How ordered are our tear lipids? For that matter, what does it mean for a tear layer to be ordered, and why does it matter? New research published in the journal Cornea explains that “ordered” lipids are like butter. Their hydrocarbon chains are packed tightly together. When lipids are disordered, they’re like olive oil: the hydrocarbon chain packing becomes less tight. Evidence suggests that more ordered lipids can create conditions that contribute to deteriorated spreading and decreased surface elasticity.

Researchers speculate that a more ordered lipid layer will lower its ability to restore the tear film lipid layer structure between blinks. Too much lipid order may also keep meibum from flowing out of the meibomian glands. This matters in particular for patients who’ve undergone hematopoietic stem cell transplants, since those patients are at an elevated risk of developing ocular surface discomfort and dry eye disease. In fact, this study shows that tear lipids are more ordered than meibum lipids, and that could have functional consequences.

The researchers measured lipid composition and structure from both tear and meibum lipids of 44 stem cell transplant patients, including those with and without meibomian gland dysfunction. Various spectroscopy tests uncovered differences in the quality of lipids from tears vs. meibum. Phospholipids and fatty acids present in tears, “which have surfactant properties, may contribute significantly toward the structure and function of the tear film lipid layer at the tear interface region,” the authors wrote.

A more ordered lipid layer may be a marker of or contribute to an unstable tear film layer, the investigators concluded, by contributing to the formation of a discontinuous, patchy tear film, which in turn results in deteriorated spreading.

Borchman D, Ramakrishnan V, Henry C, Ramasubramanian A. Differences in Meibum and Tear Lipid Composition and Conformation. Cornea. August 2, 2019. [Epub ahead of print].