The lipid layer of the preocular tear film: physiology and pathology

The preocular tear film (POTF) is composed of a deep aqueous-mucin phase th at supports a thin superficial lipid phase. The tear lipid layer (TILL), al though thin, stabilizes the POTF providing a 25% surface-tension decrease a nd a 90-95% aqueous evaporation reduction. TILL is formed from lipids secre ted by tarsal meibomian glands and spread onto the ocular surface by blinki ng. The TILL itself is composed of two phases. A thin and deep polar phase, adjacent to the aqueous-mucin layer, has a surfactant role. A thicker and superficial nonpolar phase has antievaporative properties. At the same time , tear lipocalins help the TLL spread and stabilize the lipid-aqueous inter face. For clinical examination, TILL is directly observed with the Tearscop e. POTF stability and the evaporation rate depend on the lipid layer patter n. When chronic, POTF qualitative trouble (evaporative syndrome) due to a T ILL anomaly, leads to secondary ocular surface impairment with increased te ar instability and self-propagation of ocular dryness. Meibomian gland dysf unction (MGD) results from local pathology, dermatologic disease (ocular ro sacea) or iatrogenic etiology. Cosmetic use is the other principal cause of TILL destabilization. Lid hygiene is the mainstay of MGD treatment. System ic antibiotics (cyclins) can be associated in cases of severe symptoms. Top ical treatment is useful if there is marginal lid inflammation or infection .