Assessing and Diagnosing Meibomian Gland Dysfunction
BY ERIC PAPAS, PHD, MCOPTOM, DIPCL
In the fight against dry eye, preventing tear film evaporation appears to be a critically important factor. Higher rates of evaporative loss are a feature of those who have dry eye disease (Mathers, 2004) as well as of contact lens wearers (Guillon and Maissa, 2008). The eye’s mechanism for minimizing water loss from the tear film is to create a lipid layer on its anterior surface. As this is derived from the meibomian glands, it follows that interference with the functioning of these structures may have clinical effects. Accordingly, the ability to detect and assess meibomian gland dysfunction (MGD) is an important clinical skill.
A range of tests have utility in diagnosing MGD (Foulks and Bron, 2003). Several, such as evaporimetry and chemical analysis of meibum, sit firmly in the research domain. Others, such as osmolality, are increasingly practical for in-office use thanks to more affordable and convenient instrumentation. Before resorting to these, however, clinical assessment is based on the simple procedures of taking a comprehensive history followed by a thorough biomicroscopic examination of the ocular surface and eyelids.
Positioned in a row along the lid margin, the ductal openings of the meibomian glands should be readily visible in young adults. Count the total number of ductal openings within the central 1cm portion of the lid. Obstruction to glandular secretions can appear as plugging, pouting, or narrowing of the orifices. Record the proportion of openings that are affected in this way.
Viewing the glands requires lid eversion and is aided by meibography. Originally developed as a research tool, it is now accessible to general clinicians thanks to advances in consulting room technology. Essentially, the eyelid is everted over a fiber optic transilluminator that renders the meibomian gland ducts and acini visible to the naked eye (Jester et al, 1982). Recent refinements eliminate the need for transillumination by utilizing infrared filters and imaging devices (Arita et al, 2008). This has minimized the invasiveness of the procedure.
You can view both upper and lower lids this way and store the images obtained for either record keeping or later evaluation.
Meibography’s chief advantage is the ease with which it allows you to see areas of dropout. To quantify dropout, you can count the number of whole glands or assign a score based on the proportion of the eyelid affected (Nichols et al, 2005). This latter quantity is the meiboscore (Arita et al, 2008) and is usually expressed on a 4-point scale: grade 1, no partial glands; grade 2, less than 25 percent of the image shows partial meibomian glands; grade 3, between 25 percent and 75 percent partial glands; grade 4, more than 75 percent of the image shows partial meibomian glands. Other grading definitions and increments have also been suggested. Add the values from both the upper and lower lids to produce an overall score for any particular eye.
A significant correlation between meiboscore and age has been found using these methods (Arita et al, 2008), and both GP and soft lens wearers have significantly higher scores compared to non-wearers (Arita et al, 2009). Thus, we can expect more extensive gland loss in older individuals as well as in contact lens wearers, both groups in which dryness symptoms are more prevalent.
Early evidence from studies using confocal microscopy suggests that gland morphology may also be more subtly altered in that the density of acini appears reduced in circumstances in which the meiboscore is raised (Ibrahim et al, 2010).
Once you have evaluated aspects of gland morphology, consider their functionality. This means establishing how well the oils produced in the acini are released from the gland orifices. Perhaps the least invasive way to do this is to observe the interference colors created by the precorneal lipid layer. Various authors have indicated that the thickness of this structure can be deduced from the colors or patterns observed (Guillon, 1998; Yokoi et al, 1996).
The method of meibometry permits a direct look at the oil level, and to some extent its distribution, on the eyelid. A loop of plastic tape is pressed against the lid so that oil transferred from the eyelid forms a band across its width. This band becomes more or less translucent depending on the amount of oil associated with the lid margin. You can read this by placing the tape in a densitometer.
This method has demonstrated reductions in the casual, or habitual, oil level of MGD patients relative to normals (Yokoi et al, 1999). The need for specialist equipment to read the tape reduces the technique’s appeal in most clinical situations, however.
Undoubtedly the most widely used strategy is gland expressibility. Here, digital pressure is applied to either the upper or lower lids with the aim of persuading the meibomian glands to expel a portion of their contents. The ease with which this occurs can be graded: grade 0, clear meibum is easily expressed; grade 1, cloudy meibum is expressed with mild pressure; grade 2, cloudy meibum is expressed with more than moderate pressure; and grade 3, meibum cannot be expressed even with hard pressure (Shimazaki et al, 1995). The viscosity of the expressed product also can be graded: grade 1, clear; grade 2, slightly viscous; grade 3, viscous and partly opaque; and grade 4, very thick and opaque (Mathers et al, 1996).
Clearly the higher grades indicate more severe problems, but practical details have been recognized that may affect the evaluation process. Considerable subjectivity is involved in determining what constitutes mild, as opposed to moderate or hard, pressure to the eyelid. The view on what this entails varies, not only from clinician to clinician, but probably from day to day for the same individual. Efforts have been made to remedy this situation using a device designed to apply a fixed force to the lids (Korb and Blackie, 2008). As a result, refinements have been proposed to the definition of what constitutes an expressible meibomian gland. The current proposal is that a gland be considered expressible if it produces a liquid secretion under the influence of a standard force (1.25gm/mm2) applied for a standard time of between 10 and 15 seconds.
Using these guidelines, variations in expressibility with position along the lid margin have been established so that the bulk of the secretion is contributed by the central and nasal glands, with the temporal region being relatively inactive. This finding has been interpreted to suggest that greater consistency in evaluation can be achieved by attending to only the central portion of the eyelid (McCann et al, 2009).
Lower numbers of expressible glands in an individual were also associated with more severe symptom scores, although this relationship was fairly weak and requires further confirmation.
The Most Effective?
Of course, what clinicians really would like to know is which of this array of tests best distinguishes between normals and those who have MGD. Some attempts have been made to simplify the diagnostic criteria. The results suggest that there is some merit in all if the methods. Practically speaking, however, the specialist nature of equipment, or evaluation skills required, reduces the utility of many of these for the general clinical setting. Bearing this is mind, an assessment of lower lid gland dropout may be the most appropriate single test at present (McCann et al, 2009).
Additional confidence in the diagnosis is provided by the presence of either significant symptoms or lid margin abnormalities. Examples of the former are ocular fatigue, discharge, foreign body sensation, dryness, stickiness, pain, epiphora, itching, redness, glare, excessive blinking or history of chalazion or hordeolum; and of the latter are irregularity, vascular engorgement, plugged meibomian gland orifices, and anterior or posterior replacement of the mucocutaneous junction. (Arita et al, 2008). CLS
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Associate Professor Papas is executive director of Research & Development and director of Post Graduate Studies, Brien Holden Vision Institute and Vision Cooperative Research Centre, and Senior Visiting Fellow, School of Optometry & Vision Science, University of New South Wales, Sydney, Australia. He has received research funds from Ciba Vision, Alcon, AMO, and Allergan.