How the most common preservative affects the Meibomian lipid layer

PURPOSE Effects of preservatives like benzalkonium chloride on the tear film are often hypothetized. With regard to the Meibomian lipid layer only few data are available. In order to receive precise information about the effect of benzalkonium chloride on the Meibomian lipid layer we performed in vitro experiments using a direct optical control of the films. METHODS Meibomian gland secretion was spread on a water subphase containing benzalkonium chloride in concentrations of 0.001%, 0.005%, 0.01%, 0.05%, and 0.1%, respectively. This aqueous subphase was chosen in analogy to the aqueous tear film layer. In our experimental assay, the pH was 5.5, the temperature was 20 degrees C. The spreading process and the quality of the films were controlled by the Brewster Angle Microscope. RESULTS Benzalkonium chloride was well tolerated up to concentrations of 0.005%. Concentrations > 0.005% affected the spreading process forming domains, and instable films. CONCLUSIONS. Thus it could be demonstrated that there is a critical concentration of benzalkonium chloride which destroys the spreading and alters the morphology of the Meibomian lipid layer. These results were confirmed by a modified method injecting benzalkonium chloride in the subphase after spreading the Meibomian lipid layer. Therefore eye drops with benzalkonium chloride in concentrations above 0.005% should be abandoned in clinical therapy.