Cobalt blue light provides a suitable means of exciting sodium fluorescein (orange dye) for examination of ocular surface integrity. Fluorescein mostly absorbs light of wavelengths between 485 and 500 nm, this then causes it to shines and emits a greenish light. Fluorescein has an important role in ophthalmic practice. The corneal epithelium prevents the corneal stroma to absorb the dye; and no staining is seen if the epithelium is intact. However, in corneal erosions, abrasions, ulcers, Herpes Simplex dendritic lesions, the epithelial cells are damaged or absent. This allows the stroma to absorb fluorescein, which appears as bright green. In addition, in RGP lens fitting is also important, because the amount of fluorescein indicates a good or bad fit. RGP fit is better seen using also an orange/yellow filter (Kodak Wratten #12), which significantly enhances the contrast of fluorescein staining observed with the cobalt blue light.
Equipment and set-up:
- Sterile fluorescein sodium strips
- Sterile unpreserved saline
- Orange/yellow filter (optionally)
Fluorescein is normally instilled by wetting a strip with a drop of sterile saline, having the patient look away from you, and briefly and gently touching the strip onto the bulbar conjunctiva. When the eye is dyed the patient should blink frequently not to break up the tear film. Because the fluorescein molecule is smaller than the pore size of many hydrogels lenses, therefore could be absorbed into the lens, resulting in a yellow stain. Therefore, the eye should be irrigated with sterile saline prior to contact lens insertion. If the entire cornea is stained, irrigate the eye again and re-examine.
Interpreting the results:
The prevalence of corneal staining of any degree of severity is in a population of contact lens wearers. Fluorescein should be considered as a routine procedure as a part of contact lens aftercare examination. Make notes of possible findings; help yourself with the use of different grading scales. Make a drawing of the cornea for later reference, detailing the area(s) of abnormality.