discovering dry eye
Methods to Dye for
BY JASON J. NICHOLS, OD, MPH, PHD
Several stains and dyes are available to help us manage ocular surface disease. In general, practitioners use dyes and stains for intravital
coloration of living cells and tissue.
What Green Eyes You Have
Fluorescein sodium is a highly fluorescent chemical compound that manufacturers synthesize from petroleum derivatives resorcinol and phthalic anhydride. Its molecular weight is 376.27. Fluorescein's clinical utility comes from its size (which prevents it from passing through intact layers), high water solubility, relative inertness and natural ability to fluoresce rather than incandesce. It absorbs high-energy blue light, with peak absorption and excitation occurring at wavelengths between 465nm and 490nm; it emits longer wavelengths of about 520nm to 530nm (yellow-green). Unlike phosphorescent material, fluorescein's rate of emission is almost instantaneous.
Fluorescein has a variety of applications in ophthalmic care, from the assessment of the ocular surface to tear film stability, GP lens fitting and angiography. For ocular surface purposes, I highly recommend the use of a barrier filter (Wratten #12 filter) to cut out wavelengths less than 500nm.
Clinically, practitioners most often use fluorescein to evaluate the tear film and ocular surface; it works by filling in breaks in the epithelial layer rather than by staining corneal or conjunctival cells. Fluorescein itself doesn't stain viable or degenerating cells, which makes its description as a "stain" somewhat of a misnomer.
It's best to apply fluorescein via a controlled dose (5µL of 2% solution) rather than an impregnated strip because too high of a concentration on the ocular surface will quench its fluorescence. This can also happen with patients who have severely dry eyes (and minimal tear films), in which case it's best to apply a drop of artificial tears or saline rather than more fluorescein to obtain the lower concentration needed to observe fluorescence.
Rose bengal is another water-soluble dye (molecular weight = 1017.64) that specifically stains devitalized cells and mucin and potentially stains healthy cells. Because desiccated epithelial cells become devitalized, significant staining of the corneal and conjunctival epithelium can result in the prolonged alteration or reduction of the tear film. It's best to use a controlled dose of rose bengal (5µL to 10µL, 1% solution) rather than impregnated filter strips and to apply the dose to the bulbar surface of the conjunctiva. This method will provide sufficient stain for diagnostic purposes without inducing unpleasant irritation, as rose bengal can be toxic and cause stinging.
I prefer lissamine green dye (molecular weight = 576.61), as it doesn't seem to devitalize epithelial cells and produces significantly less discomfort than does rose bengal. Further, experts believe that lissamine green stains only devitalized epithelial cells, rather than healthy cells as does rose bengal. Apply lissamine green in the same way as rose bengal.
Fluorescein is a complement for rose bengal or lissamine green because its indications are different. Fluorescein penetrates epithelial defects and can pool or diffuse through intercellular spaces. Fluorescein mainly stains lesions and defects, whereas rose bengal and lissamine green stain devitalized but intact epithelium.
Make My Brown Eyes Blue
Methylene blue (molecular weight = 373.9) stains only dead epithelial cells, while alcian blue (molecular weight = 1298.86) stains only mucus. Practitioners rarely use these in clinical practice, but could use them as a helpful adjunct to managing ocular surface disease.
Dr. Nichols is assistant professor of optometry and vision science at The Ohio State University College of Optometry.
Contact Lens Spectrum, Issue: May 2005