The casual level of meibomian lipids in humans

Sex- and gender-based differences in healthy and diseased eyes

PURPOSE

The aim of this study was to identify sex- and gender-based differences in ocular anatomy, physiology, and disease susceptibility or manifestation.

METHODS

Review of current indexed literature was conducted.

RESULTS

Sex and sex hormones influence the lacrimal system, eyelids and blinking, corneal anatomy and disease, aqueous humor dynamics and glaucoma, crystalline lens and cataract, uveitis and retinal disease, ocular circulation, and optic nerve anatomy and disease. Systemic conditions, particularly autoimmune disease, and conditions that are unique to women, such as pregnancy and menopause, further illustrate the effects of sex hormones on the eye. Gender-based differences in ocular conditions and disease should be considered within the context of the underlying physical and social environment.

CONCLUSIONS

Many sex- and gender-based differences exist in healthy and diseased eyes.

New Instruments for Dry Eye Diagnosis

Several non-invasive techniques for dry eye diagnosis have been developed in the past decade. These include quantitative assessment of tear volume, tear film stability, tear dynamics, and integrity of ocular surface epithelium. A combination of meniscometry and interferometry is useful for proving focal dry eye, by confirming whether or not tears at the meniscus have an effect on the ocular surface. Interferometer is also useful to evaluate tear dynamics on soft contact lenses. Fluorophotometry is useful for assessing the severity of dry eye from the view point of corneal epithelial barrier function and measuring the tear turnover rate. Both video-meibography and meibometry are useful for screening meibomian gland dysfunction. The advances in these techniques accumulate knowledge regarding pathophysiology of dry eye and allow precise diagnosis of dry eye. More targeted treatment will become feasible based on the clearer pathophysiology.

Meibomian gland diagnostic expressibility: correlation with dry eye symptoms and gland location

PURPOSE

To determine (1) if the number of meibomian glands yielding liquid secretion (MGYLS) is correlated with dry eye symptoms and (2) the mean number of MGYLS in the nasal, central, and temporal regions of the lower eyelid in a random clinical sample.

METHODS

Subjects presenting for routine eye examinations were recruited (n = 133; 90 females, 43 males; mean age = 50.3 +/- 14.7 years). The sample included symptomatic and asymptomatic individuals using the Standard Patient Evaluation of Eye Dryness questionnaire. Meibomian gland evaluations were performed using a standardized technique and diagnostic instrument.

RESULTS

The mean number of MGYLS in the lower eyelid correlated with dry eye symptoms, p = 0.0002. The mean numbers of MGYLS in each third of the lower eyelid were significantly different, p <or= 0.0001: temporal = 0.27 +/- 0.06, central = 2.14 +/- 0.13, and nasal = 3.10 +/- 0.15. The temporal third of the lower lid was 14 times as likely as the nasal third to have zero MGYLS; 86% of temporal versus 6% of nasal thirds had zero MGYLS.

CONCLUSIONS

This is the first report to document the following: (1) a correlation between the number of MGYLS in the lower eyelid and dry eye symptoms; (2) the number of MGYLS varies significantly across the lower eyelid, with the highest number of MGYLS in the nasal third and the lowest number of MGYLS in the temporal third of the lower eyelid; and (3) instrumentation to standardize diagnostic meibomian gland expression is desirable if not mandatory for the evaluation of meibomian gland function.

Understanding and analyzing meibomian lipids--a review

PURPOSE

This review is intended to bring to the informed reader the current state of knowledge about meibomian lipids and the art for analyzing them.

METHODS

At the forefront of any endeavor, there are controversies, and these, along with future directions in the field, are brought to the reader's attention.

RESULTS

Function and anatomy of meibomian glands are briefly covered, giving insight into possible mechanisms for secretory controls. Anatomically, some anomalies in meibomian gland distribution of different species, such as whales versus dolphins, are presented, and, for the first time, the structure of the meibomian glands in a selection of marsupials is presented. In attempting to make the literature more accessible, lipid structure and nomenclature are described, and these structures are related to their possible effects on the physicochemical properties of meibomian lipids. The advantages and disadvantages of various collection and storage techniques are described, as well as how gas chromatography and combined HPLC and mass spectrometry coupled with fragmentation are currently enabling us to determine the nature of the lipids in very small samples.

CONCLUSIONS

This review extends to discussing the lipids in tears (as opposed to meibomian gland lipids) and briefly highlights new thoughts about the interactions between proteins of the tear film and meibomian lipids. A model that includes proteins in the outer layer of the tear film is also presented. This model is currently being critically analyzed by the ocular community. It concludes briefly by highlighting possible further areas of research in this area.

Canine meibometry: Establishing baseline values for meibomian gland secretions in dogs

Meibomian lipid secretions are essential in preventing tear evaporation. Disorders of the meibomian glands may therefore play an important role in the pathogenesis of some forms of keratoconjunctivitis sicca (KCS). Until now, meibomian lipid secretions have never been quantitatively evaluated in dogs. With the aim of establishing baseline values of canine meibomian lipid secretions, meibometry was conducted in 42 healthy dogs, 16 of which were Miniature Schnauzers. The mean meibomium level in 84 eyes of the 42 dogs was 179+/-60 Meibometer units. Age, gender and side did not affect the results. However, meibomium levels were significantly lower in the Miniature Schnauzers, a breed that is susceptible to KCS, compared to other breeds. This report demonstrates that meibometry is a simple and minimally invasive technique that may be readily used in conscious dogs to quantify meibomian gland secretions and explicate tear film dynamics in normal and dry canine eyes.

Tearful relationships? Sex, hormones, the lacrimal gland, and aqueous-deficient dry eye

Sex and the endocrine system exert a significant influence on the physiology and pathophysiology of the lacrimal gland. The purpose of this article is to briefly review the nature and magnitude of these interactions between sex, hormones and lacrimal tissue, and to address how they may relate to the pathogenesis of aqueous-deficient dry eye. Towards this end, this article has a 3-fold approach: first, to summarize the influence of androgens, estrogens, glucocorticoids, mineralocorticoids, retinoic acid, prolactin, alpha-melanocyte stimulating hormone, adrenocorticotropic hormone, luteinizing hormone, follicle-stimulating hormone, growth hormone, thyroid-stimulating hormone, arginine vasopressin, oxytocin, thyroxine, parathyroid hormone, insulin, glucagon, melatonin, human chorionic gonadotropin and cholecystokinin on the structure and function of the lacrimal gland; second, to discuss the mechanism of action of each hormone on lacrimal tissue; and third, to discuss the clinical relevance of the endocrine-lacrimal gland interrelationship, with a particular focus on each hormone's role (i.e. if relevant) in the development of aqueous-tear deficiency.

Association between meibomian gland changes and aging, sex, or tear function

PURPOSE

To study changes in the lid margin and meibomian glands and their association with aging, sex, and tear function.

METHODS

We examined 354 eyes in 177 subjects (76 men and 101 women; 21-93 years; mean age, 63.0 +/- 14.3 years) with no ocular symptoms or ocular surface disorders. Anatomic changes in the lid margin were studied using slit-lamp biomicroscopy. Meibomian gland function and morphology were evaluated on the basis of meibum expression and meibography, respectively. Tear function and ocular surface epithelium were assessed with the Schirmer test, by tear film break-up time, and with a fluorescein staining test.

RESULTS

Eyes with abnormal lid margin anatomy, hyposecretion of meibum, and meibomian gland dropout were seen in 26 (7.3%), 46 (12.4%), and 68 eyes (18.6%), respectively, with a significant association between each finding and aging (P = <0.0001, 0.0498, and <0.0001, respectively). In patients < or =69 years of age, no significant association was found between meibomian gland-related findings and sex. However, a high incidence of abnormal lid margin and gland dropout was noted in men > or =70 years of age compared with women. No significant association was found between changes in the lid margin and meibomian glands and tear function in patients > or =40 years of age.

CONCLUSION

Among symptom-free subjects, we found that changes in the lid margin and meibomian glands were closely related to aging. Among elderly subjects, changes in the anatomic lid margin and meibomian gland morphology were observed more frequently in men than in women. Tear function showed no association with either changes in the lid margin or function of the meibomian glands.

The Definition and Classification of Dry Eye Disease: Report of the Definition and Classification Subcommittee of the International Dry Eye Workshop (2007)

The aim of the DEWS Definition and Classification Subcommittee was to provide a contemporary definition of dry eye disease, supported within a comprehensive classification framework. A new definition of dry eye was developed to reflect current understanding of the disease, and the committee recommended a three-part classification system. The first part is etiopathogenic and illustrates the multiple causes of dry eye. The second is mechanistic and shows how each cause of dry eye may act through a common pathway. It is stressed that any form of dry eye can interact with and exacerbate other forms of dry eye, as part of a vicious circle. Finally, a scheme is presented, based on the severity of the dry eye disease, which is expected to provide a rational basis for therapy. These guidelines are not intended to override the clinical assessment and judgment of an expert clinician in individual cases, but they should prove helpful in the conduct of clinical practice and research.

[Ocular surface investigations in dry eye]

Dry eye is a complex clinicopathological entity involving tear film, lacrimal glands, eyelids, and a wide spectrum of ocular surface cells, including epithelial, inflammatory, immune, and goblet cells. From the tightly regulated lacrimal film functions and structure, a large variety of investigations have been developed, including tear meniscus measurements, fluorophotometry, meibometry, interference pattern analysis, evaporation rate, tear osmolarity, and thermography. Dry eye conditions also interfere with the ocular surface, causing corneal irregularities that may be explored using the techniques of videokeratography and in vivo confocal microscopy, or optical impairment, as confirmed by aberrometry. At the level of ocular surface cells, impression cytology remains a standard for assessing cell alterations. It has greatly benefited from new confocal microscopy, molecular biology, and flow cytometry techniques. Biological assessment of tear proteins or other mediators is also useful. Major limits should be acknowledged, however, such as technical issues in tear film collection, especially in dry eyes, and the lack of standardization of most measurements. Tear osmolarity, electrophoresis, and dosage of normal tear proteins, such as lysozyme or lactoferrin, remain the most useful tests. Finally, some extraocular explorations such as accessory gland biopsy or serum antinuclear antibody dosage may be useful for assessing the diagnosis of Sjögren's syndrome.

Le Video tearscope: une nouvelle méthode d’évaluation du film lacrymal in vivo

PURPOSE

The lacrimal film has trophic, immunological, antimicrobial, and anti-inflammatory roles. The lipidic phase of the tear film reduces surface stresses and limits tear evaporation. We present a reproducible and noninvasive investigation of the lacrimal film, the Video Tearscope (Vi-Te), which analyzes the after-blinking tear stabilization time and the different types of blinking. Vi-Te consists of a numerical video recording of the specular reflexion produced by the Tearscope Plus adapted to a biomicroscope.

MATERIALS AND METHODS

Twenty-four normal subjects and ten patients with dry eye syndrome were included. We studied the blinking types and LFST using the Vi-Te technique. The analysis was carried out at 25 images/s, corresponding to an in-between image interval of 40 ms.

RESULTS

Four types of spontaneous blinking were observed: complete, subtotal (99%-70% occlusion), partial (69%-20%), and abortive (less than 20%). We obtained a LFST value of 0.65+/-0.12s for the control group versus 2.34+/-1.28 s for the dry eye syndrome group. The difference was significant between the two groups for the LFST (p<0.001) for complete blinking.

CONCLUSIONS

The Video Tearscope is a promising technique that provides new insights into tear film stability and physiology. The main weakness of this technique, however, is the time required for video analysis (20-45 min per case). Despite this drawback, additional studies should be undertaken in order to establish standards for evaluating ocular surface disorders in a completely noninvasive way.

Sex-related effect on gene expression in the mouse meibomian gland

PURPOSE

Sex-related differences have been identified in the anatomy and physiology of the meibomian gland. We hypothesize that these differences are due, at least in part, to variations in gene expression. This study's objective was to determine whether sex-related differences do exist in meibomian gland gene expression. We also sought to elucidate whether such differences, if any, might be (a) analogous to those known to occur in the lacrimal gland and (b) due to the effect of sex steroids.

METHODS

Meibomian glands were obtained from young adult male and female BALB/c mice (n=7 to 15 mice per sex per experiment), pooled according to sex and processed for the isolation of RNA. Samples were evaluated for differentially expressed mRNAs by using CodeLink Bioarrays and GEM 1 and 2 gene chips. Bioarray data were analyzed with GeneSifter. Net software and also compared with microarray data in GEO and GeneSifter databases.

RESULTS

Our results demonstrate that sex has a significant influence on the expression of 164 genes in the mouse meibomian gland. These genes are involved in a broad spectrum of biological processes, molecular functions, and cellular components, including such activities as metabolism, catalysis, cell growth and maintenance, membrane architecture, nucleic acid binding, transcription, and signal transduction. In addition, the nature of the sex-related variations in meibomian gland gene expression is quite different from those in the lacrimal gland and appear to be mediated in part by the action of androgens, but not estrogens or progestins.

CONCLUSIONS

These findings support our hypothesis that sex-related differences exist in gene expression of the meibomian gland.

Non-invasive methods of assessing the tear film

The interaction between the tear film and the ocular surface epithelium is crucial for the maintenance of ocular surface health; interference with this relationship may cause dry eye. Several diagnostic techniques have been developed to assess the tear film and diagnose dry eye but many of these tests are invasive and modify the parameter which they are designed to measure. Non-invasive or minimally invasive tests may overcome this problem and provide more reproducible and objective data. One test of this kind is meniscometry, which is particularly useful in assessing tear volume indirectly by measuring tear meniscus radius. The newly developed video-meniscometer, which enables calculation of the meniscus radius digitally, is useful for the diagnosis of tear-deficient dry eye. Video-meniscometry also has other applications, to the study of tear and eye drop turnover, determining the indication for punctal plugs and in demonstrating dysfunction of the tear meniscus. Interferometry of the tear film lipid layer is useful in screening and evaluating dry eye severity and in selecting dry eye candidates for punctal occlusion. It is also useful for analysing tear lipid layer pathophysiology more clearly, especially in combination with meniscometry. Meibometry is a minimally invasive technique to quantify the amount of meibomian lipid on the lid margin. Lipid is blotted onto a plastic tape and the change in optical density is used to calculate lipid uptake. Laser meibometry has increased the scope of this technique for the assessment of meibomian gland dysfunction; also, the delivery of lipids from the lid reservoir to the preocular tear film can be analysed using interferometry and laser meibometry. The present report reviews the application of these techniques to the study of tear film physiology and dry eye.

Functional aspects of the tear film lipid layer

The lipid layer is an essential component of the tear film, providing a smooth optical surface for the cornea and retarding evaporation from the eye. The meibomian lipids which compose it are well adapted for this purpose. They form a thin, smooth film whose thickness, and probably composition, influences the rate of evaporation. Their melting range ensures sufficient fluidity for delivery to the tear film from the lid margin reservoirs, while the film itself may exhibit a higher viscosity at the cooler temperature of the ocular surface. The factors governing lipid film formation during the blink are not fully understood, but one view is that the polar lipids, interacting with the aqueous sub-phase of the tear film, spread in advance of the non-polar components, which form the bulk of the film. The meibomian lipids stabilise the tear film by lowering its free energy; they carry water into the film during its formation and interact with lipid-binding proteins in the aqueous phase, such as tear lipocalin. The lipocalins, complexed with other tear components, may also contribute to the high, non-Newtonian viscosity of the tear film and its low surface tension, features which are essential for tear film stability. Formation of the lipid film is a complex process. Lipid is delivered to the tear film in the up-phase of the blink, more from the lower than the upper reservoir. The lipid layer comes to a stop well after completion of the blink and remains relatively immobile until it is compressed in the down-phase of the blink that follows. Then, it either retains its structure in a series of subsequent blinks, or is completely re-constituted after mixing with the reservoir lipids. Delivery of meibomian lipid to the marginal reservoirs is mainly the result of continuous secretion, under neural and hormonal control, supplemented by lid action. The reservoirs provide a hydrophobic barrier to tear overspill and to contamination by skin lipids which might destabilise the tear film. They probably also provide the chief route for meibomian lipid excretion.

The influence of eyelid temperature on the delivery of meibomian oil

The aim of this study is to establish the effect of heating and cooling of the eyelid on the delivery of oil from the meibomian glands, as indicated by the extent of re-establishment of the reservoir of meibomian oil on the human eyelid margin, resulting from 10 conscious blinks, following pre-cleaning by a hexane swab. A standard handling sequence was followed. Step 1. Both upper and lower eyelids of both eyes of human volunteers were cleaned by a hexane swab and measured by a Meibometer to show satisfactory removal of the pre-existing oil (initial reading less than 60 instrumental units (IU)). The volunteers were then asked to blink 10 times at their normal rate. Then the eyelid was re-measured by the Meibometer. Step 2. The closed eyelids were heated for 5 min by a 250 W infrared lamp from a distance of 50 cm. The eyelid temperature was measured by an infrared thermometer, then Step 1 was repeated. Step 3. The eyelids were cooled for 5 min by an ice-pack (crushed ice in a towel). The eyelid temperature was measured, then Step 1 was repeated. Both eyes of 20 female subjects, aged 68 +/- 3 S.E.M. (range 52-84) years were examined. The mean instrumental reading (IU +/- S.E.M.) after 10 blinks at 33.4 +/- 0.1 degrees C was 154 +/- 12. On increasing the eyelid surface temperature by 4.9 +/- 0.3 degrees C we obtained an increase of 49 +/- 9 IU. On decreasing it by 7.6 +/- 0.4 degrees C we obtained a decrease of 46 +/- 9 IU. We conclude that temperature significantly influences the delivery of the meibomian gland secretion. Our results are in agreement with the 10-blink recovery value of 191 +/- 9.3 IU previously reported. They also agree with the reported benefit of warm compresses to improve delivery from dysfunctional glands. The most likely explanation for our findings is a change in the viscosity of the meibomian oil.

The Contribution of Meibomian Disease to Dry Eye

The tear film lipid layer is the major barrier to evaporation from the ocular surface. A decrease in its thickness or functional integrity may cause evaporative dry eye (EDE). Obstructive meibomian gland dysfunction (MGD) is the most common cause of EDE and occurs as a primary disorder or secondary to acne rosacea, seborrheic or atopic dermatitis, and with cicatrizing conjunctival disorders, such as trachoma, erythema multiforme, and cicatricial pemphigoid. MGD may be an incidental finding in asymptomatic eyes, or it may be responsible for irritative lid symptoms in the absence of dry eye. MGD-dependent EDE is diagnosed on the basis of a defined degree of MGD in a symptomatic patient showing typical ocular surface damage in the absence of an aqueous tear deficiency. When MGD occurs in a background of aqueous tear deficiency (ATD), then an additional evaporative component may assumed, depending on the extent of meibomian obstruction. However, definitive criteria are not yet established. The clinical severity of dry eye is greatest when ATD and EDE occur together, particularly in Sjogren syndrome. A hypothesis is proposed to explain the steps leading to primary, simple MGD and subsequent EDE.

Tears in health and disease

PURPOSE

A brief review of normal tear function and changes resulting from disease.

METHODS

The factors contributing to normal tear physiological function are considered, and the effect of changes in composition, as reported in the literature, is surveyed, with emphasis on the physical performance of the tears. Major classes of tear tests which would reveal functional changes are listed.

RESULTS

Where possible, changes of measurable functions in disease are described. Gaps in our current knowledge are indicated.

CONCLUSIONS

Many techniques exist for examination and assessment of both normal and disease tears, but further development is needed to adapt some of these to clinical situations, and to make them more specific diagnostic tests.

Grading of corneal and conjunctival staining in the context of other dry eye tests

PURPOSE

To describe the Oxford Scheme for grading ocular surface staining in dry eye and to discuss optimization of stain detection using various dyes and filters. Also, to propose a sequence of testing for dry eye diagnosis.

METHODS

The grading of corneal and conjunctival staining is described, using the Oxford Scheme, including biomicroscopy, optical filters, illumination conditions, and the characteristics of and instillation techniques used for, selected clinical dyes.

RESULTS

A series of panels, labeled A-E, in order of increasing severity, reproducing the staining patterns encountered in dry eye, are used as a guide to grade the degree of staining seen in the patient. The amount of staining seen in each panel, represented by punctate dots, increases by 0.5 of the log of the number of dots between panels B to E. The use of the vital dyes fluorescein, lissamine green, and rose Bengal is described; fluorescein and lissamine green, used in conjunction with appropriate absorption filters, are recommended for use in clinical trials. The placement of staining in relation to the sequence of other diagnostic tests is discussed.

CONCLUSIONS

The monitoring and assessment of corneal and conjunctival staining can be greatly enhanced by the use of a grading scale, controlled instillation of dyes, and standard evaluation techniques. This is of particular benefit in clinical trials, where ocular surface staining is commonly employed as an outcome measure

Meibomian Gland Dysfunction: A Clinical Scheme for Description, Diagnosis, Classification, and Grading

Although meibomian gland disease (MGD) is one of the most common disorders encountered in ophthalmic practice, there has been no descriptive system consistently accepted to clinically characterize the anatomical and correlative biochemical alterations that occur in this condition. The purpose of this review is to synthesize a clinical description of meibomian gland disease and to provide a scheme for diagnosis, classification, and quantification that will be of value in the clinical setting and in the conduct of clinical trials.

Epithelial inclusions in association with mucin ball development in high-oxygen permeability hydrogel lenses

Debris trapped behind contact lenses may be associated with adverse reactions in extended wear. Although streaks and clumps of cellular material have been reported after overnight wear of conventional materials, recent experience with several high-oxygen permeability (Dk), silicone-containing hydrogel lenses indicates that certain participants are prone to the development of a unique back surface debris. This debris forms as spherical, translucent entities and results in depressions within the ocular surface after lens removal. Little information is known about these spherical bodies, particularly with respect to their composition and development. In this report, we provide photographic evidence of this debris (sometimes termed "mucin balls" or "lipid plugs"), discuss its differential diagnosis and describe a case in which material seems to be embedded in the epithelium as a direct consequence of their presence.

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

Meibomian gland disease--and, in particular, obstructive meibomian gland disease--makes an important contribution to ocular surface disease, in the form of meibomian keratoconjunctivitis. With improved methods for the study of meibomian oil composition and function, we are moving closer to the possibility of distinguishing the contribution of meibomian deficiency, as opposed to inflammatory events, to this disorder. More importantly, where aqueous tear deficiency and meibomian gland disease coincide in patients with dry eye, we are closer to the possibility of distinguishing their relative contributions to the dry eye state. This has implications for future therapies.

Meibomian gland dysfunction

Blepharitis is probably the most common disease entity seen in the general ophthalmologist's office. A significant proportion of these cases are secondary to meibomian gland disease. This review outlines our knowledge of the histopathology, lipid abnormalities and role of microorganisms in meibomian gland dysfunction. We will also review the physiology of meibomian gland secretion and present models of meibomian gland dysfunction which have enhanced our knowledge of this condition. The importance of diagnosing associated conditions such as aqueous tear deficiency, contact lens intolerance, rosacea, and seborrheic dermatitis is emphasized. Although this condition causes significant morbidity in the population, there are effective treatments available and these will be discussed.