TEAR ANALYSIS

And Its Significance In Contact Lens Fitting

BY HILMAR BUSSACKER, OPTOMETRIST
MAR. 1997

The quality of tears influences the compatibility of contact lenses. Here we discuss the various methods you can use to analyze and evaluate tears prior to the first contact lens fit.

To maintain a healthy cornea, we must ensure that contact lens materials have adequate oxygen transmissibility (a Dk value of greater than 150) and that the tear exchange under the lens is sufficient to supply the cornea with the necessary oxygen.

Proper tear exchange depends upon the quantity and composition of tears, the lens-to-cornea fitting relationship and the wettability of the contact lens material. On the eye, different patients' tears behave differently on the same contact lens materials due to the complex composition of each individual's tears.

There are some similarities among contact lens materials. Hydrogels, for example, have good wettability, but they tend to absorb certain components of the tears, disrupting the original good surface quality. Rigid contact lens materials contain siloxane for better oxygen transmissibility, but the more siloxane they contain, the more lipophilic and, therefore, hydrophobic they become. To counteract this, fluorocarbon is added to the material. Yet, fluorocarbon also diminishes the the elasticity modulus. Therefore, determining the quantity and composition of tears prior to the contact lens fitting is vital for choosing the appropriate size, material and wear schedule of the lenses you prescribe.

DETERMINING TEAR QUANTITY

The best way to determine tear quantity is to measure the tear meniscus. Do this at the beginning of the examination because the different actions of an examination, such as glare, visual stress or other manipulations, can influence results.

Determine size, form and extension of the tear meniscus -- the tear quantity perceived on the lower lid margin -- by using a slit lamp microscope with a horizontal narrow slit and magnification of 10x to 20x. Perform this test at different times of the day, both without auxiliary devices and with fluorescein (Fig. 1).

FIG. 1: NORMAL TEAR QUANTITY. TO DETERMINE TEAR QUANTITY, JUDGE FORM AND EXTENSION OF TEAR MENISCUS ON THE LOWER LID MARGIN.

Other procedures, such as the Schirmer test, the cotton thread test and the tear breakup time test (TBUT), do not give adequate information about tear quantity. The first two tests cause secretion impulses that falsify results, and the TBUT gives evidence about the superficial tension of the tear film only. Deficient tear quantity may contraindicate hydrogel lenses.

DETERMINING TEAR COMPOSITION

Tears are a complex mixture of saccharides, proteins, lipids and salts in a clear, watery fluid. Proteins are prone to deposit on a hydrogel contact lens. Lipid deposits on a rigid gas permeable lens cause the surface to be hydrophobic and, therefore, reduce the wettability. A deficiency of mucins, which are molecules of carbohydrate and protein, results in reduced viscosity of the tear film and wettability problems. A slit lamp and a dark field microscope will help you determine lipid concentration, viscosity and mucin content.

Most graphics in the literature do not represent the true structure of tear film. I believe the best illustration of tear film structure is the one by Royer as shown in Figure 2.

FIG. 2: ROYER'S INTERPRETATION OF TEAR FILM STRUCTURE.

LIPID CONCENTRATION

Examination of the tear film with the vertical slit of the biomicroscope set at about 10x magnification shows if the tear film is more lipidic than usual (Fig. 3). You can recognize lipid drops with both direct and indirect focal illumination. If the drops have already formed smears, inspect the lid margins further. Often, increased lipid production by the meibomian glands causes lipid drops to form at the drainage canals on the lid margin. When the patient blinks, the upper lid margin dips into these lipid drops on the lower lid margin and draws them up where they remain on the cornea as microscopic pearls. When they finally burst, they build a lipid layer that penetrates the tear film.

FIG. 3: SLIT LAMP VIEW (10X) OF LIPIDIC TEAR FILM. NOTE THE INTERFERENCE PATTERN (MCDONALD PHENOMENON) IN THE REFLEX PICTURE OF THE BULB'S GLOW.

With sufficient intensive illumination, we can observe interference phenomena on the tear surface when the lipid layer thickens. With suitable illumination and magnification of 10x, we can see interference patterns in this area when lipid production increases. The picture becomes more detailed with a higher magnification.

TEAR VISCOSITY

To view tear film viscosity, use slit lamp magnification of 10x with the vertical slit. If you observe compression when the upper lid closes and stretching when it opens, then viscosity is good. If you don't see this phenomenon, you should assume reduced viscosity and evaluate the mucin content of the tears.

MUCIN CONTENT INFLUENCES WETTABILITY

The amount of mucin in tears influences lens wettability, particularly on the back surface of the lens. The tear liquid contains mucins, which are produced by the goblet cells of the conjunctiva and also by the epithelial cells. Dilly (1985) and Brewitt (1988) proved how the endoplasmic reticulum and the Golgi apparatus of corneal epithelial cells produce vesicles of mucopolysaccharides and keratosulfates. The vesicles reach the cell surface where they ensure wettability, and are probably also responsible for the production of microvilli on the cell surface. These microvilli allow wetting particles to adhere to the surface of epithelial cells, enhancing wettability (Fig. 4). With reduced cell activity, neither vesicles nor microvilli are produced, and the cell shrinks.

FIG. 4: FORMATION OF MICROVILLI DUE TO BURSTED VESICLES IN THE CORNEAL EPITHELIAL CELL RELEASING KERATOSULFATES (BREWITT).

THE TEAR FERN TEST

The tear ferning test gives good information about the mucin content of a dried tear drop (Fig. 5).

FIG. 5: THE TEAR FERN TEST PRODUCES A FERN-SHAPED CRYSTALLIZATION OF A DRIED TEAR DROP.

This technique is based on the phenomenon that mucins produce a fern pattern in watery fluid when drying, provided that a nucleus of a crystal (e.g., sodium or potassium salt) is present. For this test, use the slit lamp and a micropipette to take tears from the lower lid. The tip of the pipette must be well-rounded to minimize contact stimulation (Fig. 6). Use a stereomicroscope with dark field illumination for this examination.

FIG. 6: USE A MICROPIPETTE TO TAKE TEARS FROM THE LOWER LID FOR THE TEAR FERN TEST.

We used fern pattern testing to measure the wettability of contact lenses worn by different patients. Our tests showed that there were no significant differences in the wettability of different rigid contact lens materials, such as Boston IV, Conflex air, Alberta XL 30, Persecon 92E or Boston RX (Figs. 7-10).

FIG. 7: SLIT LAMP VIEW (100X) OF THE INTERFERENCE PATTERN OF PATIENT A'S OILY TEAR FILM.	FIG. 8: HERE, PATIENT A IS WEARING A CONFLEX AIR CONTACT LENS. NOTE THE EVEN WETTABILITY ON THE LENS SURFACE.
FIG. 9: TEAR FERN PATTERN OF PATIENT A'S TEARS.	FIG. 10: HERE, PATIENT A IS WEARING A PERSECON 92E CONTACT LENS. NOTE THE EVEN WETTABILITY ON THE LENS SURFACE.

However, we noted wettability problems in patients with unacceptable or missing tear fern patterns when using contact lenses made of Oxycon 32, Boston Equalens, Persecon 92E, Boston RXD and Boston Equalens 2 (Figs. 11-13).

FIG. 11: IN THIS ILLUSTRATION OF PATIENT B'S DRIED TEAR, THE FERN PATTERN IS ABSENT.

FIG. 12: HERE, PATIENT B IS WEARING A PERSECON 92E CONTACT LENS. NOTE THAT THE TEAR FILM BREAKS UP.

FIG. 13: PATIENT B'S TEAR FILM ON AN EQUALENS II CONTACT LENS SHOWS PARTIAL WETTABILITY.

The tear fern test seems to be a good method to determine how wettability will influence the expected behavior of tears on contact lenses prior to the fitting. However, this method is still quite new, and extensive experiences and examinations will be necessary to support this statement. CLS

References are available upon written request to the editors at Contact Lens Spectrum. To receive references via fax, call (800) 239-4684 and request document #22.

Dr. Bussacker is dean of the Swiss College of Optometry in Olten, Switzerland, and director of the Contact Lens Clinic. He is a graduate of the Berlin School of Optometry.