Why Use Diagnostic Dyes?
The ocular surface often shows pathology that is invisible to the naked eye. Diagnostic dyes highlight areas of damage, disease, or dysfunction by selectively staining specific types of tissue. Each dye has unique staining properties that provide different clinical information. Understanding when and why to use each dye is a core competency for ophthalmic assistants.
Fluorescein
Sodium fluorescein is the most widely used ophthalmic dye. It is available as preserved solution drops, impregnated paper strips (most common), or as a combination with anesthetic (Fluress).
How It Works
Fluorescein does not actually stain cells. Instead, it fills in areas where the corneal epithelium is disrupted or missing. Where the tight junctions between epithelial cells are broken, the dye seeps into the intercellular spaces and the underlying stroma, creating visible fluorescence.
Viewing
Fluorescein is best viewed with a cobalt blue filter on the slit lamp. Under blue light, the dye fluoresces bright yellow-green, making even small areas of epithelial disruption clearly visible. A Wratten #12 yellow barrier filter further enhances contrast.
Clinical Applications
- Corneal abrasions: Sharp-bordered, bright green staining where epithelium is missing
- Corneal ulcers: Staining reveals the size and depth of the ulcer bed
- Foreign body identification: Fluorescein pools around embedded foreign bodies
- Contact lens fitting evaluation: Shows the fluorescein pattern between the lens and cornea, revealing areas of bearing, clearance, and edge lift
- Seidel test: Concentrated fluorescein applied to a suspected wound; aqueous leaking through dilutes the dye, creating a stream of lighter green against the darker concentrated dye
- Tear breakup time (TBUT): After instilling fluorescein, the time until dry spots appear in the tear film indicates tear film stability. Normal is greater than 10 seconds.
Rose Bengal
Rose bengal stains devitalized (dead or dying) cells and mucus on both the cornea and conjunctiva. Unlike fluorescein, which highlights missing cells, rose bengal highlights cells that are still present but unhealthy.
- Stains areas not protected by a healthy mucin layer
- Particularly useful for diagnosing dry eye disease and keratoconjunctivitis sicca
- Shows characteristic staining patterns in Sjogren's syndrome
- Disadvantage: Causes significant stinging and discomfort, which limits its use
Lissamine Green
Lissamine green stains the same structures as rose bengal (devitalized cells and mucus) but causes significantly less discomfort. For this reason, it has largely replaced rose bengal in clinical practice.
- Best viewed with a red filter or white light on the slit lamp
- Excellent for evaluating conjunctival staining in dry eye
- Commonly used in dry eye clinical trials and grading systems (Oxford, DEWS)
Comparison of Diagnostic Dyes
| Feature | Fluorescein | Rose Bengal | Lissamine Green |
|---|---|---|---|
| What it stains | Epithelial defects (missing cells) | Dead/dying cells, mucus | Dead/dying cells, mucus |
| Best filter | Cobalt blue | White light | Red filter or white light |
| Patient comfort | Minimal stinging | Significant stinging | Minimal stinging |
| Primary use | Corneal abrasions, ulcers, CL fitting | Dry eye (less used now) | Dry eye, conjunctival staining |
| Stains conjunctiva | Poorly | Yes | Yes |
Key Takeaways
- Fluorescein highlights corneal epithelial defects (missing cells) and is viewed under cobalt blue light
- Rose bengal and lissamine green stain devitalized cells and mucus on both cornea and conjunctiva
- Lissamine green has largely replaced rose bengal because it provides similar diagnostic information with much less patient discomfort
- Each dye has specific clinical applications: fluorescein for abrasions, ulcers, and CL fitting; lissamine green for dry eye assessment
- Tear breakup time (TBUT) using fluorescein assesses tear film stability (normal greater than 10 seconds)