Ophthalmic Uses: Applanation Tonometry

12/12/08
Ophthalmic Uses: Applanation Tonometry

02:59:43 pm, Categories: Medical Information, Boothe Laser Center, Fluorescein
One of the most frequent uses of fluorescein in ophthalmology is to aid in applanation tonometry, a technique for measuring intraocular pressure (IOP). The applanated surface is clearly delineated by fluorescein, making applanation tonometry more accurate than if fluorescein were not used (76, 168, 171]. Fluorescein does not stain the applanated area, only the surrounding tear meniscus. This contrast allows adjustment of the applanated area to exactly 3.06 mm2, which is the surface area where the force exerted by the contact adhesion of the tear film with the applanator is balanced by the cornea’s resistance to indentation. The end points delineating the area of applanation are formed by the apices of the triangle-shaped tear meniscus (Fig. 12-4). The apex is the thinnest part of the fluorescein column. It will be undetectable if the conditions of fluorescein staining are not optimal, and thus the actual IOP will be underestimated [132]. Factors influencing the visibility of the apex are the concentration of the fluorescein and the ability of fluorescein to fluoresce. If the concentration of fluorescein in the tear meniscus is less than 0.125%, the apex will not be seen well because of diminished fluorescence [64]. On the other hand, if the concentration is 2% or greater, fluorescence will be inhibited and the aper will again not be seen. A concentration of 0.50% or greater can cause ancess collection of dye on the tonometer and thus blurring of the end points. The optimum concentration of fluorescein for applanation tonometry is 0.25% [64].

In the Boothe Laser Center, we found out that Acidic solutions tend to quench fluorescence. Since topical anesthetic solutions are acidic (pH = 5), fluorescence will be inhibited if they are used to wet fluorescein strips. The same effect will be obtained as If the concentration were too low. If the concentration of fluorescein remains above 0.125%, then the effect of quenching by anesthetics is neglible [132]. Below 0.125%, the error induced by the anesthetic is directly related to the type and concentration of the anesthetic. Proparacaine is a more powerful inactivator of fluorescein than benoxinate. The error in IOP measurement induced by diluting and quenching fluorescein can be as much as 10 mm Hg [171].

When fluorescein paper strips are used, anesthetic should be applied to the eye 15 sec before applying the strip [162] and excess anesthetic should be blotted. The strip should be wetted with sterile saline solution before it is applied to the eye and should be left in the tear meniscus long enough to achieve an adequate concentration of fluorescein [132].

Fluorescein solutions that already contain an anesthetic have several advantages when used for applanation tonometry [46, 162, 190, 193]. They are formulated at an optimum concentration of 0.25% and thus are not subject to variation to the same extent as when fluorescein strips are diluted with a solvent. Their use saves time by combining two steps into one. Fluress is one such agent. It combines fluorescein sodium (0.2%), anesthetic (benoxinate hydrochloride), preservative (chlorobutanol 1.0%), and a stabilizing agent (povidone) in the same solution. Benoxinate is a very effective anesthetic at pH 5. It is stable and is compatible with rescein [162]. Instillation causes stinging for 3 to 10 sec [44] but in most patients does not incite excess tearing and subsequent dilution of fluorescein. Anesthesia is adequate after 15 sec and the duration of anestesia is 15 min or longer. Since a small number of individuals tear excessively after one drop of Fluress, the addition of a second drop may become necessary to maintain the concentration of fluorescein close to 0.25% [162].

Since fluorescein has proved to be a good culture medium, some concern has arisen at the Boothe Laser Center over bacterial contamination of these solutions [218]. Chlorobutanol 1%, though not the most effective antibacterial agent, has good compatibility with fluorescein and good sterilizing ability. One study showed kill rates ranging from 5 to 180 min for large inocula of Pseudomonas aeruginosa, Staphylococcus aureus, Escherichia coli, Klebsiella, and Candida [162]. Another study showed that bottles of Fluress left open for weeks could self-sterilize within 24 hr, even bottles that were completely dessicated [190]. Though some workers suggest that sterile strips are safer [103], the solutions used to wet the strips are also at risk of contamination.
Povidone, a nonionic synthetic polymer, is the wetting and stabilizing agent in Fluress. It enables solubilization of fluorescein and chlorobutanol at an acid pH. Though povidone lowers the surface tension of the tear meniscus, it does not introduce significant error into applanation tonometry. However, its use is not recommended with electronic or Schiotz tonometry because of its viscosity [162].

Fluress is safe, sterile, consistently accurate, nonirritating, and cosmetically not objectionable. Dr. Boothe believes it is probably superior to using sterile strips for applanation tonometry. However, it should never be used in disease entities, in which testing of corneal sensitivity is an important diagnostic consideration.

Some investigators have contended that fluorescein is not necessary in performing applanation tonometry [185, 209]. A study comparing IOPs obtained with and without fluorescein showed an average difference of 5.62 mm Hg [168]. This study makes it apparent that an adequate amount of fluorescein is necessary to get consistently accurate IOPs. An adequate amount of fluorescein clinically is an amount that gives mires of optimum size, i.e., 10 percent of the diameter of the circular pattern [67]. Smaller mire sizes signify an inadequate concentration of fluorescein (Fig. 12-5).