The meibomian glands and tear film lipids. Structure, function, and control

Smart Contact Lenses as Wearable Ophthalmic Devices for Disease Monitoring and Health Management

The eye contains a complex network of physiological information and biomarkers for monitoring disease and managing health, and ocular devices can be used to effectively perform point-of-care diagnosis and disease management. This comprehensive review describes the target biomarkers and various diseases, including ophthalmic diseases, metabolic diseases, and neurological diseases, based on the physiological and anatomical background of the eye. This review also includes the recent technologies utilized in eye-wearable medical devices and the latest trends in wearable ophthalmic devices, specifically smart contact lenses for the purpose of disease management. After introducing other ocular devices such as the retinal prosthesis, we further discuss the current challenges and potential possibilities of smart contact lenses.

Molecular nature of ocular surface barrier function, diseases that affect it, and its relevance for ocular drug delivery

The structural and functional integrity of the ocular surface, a continuous epithelial structure comprised of the cornea, the conjunctiva, and the ductal surface of the lacrimal as well as meibomian glands, is crucial for proper vision. The ocular surface barrier function (OSBF), sum of the different types of protective mechanisms that exist at the ocular surface, is essential to protect the rest of the eye from vision-threatening physical, chemical, and biological insults. OSBF helps maintain the immune privileged nature of the cornea and the aqueous humor by preventing entry of infectious agents, allergens, and noxious chemicals. Disruption of OSBF exposes the dense nerve endings of the cornea to these stimuli, resulting in discomfort and pain. This review summarizes the status of our knowledge related to the molecular nature of OSBF, describes the effect of different ocular surface disorders on OSBF, and examines the relevance of this knowledge for ocular drug delivery.

Induction of meibocyte differentiation by three-dimensional, matrigel culture of immortalized human meibomian gland epithelial cells to form acinar organoids

PURPOSE

Recent studies have shown that two-dimensional (2D) culture of primary rabbit and immortalized human meibomian gland epithelial cells (iHMGEC) do not recapitulate normal meibocyte differentiation and fail to express critical enzymes necessary for synthesis of meibum lipids. The purpose of this study was to test the hypothesis that 3D-spheroid culture of iHMGEC can facilitate meibocyte differentiation and induce the expression of acyl-CoA wax-alcohol acyltransferase 2 (AWAT2), shown to be required for synthesis of meibum wax esters.

METHODS

iHMGEC were suspended in matrigel/basement membrane matrix and grown in proliferation media to form distinct cell clusters or spheroids. Cells were then treated with serum-free, differentiation media (advanced DMEM/F12) with and without FGF10 and synthetic agonists for the nuclear lipid receptor, peroxisome proliferator activator receptor gamma (PPARγ). Cells were then evaluated for differentiation markers using western blotting, immunocytochemistry (ICC) and real-time PCR. Control cells were grown in standard 2D culture systems.

RESULTS

Under proliferative conditions, 3D culture induced the formation of KRT5+ spheroids that contained a Ki67+/P63+ undifferentiated, basal cell population. When spheroids were switched to differentiation media containing PPARγ agonists, two different organoid populations were detected, a KRT6low population that was AWAT2+/PPARγ+ and a KRT6high population that was AWAT2-/PPARγ-, suggesting that iHMGEC exhibit a dual differentiation potential toward either a ductal or meibocyte organoid phenotype.

CONCLUSION

The 3D culturing of iHMGEC can induce the formation of both meibocyte and ductal organoids and may thus serve as a better in vitro model system for studying the regulatory mechanisms controlling meibomian gland function.

Protein and polypeptide mediated delivery to the eye

Hybrid or recombinant protein-polymers, peptide-based biomaterials, and antibody-targeted therapeutics are widely explored for various ocular conditions and vision correction. They have been noted for their potential biocompatibility, potency, adaptability, and opportunities for sustained drug delivery. Unique to peptide and protein therapeutics, their production by cellular translation allows their precise modification through genetic engineering. To a greater extent than drug delivery to other systems, delivery to the eye can benefit from the combination of locally-targeted administration and protein-based specificity. Consequently, a range of delivery platforms and administration methods have been exploited to address the ocular delivery of peptide and protein biomaterials. This review discusses a sample of preclinical and clinical opportunities for peptide-based drug delivery to the eye.

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.

[Thermal pulsation system (LipiFlow®) for treatment of meibomian gland dysfunction (MGD) from the perspective of an ophthalmologist in private practice]

PURPOSE

Thermal pulsation (LipiFlow®, Johnson&Johnson, Santa Ana, CA, USA) has been advocated for meibomian gland dysfunction (MGD) treatment and was found to be useful in many studies. The aim of this study was to show the efficacy of the method and to compare it to a daily eyelid margin massage in a non-university institution.

METHODS

A non-blinded, single-center interventional study comparing thermal pulsation with eyelid margin massage for the treatment of MDD. In this study 30 patients were recruited during daily office hours. Symptoms (OSDI) and ocular surface (NIK-BUT, tear ferning test, tear meniscus height, LIPCO folds, meibography, meibomian gland evaluator) were assessed before treatment. A total of 15 patients (9 women) underwent thermal pulsation (single session), while 15 patients (8 women) performed eyelid margin massage (once daily) as instructed for 3 months.

RESULTS

Before the two treatment methods, there were no differences in the above parameters, gender and age were also normally distributed. After treatment, both subjective and objective criteria improved in the two groups but significantly more in the thermal pulsation arm. In particular, the limited compliance of 30% in the eyelid margin massage arm should be noted. Safety parameters, such as visual acuity and intraocular pressure (IOP) remained normal in all patients.

CONCLUSION

A single session of thermal pulsation showed significantly better results in the efficacy and safety profile after 3 months compared to eyelid margin massage once daily; however, the high costs for the patients due to the single use mode of the activators must be taken in account.

Einfluss von Permanent Make-up auf die Meibomdrüsen und den Tränenfilm

Hintergrund

Ziel dieser Studie war es, Veränderungen hinsichtlich der Meibomdrüsen, der Augenoberfläche und des Tränenfilms durch Lidrandtätowierungen zu ermitteln.

Methodik

Querschnittsstudie: 100 Augen mit Permanent-Make-up von 50 Patientinnen wurden 100 Augen ohne Permanent-Make-up von 50 Patientinnen gegenübergestellt. Die subjektive Symptomabfrage erfolgte mittels OSDI-Fragebogen und wurde mit den objektiven Testergebnissen – erhoben durch die klinische Untersuchung an der Spaltlampe sowie am Keratographen 5M (Oculus, Germany) – verglichen.

Resultate

Es zeigte sich ein statistisch signifikanter Unterschied zwischen den beiden Gruppen hinsichtlich des OSD-Index, der NIK-BUT, der Meibomdrüsen und der LIPKO-Falten. Keinen Unterschied zeigte sich in der bulbären Injektion und der Tränenmeniskushöhe.

Schlussfolgerungen

In der durchgeführten Studie zeigte sich, dass Permanent-Make-up an den Lidrändern zu anatomischen und funktionellen Veränderungen der Meibomdrüsen und des Tränenfilms führt. Diese Ergebnisse korrelieren auch stark mit einem subjektiven okulären Dyskomfort der Patientinnen.

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.

[In vivo Meibomian gland imaging techniques: A review of the literature (French translation of the article)]

Meibomian gland dysfunction (MGD) encompasses a group of complex pathologies of the ocular surface. They represent one of the main etiologies of dry eye but also one of the leading causes of consultation in ophthalmology. Conventional clinical tests (dry eye symptoms, tear film rupture time, glandular expressiveness assessment, or Schirmer's test) allow only an indirect assessment of Meibomian gland function and physiology. Various in vivo investigation methods have therefore been developed to image the meibomian glands such as meibography, optical coherence tomography, ultrasound or in vivo confocal microscopy. Some are accessible in clinical practice, while others remain in the field of clinical research. All these techniques aim to develop a direct structural analysis of the Meibomian glands to help in the diagnosis of DGM but also to better understand the pathophysiology of Meibomian glands. This review of the literature aims to provide an overview of existing imaging modalities and their interest in the evaluation of Meibomian glands and MGD.

Conjunctival staining with lissamine green as a predictor of tear film deficiency in dogs

OBJECTIVE

To evaluate a grading scheme for conjunctival staining patterns with lissamine green ocular dye in the diagnosis of tear film deficiencies in dogs.

PROCEDURES

Client-owned and research colony dogs were enrolled in a prospective study between February and October 2018 in which slit-lamp biomicroscopy, Schirmer tear test (STT), tear film breakup time (TFBUT), conjunctival lissamine green staining (LGS), and intraocular pressure (IOP) measurement were performed in both eyes of all dogs. Lissamine green staining of the temporal bulbar conjunctiva was graded from 0-3, with a higher grade corresponding to an increased stain intensity.

RESULTS

Fifty-four dogs (107 eyes), comprising 31 males and 23 females with a mean age of 5.0 ± 3.9 years (range 0.5-14.3), were enrolled in the study. STT was <15 mm/min in 21 eyes and ≥15 mm/min in 86 eyes. Lissamine green staining grade for eyes with a STT of <15 mm/min (2.0 ± 0.9) was significantly higher than for eyes with a STT ≥15 mm/min (0.2 ± 0.7) (P < .001). TFBUT for eyes with a STT <15 mm/min (6.5 ± 4.4 seconds) was significantly shorter than for eyes with a STT ≥ 15 mm/min (16.1 ± 3.6 seconds) (P < .001). As LGS grade increased, both STT (P < .001) and TFBUT (P < .001) significantly decreased.

CONCLUSIONS

A higher LGS grade was significantly associated with a lower STT and more rapid TFBUT in dogs. Lissamine green ocular dye can be considered as an adjunctive diagnostic test when evaluating tear film deficiency in dogs.

How I do it: The Neuro-Ophthalmological Assessment in Parkinson's Disease

Visual disorders like double vision, dry eyes, and visual field deficits are common but frequently missed in Parkinson’s disease. Here, we aim to increase awareness for these visual disorders in Parkinson patients by discussing several common problems that can be easily diagnosed using comprehensive history taking and a basic neuro-ophthalmological examination. We offer practical guidance for the patient interview and physical exam that can facilitate a timelier recognition of visual disorders. Such recognition has immediate therapeutic relevance, because Parkinson patients are strongly dependent on an adequate vision, for example to optimally benefit from visual cueing strategies.

Histological and immunohistochemical characterization of the porcine ocular surface

The ocular surface of the white domestic pig (Sus scrofa domestica) is used as a helpful model of the human ocular surface; however, a complete histological description has yet to be published. In this work, we studied porcine eyeballs with intact eyelids to describe and characterize the different structures that form the ocular surface, including the cornea and conjunctiva that covers the bulbar sclera, tarsi, and the nictitating membrane. We determined the distribution of goblet cells of different types over the conjunctiva and analyzed the conjunctival-associated lymphoid tissue (CALT). Porcine eyeballs were obtained from a local slaughterhouse, fixed, processed, and embedded in paraffin blocks. Tissue sections (4 μm) were stained with hematoxylin/eosin, Alcian blue/Periodic Acid Schiff, and Giemsa. Slides were also stained with lectins from Arachis hypogaea (PNA) and Helix pomatia (HPA) agglutinins and immunostained with rabbit anti-CD3. We found that the porcine cornea was composed of 6–8 epithelial cell layers, stroma, Descemet’s membrane, and an endothelial monolayer. The total corneal thickness was 1131.0±87.5 μm (mean±standard error of the mean) in the center and increased to 1496.9±138.2 μm at the limbus. The goblet cell density was 71.25±12.29 cells/mm, ranging from the highest density (113.04±37.21 cells/mm) in the lower palpebral conjunctiva to the lowest density (12.69±4.29 cells/mm) in the bulbar conjunctiva. The CALT was distributed in the form of intraepithelial lymphocytes and subepithelial diffuse lymphoid tissue. Lenticular-shaped lymphoid follicles, about 8 per histological section, were also present within the conjunctival areas. In conclusion, we demonstrated that the analyzed porcine ocular structures are similar to those of humans, confirming the potential usefulness of pig eyes to study ocular surface physiology and pathophysiology.

In vivo Meibomian gland imaging techniques: A review of the literature

Meibomian gland dysfunction (MGD) includes a group of complex disorders of the ocular surface. It represents one of the main etiologies of dry eye as well as one of the main reasons for patient visits to the ophthalmologist. Classic clinical tests (dry eye symptoms, tear film break-up time, evaluation of gland expressibility or Schirmer's testing) only provide an indirect assessment of the function of the Meibomian glands and the meibum. Various in vivo testing methods have therefore been developed to image the Meibomian glands, such as Meibography, optical coherence tomography, ultrasound, or even in vivo confocal microscopy. Some are accessible in clinical practice, while others are limited to the realm of clinical research. All of these techniques aim to develop a direct structural analysis of the Meibomian glands so as to assist in the diagnosis of MGD as well as to better understand the pathophysiology of the Meibomian glands. This review of the literature hopes to provide an overview of the current imaging modalities and their role in the evaluation of the Meibomian glands and MGD.

Conjunctival goblet cells: Ocular surface functions, disorders that affect them, and the potential for their regeneration

Conjunctival goblet cells (CGCs) are specialized cells that produce and secrete soluble mucins to the tear film that bathes the ocular surface. CGC numbers and functions are affected in various ocular surface diseases including dry eye disease with diverse etiologies. In this review we will (i) summarize the important functions of CGCs in ocular surface health, (ii) describe the ocular surface diseases that affect CGC numbers and function, (iii) provide an update on recent research outcomes that elucidate CGC differentiation, gene expression and functions, and (iv) present evidence in support of the prediction that restoring CGC numbers and/or functions is a viable strategy for alleviating ocular surface disorders that impact the CGCs.

Myoepithelial cell-driven acini contraction in response to oxytocin receptor stimulation is impaired in lacrimal glands of Sjögren's syndrome animal models

The purpose of the present studies was to investigate the impact of chronic inflammation of the lacrimal gland, as occurs in Sjögren’s syndrome, on the morphology and function of myoepithelial cells (MECs). In spite of the importance of MECs for lacrimal gland function, the effect of inflammation on MECs has not been well defined. We studied changes in MEC structure and function in two animal models of aqueous deficient dry eye, NOD and MRL/lpr mice. We found a statistically significant reduction in the size of MECs in diseased compared to control lacrimal glands. We also found that oxytocin receptor was highly expressed in MECs of mouse and human lacrimal glands and that its expression was strongly reduced in diseased glands. Furthermore, we found a significant decrease in the amount of two MEC contractile proteins, α-smooth muscle actin (SMA) and calponin. Finally, oxytocin-mediated contraction was impaired in lacrimal gland acini from diseased glands. We conclude that chronic inflammation of the lacrimal gland leads to a substantial thinning of MECs, down-regulation of contractile proteins and oxytocin receptor expression, and therefore impaired acini contraction. This is the first study highlighting the role of oxytocin mediated MEC contraction on lacrimal gland function.

Semifluorinated Alkane Eye Drops for Treatment of Dry Eye Disease Due to Meibomian Gland Disease

Purpose: Meibomian gland disease is generally accepted as the leading cause for evaporative dry eye disease (DED). In a previous study, perfluorohexyloctane, a semifluorinated alkane, has been demonstrated to significantly increase tear film breakup time and to reduce corneal fluorescein staining in patients with evaporative DED, thereby vastly reducing dry eye-related symptoms. This study was set up to evaluate perfluorohexyloctane in a larger population of patients with Meibomian gland dysfunction.

Methods: Seventy-two patients with Meibomian gland disease and associated dry eye received 1 drop of perfluorohexyloctane 4 times daily during an observational, prospective, multicenter, 6–8-week study. Clinical assessment included best-corrected visual acuity, intraocular pressure, Schirmer test I, tear film breakup time, anterior and posterior blepharitis assessment, number of expressible Meibomian glands, meibum quality and quantity, ocular surface fluorescein staining, lid margin and symptom assessment, and Ocular Surface Disease Index (OSDI^©).

Results: From the 72 patients recruited, 61 completed the trial per protocol. Nine patients did not apply the medication as recommended and 2 patients were lost to follow-up. Tear film breakup time, corneal and conjunctival fluorescein staining, number of expressible Meibomian glands, and severity of anterior and posterior blepharitis significantly improved after 6–8 weeks of perfluorohexyloctane application. In addition, symptoms improved as demonstrated by a significant decrease of OSDI-values from 37 (±13) to 26 (±16).

Conclusions: In concordance with previous findings, 6–8 weeks of topical application of perfluorohexyloctane significantly improves clinical signs of Meibomian gland disease and associated mild to moderate DED.

Disturbing the balance: effect of contact lens use on the ocular proteome and microbiome

Contact lens wear is a popular, convenient and effective method for vision correction. In recent years, contact lens practice has expanded to include new paradigms, including orthokeratology; however, their use is not entirely without risk, as the incidence of infection has consistently been reported to be higher in contact lens wearers. The explanations for this increased susceptibility have largely focused on physical damage, especially to the cornea, due to a combination of hypoxia, mechanical trauma, deposits and solution cytotoxicity, as well as poor compliance with care routines leading to introduction of pathogens into the ocular environment. However, in recent years, with the increasing availability and reduced cost of molecular techniques, the ocular environment has received greater attention with in-depth studies of proteins and other components. Numerous proteins were found to be present in the tears and their functions and interactions indicate that the tears are far more complex than formerly presumed. In addition, the concept of a sterile or limited microbial population on the ocular surface has been challenged by analysis of the microbiome. Ocular microbiome was not considered as one of the key sites for the Human Microbiome Project, as it was thought to be limited compared to other body sites. This was proven to be fallacious, as a wide variety of micro-organisms were identified in the analyses of human tears. Thus, the ocular environment is now recognised to be more complicated and interference with this ecological balance may lead to adverse effects. The use of contact lenses clearly changes the situation at the ocular surface, which may result in consequences which disturb the balance in the healthy eye.

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.

Meibomian glands, meibum, and meibogenesis

Meibum is a lipid-rich secretion that is produced by fully differentiated meibocytes in the holocrine Meibomian glands (MG) of humans and most mammals. The secretion is a part of a defense mechanism that protects the ocular surface from hazardous environmental factors, and from desiccation. Meibomian lipids that have been identified in meibum are very diverse and unique in nature. The lipid composition of meibum is different from virtually any other lipid pool found in the human body. In fact, meibum is quite different from sebum, which is the closest secretion that is produced by anatomically, physiologically, and biochemically related sebaceous glands. However, meibum of mice have been shown to closely resemble that of humans, implying similar biosynthetic mechanisms in MG of both species. By analyzing available genomic, immunohistochemical, and lipidomic data, we have envisioned a unifying network of enzymatic reactions that are responsible for biosynthesis of meibum, which we call meibogenesis. Our current theory is based on an assumption that most of the biosynthetic reactions of meibogenesis are catalyzed by known enzymes. However, the main features that make meibum unique - the ratio of identified classes of lipids, the extreme length of its components, extensive ω-hydroxylation of fatty acids and alcohols, iso- and anteiso-branching of meibomian lipids (e.g. waxes), and the presence of rather unique complex lipids with several ester bonds - make it possible that either the activity of known enzymes is altered in MG, or some unknown enzymes contribute to the processes of meibogenesis, or both. Studies are in progress to elucidate meibogenesis on molecular level.

Semifluorinated Alkane Eye Drops for Treatment of Dry Eye Disease--A Prospective, Multicenter Noninterventional Study

Purpose: Evaporation of the tear film is heavily discussed as one core reason for dry eye disease (DED). Subsequently, new artificial tear products are developed that specifically target this pathomechanism. Perfluorohexyloctane (F6H8, NovaTears^®) from the family of semifluorinated alkanes is a novel substance that has been approved as a medical device, as a nonblurring wetting agent for the ocular surface.

Methods: Thirty patients with hyperevaporative dry eye received F6H8 during a prospective, multicenter, observational 6-week study. Patients were advised to apply 1 drop 4 times daily in both eyes. Parameters assessed included best corrected visual acuity, intraocular pressure, Schirmer I test, tear fluid, tear film breakup time (TFBUT), corneal staining, meibum secretion, and Ocular Surface Disease Index (OSDI^©).

Results: From the 30 patients recruited, 25 completed the trial per protocol. Four patients discontinued F6H8 and 1 patient did not present for follow-up. F6H8 treatment led to significant reduction of corneal staining and significant increase of Schirmer I and TFBUT. In addition, OSDI score dropped significantly from a mean of 55 (±23.0) to 34 (±22.4). Visual acuity and ocular pressure did not change.

Conclusions: This prospective observational study shows significant beneficial effects in patients suffering from evaporative DED, using F6H8 in all the relevant parameters tested. The decrease of the OSDI by a mean of 21 points was particularly remarkable and clearly exceeds minimal, clinical important differences for mild or moderate and severe disease. Overall, F6H8 (NovaTears) seems to be safe and effective in treating mild to moderate hyperevaporative DED.

The tear ferning test: a simple clinical technique to evaluate the ocular tear film

A healthy tear film is very important for many major functions of the ocular surface. Dry eye disease is a significant clinical problem that needs to be solved but the poor correlation between clinical signs and reported symptoms makes it difficult for the clinician to apply a scientific basis to his clinical management. The problem is compounded by the difficulties of evaluating the tear film due to its transparency, small volume and complex composition. Practical insight into tear film composition would be very useful to the clinician for patient diagnosis and treatment but detailed analysis is restricted to expensive, laboratory-based systems. There is a pressing need for a simple test. The tear ferning test is a laboratory test but it has the potential to be applied in the clinic setting to investigate the tear film in a simple way. Drying a small sample of tear fluid onto a clean, glass microscope slide produces a characteristic crystallisation pattern, described as a 'tear fern'. This test is currently not widely used because of some limitations that need to be overcome but several studies have demonstrated its potential. Such limitations need to be resolved so that tear ferning could be used in the clinic setting to assess the tear film.

Granular corneal dystrophy type 2 is associated with morphological abnormalities of meibomian glands

AIMS

To investigate morphological changes in meibomian glands in patients with granular corneal dystrophy type 2 (GCD2) using non-invasive meibography.

METHODS

Eleven patients (3 men and 8 women) with GCD2, and sex-matched and age-matched healthy volunteers as a controls were enrolled in this study. The diagnosis of GCD2 was confirmed by transforming growth factor β-induced (TGFBI) gene analysis using direct sequencing in exon 4 of TGFBI gene. Meibography was performed in the right eye of the studied cases. Meiboscore was determined according to the morphology of meibomian gland and classified into four grades; grade 0 (no meibomian gland loss), grade 1 (loss less than one-third the total area of meibomian glands), grade 2 (area loss between one-third and two-thirds of the total area), and grade 3 (area loss greater than two-thirds of the total).

RESULTS

R124H mutation was detected in all patients with GCD2. Extinguishing or shortening of the meibomian glands was observed in patients with GCD2. The meiboscore was 3.8±1.3 in patients with GCD2 and 1.3±1.1 in the control group, showing significant difference between two groups (Mann-Whitney U-test, p=0.042).

CONCLUSIONS

In GCD2, corneal deposits, and also morphological abnormalities of meibomian glands, such as obstruction or shortening, were found. Since abnormal phospholipid deposition is noted in GCD2, these results are interesting because phospholipid is possibly secreted from the meibomian gland.

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.

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.

Differences in human meibum lipid composition with meibomian gland dysfunction using NMR and principal component analysis

PURPOSE

Nuclear magnetic resonance (NMR) spectroscopy has been used to quantify lipid wax, cholesterol ester terpenoid and glyceride composition, saturation, oxidation, and CH₂ and CH₃ moiety distribution. This tool was used to measure changes in human meibum composition with meibomian gland dysfunction (MGD).

METHODS

(1)H-NMR spectra of meibum from 39 donors with meibomian gland dysfunction (Md) were compared to meibum from 33 normal donors (Mn).

RESULTS

Principal component analysis (PCA) was applied to the CH₂/CH₃ regions of a set of training NMR spectra of human meibum. PCA discriminated between Mn and Md with an accuracy of 86%. There was a bias toward more accurately predicting normal samples (92%) compared with predicting MGD samples (78%). When the NMR spectra of Md were compared with those of Mn, three statistically significant decreases were observed in the relative amounts of CH₃ moieties at 1.26 ppm, the products of lipid oxidation above 7 ppm, and the =CH moieties at 5.2 ppm associated with terpenoids.

CONCLUSIONS

Loss of the terpenoids could be deleterious to meibum since they exhibit a plethora of mostly positive biological functions and could account for the lower level of cholesterol esters observed in Md compared with Mn. All three changes could account for the higher degree of lipid order of Md compared with age-matched Mn. In addition to the power of NMR spectroscopy to detect differences in the composition of meibum, it is promising that NMR can be used as a diagnostic tool.

Changes in the evaporation rate of tear film after digital expression of meibomian glands in patients with and without dry eye

PURPOSE

To evaluate the effect of excess meibum on tear evaporation rate in patients with and without dry eye.

METHODS

Eleven healthy subjects and 16 patients with dry eye were tested. The dry eye group was divided into 2 subgroups: classic keratoconjunctivitis sicca (KCS) with clear and easily expressed meibum and KCS with meibomian gland dysfunction (MGD) with turbid secretions and difficult-to-express meibum. Evaporative measurements were performed at baseline and after digital expression of meibomian glands at 12, 24, 36, and 48 minutes. Two ranges of relative humidity were used, 25% to 35% and 35% to 45%. The data were expressed as microliters per square centimeter per minute.

RESULTS

An increase in the evaporation rate of the tear film was noted for all measurements at both relative humidities in the classic KCS and KCS with MGD groups compared with healthy subjects (P < 0.05). The average evaporation rates at relative humidities of 25% to 35% and 35% to 45% were 0.056 ± 0.016 and 0.040 ± 0.008 for the classic KCS group; 0.055 ± 0.026 and 0.037 ± 0.019 for the KCS with MGD group and 0.033 ± 0.012 and 0.023 ± 0.008 for the healthy group. Also, a decrease in the evaporation rate was observed in the healthy and KCS with MGD groups between baseline and the first measurement after digital expression for both relative humidities (P < 0.05). The classic KCS group did not show any changes after expression.

CONCLUSIONS

Classic KCS and KCS with MGD groups showed an increase in tear evaporation rates compared with the healthy group. Aqueous tear evaporation diminished in the healthy and KCS with MGD groups after expression of meibomian glands. However, this effect was transient and negligible after the second measurement.

Analysis of the composition of lipid in human meibum from normal infants, children, adolescents, adults, and adults with meibomian gland dysfunction using ¹H-NMR spectroscopy

PURPOSE

This study represents a first step toward the evaluation of possible compositional differences in meibum from normal donors (Mn) and donors with meibomian gland dysfunction (Md) by (1)H-NMR spectroscopy. The results highlight the applicability of (1)H-NMR spectroscopy for the quantitative analysis of waxes, cholesteryl esters, and glycerides in meibum lipid (ML).

METHODS

Meibum was obtained from 41 normal donors and 51 donors with meibomian gland dysfunction (MGD). (1)H-NMR spectroscopy was used to quantify the amount of waxes, glycerides, and cholesteryl esters in human meibum.

RESULTS

The relative amount of cholesteryl esters in Mn increased with age and was 40% (P < 0.05) lower in Md. Interestingly, the relative levels of cholesteryl esters in infant meibum were comparable to those in Md. The relative amounts of glycerides were not affected significantly by age or MGD.

CONCLUSIONS

The changes in cholesteryl ester could be used as a molecular marker for MGD and could potentially be applied to follow the efficacy of drug therapy in the treatment of MGD. The similarity of the levels of cholesteryl esters in infant meibum and Md suggests that the relative amounts of these meibum components alone are unlikely to be responsible for the increased stability of the infant tear film and decreased stability of the tear film with MGD. This study reveals the complexity of human MLs and the changes that occur with age and disease. Understanding the factors that lead to such variations is of utmost relevance in the design of effective therapies.

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.

Tear film volume and protein analysis in full-term newborn infants

PURPOSE

To investigate infant tear film secretion and protein profile, and to compare major tear proteins, such as immunoglobulin A, lactoferrin, and lysozyme, with those of adult controls.

METHODS

Tears were collected, with a cellulose rod, from 40 healthy infants (19 female infants and 21 male infants, gestational duration: 39.71 ± 1.27 weeks) within 48 hours of birth and 22 adults (10 female infants and 12 male infants, mean age: 24.95 ± 3.63 years). A second collection was obtained from 14 of the infants (8 female infants and 6 male infants, postnatal age: 7.76 ± 6.14 weeks). The tear volume was measured, and protein in the samples was analyzed by Bradford assay and gel electrophoresis.

RESULTS

Median tear volume (interquartile range) was 0.5 μL (0.6-2 μL) for newborn infants, 2.5 μL (1.4-7.75 μL) for these infants at an older age, and 6 μL (2.73-12.75 μL) in adults (P < 0.001, Kruskall-Wallis test). Immunoglobulin A concentration was significantly lower in newborns (P < 0.001, analysis of variance). Lipocalin was present in 36% of the newborn tear samples, whereas serum albumin was found in 86%. Mean protein concentration (μg/μL ± SD) was 10.95 ± 5.51 in the newborns, 12.93 ± 3.99 in the older infants, and 13.04 ± 3.46 in the adults (P > 0.5, analysis of variance).

CONCLUSIONS

This is the first study reporting an investigation of unstimulated infant tears, using a noninvasive collection method. Tear protein content demonstrated that the infant tear film is different to that in adults.

Human meibum lipid conformation and thermodynamic changes with meibomian-gland dysfunction

PURPOSE

Instability of the tear film with rapid tear break-up time is a common feature of aqueous-deficient and evaporative dry eye diseases, suggesting that there may be a shared structural abnormality of the tear film that is responsible for the instability. It may be that a change in the normal meibum lipid composition and conformation causes this abnormality. Principle component analyses of infrared spectra of human meibum indicate that human meibum collected from normal donors (Mn) is less ordered than meibum from donors with meibomian gland dysfunction (Md). In this study the conformation of Md was quantified to test this finding.

METHODS

Changes in lipid conformation with temperature were measured by infrared spectroscopy. There were two phases to our study. In phase 1, the phase transitions of human samples, Mn and Md, were measured. In phase 2, the phase transitions of model lipid standards composed of different waxes and cholesterol esters were measured.

RESULTS

The phase-transition temperature was significantly higher (4°C) for the Md compared with the Mn of age-matched donors with no history of dry-eye symptoms. Most (82%) of the phase-transition temperatures measured for Md were above the values for Mn. The small change in the transition temperature was amplified in the average lipid order (stiffness) at 33.4°C. The average lipid order at 33.4°C for Md was significantly higher (30%, P = 0.004) than for Mn. The strength of lipid-lipid interactions was 72% higher for Md than for Mn. The ability of one lipid to influence the melting of adjacent lipids is termed cooperativity. There were no significant differences between Mn and Md in phase-transition cooperativity, nor was there a difference between Mn and Md in the minimum order or maximum order that Mn and Md achieved at very low and very high temperatures, respectively. The model wax studies showed that the phase transition of complex mixtures of natural lipids was set by the level of unsaturation. A double bond decreased the phase-transition temperature by approximately 40°C. The addition of a second CH CH moiety decreased the phase-transition temperature by approximately 19°C. Unsaturated waxes were miscible with saturated waxes. When a saturated wax was mixed with an unsaturated one, the saturated wax disproportionately increased the phase transition of the mixture by approximately 30°C compared with the saturated wax alone. Cholesterol ester had little effect on the phase-transition temperature of the waxes. Model studies indicated that changes in the amount of lipid saturation, rather than the amount of cholesterol esters, could be a factor in the observed conformational changes.

CONCLUSIONS

Meibum lipid compositional changes with meibomian gland dysfunction reflect changes in hydrocarbon chain conformation and lipid-lipid interaction strength. Spectroscopic techniques are useful in studying the lipid-lipid interactions and conformation of lipid from individual patients. (ClinicalTrials.gov number, NCT00803452.).

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.

Picosecond spectral coherent anti-Stokes Raman scattering imaging with principal component analysis of meibomian glands

The lipid distribution in the mouse meibomian gland was examined with picosecond spectral anti-Stokes Raman scattering (CARS) imaging. Spectral CARS data sets were generated by imaging specific localized regions of the gland within tissue sections at consecutive Raman shifts in the CH(2) stretching vibrational range. Spectral differences between the location specific CARS spectra obtained in the lipid-rich regions of the acinus and the central duct were observed, which were confirmed with a Raman microspectroscopic analysis, and attributed to meibum lipid modifications within the gland. A principal component analysis of the spectral data set reveals changes in the CARS spectrum when transitioning from the acini to the central duct. These results demonstrate the utility of picosecond spectral CARS imaging combined with multivariate analysis for assessing differences in the distribution and composition of lipids in tissues.

Identification of phospholipids in human meibum by nano-electrospray ionisation tandem mass spectrometry

Meibum is believed to be the major source of tear film lipids, which are vital in the prevention of excess evaporation of the aqueous phase. The complete lipid composition of meibum has yet to be established. While earlier studies reported the presence of phospholipids in human meibum, recent mass spectrometric studies have not detected them. In this study we use electrospray ionisation tandem mass spectrometry to investigate the presence of phospholipids in meibum and provide comparison to the phospholipid profile of tears. Lipids were extracted from human meibum and tear samples using standard biphasic methods and analysed by nano-electrospray ionisation tandem mass spectrometry using targeted ion scans. A total of 35 choline-containing phospholipids were identified in meibum and the profile of these was similar to that observed in tears, suggesting tear lipids are derived from meibum. The results shown here highlight the need for a combination of optimised techniques to enable the identification of the large range of lipid classes in meibum.

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.

[Meibomian glands. Part I: anatomy, embryology and histology of the Meibomian glands]

The Meibomian glands are large sebaceous glands that are located as separate gland strands in parallel arrangement within the tarsal plates of the eyelids. Their oily product (meibum) is secreted by a holocrine mechanism during which the secretory cells (meibocytes) are completely transformed into the meibum after synthesis and accumulation of lipids. After production in the gland acini, meibum is transported through the ductal system via the connecting duct (ductule) and the central duct towards the orifice at the free lid margin close to the inner lid border. The embryological development of the Meibomian glands takes place during the differentiation of the eyelids in the sealing phase of the eyelids. They are not directly associated with hair follicles but share important similarities in embryology, structure and keratinization potency with the cilia. Similar to the sebaceous glands Meibomian glands are regulated via sex hormones and androgens have a supporting function whereas estrogens act antagonistically. However, in contrast to other sebaceous glands they also have a distinct innervation, apart from sympathetic and sensory primarily by parasympathetic fibers that share the innervation pattern of the lacrimal glands. The anatomy, embryology and histology of the Meibomian glands are explained here, mainly with respect to humans, in an extensive review.