Fluorescein strips are a simple, yet highly effective, diagnostic tool used to assess the integrity of the eye’s surface. These small, sterile paper strips are impregnated with fluorescein sodium, a water-soluble dye that reveals damage or irregularities on the cornea and conjunctiva that are otherwise invisible to the naked eye. When introduced to the tear film, the dye flows across the ocular surface, temporarily highlighting areas where the protective epithelial layer is disrupted. This method allows for the rapid identification of scratches, abrasions, or the presence of foreign material on the eye.
Essential Preparation and Required Tools
Proper hand hygiene is necessary before beginning the examination to prevent contamination. The patient should be comfortably seated with their head supported, ideally resting on the chin and forehead rests of a slit lamp, to ensure stability. Required equipment includes the fluorescein strip, sterile saline or water for activation, and a light source equipped with a cobalt blue filter.
The cobalt blue light is required because the fluorescein dye only becomes visible, emitting a bright, yellowish-green color when illuminated by the correct wavelength. Some procedures require a topical anesthetic drop, applied directly to the eye before the dye. The anesthetic numbs the surface, reducing discomfort and involuntary blinking, which facilitates application. Applying the anesthetic before the dye is crucial, as applying the dye first can interfere with the anesthetic’s efficacy.
Applying the Fluorescein Strip: A Step-by-Step Guide
The application process begins by activating the fluorescein strip. The tip of the strip is gently moistened using a single drop of sterile saline solution or sterile water; it should be damp enough to release the dye but not dripping wet. It is important to avoid using the topical anesthetic solution to wet the strip, as this can potentially affect the dye’s properties and lead to inconsistent results.
With the patient gazing upward, the lower eyelid is gently pulled down to expose the inferior palpebral conjunctiva, the delicate membrane lining the inside of the lower lid. The moistened strip tip is lightly touched to this exposed conjunctiva for one to two seconds, transferring the orange dye into the tear film. The strip should never be touched directly to the cornea, as this can cause discomfort or a minor abrasion.
Once the dye is transferred, the patient is instructed to blink several times to encourage uniform distribution across the entire ocular surface. This allows the dye to mix thoroughly with tears and flow into any existing defects. After the dye is applied, the used strip must be immediately discarded into an appropriate waste receptacle to maintain sterility and prevent cross-contamination. The eye is then ready for examination under the specialized lighting conditions.
Understanding the Visual Results
The examination is performed immediately after dye distribution using the cobalt blue filter. When viewed through this specific light, any areas of epithelial disruption absorb the fluorescein and glow with a distinct, bright yellowish-green fluorescence. This staining occurs because the dye penetrates the exposed stromal layer beneath the damaged surface cells, creating a high-contrast visual display of the defect’s size and shape.
Common findings include punctate epithelial erosions, which appear as small, scattered dots of fluorescence often caused by dry eye. Larger, defined areas of staining typically indicate a corneal abrasion, such as a scratch from a foreign object. If a foreign body is suspected, the object itself often appears dark, while the surrounding area shows a fluorescent ring or track where the object scraped the surface.
The Seidel test is a specific application of fluorescein used to check for globe perforation, such as a deep wound penetrating the full thickness of the cornea or sclera. In a positive Seidel test, aqueous humor leaks through the perforation, washing away the fluorescein dye in a localized stream. This appears as a dark, non-fluorescent flow against the surrounding green tear film. Contact lens wearers may also show mild, diffuse staining patterns due to irritation or solution toxicity, which should not be confused with a significant injury.