Natural fat in external eye. Vital-stained by Sudan III powder

Temporal variations in meibomian gland structure-A pilot study

PURPOSE

To investigate whether there is a measurable change in meibomian gland morphological characteristics over the course of a day (12 h) and over a month.

METHODS

The study enrolled 15 participants who attended a total of 11 study visits spanning a 5-week period. To assess diurnal changes in meibomian glands, seven visits were conducted on a single day, each 2 h apart. For monthly assessment, participants attended an additional visit at the same time of the day every week for three consecutive weeks. Meibography using the LipiView® II system was performed at each visit, and meibomian gland morphological parameters were calculated using custom semi-automated software. Specifically, six central glands were analysed for gland length ratio, gland width, gland area, gland intensity and gland tortuosity.

RESULTS

The average meibomian gland morphological metrics did not exhibit significant changes during the course of a day or over a month. Nonetheless, certain individual gland metrics demonstrated notable variation over time, both diurnally and monthly. Specifically, meibomian gland length ratio, area, width and tortuosity exhibited significant changes both diurnally and monthly when assessed on a gland-by-gland basis.

CONCLUSIONS

Meibomian glands demonstrated measurable structural change over short periods of time (hours and days). These results have implications for innovation in gland imaging and for developing precision monitoring of gland structure to assess meibomian gland health more accurately.

Evaluation of Meibomian gland structure and appearance after therapeutic Meibomian gland expression

CLINICAL RELEVANCE

Evaluating how Meibomian glands can change in appearance has the potential to advance the understanding of Meibomian gland health and may lead to enhanced diagnosis and therapy.

BACKGROUND

This work aimed to investigate Meibomian gland appearance after therapeutic Meibomian gland expression.

METHODS

Fifteen subjects attended three study visits over a two-week period. Meibography was performed before and after therapeutic Meibomian gland expression, the following day, and 2 weeks after expression. Six central glands were used to calculate Meibomian gland morphological parameters such as gland length ratio, gland width, gland area, gland tortuosity, and gland contrast. A custom semi-automated image analysis software was used to calculate Meibomian gland metrics. Furthermore, a high-resolution imaging system was developed to capture clear images of the Meibomian glands, free of any artefacts, which were used for precise calculations of Meibomian gland contrast.

RESULTS

The expression procedure had a significant impact on Meibomian gland contrast and length ratio immediately afterwards. The least square mean difference (95% CI) from baseline for Michelson contrast was -0.006 (-0.010, -0.001) and -1.048 (-2.063, -0.033) for simple contrast. The least square mean ratio of the gland length ratio immediately after the expression to baseline was 0.758 (0.618, 0.931).

CONCLUSIONS

Following therapeutic expression, Meibomian glands exhibit reduced brightness and length. However, within 24 h, they appear to recover and return to their baseline state, indicating a relatively short recovery time. This sheds light on whether meibography is solely focused on capturing gland structure or if it also captures acinar activity. The hyperreflective properties of lipids suggest that the decrease in contrast observed after expression could be attributed to a reduction in the visualisation of acini activity. A decrease in Meibomian gland length ratio implies that the loss of gland structure following treatment may be indicative of a temporary structural alteration.

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.

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.

Nanoscale phase dynamics of the normal tear film

The tear film is a dynamic multilayered structure. The interactions and the interfacial dynamics between the layers that occur during a blink cycle must be such that they allow for maintenance of a stable tear film. Attempts to understand these dynamics have been limited by the techniques and biomarkers used. Quantum dots (qdots) offer a new potential to monitor the dynamics of the tear film layers in vivo without the drawbacks of previously used methodologies. Indium phosphide-gallium qdots were used to differentially assess the dynamics of the lipid and aqueous layers of the tear film in real time. In the aqueous, qdots dispersed to form a stable local region that was swept away into the upper and lower menisci during a blink. They did not redisperse onto the ocular surface but were progressively removed from the menisci through the puncta. Some of these qdots adhered to the mucin layer on the ocular surface in a meshlike pattern and remained there for five to six blinks before they were removed. The organic qdots dispersed quickly but patchily over the whole outer surface of the tear film. They also strongly marked both eyelid margins and slowly dispersed onto the skin and eyelashes and not through the puncta. Some were trapped in the menisci as blobs that rolled along the meniscus. These data support the view of a distinct three-layered tear film: an inner mucin layer attached to the epithelial cells, a fluid aqueous layer, and an outer viscoelastic lipid layer.

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.

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.

The casual level of meibomian lipids in humans

Using a modified skin surface lipid measuring instrument, the Meibometer, the amounts of meibomian lipid on the lid margins (the casual levels) of 421 subjects aged 1 to 94 years were measured. The lowest levels were found in children younger than 14 years (means +/- S.E.: males = 1.48 +/- 0.17, females = 1.53 +/- 0.17 micrograms lipid/mm2 lid margin surface) and rose with age, the highest levels being found in males aged 60-69 years (mean +/- S.E. = 3.26 +/- 0.18 micrograms lipid/mm2 lid margin surface). Male levels were higher than females in the 20-29 years age group but male and female levels became indistinguishable above the age of 50. These age- and sex-related changes differed markedly from those seen in skin surface lipid levels. The age-related changes in the casual levels of lid lipids remain unexplained although an endocrinological mechanism modified by morphological factors is considered.

An instrument for quantifying meibomian lipid on the lid margin: the Meibometer

An instrument, the Meibometer, is described for estimating the casual level of meibomian lipid on the human eyelid margins, adapted from a commercially-available instrument used for measurement of skin surface lipid. A loop of plastic tape is pressed onto the everted lower lid margin to lift off a blot of lipid. The resultant change in light transmission of the tape is read by a photometer. Readings are not affected by side (R or L), time of day or lid surface temperature. After cleaning lipid from the lid margins with hexane, the rate of recovery per 10 blinks, as a percentage of the pre-cleaned level, was measured as 33.7 +/- 5.8 (mean +/- SE). This rate of delivery appears to provide enough lipid for complete resurfacing of the preocular tear film with every blink. Over short periods no detectable lipid was delivered in the absence of blinking.

Meibomian gland disease. Classification and grading of lid changes

In recent years attention has been paid to meibomian gland dysfunction (MGD) as a distinct clinical entity responsible for chronic symptoms and signs and occurring independently or in association with atopy, cicatrising mucosal disorders and rosacea. Attempts to correlate MGD with microbiological and lipid biochemical changes are confounded by the absence of a clear descriptive language for the disorder and its associated changes. Such a language is crucial for the conduct of cross-sectional and natural history studies and therapeutic clinical trials. We present a comprehensive classification and grading scheme of meibomian gland disease, supporting our observations with illustrations.

Foam in the external part of the eye

Foam in the external part of the eye has been measured at the slit-lamp on 274 normal subjects. Foam occurred in 78%, most frequently in the outer canthus, on the skin round this, and in the lateral half of the lid margin. The age group of 30-50 years predominated. The foam-covered areas averaged 1.1 mm2, the largest amount always being found laterally (0.93 mm2). The lateral amount of foam was maximal in the age group of 30-50 years, while the medial maximum was seen in the age group of 60-70 years. The incidence of foam was not correlated to the lipid layer on the precorneal film, expressibility of Meibomian secretion, scales on the lid margin, or greasy hair. A hypothesis has been advanced for the mainly lateral position of the foam: blinking whips up the foam, which possibly has sebum admixed.

Congenital deficiency of meibomian glands

A 16-year-old girl presented with contact lens intolerance. She was found to have a marked deficiency of meibomian glands in the upper lids and almost total absence in the lower lids. Evidence of tear film instability was found and attributed to deficient lid oil production. A daily wear soft contact lens was later fitted and tolerated.

Meibomian orifices and Marx's line. Studied by triple vital staining

The ciliary margins of the lower lids have been vital stained by the lipid-specific Sudan III powder, fluorescein 0.1% and the bottom of the lacrimal river (Marx's line) by lissamine green 1% in 100 cases. The Meibomian orifices are situated in a straight row just in front of the Marx's line in the lipid phase. With increasing age (greater than 50 years) the orifices are more often displaced and also discharge their lipid in the depth of the aqueous phase. The number averaged 21.5 in the lipid phase and 1.7 in the aqueous phase. Active orifices staining with lipid were found in 45% of all orifices in normals, independent of age, and were increased in conjunctivitis in the lipid phase. Lissamine green-stained orifices were independent of age, phase and diagnosis. The anterior edge of Marx's line may run an irregular course in elderly normals (greater than 50 years), significantly more often in conjunctivitis and blepharitis.