Article

CLINICAL: DRY EYE

LAYERING THE TEAR FILM

THE TFOS DEWS II REPORT PRESENTS A NEW MODEL OF PRE-CORNEAL TEAR FILM

AS A kid, making a solar system model for a science project was a rite of passage. Styrofoam planets rotated around a centered, big, orange Styrofoam sun, and the number of planets was a constant: They started with Mercury and ended with Pluto, that is, until 2006, when Pluto was named a “dwarf planet.” It seems unthinkable to change a basic scientific fact taught for decades, but science and its discoveries evolve, and the science regarding the layers of the tear film has evolved too.

WHAT WE LEARNED

Although optometry schools have taught for years that the tear film has three layers, that may be less than accurate, per the Tear Film and Ocular Surface Society (TFOS) Dry Eye Workshop II report (DEWS II), which identifies a two-layered tear film model — the lipid layer and the mucoaqueous layer — as the more accurate description. And both of which have partial integration.

Healthy Tear Film Requirements

The report further recognizes the necessity of tear film stability and ocular surface wettability to produce a healthy tear film.

Without stability and wettability, patients may experience blur, loss of the tear’s homeostasis, exhibited by increase in osmolarity, and increase in tear evaporation. The imbalance of the tear film leads to the signs and symptoms of dry eye disease.

THE LIPID LAYER

This layer is provided by lid margin glands secreting meibum and, thus, plays a significant role in tear stabilization. Specifically, it spreads across the eye with each blink and is driven by surface-tension forces, making it likely responsible for resistance to evaporation.

While the lipid layer’s mechanism has not been entirely elucidated, it’s believed that surfactant molecules interact with the mucoaqueous interface, and lipophilic molecules interact with the air interface.

The lipid layer can be assessed clinically by interferometry techniques, which may be automated, as in the use of corneal topography, an ocular surface interferometer or placido disc.

The updated description of the lipid layer maintains some consistency with previous thoughts. For example, it’s the thinnest layer, measuring 15 nm to 157 nm thick (average 40 nm).

THE MUCOAQUEOUS LAYER

This layer was considered two interdependent, but distinct, layers, the aqueous (water) layer and the mucin layer. The TFOS DEWS II marries the layers, creating a new pre-corneal tear film model.

The apical surfaces of the corneal and conjunctival epithelia possess transmembrane mucins, or microplicae, to increase the surface adhesion tension of water, which helps the spread of the tear across the ocular surface.

The mucoaqueous layer contains a mix of proteins, peptides, oxygen, metabolites, electrolytes, immunoglobulins and phospholipids.

EFFECTS ON THE PRACTICE

Pluto’s designation as a “dwarf planet,” was big scientific news. This tear film model is big eye care news, as it requires we change how we provide dry eye disease education. Stay current, or risk falling behind. OM