Abstract
Purpose: :
It is believed that a compositional change in meibomian lipids will cause a change to the physical characteristics of the lipid layer of the tear film and this may be a cause of dry eye. Therefore, the physical properties and biochemical composition of lipids collected from dry eye patients have been compared.
Methods: :
Patients (N=20) identified as having dry eye were recruited and gave informed consent in accordance with the Tenets of Helsinki. Lipids, soft squeezed from their meibomian glands, were dissolved in chloroform and analysed for their physical characteristics using a spread film on a Langmuir trough. Targeted triple quadruple precursor ion and neutral loss mass spectrometry scans were used to identify the majority of lipid molecules in the same samples. Quantification of six lipid classes for a total of 129 species, including omega-acyl hydroxyl fatty acids (OAHFA), cholesterol esters (CE), wax esters (WE), triacylglycerides (TAG), phosphatidylcholines and sphingomyelins was performed by comparison to internal synthetic standards.
Results: :
Pressure area isocycles of spread films showed two distinct patterns: lipid films that had little hysteresis, and films that showed distinct hysteresis. No equivalent biphasic pattern of lipid profiles was observed. Lipid profiles showed a dominance of long-chain CE (~74%), and WE (~23%), with less amounts of OAHFA (~5%), TAG (~2%) and phospholipids (<0.1%).
Conclusions: :
Variations of the pressure-area isotherms of meibomian lipids from dry eye patients do not correlate with major lipid class profiles. This indicates that either a specific lipid species or other type of molecular species e.g. a protein, is responsible for the variation. This variation could also occur from the collection technique because hard squeezed meibomian lipids tend to show larger hysteresis in pressure area isotherms. Consistent with the findings of others, OAHFAs are the most abundant surfactant molecule in meibomian lipids, even in dry eye patients and phospholipids, if present, are at very low levels.
Keywords: lipids • cornea: tears/tear film/dry eye • cornea: basic science