens for continuous non-invasive measurements of sodium and chloride ion concentrations in tears

Rapid and non-invasive measurement of hydration status is medically important because even mild levels of dehydration can have a significant impact on physical and cognitive performance. Despite the potential value of determining whole-body hydration based on the electrolytes found in tears, very few tests are available. An area of intense interest is the development of a contact lens which could measure ion concentrations in tears, specifically that of sodium (Na⁺) and chloride (Cl^-) ions, the dominant electrolytes in blood plasma and tears. Here, we describe a method to make fluorescent contact lenses which allow determination of Na⁺ and Cl^- ion concentrations in tears. Fluorophores known to be sensitive to Na⁺ and Cl^- were derivatized to bind non-covalently to two commercially-available silicone hydrogel (SiHG) contact lenses-the Biofinity (Comfilcon A) or MyDay (Stenfilcon A) lenses. The sodium- and chloride-sensitive fluorophores displayed spectral changes in the physiological range for Na⁺ and Cl^- ions in tears. The lenses for both Na⁺ and Cl^- ions were completely reversible. The sodium responses were not sensitive to protein interference including human lysozyme, human serum albumin and mucin type 2. The chloride sensitivity was similar with both lenses, but the sodium-sensitive range was different in the Biofinity and MyDay lenses. We also fabricated a lens with both the Na⁺ and Cl^- probes in a single MyDay lens resulting in a contact lens that independently measured Na⁺ and Cl^- concentrations without physical separation of the fluorophores. Our findings indicated that a sodium and chloride-sensitive contact lens (NaCl-lens) could be used for rapid non-invasive detection of whole-body hydration, as well as associated diseases or other infections.

Nanoscale Characteristics of Ocular Lipid Thin Films Using Kelvin Probe Force Microscopy

Purpose

To describe the use of Kelvin probe force microscopy (KPFM) to investigate the electrical surface potential of human meibum and to demonstrate successful use of this instrument on both human meibum and a meibum model system (six-lipid stock [6LS]) to elucidate nanoscale surface chemistry and self-assembly characteristics.

Materials and Methods

6LS and meibum were analyzed in this study. Mica-supported thin films were created using the Langmuir-Blodgett trough. Topography and electrical surface potential were quantified using simultaneous atomic force microscopy/KPFM imaging.

Results

Both lipid mixtures formed thin film patches on the surface of the mica substrate, with large aggregates resting atop. The 6LS had aggregate heights ranging from 41 to 153 nm. The range in surface potential was 33.0 to 125.9 mV. The meibum thin films at P = 5 mN/m had aggregates of 170 to 459 nm in height and surface potential ranging from 15.9 to 76.1 mV, while thin films at P = 10 mN/m showed an aggregate size range of 147 to 407 nm and a surface potential range of 11.5 to 255.1 mV.

Conclusions

This study showed imaging of the differences in electrical surface potential of meibum via KPFM and showed similarities in nanoscale topography. 6LS was also successfully analyzed, showing the capabilities of this method for use in both in vitro and ex vivo ocular research.

Translational Relevance

This study describes the use of KPFM for the study of ocular surface lipids for the first time and outlines possibilities for future studies to be carried out using this concept.

Prophylactic action of lipid and non-lipid tear supplements in adverse environmental conditions: A randomised crossover trial

PURPOSE

To evaluate the prophylactic benefits of lipid-based and non-lipid-based artificial tear lubricants, in dry eye disease, after adverse environmental exposure.

METHODS

Twenty-eight participants with dry eye disease were recruited in a prospective, double-masked, randomised crossover trial. On separate days, participants were randomised to receive a single application of a lipid-containing tear supplement (Systane Complete) to one eye, and a non-lipid containing eye drop (Systane Ultra) to the contralateral eye. Participants were then exposed to a previously validated simulated adverse environment. Symptoms, non-invasive tear film breakup time, lipid layer grade, and tear meniscus height were assessed at three time points; baseline, following eye drop instillation, and after exposure to the adverse environment.

RESULTS

Both treatments effected improvements in symptoms and non-invasive tear film stability following instillation (all p < 0.05), although an improvement in lipid layer quality was limited to the lipid-containing nano-emulsion tear supplement (p = 0.003). Although protective effects were conferred by both treatments following exposure to the simulated adverse environment, more favourable symptomology scores, non-invasive tear film stability, and lipid layer quality were observed in the lipid-containing tear supplement group (all p < 0.05). No significant changes were observed in tear meniscus height in both treatment groups (all p > 0.05).

CONCLUSIONS

Both the lipid and non-lipid-based artificial tear supplement demonstrated prophylactic benefits in a simulated adverse environment. However, the ability to preserve tear film quality and reduce dry eye symptomology was greater with the lipid-containing eye drop.

TRIAL REGISTRATION NUMBER

ACTRN12619000361101.

Why Chain Length of Hyaluronan in Eye Drops Matters

The chain length of hyaluronan (HA) determines its physical as well as its physiological properties. Results of clinical research on HA eye drops are not comparable without this parameter. In this article methods for the assessment of the average molecular weight of HA in eye drops and a terminology for molecular weight ranges are proposed. The classification of HA eye drops according to their zero shear viscosity and viscosity at 1000 s-1 shear rate is presented. Based on the gradient of mucin MUC5AC concentration within the mucoaqueous layer of the tear film a hypothesis on the consequences of this gradient on the rheological properties of the tear film is provided. The mucoadhesive properties of HA and their dependence on chain length are explained. The ability of HA to bind to receptors on the ocular epithelial cells, and in particular the potential consequences of the interaction between HA and the receptor HARE, responsible for HA endocytosis by corneal epithelial cells is discussed. The physiological function of HA in the framework of ocular surface homeostasis and wound healing are outlined, and the influence of the chain length of HA on the clinical performance of HA eye drops is illustrated. The use of very high molecular weight HA (hylan A) eye drops as drug vehicle for the next generation of ophthalmic drugs with minimized side effects is proposed and its advantages elucidated. Consequences of the diagnosis and treatment of ocular surface disease are discussed.

Provocation of the ocular surface to investigate the evaporative pathophysiology of dry eye disease

PURPOSE

To investigate whether standard clinical measures of tear film stability, meniscus height and symptomology reflect changes in evaporation rate induced by ocular surface provocations.

METHODS

Forty participants (23.8 ± 4.5 years, 53 % female) with healthy to mild dry eyes underwent two tear film provocations in random sequence on separate occasions: playing a tablet computer high concentration game (http://slither.io/) for 30 min; and receiving treatment with humidity goggles for 10 min followed by liposomal spray application. Measures at baseline and 30 min later were: Symptom Assessment iN Dry Eye (SANDE) questionnaire, tear film lipid layer thickness (LLT), non-invasive tear film break-up time (NIBUT), tear meniscus height (TMH), dynamic tear film lipid layer pattern (DLP) and tear film evaporation (TFE).

RESULTS

There were no differences in the baseline measurements before each provocation (p > 0.05). Dry eye symptoms significantly worsened with concentration task (p < 0.001) and improved with treatment (p < 0.001). DLP and LLT significantly increased with treatment (p < 0.05), but was unaffected with the concentration task (p > 0.05). NIBUT declined with the concentration task (p = 0.015), but was not enhanced with treatment (p = 0.142). TMH increased after treatment (p = 0.001) and decreased with the concentration task (p = 0.006). While evaporation decreased with the concentration task (p < 0.001), treatment had no effect (p = 0.333). LLT was associated with evaporation (p = 0.036) and additionally with symptom severity (p = 0.002) and tear volume (p = 0.017).

CONCLUSIONS

Sub-classifying dry eye based on an 'evaporative' component to inform treatment seems over-simplistic. However objective TMH, NIBUT and LLT seem to be the key clinical metrics that drive ocular comfort.

Mixed polar-nonpolar lipid films as minimalistic models of Tear Film Lipid Layer: A Langmuir trough and fluorescence microscopy study

The Tear Film Lipid Layer (TFLL) covering the surface of the aqueous film at human cornea forms a first barrier between the eye and environment. Its alterations are related to dry eye disease. TFLL is formed by a complex mixture of lipids, with an excess of nonpolar components and a minor fraction of polar molecules. Its thickness is up to 160 nm, hence a multilayer-like structure of TFLL is assumed. However, details of TFLL organization are mostly unavailable in vivo due to the dynamic nature of the human tear film. To overcome this issue, we employ a minimalistic in vitro lipid model of TFLL. We study its biophysical characteristics by using a combination of the Langmuir trough with fluorescence microscopy. The model consists of two-component polar-nonpolar lipid films with a varying component ratio spread on the aqueous subphase at physiologically relevant temperature. We demonstrate that the model lipid mixture undergoes substantial structural reorganization as a function of lateral pressure and polar to nonpolar lipid ratio. In particular, the film is one-molecule-thick and homogenous under low lateral pressure. Upon compression, it transforms into a multilayer structure with inhomogeneities in the form of polar-nonpolar lipid assemblies. Based on this model, we hypothesize that TFLL in vivo has a duplex polar-nonpolar structure and it contains numerous mixed lipid aggregates formed because of film restructuring. These findings, despite the simplified character of the model, seem relevant for TFLL physiology as well as for understanding pathological conditions related to the lipids of the tear film.

Concentration dependent cholesteryl-ester and wax-ester structural relationships and meibomian gland dysfunction

Background

With dry eye, the ratio of cholesteryl ester (CE) to wax ester (WE) decreases substantially in meibum, but the functional and structural consequences of this change are speculative. The aim of this study is to confirm this finding and to bridge this gap in knowledge by investigating the effect of varying CE/WE ratios on lipid structure and thermodynamics.

Methods

Infrared spectroscopy was use to quantify CE and WE in human meibum and to measure hydrocarbon chain conformation and thermodynamics in a cholesteryl behenate, stearyl stearate model system.

Results

The CE/WE molar ratio was 36% lower for meibum from donors with dry eye due to meibomian gland dysfunction compared with meibum from donors without dry eye. CE (5 mol %) dramatically increased the phase transition temperature of pure WE from -0.12 °C to 63 °C in the mixture. Above 5 mol % CB, the phase transition temperature increased linearly, from 68.5 °C to 85 °C. In the ordered state, CE caused an increase in lipid order from about 72% trans rotamers to about 86% trans rotamers. Above 10% CE, the hydrocarbon chains were arranged in a monoclinic geometry.

Conclusions

The CE/WE is lower in meibum from donors with dry eye due to meibomian-gland dysfunction. Major conformational changes in the hydrocarbon chains of wax and cholesteryl ester mixtures begin to occur with just 5% CB and above.

General significance

CE-WE interactions may be important for in understanding lipid layer structure and functional relationships on the surface of tears, skin and plants.

•

The CE/WE is lower in meibum from donors with meibomian-gland dysfunction.

•

CE may be important for the tear film lipid layer structure and function.

•

CE-WE interactions may be important on the surface of tears, skin and plants.

•

Conformational changes in WE and CE mixtures begin to occur with just 5% CE.

•

CE, cholesteryl ester; WE, wax ester.

Differences in Meibum and Tear Lipid Composition and Conformation

PURPOSE

The compositional, structural, and functional relationships of meibum may provide insights into the loss of tear film stability. Although the conformation of meibum lipids has been studied rigorously, that of tear lipids has not.

METHODS

Tear lipids (TLHSCT) and meibum (MHSCT) from patients who had hematopoietic stem cell transplantation were pooled prospectively. The infrared spectra of meibum from donors with (MMGD) and without (Mn) meibomian gland dysfunction were retrospectively analyzed to measure the lipid composition and structure. The infrared CH stretching region was used to measure the relative content of CH3 and CH2 moieties in the meibum.

RESULTS

The 3 major findings of the current study are as follows: 1) compared with Mn, MHSCT and MMGD had 18% fewer CH3 moieties; 2) compared with MHSCT, the phase transition temperature, cooperativity, and order were approximately 20% greater for TLHSCT; and 3) compared with Mn and MMGD, MHSCT and TLHSCT contained fewer double bonds.

CONCLUSIONS

Tear lipids are more ordered than meibum lipids, which could have functional consequences. The human meibum peak height ratio of the CH3/CH2 bands is not a factor related to tear film stability with age or sex. The amount of CH3 moieties relative to CH2 moieties and saturation could contribute to a higher meibum lipid order associated with a younger age, meibomian gland dysfunction, and dry eye from hematopoietic stem cell transplantation. Therefore, the hydrocarbon order may be a marker of or contribute to an unstable tear film layer.

Meibomian gland dysfunction (MGD), as diagnosed by non-contact infrared Meibography, in dogs with ocular surface disorders (OSD): a retrospective study

Background

Meibomian gland dysfunction (MGD) is one of the possible conditions underlying ocular surface disorders (OSD). Prevalence of MGD in dogs affected by OSD has not yet been reported. We aimed to evaluate the prevalence of MGD among OSD canine patients, which had been assessed by non-contact infrared meibography and interferometry, and to identify MGD associated factors that might guide its diagnosis. Medical records of canine patients examined for OSD between 2016 and 2019 were reviewed. The frequency of MGD was evaluated within different categories (skull conformation, gender, eye and STT-1). The putative MGD risk factors and frequency of MGD within grades of interferometry were evaluated in a regression analysis model and reported as odd ratios (ORs).

Results

One hundred fifty eyes from 81 dogs with OSD were included with median age 75 months (range 3–192) and female representation with 52%. MGD was present in 70% of the examined eyes. MGD risk was higher in males OR_adj = 3.015 (95% CI: 1.395–6.514) (P = 0.005) and older patients OR_adj = 1.207 (95% CI: 1.081–1.348) (P = 0.001). No significant differences were found between left and right eyes (P = 0.66) or between the two types of skull conformation (P = 0.477) and MGD presence. MGD was associated to the lowest lipid layer (LL) thickness, as assessed by interferometry (grade 0) OR = 16.00 (95% CI: 2.104–121.68) (P < 0.001). STT values were not significantly associated with the presence of MGD (P > 0.05).

Conclusions

MGD is a common underlying pathology in OSD. Being male and higher age are risk factors for MGD. An interferometry grade 0 may guide OSD diagnosis towards MGD.

Structural and Molecular Tear Film Changes in Glaucoma

The Tear Film (TF) is a trilaminar and dynamic fluid covering the entire Ocular Surface (OS), consisting of a mucus, aqueous, and lipid layer deeply interacting between them. Because of its structure and functions, TF plays a pivotal role in the preservation of the OS integrity and the quality of vision. Medical therapy for glaucoma is recognized to profoundly disturb the OS homeostasis by altering all components of the ocular surface unit, including TF. The presence of preservatives, the number of daily eye drops instillations, and the duration of therapy are the main contributors to TF changes. From the physio-pathological side, TF alterations are induced by toxic and allergic mechanisms and result from goblet cell and Meibomian gland loss, dysfunction of accessory lacrimal glands, and epithelial disruption. In detail, TF changes are represented by mucus layer thinning, reduced mucin concentration, aqueous layer volume reduction, and lipid layer thinning with increased tear evaporation. Hyper- osmolarity and instability represent the main hallmarks of these changes and are an expression of a iatrogenic form of dry eye. TF undergoes also molecular modifications that primarily reflect a therapy- or disease-induced inflammatory status of the OS. Over the last years, this field of research aimed a progressively growing interest since molecular variations may be considered as potential candidate biomarkers of glaucoma. The aim of this review is to report the main TF changes occurring during glaucoma, exploring the relationship they may have with the glaucoma-related ocular surface disease and the patient quality of life, and their utility as potential biomarkers of disease.

Human Meibum Cholesteryl and Wax Ester Variability With Age, Sex, and Meibomian Gland Dysfunction

Purpose

Relationships between tear film lipid (TFL) layer composition, structure, and function could provide insight into the etiology of dry eye. The molar ratio of cholesteryl ester (CE)/wax ester (WE) was measured in meibum from normal donors (M_n) and compared with meibum from donors with meibomian gland dysfunction (M_MGD).

Methods

CE/WE was measured using nuclear magnetic resonance spectroscopy.

Results

CE/WE was distributed into two populations with 81% distributed near 0.55 and 19% near 0.3. CE/WE were higher in donors 13 to 19 years old compared with donors 1 to 12 years old and 20 to 88 years old. CE/WE for M_MGD was 30% lower, 0.34 ± 0.04, compared with M_n, 0.49 ± 0.04. There were no sex differences in CE/WE. There were no significant racial differences between the CE/WE ratios for Asians and Caucasians. The CE/WE ratio was higher for blacks and lower for Hispanics compared to Caucasians. Due to the small number sampled, confirmation of the later racial results is needed. The packing of CE and WE in the TFL layer was proposed.

Conclusions

Although M_MGD contains much less CE than M_n, factors other than the CE content, such as the levels of saturation and/or proteins, may be responsible for the higher order of M_MGD. In addition to saturation, CE could contribute to the increase in order of M_n between 0 and 20 years of age. Observed changes in the meibum content of CE alone is not likely to influence tear film stability.