Purpose: Instability of the tear film with rapid tear break-up time is a common feature of aqueous-deficient and evaporative dry eye diseases, suggesting that there may be a shared structural abnormality of the tear film that is responsible for the instability. It may be that a change in the normal meibum lipid composition and conformation causes this abnormality. Principle component analyses of infrared spectra of human meibum indicate that human meibum collected from normal donors (Mn) is less ordered than meibum from donors with meibomian gland dysfunction (Md). In this study the conformation of Md was quantified to test this finding.
Methods: Changes in lipid conformation with temperature were measured by infrared spectroscopy. There were two phases to our study. In phase 1, the phase transitions of human samples, Mn and Md, were measured. In phase 2, the phase transitions of model lipid standards composed of different waxes and cholesterol esters were measured.
Results: The phase-transition temperature was significantly higher (4°C) for the Md compared with the Mn of age-matched donors with no history of dry-eye symptoms. Most (82%) of the phase-transition temperatures measured for Md were above the values for Mn. The small change in the transition temperature was amplified in the average lipid order (stiffness) at 33.4°C. The average lipid order at 33.4°C for Md was significantly higher (30%, P = 0.004) than for Mn. The strength of lipid-lipid interactions was 72% higher for Md than for Mn. The ability of one lipid to influence the melting of adjacent lipids is termed cooperativity. There were no significant differences between Mn and Md in phase-transition cooperativity, nor was there a difference between Mn and Md in the minimum order or maximum order that Mn and Md achieved at very low and very high temperatures, respectively. The model wax studies showed that the phase transition of complex mixtures of natural lipids was set by the level of unsaturation. A double bond decreased the phase-transition temperature by approximately 40°C. The addition of a second CH CH moiety decreased the phase-transition temperature by approximately 19°C. Unsaturated waxes were miscible with saturated waxes. When a saturated wax was mixed with an unsaturated one, the saturated wax disproportionately increased the phase transition of the mixture by approximately 30°C compared with the saturated wax alone. Cholesterol ester had little effect on the phase-transition temperature of the waxes. Model studies indicated that changes in the amount of lipid saturation, rather than the amount of cholesterol esters, could be a factor in the observed conformational changes.
Conclusions: Meibum lipid compositional changes with meibomian gland dysfunction reflect changes in hydrocarbon chain conformation and lipid-lipid interaction strength. Spectroscopic techniques are useful in studying the lipid-lipid interactions and conformation of lipid from individual patients. (ClinicalTrials.gov number, NCT00803452.).