discovering dry eye

Assessing the Lipid Layer of the Tears

BY BARBARA A. FINK, OD, FAAO
April 2001

There are multiple causes of the signs and symptoms of dry eye. Use Farris's mnemonic "LAMBS" to review the elements of the tear system that may be causing dry eye problems. In this mnemonic, "L" stands for lipid abnormality, "A" represents aqueous abnormality, "M" is mucin abnormality, "B" stands for base abnormality (epitheliopathy), and "S" is for surfacing abnormality (eyelid problems or problems resulting from exposure or non-wetting of the ocular surface).  Abnormalities in one or more of these elements can result in the signs and symptoms of dry eye for a given patient.

Dry Eye Testing

Tests that assess the structure or function of the meibomian glands or the lipid layer include: slit lamp examination of the meibomian glands and tears, meibography, tear evaporimetry, assessment of tear viscosity and interferometric measurement of lipid layer thickness.

During the slit lamp examination of the meibomian glands, evaluate the lid margin, mucocutaneous junction, orifices and secretions expressed. Meibography involves photographing or videotaping the meibomian glands at a photo slit lamp, using high-speed infrared film, while the lid is everted over a transilluminator probe. Meibomian gland size, number and morphology can then be evaluated.

Tear evaporimetry involves installing humidity and temperature sensors inside the chambers of goggles and fitting the goggles to the eyes of the patient so that the goggles are well sealed against the face. The humidity is then sampled for a predetermined amount of time with the eyes open. The increase in relative humidity is due to water evaporation from tears. Tear evaporation is increased for patients with meibomian gland dysfunction. It can be reduced for patients who have dry eye due to reduction in aqueous production.

Tear Viscosity

Tear viscosity is the property of resistance to flow, and it increases with increases in protein and lipid content. Subjectively grade tear viscosity at the slit lamp biomicroscope by watching the rates of particle movement on the surface and deep within the lower tear prism. Tear viscosity grading can be made from one to five. Grade 1 demonstrates watery tears, with no detectable drag of surface particles and very rapid flow at all levels of the tear prism. In Grade 3, there is normal, easily detectable flow of surface and deep particles within the prism, with the surface particles showing readily detectable drag. Grade 5 indicates very viscous tears, with no detectable flow of tears within the lower tear prism toward the puncta; it is a very oily tear film with much debris. Tear viscosity can be quantified by comparing the time for tear samples vs. distilled water to travel through a precision capillary tube.

Three types of interference methods measure the thickness of different layers of the tear film: wavelength-dependent fringes (WDFs), thickness-dependent fringes (TDFs) and angle-dependent fringes (ADFs). WDFs can accurately measure both the lipid layer and the tear film thicknesses at a single location. TDFs record the two-dimensional distribution of tear thickness over the surface of the cornea. ADFs are best used for studying relatively thick tear films.

Dr. Fink is an associate professor at The Ohio State University and a principal investigator for the OSU CLEK clinic.