Tears nourish the cornea, provide lubrication and act as a refracting surface for light. The tear film lipid layer provides a hydrophobic barrier that stabilises the tear film and retards evaporation of the aqueous layer.1 Insufficient or inferior quality meibum leads to increased tear evaporation rate, the spilling of tears over the lid margin and a poorer refracting surface.2 Other functions of meibum include sealing the lid margins during sleep1 and providing a protective barrier against micro-organisms that may contaminate the eye.3
[CaptionComponent="1016"][CaptionComponent="1017"]The meibomian glands are located in the upper and lower tarsal plates of the eyelids. The median number of distinct glands in the upper eyelid is 31, while the number in the lower eyelid was found to be 26.4
A single meibomian gland consists of 10-15 spherical acini (between 150 and 200 microns in diameter) clustered around a central duct. The acini, which are filled with secretory meibocytes, are each connected to the central duct via a smaller ductule. The central duct spans the length of the gland and roughly the length of the tarsus, before joining with the excretory duct and forming the gland orifice.5
The delivery of meibum to the tear film is driven by constant secretion from the acini, building up a secretory pressure which then expels the meibum into the ductules and toward the gland orifice.6 Also, the muscle fibres of the orbicularis and the muscle of Riolan, act to compress the gland so that, upon blinking, a force of approximately 1.25g/mm2 is applied by the lids to the globe7 squeezing the meibum into the central duct and then towards the gland orifice, which lies just anterior to the mucocutaneous junction.6 The action of the eyelids then spreads the lipid across the ocular surface.
The excretory duct of the meibomian glands may be obstructed if there is an increase in viscosity of meibum and this, coupled with hyperkeratinisation8 of the epithelium of the duct, will lead to dilation of the ducts and acini. This process will eventually lead to gland atrophy, known as meibomian gland dropout.9 Following gland dropout, there will be a decrease in meibum available, leading to an increase in tear evaporation and tear film instability. The end results are an intensification of dry eye symptoms and inflammation which, in turn, will exacerbate the original problem and cause further gland dropout.10
Several factors contribute to meibomian gland dropout, including age11 and contact lens use, with the decrease in functional glands being proportional to the duration of lens wear.12 Also, patients suffering from conditions such as blepharitis, trachoma or Stevens Johnson syndrome were found to have significant meibomian gland dropout compared with normal patients.13
Meibography
The practice of examining the glands, meibography, is a useful tool, especially in the investigation of ‘non-obvious’ meibomian gland dysfunction or MGD,14 where there are few or no signs of inflammation (ie meibomitis). It provides a way to assess the efficacy of treatments as well as changes in gland morphology with the progression of MGD.
Meibomian glands can be visualised in a variety of ways including keratography,15 confocal microscopy,16 optical coherence tomography17 or using ultrasound.18 The glands may also be examined using a simple white light device such as the Welch Allyn transilluminator. Although devices such as these are widely available, they only allow two or three glands to be examined at a time19 so the process is time-consuming and uncomfortable for the patient. The light is passed through the eyelid in order for the practitioner to inspect the silhouette of the gland. Examining the glands in this manner requires skill and experience on the part of the practitioner11 as there may be poor contrast between the meibomian glands and the surrounding tissues.
[CaptionComponent="1018"][CaptionComponent="1019"]One way to increase the contrast is to use infrared (IR) light to transilluminate the glands. Meibum absorbs light in the infrared portion of the spectrum while the eyelid transmits it. The resulting image is X-ray-like, with the meibomian glands dark against the bright background of the eyelid.
Infrared transillumination requires an infrared sensitive camera to view the glands. Researchers have previously used IR film20 and video meibography with an infrared probe19 or ‘security cameras’ (an infrared sensitive camera with a ring of IR LEDs around the lens) to capture images of the glands.21
Working on the same principle of a transillumination device that is both easy and fast for the practitioner to use and comfortable for the patient, an IR meibography system was developed. The transillumination device (wand) design was based on a converted pen torch and the imaging system used was a modified digital camera. The camera was made infrared sensitive by removing the IR blocking glass plate from the front of the sensor and introducing a filter in front of the lens to minimise transmission of visible light.
This system images the lower eyelid only, which is more comfortable for the patient but can be used as indicator of dropout in the entire eye.22
During testing, it was found that skin pigmentation did not affect image quality, demonstrating that the transillumination system is suitable for use on subjects of all ethnicities.
Currently, slit-lamp biomicroscopy is conducted before fitting contact lenses and, as part of the examination, the practitioner looks at the meibomian gland orifices for evidence of MGD or posterior blepharitis. This check will miss those patients with non-obvious MGD14 or congenital gland deficiency.23
The transillumination system could be used in optometric practice to screen patients before contact lens fitting and to tailor lens choice. It can also be used during contact lens aftercare examinations to determine if lens wear is detrimentally affecting the meibomian glands and to allow a management plan to be constructed that would minimise future gland dropout.
Data on morphological changes of the meibomian glands throughout life could be collected if the IR transillumination system was made commercially available to optometrists working in practice. Photographs of the meibomian glands could be taken routinely at each eye examination and stored for future reference, like fundus images. This would also allow the impact of contact lens wear on the meibomian glands to be investigated as images could be obtained of the glands before wear and at each aftercare appointment.
The system may also be used in a hospital setting to grade severity of gland dropout and help in both the diagnosis of meibomian gland dysfunction and its treatment. A grading scale, such as the one developed by Pult,24 could be used by optometrists to quantify the degree of drop-out during eye examinations, contact lens fittings and contact lens aftercares. The speed of data collection and its non-invasive nature make it ideal for screening.
It may also be possible to tell the difference between meibomian glands missing through inflammatory processes and those that have never fully developed. Glands that were once patent but now contain little or no meibum have a faint, ghost-like appearance (Figure 3) while areas of glands that simply have not developed would lack such structures.
[CaptionComponent="1020"]In summary, the ease of use of this system and its ability to capture images of the glands would give a dry eye specialist or contact lens clinician a better understanding of their patients’ comfort. It would also be an invaluable tool for patient education and may increase compliance in those with evaporative dry eye.
This technique was developed at Glasgow Caledonian University under the guidance of Dr Ian Pearce.
References
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Michelle Snowball is is an optometrist practising in Scotland