A Fresh Look at Tear Film Structure and Dynamics

Abstract

PurposeTo determine the structure of the precorneal tear film from its behaviour during blinking and eye movements.MethodsThe tear film lipid layer (TFLL) and the fluorescein‐stained mucoaqueous subphase (MAS) were studied during spontaneous blinking and after vertical or horizontal saccades, using the DR‐1TM video‐interferometer and video‐biomicroscope, respectively.ResultsDuring horizontal saccades, the TFLL and MAS behave as a ‘fluid shell’, that moves with the cornea (Yokoi et al. Ocul Surf 2014). This cohesion is lost during blinking. In the downstroke, the TFLL is stripped from the MAS and compressed. In the upstroke it spreads, driven by a surface tension gradient, dragging with it the newly deposited MAS. As these layers spread, they are transiently dissociated and also, the MAS is dissociated from the glycocalyx. Cohesion is restored as the tear film stabilises before the next blink. When the tear film forms in the upstroke of the blink, the menisci draw fluid from the tear film until they are separated by a zone of meniscus‐induced thinning (MIT) – the “black line” seen in the fluorescein‐stained tear film. During pauses at the end of any saccade, this MIT is imprinted onto the MAS and made visible in the fluorescein‐stained tear film as dark arcs on return from horizontal saccades, and dark bands after vertical saccades. Their persistence in the blink interval, implies that the MAS has viscous, gel‐like properties.ConclusionsJust prior to each blink there is cohesion between the tear film and glycocalyx and between each layer of the tear film. Cohesion is broken transiently by blinking. Most of the water intervening between the gel‐like MAS and TFLL is drawn off into the menisci at the time of tear film formation.