This study confirms the presence of several lipid classes and species in the FR during SL wear and suggests that nonpolar hydrophobic lipids, specifically WEs, CEs, and TAGs, contribute to MDF. The additional detection of lipids common to cell membranes—namely PC, PE, and SM—suggests that cellular debris exists in the FR independent of MDF severity.
There is no universal method for quantifying MDF, although other techniques have been reported. Carracedo et al.
34 measured particles per mm
2 from OCT images, rather than net gray value as was done here, and found that FR turbidity increased by a factor of 8 after 6 to 9 hours of SL wear. Schornack and Nau
33 measured Scheimpflug tomography images and found a doubling of optical density from 5% to 10% after 2 hours of SL wear. Neither of these studies assessed the FR composition or specified clinically significant MDF, but both are consistent with the findings here that show a range of FR turbidity after several hours of SL wear. The method used to quantify MDF in this study was shown to be highly repeatable both within and between observers and illustrates that MDF is not binary but rather exists as a continuous variable of severity but always a nonzero value. Using a quantitative scale, rather than the more common strategy of categorical assignment,
51,52,54,75 allows more precision when grading MDF and could be useful in future studies. However, as seen here, it is also helpful to group participants into MDF and non-MDF for some analyses, in which case determining a cutoff may be appropriate.
Lipids across five major lipid categories were detected in the FR samples. The WEs, a class of nonpolar fatty acyl lipids that are primarily secreted by the Meibomian glands,
41,44,76–78 showed the greatest correlations to MDF. These normally comprise between 13% and 68% of total lipids in the natural tear film,
41,42,76,79 are characterized by long fatty acid and fatty alcohol chains, and are known for their hydrophobic properties.
43–47,78,80–84 In their pure state, they have melting points above physiologic temperatures that increase with each additional carbon atom,
71,85 which could explain why longer chain WEs (i.e., >40 carbons) were most significantly increased in MDF. In vivo, the WEs interact with other lipids (i.e., CEs) to lower their melting point and facilitate transparency,
70,71 a feature that may be altered when a SL is applied to the eye with aqueous-based (e.g., saline) solutions.
Several species of CEs, which comprise 8% to 39% of the total natural tear lipids,
41,42,76,79 were elevated in MDF. Like WEs, they are nonpolar, hydrophobic, and primarily secreted by Meibomian glands.
76,77,86 They have a tendency to change phase in aqueous solution,
44 existing as a solid, a liquid, or an intermediate crystalline state, the latter of which they appear turbid rather than clear.
69 This state can be affected by temperature, pressure, and interactions with other lipids and surrounding fluid,
69,71 so it is plausible that the physical characteristics of the FR change the state of CEs in the FR, contributing to the turbidity observed during MDF.
The TAGs, which are typically the third most abundant nonpolar lipid in the tear film (after WEs and CEs
87,88), comprising between 0.05% and 6% of total tear lipids,
41,42,76,79 were increased in MDF and specifically in the unsaturated form, consistent with the findings of mostly unsaturated TAGs in the natural tear film.
76,89 Similar to WEs and CEs, although not as abundant and less understood functionally, the TAGs are believed to contribute to the hydrophobic properties and reduce evaporative loss of the tear film.
44,88 Collectively, the correlations of these three classes of hydrophobic nonpolar lipids to MDF provide logical evidence of an alteration in physical form, or dissociation of nonpolar lipids, that creates the turbidity in the aqueous FR observed in MDF.
Polar lipids are far less abundant than nonpolar lipids in the natural tear film, ranging from <1% to 16% of total tear film lipids.
41,42,76,77,90–92 Their proposed functionality is to act as an interface between nonpolar lipids and aqueous tears,
74,83,90,93 helping to maintain the structure of the tear film.
76,79 Although no polar lipids were correlated to MDF, they were present in a greater proportion than expected. For example, PC lipids, which comprise about 0.015% to 0.2% of total natural tear lipids,
44,77,89,90 were 50% to 90% of total lipids detected in the FR. The PE and SM have likewise been found in variable low concentrations or not at all in natural tears and meibum
87,89,90,92,94 but collectively comprised over 10% of the lipids in the FR. The most likely origin of these polar lipids is from cellular debris, as these are common lipids found in cell membranes.
80,95 Possible cellular origins are corneal epithelial or perilimbal conjunctival cells, as well as immune cells, which are abundant in the tear film in the morning
96–101 and could become trapped when the SL is applied. Since polar lipids are by nature hydrophilic, they likely behave more miscibly with the aqueous FR and are not found to contribute to MDF. There was, however, a trend toward greater SM in MDF that could indicate that there is some aspect of cellular debris that could contribute to MDF, which future studies with larger sample sizes could investigate.
The findings of this study can inform the clinical management of MDF. The Meibomian glands produce most of the lipids in the tears, specifically the nonpolar WEs, CEs, and TAGs that were increased in MDF, so it is important to consider them specifically. Many ocular surface diseases—namely, dry eye diseases such as Meibomian gland dysfunction (MGD)—have altered lipid composition.
86,89,102–106 Anecdotally, clinicians have linked the occurrence of MDF to patients with dry eye and MGD, and several studies have found alterations in WE and CE lipids in patients with MGD.
86,104,107 This association could suggest that patients with MGD may have a lipid profile that is susceptible to developing MDF. One limitation of this study is that lipid profiles were not evaluated prior to any SL wear, which could be done in future studies to understand if people with altered proportions of select lipids are more susceptible to MDF. Clinically, MGD and other eyelid inflammatory conditions (e.g., blepharitis) should be evaluated and treated accordingly in patients experiencing MDF with SL wear.
There are no strong evidence-based guidelines for managing the SL fit in MDF; however, practitioners will often make several modifications to minimize MDF. Postnikoff et al.
51 found that greater lens vault was a risk factor for MDF, but the impact of reducing vault or any other SL parameters (i.e., diameter, limbal curves) has yet to be explored in MDF. The SLs in this study were fit similarly on each eye; therefore, no conclusions can be drawn on whether lens modifications could reduce MDF, although there was a greater proportion of eyes with excessive limbal clearance in the MDF group (57%) compared to the non-MDF group (11%). The impact of tear exchange, although it is expected to be minimal,
35,48,49,108 was not explored here but could certainly impact the FR composition and would be a meaningful future investigation.
Another approach that practitioners employ to reduce MDF is changing the lens application solution,
28,109,110 often to a more viscous solution (i.e., preservative-free artificial tears), although this has not been studied. The nonpolar nature of the lipids correlated to MDF suggests that using an application solution that is more lipophilic (e.g., artificial tear solutions) could reduce the precipitation of nonpolar lipids into the FR. Current SL application solutions approved by the Food and Drug Administration are all aqueous saline-based solutions, so future studies may consider evaluating the use of preservative-free formulations such as artificial tears with nonpolar components as application solutions to reduce MDF during SL wear.
To our knowledge, this is the first study to analyze lipids in the FR during SL wear and can be used to improve future study designs. This was a pilot study with a small sample size, which should be expanded in future studies to include more subjects with a broader range of MDF, perhaps with a deeper investigation to some of the nonpolar lipids found here. Future studies could also attempt to differentiate MDF beyond level of grayscale turbidity, as the clinical presentation of MDF can vary in appearance, sometimes looking more milky and white, other times having a darker, browner hue.
109 Studies could also consider additional factors that could contribute to MDF, such as differences in participant behavior when applying their lenses (e.g., amount of force applied, whether lid interaction occurred), as this was not specifically controlled here.
In addition to lipids in the FR, soluble proteins, metabolites, and cells in the fluid could contribute to MDF. We previously evaluated cytokines in the FR
32 but not specifically in MDF, and leukocytes have been detected in both MDF and non-MDF,
51 but additional proteins or further specification of the cell types in the FR are lacking. We have collected MDF samples that indicate sloughed corneal epithelial cells are also present in the FR (unpublished data), but it is unclear if they contribute to MDF. Future studies should evaluate these additional components of the FR that could be contributory to MDF and assess interventions such as SL modifications or application solutions to determine the best way to mitigate MDF during SL wear.