Tear Film Lipid Layer Thickness: Measurement Techniques, Normative Values and Alteration in Ocular Surface Diseases.

Purpose: Tear film instability is one of the principal pathomechanisms of dry eye disease (DED), arising from disrupted tear homeostasis, reduced tear volume, or lipid layer alterations. The lipid layer, secreted by the meibomian glands, can be imaged qualitatively and quantitatively. This review summarizes different lipid layer thickness (LLT) measurement techniques, normative values, and alterations in ocular surface disorders.

Method: Full-texts of 88 of 236 searched articles from PubMed, ScienceDirect, and Google Scholar using the terms 'Tear film lipid layer thickness,' 'Lipid layer thickness', 'Tear film lipids,' 'Dry eye disease,' 'Meibomian gland dysfunction,' 'Meibography,' and 'Interferometry' were reviewed.

Result: Advanced imaging techniques, such as LipiView, Oculus Keratograph 5M, and Tearscope, utilize interference principles, but are not interchangeable for measuring LLT. LLT ranges from 13 to 170 nm in healthy individuals, depending on the instrument and grading scale used. The reported central LLT values in DED vary from 33 to 84 nm, varying with disease severity. There are conflicting reports on the association between age, gender, and LLT values. LLT positively correlates with expressible meibomian glands, and negatively with gland loss. LLT has been associated with tear film breakup time, MGD severity, and blink pattern irregularities.

Conclusion: The significant overlap in LLT values between the normal individuals and DED patients precludes its role in the diagnosis of DED. Future studies exploring LLT variations under different environmental conditions and across larger cohorts with varying DED severity are essential to enhance clinical applicability.