Fluorescein staining is a common, non-invasive diagnostic technique used in ophthalmology to assess the integrity of the eye’s outer surface. This procedure involves applying sodium fluorescein, a water-soluble, organic fluorescent dye, to the tear film covering the cornea and conjunctiva. The dye glows intensely when exposed to light of a specific wavelength. It highlights damage or defects in the protective epithelial layer of the ocular surface by adhering to and pooling in areas where epithelial cells are missing, allowing practitioners to detect otherwise invisible issues.
Diagnostic Purposes of Fluorescein Staining
A practitioner uses fluorescein stain to identify a wide range of conditions affecting the front of the eye. The dye is particularly effective at revealing mechanical injuries, such as a corneal abrasion (a scratch on the clear surface of the eye), or confirming the presence and location of a foreign body lodged on the cornea or under the eyelid.
The staining helps diagnose various infections and inflammatory conditions, including corneal ulcers caused by bacteria or viruses like Herpes Simplex Keratitis. The test is also used to detect the tiny, scattered surface defects characteristic of dry eye syndrome. Fluorescein plays a role in contact lens fittings, especially with rigid gas permeable lenses, by revealing how the lens interacts with and distributes the tear film across the cornea.
Step-by-Step Procedure for Application
The process begins with patient preparation, usually positioning them comfortably at a slit lamp microscope. The dye is most commonly delivered using a small, sterile paper strip impregnated with sodium fluorescein. This strip is first lightly moistened with a drop of sterile saline solution to activate the dye and ensure a controlled amount is administered.
The moistened tip of the strip is gently touched to the inner surface of the lower eyelid (inferior tarsal conjunctiva or lower fornix). Alternatively, a preservative-free liquid drop of fluorescein may be instilled directly into the eye. The patient is instructed to blink a few times to distribute the dye uniformly across the tear film and ocular surface.
After a short waiting period, the examination proceeds using a specialized light source. The practitioner uses either a slit lamp equipped with a cobalt blue filter or a handheld blue light. Under this blue light, the dye fluoresces brightly, emitting a yellow-green light. This fluorescence makes any surface defect visible, as the dye is retained in areas where epithelial cells have been damaged or lost.
Understanding the Staining Patterns
The appearance and pattern of the bright green fluorescence provide specific clues about the underlying problem. One common finding is punctate staining, which appears as numerous tiny, scattered dots across the corneal surface, often associated with dry eye syndrome or mild chemical irritants.
A more severe finding is geographic staining, presenting as a large, continuous area of fluorescence with distinct borders. This indicates a significant epithelial defect, such as a corneal abrasion caused by trauma. A highly specific pattern is dendritic staining, characterized by a branching, tree-like structure, which is strongly indicative of a viral infection, specifically Herpes Simplex Keratitis.
The dye is also used in the Seidel test to detect leakage from the inside of the eye. In a positive Seidel sign, internal aqueous humor streams out through a corneal wound and washes away the fluorescein. This creates a dark, non-fluorescing rivulet against the bright green background, confirming a full-thickness perforation. Negative staining describes areas where the dye is repelled, such as over raised lesions or bubbles under a contact lens, appearing as a dark area within the fluorescing tear film layer.
Safety and Post-Procedure Guidance
Fluorescein staining is a safe procedure. The dye is well-tolerated because its application is topical, staying on the eye’s surface with minimal systemic absorption. The most common temporary effect is a transient yellow discoloration of the tears or any soft contact lenses not removed prior to the test.
The dye may also cause slight, brief discoloration of the skin or clothing if it runs down the cheek. The dye is naturally cleared from the ocular surface quickly through the tear drainage system. The only absolute contraindication is a known history of a severe allergic or anaphylactic reaction to fluorescein, underscoring the need for a thorough patient history before administration.