Dry eye disease affects approximately 12 million people in the US. The integrity of the tear film lipid and aqueous layers is critical for proper tear film function and avoidance of dry eye. Wavelength-dependent optical interferometry has been used to simultaneously measure tear film lipid and aqueous layers thicknesses at a single point at the apex of the cornea over a period of time. Such measurements can be used to diagnose sub categories of dry eye from (a) either lipid or aqueous deficiencies, or (b) time-profile changes in lipid and aqueous layer thicknesses after a blink.
Accurate and precise determination of lipid and aqueous layer thicknesses are critical for proper diagnosis of dry eye sub categories. Previous lipid and aqueous layer thickness calculation methods using interferometry data were conducted separately. This was due to the large differences in layer thicknesses (aqueous: 1-5 microns, lipid: 20-120 nanometers), basing aqueous thickness calculations upon spectral interference oscillations and absence of spectral oscillations from the lipid layer.
The latter absence of spectral oscillations from the lipid layer presents a difficulty in calculating tear film lipid layer thickness. Thus, there is a need for improvement in the accurate and precise determination of the thickness of the lipid layer. This improvement would be beneficial to the physician, and would allow researchers to better design lubricant eye drops for dry eye patients with meibomian gland dysfunction (MGD). Improved measurement methods may also allow researchers to design more comfortable contact lens and lens care solutions for contact lens wearers. Contact lens wear adversely affects the tear film lipid layer, which is thought to be responsible for the thinner, less stable tear film and resulting dryness among contact lens wearers than non-lens wearers.
Lastly, there are no known non-invasive diagnostic methods in the art for measuring the refractive index of the corneal epithelium at the precise boundary of the aqueous layer and the anterior surface of the corneal epithelium, known as the corneal glycocalyx. Anterior corneal surface refractive index is believed to be a parametric measurement directly related to the structure of the glycocalyx. The glycocalyx is comprised of transmembrane mucins and the integrity of the corneal glycocalyx is believed to be required for tear film stability and prevention of dry eye. Having such a diagnostic tool would assist researchers in the diagnosis of dry eye and development of compositions which could assist or restore a patient's glycocalyx function. Simultaneous in vivo measurements of this interface, along with tear film aqueous and lipid layer measurements, all of which comprise the ocular environment, may prove to be of use in dry eye diagnosis or elucidating dry eye etiology or ocular effects of topically-applied solutions.