The lipid secretion of the meibomian glands

Differential characteristics among asymptomatic and symptomatic meibomian gland dysfunction and those with dry eye

PURPOSE

To identify the characteristics of asymptomatic meibomian gland dysfunction (MGD), symptomatic MGD, and MGD coexisting with dry eye disease (DED).

METHODS

This cross sectional study enrolled a total of 153 eyes of 87 MGD patients. Participants filled in ocular surface disease index (OSDI) questionnaires. Age, gender, Schirmer's test, meibomian gland (MG) related parameters, lipid layer thickness (LLT) and blinking were compared among patients with asymptomatic MGD, symptomatic MGD, and MGD with DED. Multivariate regression was used to analyze the significant factor of DED in MGD. Spearman's rank correlation analysis was used to evaluate the association between the significant factors and MG function.

RESULTS

There was no difference in age, Schirmer's test, lid changes, MG secretion, and MG morphology among three groups. The OSDI of asymptomatic MGD, symptomatic MGD and MGD coexisting with DED were 8.5 ± 2.9, 28.5 ± 12.8 and 27.9 ± 10.5, respectively. Patients with MGD coexisting with DED exhibited more frequent eye blinking than that of patients with asymptomatic MGD (8.1 ± 4.1 vs. 6.1 ± 3.5 blinks/20 sec, P = 0.022), and reduced LLT than that of patients with asymptomatic MGD (68.6 ± 17.2 vs. 77.6 ± 14.5 nm, P = 0.010) and symptomatic MGD (78.0 ± 17.1 nm, P = 0.015). Multivariate analysis identified LLT (per nm, OR = 0.96, 95% CI = 0.93-0.99, P = 0.002) as a significant factor associated with DED development in MGD. The number of expressible MG was positively correlated with LLT (Spearman's correlation coefficient = 0.299, P = 0.016) but negatively correlated with the number of blinking (Spearman's correlation coefficient = -0.298, P = 0.016) in MGD patients with DED, and these findings were not identified in those without DED.

CONCLUSIONS

Asymptomatic MGD, symptomatic MGD, and MGD coexisting with DED share similar characteristics, including meibum secretion and morphology, but MGD patients coexisting with DED exhibited significantly reduced LLT.

The Lowly Chalazion

Chalazia are localized cysts of chronic lipogranulomatous inflammation arising from the obstruction of sebaceous glands of the eyelid tarsal plate, including the Meibomian gland (deep chalazion) or Zeis gland (superficial chalazion). This disease entity is differentiated from the hordeolum (stye), an acute purulent localized swelling of the eyelid often associated with an eyelash follicle, Zeis gland, or Moll gland obstruction and infection. Ambiguously, the chalazion, hordeolum, and blepharitis are commonly categorized and described on a continuum in the literature. While it is one of the most common eyelid disorders across all age demographics, the chalazion remains largely understudied and pathophysiological, epidemiological, and therapeutic findings exist fragmented in the literature. We discuss current understandings of the chalazion and provide current best practice guidelines supported by clinical anecdotal evidence.

Antimicrobial Tear Lipids in the Ocular Surface Defense

The concept of antimicrobial lipids as effectors of innate host defense is an emerging field. There is limited knowledge on the antimicrobial role of lipids in the ocular environment. Tears act as first line of defense to protect the ocular surface from infections. Antimicrobial effects of tear lipids have been demonstrated using meibomian lipids that are the source of majority of lipids in tears. This article describes the knowledge available on the antimicrobial role of tear lipids at the ocular surface and the antimicrobial potential of various lipid classes present in tears that can contribute to antimicrobial protection of the eye. Like other mucosal secretions, tears contain many proteins and lipids with known antimicrobial effects. The antimicrobial defense of tears is far stronger than can be demonstrated by the effects of individual compounds many of which are present in low concentrations but synergistic and additive interactions between them provide substantial antimicrobial protection to the ocular surface. It is inferred that antimicrobial lipids play important role in innate defense of tears, and cooperative interactions between various antimicrobial lipids and proteins in tears provide a potent host defense mechanism that is effective against a broad spectrum of pathogens and renders self-sterilizing properties to tears for keeping the microbial load low at the ocular surface.

Quantitative measurement of optical properties and Hb concentration in a rodent model of inflammatory Meibomian gland dysfunction using spatial frequency domain imaging

Herein, to investigate a new diagnostic method for Meibomian gland dysfunction (MGD) induced by eyelid inflammation, optical properties and deoxy-hemoglobin (Hb) concentrations in rodent eyelid tissues, including Meibomian glands(MGs), were measured using spatial frequency domain imaging (SFDI). Complete Freund's adjuvant solutions were injected into the eyelid margins of Sprague-Dawley rats to induce MGD. After three weeks, the optical properties and Hb of the MG and non-MG regions of the eyelids were measured ex-vivo using an SFDI system. The comparison of Hb showed that the MGD group exhibited significantly higher values than those of the control group in both regions. The optical properties at 730 and 850 nm for the MG regions in the MGD group were significantly different from those in the control group. In addition, the 630 nm absorption coefficients of both regions were significantly higher in the MGD group than in the control group. Thus, the SFDI technique can detect the increased Hb concentration and changes in the optical properties of the eyelids due to inflammatory MGD in a noncontact manner and has the potential to be used as a novel quantitative diagnostic method for the occurrence of MGD.

Effects of Aging on Human Meibum

Purpose

The purpose of this study was to determine if aging affects meibum lipid composition in non-meibomian gland dysfunction (MGD)/non-dry eye (DE) population. Aging has been repeatedly linked to pathological changes in various tissues and organs, including the onset of MGD and DE, in a number of clinical and population-wide surveys. Both conditions have been associated with abnormal meibum secretion and composition, among other factors. However, the chemical basis for such a connection has not been established yet.

Methods

To identify and characterize possible changes in the meibum and meibogenesis with aging, lipidomic analyses of meibum samples collected from human subjects of two age groups - young (29 ± 5 years, n = 21) and elderly (68 ± 7 years, n = 29) - with similar male to female ratios in each group were conducted. Intact lipid species from major lipid groups of meibum (such as wax esters, cholesteryl esters, free cholesterol, triacylglycerols, etc.) were compared using lipidome-wide untargeted (such as Principal Component Analysis) and targeted (such as Orthogonal Projections to Latent Structures Discriminant Analysis) approaches, along with focused analyses of specific lipid species in liquid-chromatography mass spectrometry (LC-MS) and tandem mass spectrometry (MS-MS) experiments.

Results

Extremely high similarities of meibum lipids in the two age groups were observed, with only minor changes in the individual lipid species. The magnitude of the intergroup variability for tested lipid species was comparable to the intragroup variability for the same meibum components. No statistically significant differences in the lipid esterification, elongation, and unsaturation patterns were observed.

Conclusions

Chronological aging itself seems to have only minor effect on meibogenesis in healthy, non-MGD/non-DE subjects.

The effect of vertical gaze position on ocular tear film stability

BACKGROUND

An enlarged exposed ocular surface area is known to degrade tear film stability. Little is known of how tear film stability alters with vertical gaze and the change of palpebral fissure height.

METHODS

Tear film break-up time (TBUT; fluorescent dye) and palpebral fissure height were measured in three vertical gaze positions (primary position and 20 degrees up and 20 degrees down) on 23 healthy volunteers. The effect of gaze direction on TBUT was analysed by ANOVA.

RESULTS

Gaze had a significant influence on tear film stability [F(2,44)=27.6, p<0.001]. In up-gaze with an enlarged palpebral fissure height (11.1 +/- 1.4 mm), TBUT was degraded (17.3 +/- 12.7 seconds). In down-gaze and decreased palpebral fissure height (6.7 +/- 1.2 mm) TBUT was extended (43.5 +/- 35.4 seconds).

CONCLUSION

Ocular tear film stability alters with vertical gaze. Our hypothesis is that the change of the palpebral fissure height and the exposed ocular surface area alters the thickness of the lipid layer and with it tear film evaporation.

Lipid deposition on contact lenses in symptomatic and asymptomatic contact lens wearers

PURPOSE

Lipid deposition on contact lenses (CL) has traditionally been believed to reduce comfort during CL wear. The purpose of this study was to quantify lipid deposition on CL in a group of symptomatic and asymptomatic adapted CL wearers.

METHODS

This was a single-masked, randomized clinical trial. Only confirmed symptomatic (comfortable lens wear time (CWT) < 8 h and a noticeable reduction in comfort over the course of the day) and asymptomatic (CWT > 10 h and minimal reduction in comfort over the course of the day) participants were recruited to participate in the study. Participants wore senofilcon A lenses in combination with a polyquaternium-based care solution (OPTI-FREE Replenish). Worn CL samples were collected on Day 14. Deposited lipid amounts from the lenses (including cholesteryl ester, cholesterol and triolein) were quantified using a liquid chromatography-mass spectrometry technique.

RESULTS

Lipid deposition was significantly higher in CL extracts of asymptomatic wearers compared to the symptomatic wearers for all lipid types quantified, including cholesteryl ester (2.1 ± 0.6 vs 1.6 ± 0.5 log μg/lens), cholesterol (1.5 ± 0.3 vs 1.1 ± 0.3 log μg/lens) and triolein (0.3 ± 0.2 vs 0.1 ± 0.1 log μg/lens) (all p < 0.002). The amount of cholesteryl ester deposited was greatest (p = 0.0001), followed by cholesterol, then triolein, for both the asymptomatic and symptomatic groups (both p = 0.0001).

CONCLUSION

This study demonstrated that the asymptomatic group deposited a significantly greater amount of lipid on their CL. Although lipid levels measured are considered low to trigger any observable clinical deposition, they may influence other clinical outcomes, particularly comfort.

Infrared meibography allows detection of dimensional changes in meibomian glands following intranasal neurostimulation

PURPOSE

Patients with dry eye disease (DED) may suffer from decreased tear break-up time due to meibomian gland (MG) dysfunction. Infrared meibography (IR Meibography) uses infrared wavelength light to visualize meibomian glands in vivo. We aimed to explore the feasibility of using serial IR Meibography imaging to assess morphological changes in MGs as an indirect measure of functionality, following intranasal neurostimulation (ITN).

METHODS

Fifteen DED subjects were prospectively enrolled in a single-center, single-arm study. Changes in MGs were captured using IR meibography (RTVUE-XR, Optovue, Inc. Fremont, CA, USA) on the lower eyelids before and after 3 min of ITN (TrueTear®, Allergan, Dublin, Ireland) use that delivers a microcurrent to sensory neurons of the nasal cavity. The same MGs were selected pre- and post-stimulation, and MG area and perimeter were analyzed by two masked observers.

RESULTS

Mean (±SD) pre- and post-stimulation MG areas were 2,187.60 ± 635.88 μm² and 1,933.20 ± 538.55 μm², respectively. The mean change in area, 254.49 μm², representing an 11.6% reduction following ITN use, was statistically significant (p = 0.001). Mean (±SD) pre- and post-stimulation MG perimeters were 235.9 ± 51.38 μm and 222.2 ± 47.72 μm, respectively. The mean change in perimeter, 13.7 μm, representing a 5.81% reduction following ITN use, was statistically significant (p = 0.012).

CONCLUSIONS

Our study shows that IR meibography can be used to detect immediate changes in gland area and perimeter, an indirect measure of MG activity following intervention by ITN.

Risk factors for severe Meibomian gland atrophy in a young adult population: A cross-sectional study

PURPOSE

Assess potential risk factors for severe Meibomian gland atrophy (SMGA) in a young adult population.

METHODS

Cross-sectional study using medical history and ocular surface examination to evaluate relationships with study outcomes: SMGA, tear lipid layer (TLL) thickness, non-invasive (NITBUT) and fluorescein (FTBUT) tear breakup times, and symptoms using the Standard Patient Evaluation of Eye Dryness (SPEED) questionnaire.

RESULTS

One hundred one participants (101; 202 eyes; Age: mean±SD = 22.3±4.0 years) completed the study. Hormonal birth control (HBC) use was the only significant risk factor for SMGA (p = 0.028). Female HBC users had 4.8 times greater odds of having SMGA compared to female HBC non-users (p = 0.028), but the odds of having SMGA was similar between female HBC non-users and males (p = 0.885). Multivariable analysis suggested that the relationship between SMGA and TLL thickness was dependent on HBC use. Compared to female HBC non-users without SMGA, TLL thickness for HBC users was estimated to be 10 nm thinner if SMGA was absent (p = 0.007) and 21 nm thinner if SMGA was present (p<0.001). SMGA status had no significant impact on TLL thickness among female HBC non-users (p = 0.552). The effect of TLL thickness on FTBUT was small but significant (p = 0.026). TLL thickness was not significantly associated with NITBUT (p = 0.349). Neither FTBUT nor NITBUT was significantly associated with the SPEED score.

CONCLUSION

HBC use may be associated with SMGA, supporting the hypothesis that SMGA could lead to thinner TLL. However, less evidence was present to support that thin TLL could lead to clinically detectable tear film instability and subsequently to increased ocular dryness symptoms. Further investigation with a larger sample size is warranted to confirm these findings.

TFOS DEWS II pathophysiology report

The TFOS DEWS II Pathophysiology Subcommittee reviewed the mechanisms involved in the initiation and perpetuation of dry eye disease. Its central mechanism is evaporative water loss leading to hyperosmolar tissue damage. Research in human disease and in animal models has shown that this, either directly or by inducing inflammation, causes a loss of both epithelial and goblet cells. The consequent decrease in surface wettability leads to early tear film breakup and amplifies hyperosmolarity via a Vicious Circle. Pain in dry eye is caused by tear hyperosmolarity, loss of lubrication, inflammatory mediators and neurosensory factors, while visual symptoms arise from tear and ocular surface irregularity. Increased friction targets damage to the lids and ocular surface, resulting in characteristic punctate epithelial keratitis, superior limbic keratoconjunctivitis, filamentary keratitis, lid parallel conjunctival folds, and lid wiper epitheliopathy. Hybrid dry eye disease, with features of both aqueous deficiency and increased evaporation, is common and efforts should be made to determine the relative contribution of each form to the total picture. To this end, practical methods are needed to measure tear evaporation in the clinic, and similarly, methods are needed to measure osmolarity at the tissue level across the ocular surface, to better determine the severity of dry eye. Areas for future research include the role of genetic mechanisms in non-Sjögren syndrome dry eye, the targeting of the terminal duct in meibomian gland disease and the influence of gaze dynamics and the closed eye state on tear stability and ocular surface inflammation.

TFOS DEWS II Tear Film Report

The members of the Tear Film Subcommittee reviewed the role of the tear film in dry eye disease (DED). The Subcommittee reviewed biophysical and biochemical aspects of tears and how these change in DED. Clinically, DED is characterized by loss of tear volume, more rapid breakup of the tear film and increased evaporation of tears from the ocular surface. The tear film is composed of many substances including lipids, proteins, mucins and electrolytes. All of these contribute to the integrity of the tear film but exactly how they interact is still an area of active research. Tear film osmolarity increases in DED. Changes to other components such as proteins and mucins can be used as biomarkers for DED. The Subcommittee recommended areas for future research to advance our understanding of the tear film and how this changes with DED. The final report was written after review by all Subcommittee members and the entire TFOS DEWS II membership.

Sebum/Meibum Surface Film Interactions and Phase Transitional Differences

Purpose

Sebum may contribute to the composition of the tear film lipid layer naturally or as a contaminant artifact from collection. The aims of this study were to determine: if sebum changes the rheology of meibum surface films; if the resonance near 5.2 ppm in the ¹H-NMR spectra of sebum is due to squalene (SQ); and if sebum or SQ, a major component of sebum, interacts with human meibum.

Methods

Human meibum was collected from the lid margin with a platinum spatula. Human sebum was collected using lipid absorbent tape. Langmuir trough technology was used to measure the rheology of surface films. Infrared spectroscopy was used to measure lipid conformation and phase transitions. We used ¹H-NMR to measure composition and confirm the primary structure of SQ.

Results

The NMR resonance near 5.2 ppm in the spectra of human sebum was from SQ which composed 28 mole percent of sebum. Both sebum and SQ lowered the lipid order of meibum. Sebum expanded meibum films at lower concentrations and condensed meibum films at higher concentrations. Sebum caused meibum to be more stable at higher pressures (greater maximum surface pressure).

Conclusions

Physiological levels of sebum would be expected to expand or fluidize meibum making it spread better and be more surface active (qualities beneficial for tear film stability). Sebum would also be expected to stabilize the tear film lipid layer, which may allow it to withstand the high shear pressure of a blink.

In Vivo Laser Scanning Confocal Microscopy of Human Meibomian Glands in Aging and Ocular Surface Diseases

Meibomian glands (MGs) play a crucial role in the ocular surface homeostasis by providing lipids to the superficial tear film. Their dysfunction destabilizes the tear film leading to a progressive loss of the ocular surface equilibrium and increasing the risk for dry eye. In fact, nowadays, the meibomian gland dysfunction is one of the leading causes of dry eye. Over the past decades, MGs have been mainly studied by using meibography, which, however, cannot image the glandular structure at a cellular level. The diffusion of the in vivo laser scanning confocal microscopy (LSCM) provided a new approach for the structural assessment of MGs permitting a major step in the noninvasive evaluation of these structures. LSCM is capable of showing MGs modifications during aging and in the most diffuse ocular surface diseases such as dry eye, allergy, and autoimmune conditions and in the drug-induced ocular surface disease. On the other hand, LSCM may help clinicians in monitoring the tissue response to therapy. In this review, we summarized the current knowledge about the role of in vivo LSCM in the assessment of MGs during aging and in the most diffuse ocular surface diseases.

Surface Properties of Squalene/Meibum Films and NMR Confirmation of Squalene in Tears

Squalene (SQ) possesses a wide range of pharmacological activities (antioxidant, drug carrier, detoxifier, hydrating, emollient) that can be of benefit to the ocular surface. It can come in contact with human meibum (hMGS; the most abundant component of the tear film lipid layer) as an endogenous tear lipid or from exogenous sources as eyelid sebum or pharmaceuticals. The aims of this study were to determine (i) if SQ is in tear lipids and (ii) its influence on the surface properties of hMGS films. Heteronuclear single quantum correlation NMR confirmed 7 mol % SQ in Schirmer’s strips extracts. The properties of SQ/hMGS pseudo-binary films at the air/water interface were studied with Langmuir surface balance, stress-relaxation dilatational rheology and Brewster angle microscopy. SQ does not possess surfactant properties. When mixed with hMGS squalene (i) localized over the layers’ thinner regions and (ii) did not affect the film pressure at high compression. Therefore, tear SQ is unlikely to instigate dry eye, and SQ can be used as a safe and “inert” ingredient in formulations to protect against dry eye. The layering of SQ over the thinner film regions in addition to its pharmacological properties could contribute to the protection of the ocular surface.

A novel technique of contrast-enhanced optical coherence tomography imaging in evaluation of clearance of lipids in human tears

Purpose

The aim of this work was to gather preliminary data in different conditions of healthy eyes, aqueous tear deficient dry eyes, obstructive meibomian gland disease (MGD) and non-obvious obstructive MGD (NOMGD) individuals, using a new, contrast-enhanced optical coherence tomography (OCT) imaging method to evaluate the clearance of lipids in human tears.

Methods

Eighty-two adult patients presenting with complaints of ocular irritation were studied for abnormalities of the ocular surface and classified as healthy (n = 21), aqueous tear deficient dry eyes (n = 20), obstructive MGD (n = 15) and NOMGD (n = 26) individuals. A lipid-based tracer, containing an oil-in-water emulsion, was used to obtain an enhanced OCT imaging of the lower tear meniscus. After instillation, a dramatic initial increase of reflectivity of the lower tear meniscus was detected by OCT, followed by a decay back to baseline values over time. Based on this finding, the clearance of lipids was measured in real-time by Fourier-domain anterior segment OCT.

Results

The differences in the clearance of lipids among the four groups as well as the correlations between symptom questionnaire score, standardized visual scale test, fluorescein break-up time, ocular surface fluorescein staining score, Schirmer I test scores were found to be statistically significant. The individual areas under the curve of the clearance of lipids calculated by the receiver operating characteristic curve technique ranged from 0.66 to 0.98, suggesting reliable sensitivity and specificity of lipid-enhanced OCT imaging.

Conclusions

This new technique of contrast-enhanced OCT imaging of the tear film following lipid-based tracer instillation provides a measure of the clearance of lipids. The quantitative values found are in agreement with other methods of evaluation of the lacrimal system. An improvement of the clinician's ability in the diagnosis and understanding of abnormalities of the ocular surface may be achieved by this simple approach.

Evaporation-driven instability of the precorneal tear film

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

Rethinking dry eye disease: a perspective on clinical implications

Publication of the DEWS report in 2007 established the state of the science of dry eye disease (DED). Since that time, new evidence suggests that a rethinking of traditional concepts of dry eye disease is in order. Specifically, new evidence on the epidemiology of the disease, as well as strategies for diagnosis, have changed the understanding of DED, which is a heterogeneous disease associated with considerable variability in presentation. These advances, along with implications for clinical care, are summarized herein. The most widely used signs of DED are poorly correlated with each other and with symptoms. While symptoms are thought to be characteristic of DED, recent studies have shown that less than 60% of subjects with other objective evidence of DED are symptomatic. Thus the use of symptoms alone in diagnosis will likely result in missing a significant percentage of DED patients, particularly with early/mild disease. This could have considerable impact in patients undergoing cataract or refractive surgery as patients with DED have less than optimal visual results. The most widely used objective signs for diagnosing DED all show greater variability between eyes and in the same eye over time compared with normal subjects. This variability is thought to be a manifestation of tear film instability which results in rapid breakup of the tearfilm between blinks and is an identifier of patients with DED. This feature emphasizes the bilateral nature of the disease in most subjects not suffering from unilateral lid or other unilateral destabilizing surface disorders. Instability of the composition of the tears also occurs in dry eye disease and shows the same variance between eyes. Finally, elevated tear osmolarity has been reported to be a global marker (present in both subtypes of the disease- aqueous-deficient dry eye and evaporative dry eye). Clinically, osmolarity has been shown to be the best single metric for diagnosis of DED and is directly related to increasing severity of disease. Clinical examination and other assessments differentiate which subtype of disease is present. With effective treatment, the tear osmolarity returns to normal, and its variability between eyes and with time disappears. Other promising markers include objective measures of visual deficits, proinflammatory molecular markers and other molecular markers, specific to each disease subtype, and panels of tear proteins. As yet, however, no single protein or panel of markers has been shown to discriminate between the major forms of DED. With the advent of new tests and technology, improved endpoints for clinical trials may be established, which in turn may allow new therapeutic agents to emerge in the foreseeable future. Accurate recognition of disease is now possible and successful management of DED appears to be within our grasp, for a majority of our patients.

Meibography: A review of techniques and technologies

Meibomian glands play a significant role in tear production by contributing lipids to the superficial tear film.(1) Dysfunction of the meibomian glands destabilizes tears resulting in evaporative dry eye.(2,3) Historically, the meibomian glands were assessed in an ex vivo fashion through histologic studies. However, innovations in ocular imaging have advanced significantly in recent decades to include meibography. Meibography is an imaging study developed 35 years ago exclusively for the purpose of observing the morphology of meibomian glands in vivo.(4,5) In this review of meibography, we briefly describe the etiology of meibomian gland dysfunction and then discuss various meibography techniques, technologies, and methods of image analysis. We close with a review of the literature, crediting various studies for the significant contributions made toward our current understanding of the meibomian glands.

Tear film lipids

Human meibomian gland secretions (MGS, or meibum) are formed from a complex mixture of lipids of different classes such as wax esters, cholesteryl esters, (O-acyl)-ω-hydroxy fatty acids (OAHFA) and their esters, acylglycerols, diacylated diols, free fatty acids, cholesterol, and a smaller amount of other polar and nonpolar lipids, whose chemical nature and the very presence in MGS have been a matter of intense debates. The purpose of this review is to discuss recent results that were obtained using different experimental techniques, estimate limitations of their usability, and discuss their biochemical, biophysical, and physiological implications. To create a lipid map of MGS and tears, the results obtained in the author's laboratory were integrated with available information on chemical composition of MGS and tears. The most informative approaches that are available today to researchers, such as HPLC-MS, GC-MS, and proton NMR, are discussed in details. A map of the meibomian lipidome (as it is seen in reverse phase liquid chromatography/mass spectrometry experiments) is presented. Directions of future efforts in the area are outlined.

Making, baking, and breaking: the synthesis, storage, and hydrolysis of neutral lipids

The esterification of amphiphilic alcohols with fatty acids is a ubiquitous strategy implemented by eukaryotes and some prokaryotes to conserve energy and membrane progenitors and simultaneously detoxify fatty acids and other lipids. This key reaction is performed by at least four evolutionarily unrelated multigene families. The synthesis of this "neutral lipid" leads to the formation of a lipid droplet, which despite the clear selective advantage it confers is also a harbinger of cellular and organismal malaise. Neutral lipid deposition as a cytoplasmic lipid droplet may be thermodynamically favored but nevertheless is elaborately regulated. Optimal utilization of these resources by lipolysis is similarly multigenic in determination and regulation. We present here a perspective on these processes that originates from studies in model organisms, and we include our thoughts on interventions that target reductions in neutral lipids as therapeutics for human diseases such as obesity and diabetes.

Disruption and destabilization of meibomian lipid films caused by increasing amounts of ceramides and cholesterol

PURPOSE

We evaluated quantitatively direct effects of ceramide (Cer) and free cholesterol (FC) on meibomian lipid films (MLF) using a Langmuir trough (LT) and a Brewster angle microscope (BAM).

METHODS

Meibum was obtained from healthy volunteers. A series of mixtures of meibum with Cer or FC (mixed MLF) taken in different ratios were tested. Standard rheologic parameters, such as elasticity and hysteresis of MLF, were computed. BAM was used to study the morphology of MLF.

RESULTS

Pure MLF were capable of withstanding multiple compression/expansion cycles with little hysteresis observed (1.9 J/G meibum). The films made of either pure CER or pure FC were clearly collapsible, and had much higher rigidity and hysteresis than pure meibum. Adding progressively higher amounts of CER or FC to meibum had a strong impact on the rigidity, stability, and morphology of the mixed MLF: their hysteresis increased many fold compared to pure meibum. A concomitant increase in the rigidity and collapsibility of the mixed MLF was observed.

CONCLUSIONS

Cer and FC changed the surface properties of mixed MLF in a way that implied their destabilization and/or disruption. One of the mechanisms that might lead to these effects is strong aggregation of meibum lipids with FC or Cer that leads to the formation of smaller particles of meibum surrounded by a thinner layer of FC or Cer. As Cer and FC can be elevated in meibum and the tear film because of certain pathologic processes, or can be of exogenous nature, our results can explain (partially) a less stable tear film in those subjects.

How to promote and preserve eyelid health

Disorders of the lacrimal functional unit are common in ophthalmological practice, with meibomian gland dysfunction, blepharitis, and dry eye forming a significant part of the general ophthalmologist’s practice. The eyelid and its associated structures form a complex organ designed to protect the fragile corneal surface and improve visual acuity. This organ is subject to a number of disorders, including meibomian gland dysfunction, dry eye syndrome, anterior blepharitis, allergic and dermatological conditions, and disorders associated with contact lens use. Although commonly described separately, disorders of the lacrimal function unit are better considered as a group of interacting pathologies that have inflammatory mediators as a central feature. Eyelid hygiene, in the sense of routine cleansing and massage of the eyelids, is well accepted in the management of many disorders of the eyelid. However, a broader concept of eyelid health may be appropriate, in which eyelid cleansing is but a part of a more complete program of care that includes screening and risk assessment, patient education, and coaching. The ophthalmologist has an important role to play in helping patients persist with routine eyelid care that may be long-term or lifelong. A number of preparations exist to make routine eyelid care both more effective and more pleasant, and might also improve compliance. Several such preparations have been devised, and are being assessed in clinical studies, and appear to be effective and preferred by patients over traditional soap and water or baby shampoo.

Consequences of interfacial viscoelasticity on thin film stability

The phenomenon of dewetting is frequently observed in our everyday life. It is of central importance in many technological applications as well as in a variety of physical and biological systems. The presence of nonsoluble surfactants at an air/liquid interface may affect the dewetting properties of the aqueous layer. An important example is the tear film, which comprises an aqueous layer covered with a ∼100-nm-thick blanket of lipids, known as the meibomian lipids. Interfacial rheological measurements of meibomian lipids reveal that these films are remarkably viscoelastic. Tear film dewetting is of central importance to understanding tear film stability. To better understand the role of surface viscoelasticity in tear film stability, we have developed a methodology to systematically control interfacial rheology of thin aqueous layers at the onset of dewetting events. The apparatus allows control over the surface pressure of the monolayer, which is a key feature since this variable controls the surface viscoelasticity. Three insoluble monolayer materials were used: newtonian arachidyl alcohol (AA), DPPC, a phospholipid that is slightly viscoelastic, and meibum, which produces a strongly viscoelastic monolayer. It is reported that monolayers of viscoelastic surfactants are able to stabilize thin films against spontaneous dewetting. As the surface pressure of these layers is increased, their effectiveness is enhanced. Moreover, these surfactants are able to reduce the critical film thickness for dewetting. Meibum is particularly effective in stabilizing thin films. Our results suggest that the meibomian lipids play a vital role in maintaining tear film stability in addition to suppressing evaporation.

Evaluation of dry eye

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

The significance of hand wash compliance on the transfer of dermal lipids in contact lens wear

AIM

The aim of this study was to assess the impact of hand washing regimes on lipid transference to contact lenses. The presence of lipids on contact lenses can affect visual acuity and enhance spoilation. Additionally, they may even mediate and foster microbial transfer and serve as a marker of potential dermal contamination.

METHODS AND MATERIALS

A social hand wash and the Royal College of Nursing (RCN) hand wash were investigated. A 'no-wash regime' was used as control. The transfer of lipids from the hand was assessed by Thin Layer Chromatography (TLC). Lipid transference to the contact lenses was studied through fluorescence spectroscopy (FS).

RESULTS

Iodine staining, for presence of lipids, on TLC plates indicated the 'no-wash regime' score averaged at 3.4±0.8, the social wash averaged at 2.2±0.9 and the RCN averaged at 1.2±0.3 on a scale of 1-4. The FS of lipids on contact lenses for 'no washing' presented an average of 28.47±10.54 fluorescence units (FU), the social wash presented an average of 13.52±11.12 FU and the RCN wash presented a much lower average 6.47±4.26 FU.

CONCLUSIONS

This work demonstrates how the method used for washing the hands can affect the concentration of lipids, and the transfer of these lipids onto contact lenses. A regime of hand washing for contact lens users should be standardised to help reduce potentially transferable species present on the hands.

[Metronidazole: alternative treatment for ocular and cutaneous rosacea in the pediatric population]

OBJECTIVE

To assess the effectiveness and tolerance of systemic metronidazole in the treatment of childhood ocular and cutaneous rosacea.

METHOD

Single-center multidisciplinary retrospective study.

PATIENTS

Children aged between 1 and 15, with ocular and/or cutaneous rosacea, treated in the pediatric ophthalmology and dermatology department of Bordeaux, France, from January 1996 to September 2009.

RESULTS

Eleven patients out of 20 had ocular and cutaneous rosacea, three had ocular symptoms only, and six had cutaneous symptoms only. In 11 patients (55%), the ocular symptoms preceded the skin disease. Meibomian cyst and phlyctenular conjunctivitis were the main symptoms. Keratitis was seen in four patients and lower corneal ulcer in two cases. The papulopustular form was the most frequent dermatologic form. All patients with ocular involvement received first-line treatment of eyelid hygiene. No topical ophthalmic treatment such as corticosteroid or cyclosporine 0.5% or 2% was used. Thirteen patients who showed no improvement despite eyelid treatment, the association of ocular and cutaneous rosacea, severe ocular involvement with keratitis, and severe recurrent cutaneous rosacea were treated orally. Two patients, aged between 12 and 14 years, received treatment with an anti-inflammatory dose of doxycycline for 2 to 3 months and achieved complete remission. One 22-month-old patient received oral treatment with erythromycin at a dose of 250 mg three times daily for 4 months. Ten patients, aged 12 to 64 months, were treated with systemic Metronidazole. Treatment lasting at least 3 months at a dose between 20 and 30 mg/kg per day was necessary to obtain complete and lasting remission. An early cessation of treatment, before 3 months, seems associated with partial remission of the disease and early recurrence. In cases complicated by ocular keratitis and corneal ulcer, prolonged treatment lasting 6 months led to clinical remission. The short courses (3-6 months) were preferred to long-term administration to prevent neurological toxicity. Maintenance therapy was based on eyelid hygiene. No recurrences and no toxic effects were observed at a median of 48 ± 6 months.

CONCLUSION

Childhood ocular rosacea is not rare, but is often misdiagnosed. It often precedes skin symptoms but it can remain isolated. Metronidazole could be alternative treatment for ocular and cutaneous rosacea in the pediatric population.

Examination of human meibum collection and extraction techniques

PURPOSE

To compare various meibum collection methods and extraction techniques.

METHODS

Sixty subjects, all successful contact lens wearers, were seen on two visits. Meibum was collected from the lower lid of the right eye with a glass microcapillary tube, and with a Dacron swab, cytology microbrush, or spatula from the left eye. Extraction with 2:1 chloroform:methanol was done either immediately or after data collection was complete. Individual samples were divided into four equal aliquots for analysis of total lipids, cholesterol, and inorganic phosphates by assay-based techniques. Effects of collection method, extraction, and dry eye status were examined using repeated measures analysis of variance and logistic regression.

RESULTS

Total lipids showed significance for collection device (p < 0.0001) but not for extraction technique (p = 0.13) or dry eye status (p = 0.97). Dacron swab collection was associated with more total lipid on average than each other collection device (p < 0.0001). The cholesterol assay showed significance of collection device (p < 0.0001) and extraction technique (p = 0.0002) but not dry eye status (p = 0.55). Spatula collection was associated with more cholesterol on average than each other collection device (p < 0.0001). For inorganic phosphates, immediate extraction (p < 0.0001), cytology microbrush collection (p < 0.0001), and non-dry eye status (p = 0.03) were associated with the greater likelihood of detection.

CONCLUSIONS

Dacron swab collection was associated with the highest average amount of total lipid detected, whereas spatula collection and immediate extraction was associated with the highest average amount of cholesterol detected. Cytology microbrush collection with immediate extraction on non-dry eye subjects was associated with the highest probability of detection of inorganic phosphates.

The challenge of dry eye diagnosis

The currently available methods for the diagnosis of dry eye are still far from being perfect for a variety of reasons. This review attempts to highlight the advantages and disadvantages of both traditional tests (such as Schirmer’s test, break-up time and ocular surface staining) and innovative noninvasive procedures, including tear meniscus height measurement, corneal topography, functional visual acuity, tear interferometry, tear evaporimetry and tear osmolarity assessment.

Effects of free fatty acids on meibomian lipid films

The purpose of this study was to evaluate the impact of free fatty acids (FFA), namely oleic (OA) and linoleic (LA) ones, on meibomian lipid films (MLF) using a Langmuir trough (LT) and a Brewster angle microscope (BAM). Human meibum was collected from healthy volunteers. A Tris-buffered saline (TBS, pH 7.4) was used as the control aqueous subphase for LT experiments. Then, varying amounts of OA and LA were dissolved in TBS to make FFA-containing subphases. Predetermined amounts of meibum were loaded onto the surface of the (TBS/±FFA) subphases to form MLF. Then, surface pressure-area (π/A) isotherms of MLF were recorded. Standard rheological parameters such as rigidity, elasticity, and hysteresis, were computed. In a separate experiment, OA and LA were pre-mixed with meibum at different weight ratios prior their spreading onto the control TBS subphase, and the (π/A) isotherms of the resulting mixed films of meibum and FFA were studied and analyzed in the same fashion as described above. When studied at the normal corneal temperature of 34 °C with the (TBS/-FFA) subphase, meibum formed stable films. When (TBS/+FFA) subphase was used, both FFA quickly disrupted the MLF, acting in a similar fashion. BAM revealed that the most dramatic changes in the structure of MLF occurred in the range of OA concentrations between 5 and 15 μM. However, this effect was apparent even with 2.5 μM OA. When OA was pre-mixed with meibum, but was absent from the subphase, it caused gradual concentration-dependent changes in the (π/A) isotherms, but the MLF did not disappear from the surface. Thus, tested FFA showed a remarkable ability to disrupt, and/or prevent the formation of, human MLF, which could contribute to the onset of those forms of dry eye disease that are associated with enhanced activity of lipolytic enzymes, such as chronic blepharitis.

Lipidomics of human Meibomian gland secretions: Chemistry, biophysics, and physiological role of Meibomian lipids

Human Meibomian gland secretions (MGS) are a complex mixture of diverse lipids that are produced by Meibomian glands that are located in the upper and the lower eyelids. During blinking, MGS are excreted onto the ocular surface, spread and mix with aqueous tears that are produced by lachrymal glands, and form an outermost part of an ocular structure called "the tear film" (TF). The main physiological role of TF is to protect delicate ocular structures (such as cornea and conjunctiva) from desiccating. Lipids that are produced by Meibomian glands are believed to "seal" the aqueous portion of TF by creating a hydrophobic barrier and, thus, retard evaporation of water from the ocular surface, which enhances the protective properties of TF. As lipids of MGS are interacting with underlying aqueous sublayer of TF, the chemical composition of MGS is critical for maintaining the overall stability of TF. There is a consensus that a small, but important part of Meibomian lipids, namely polar, or amphiphilic lipids, is of especial importance as it forms an intermediate layer between the aqueous layer of TF and its upper (and much thicker) lipid layer formed mostly of very nonpolar lipids, such as wax esters and cholesteryl esters. The purpose of this review is to summarize the current knowledge on the lipidomics of human MGS, including the discussions of the most effective modern analytical techniques, chemical composition of MGS, biophysical properties of Meibomian lipid films, and their relevance for the physiology of TF. Previously published results obtained in numerous laboratories, as well as novel data generated in the author's laboratory, are discussed. It is concluded that despite a substantial progress in the area of Meibomian glands lipidomics, there are large areas of uncertainty that need to be addressed in future experiments.

Nonobvious obstructive meibomian gland dysfunction

This review presents the rationale and supporting data for a recent paradigm shift in our understanding of meibomian gland dysfunction (MGD). The historical understanding of MGD has been that of an infectious hypersecretory disorder with obvious signs of inflammation, hypersecretion, and purulent excreta. The current understanding of MGD now includes the polar concept of a less obvious or nonobvious type of hyposecretory obstructive MGD, where inflammation and other signs of pathology may be absent unless special examination techniques are employed. A new term, nonobvious obstructive MGD (NOMGD), is used to describe what may be the most common form of obstructive MGD. Obstructive MGD is an area of growing importance because obstructive MGD is now recognized to be the most common cause of evaporative dry eye, and because NOMGD seems to be the precursor to obvious obstructive MGD, it is also an important area to understand. The prevalence of NOMGD seems to be very high but currently significantly underdiagnosed. This review presents the relevant anatomy and physiology, concepts of obstructive MGD, the usual absence of inflammation in obstructive MGD, nomenclature and classification of obstructive and NOMGD, clinical diagnosis of NOMGD emphasizing the necessity for diagnostic expression, the use of a new instrument for diagnostic expression providing a standardized method of assessing meibomian gland functionality, the complementary roles of the aqueous and lipid layers, and the specific treatment of NOMGD, emphasizing that the success of treatment of all forms of obstructive MGD is dependent on the relief of the obstruction.

Pharmacotherapy of dry eye

INTRODUCTION

based on data from the largest studies of dry eye to date - the Women's Health Study (WHS) and the Physicians' Health Study (PHS) - and other studies, it has been estimated that about 3.23 million women and 1.68 million men, for a total of 4.91 million Americans aged ≥ 50 years, have dry eye. Tens of millions more have less severe symptoms and probably a more episodic manifestation of the disease that is notable only during contact with some adverse contributing factor(s), such as low humidity or contact lens wear. Dry eye disease is a common yet frequently under-recognized public health problem whose etiology and management challenge clinicians and researchers involved in this field.

AREAS COVERED

advances in the understanding of the disease have been made over the past 10 years in areas of epidemiology, pathogenesis, clinical manifestation, and possible therapy. Historical aspects and recent information in relation to the use of artificial tear substitutes and anti-inflammatory agents in dry eye disease, including topical cyclosporin and corticosteroids, autologous serum, tetracyclines and systemic immunosuppressants, are covered in this review. The reader will gain insight into the recent views on the pharmacological menu of treatments for dry eyes following the recommendations of the 2007 International Dry Eye Workshop.

EXPERT OPINION

dry eye is a visually disabling disease, the treatment of which needs tailoring according to the type and severity of dry eye disease.

Fatty acid composition of cholesteryl esters of human meibomian gland secretions

Very long chain cholesteryl esters (CE) are a major group of lipids found in meibomian gland secretions (MGS, also called meibum). MGS are produced by the meibomian glands of human and animal eyelids. They are a critical part of the tear film which covers the exposed ocular surface and serves various physiological roles. The composition of CE of MGS is complex, and still remains poorly understood. Here, a liquid chromatography-ion trap mass spectrometry (LC-MS) procedure developed to analyze CE is described, and a detailed composition of human meibomian CE is reported. MGS were collected from donors, analyzed without any modifications by LC-MS in positive and negative ion modes (PIM and NIM), and quantified using lipid standards where available. CE comprised about 30% of human meibum by mass. More than 40 individual CE species were found and characterized. In PIM, CE were observed as spontaneously in-source generated product ions m/z 369. The signals of the proton adducts of intact CE (M+H)(+) were of very low intensity. In NIM, all tested CE spontaneously fragmented in-source producing signals of their respective FA. By combining the LC and MS information, the most abundant CE were found to be based on FA ranging from C(16) to at least C(32) in the following order C(26:0)>C(25:0)>C(24:0)>C(27:0)>C(24:1)=C(18:1)=C(20:0)>other CE. We conclude that the FA composition of CE can be successfully established in LC-MS experiments conducted in NIM. Meibomian CE have a large presence of both saturated and unsaturated FA with an average molar ratio of 4 to 1, respectively.

Meibomian lipid films and the impact of temperature

PURPOSE

There is evidence that, in cold conditions, the temperature of human eyelids and of the ocular surface drops well below normal physiological levels. This may have a detrimental impact on the stability and functionality of the human tear film and the tear film lipid layer. The goal of this project was to quantitatively examine the possible impact of temperature on the latter.

METHODS

Meibum samples were collected by using a soft-squeezing technique and were studied in a Langmuir trough. The obtained surface pressure and area isotherms were analyzed to determine the biophysical parameters of thin meibomian lipid film (MLF): the lift-off area, collapse pressure, two-dimensional elasticity, and hysteresis and their dependence on temperature.

RESULTS

MLF was found to be highly susceptible to changes in temperature. At temperatures below the physiological level, the MLF became stiff and shrank considerably. The shrinkage left a large portion of the air-water interface uncovered with lipid molecules. The effect was shown to be reversible. On reheating, the lipids melted and respread to restore the original film. There was a fundamental difference observed between three-dimensional melting of dry meibum in bulk and the two-dimensional melting in MLF at the air-water interface. Bulk meibum melted in a narrower temperature range and showed a much higher cooperativity of melting.

CONCLUSIONS

Temperature critically influences MLF. Low temperature leads to stiffening of the film, which loses its ability to form continuous layers at the air-water interface. These effects were shown be of a cooperative nature, manifesting in relatively narrow concentration and temperature ranges.

Significance of lipid mediators in corneal injury and repair

Corneal injury induces an inflammatory reaction and damages the sensory nerves that exert trophic influences in the corneal epithelium. Alterations in normal healing disrupt the integrity and function of the tissue with undesirable consequences, ranging from dry eye and loss of transparency to ulceration and perforation. Lipids play important roles in this complex process. Whereas lipid mediators such as platelet activating factor (PAF) and cyclooxygenease-2 metabolites contribute to tissue damage and neovascularization, other mediators, such as the lipoxygenase (LOX) derivatives from arachidonic acid, 12- and 15-hydroxy/hydroperoxyeicosatetraenoic acids, and lipoxin A4, act as second messengers for epidermal growth factor to promote proliferation and repair. Stimulation of the cornea with pigment epithelial derived factor in the presence of docosahexaenoic acid gives rise to the synthesis of neuroprotectin D1, a derivative of LOX activity, and increases regeneration of corneal nerves. More knowledge about the role that lipids play in corneal wound healing can provide insight into the development of new therapeutic approaches for treating corneal injuries. PAF antagonists, lipoxins, and neuroprotectins can be effective therapeutic tools for maintaining the integrity of the cornea.

Retinal very long-chain PUFAs: new insights from studies on ELOVL4 protein

Compared with other mammalian tissues, retina is highly enriched in PUFA. Long-chain PUFA (LC-PUFA; C18-C24) are essential FAs that are enriched in the retina and are necessary for maintenance of normal retinal development and function. The retina, brain, and sperm also contain very LC-PUFA (VLC-PUFA; >C24). Although VLC-PUFA were discovered more than two decades ago, very little is known about their biosynthesis and functional roles in the retina. This is due mainly to intrinsic difficulties associated with working on these unusually long polyunsaturated hydrocarbon chains and their existence in small amounts. Recent studies on the FA elongase elongation of very long chain fatty acids-4 (ELOVL4) protein, however, suggest that VLC-PUFA probably play some uniquely important roles in the retina as well as the other tissues. Mutations in the ELOVL4 gene are found in patients with autosomal dominant Stargardt disease. Here, we review the recent literature on VLC-PUFA with special emphasis on the elongases responsible for their synthesis. We focus on a novel elongase, ELOVL4, involved in the synthesis of VLC-PUFA, and the importance of these FAs in maintaining the structural and functional integrity of retinal photoreceptors.