The Slit Lamp in Contact Lens Practice
The slit lamp biomicroscope is the most important instrument for evaluating contact lens fit and monitoring the ocular surface for lens-related complications. It provides magnified, illuminated views of the anterior eye structures, allowing you to assess how a contact lens interacts with the cornea, tear film, and surrounding tissues.
Evaluating RGP Lenses with Fluorescein
For rigid gas permeable (RGP) lens evaluation, sodium fluorescein (NaFl) is the essential diagnostic tool. When instilled into the tear film and viewed under cobalt blue illumination, fluorescein reveals the tear layer pattern between the lens and the cornea.
How Fluorescein Patterns Work
Fluorescein absorbs blue light and emits green-yellow fluorescence. Under the cobalt blue filter:
- Bright green areas: Indicate pooling of tears (a thicker tear layer between the lens and cornea)
- Dark areas (absence of fluorescence): Indicate touch or bearing (the lens is in close contact with the cornea, with minimal or no tear layer)
- Thin green band: Indicates alignment (the lens parallels the corneal surface with a thin, even tear layer)
Ideal Fluorescein Pattern
A well-fitting RGP lens typically shows:
- Light central touch or alignment: Thin, even fluorescence centrally
- Mid-peripheral alignment: Gradual transition zone
- Peripheral edge clearance: A bright green ring at the lens edge where tears pool under the edge lift
Common Fluorescein Patterns
- Steep (tight) fit: Central pooling (bright green center) with minimal or no edge clearance. The tear layer is thickest under the center and thin at the periphery. The lens may show minimal movement
- Flat (loose) fit: Central touch or bearing (dark center) with excessive edge lift and wide peripheral pooling. The lens may show excessive movement and decentration
- Alignment fit: Even fluorescence across the lens with mild edge clearance. This is generally the target for most RGP fits
Using a Wratten Filter
Adding a yellow Wratten filter (#12) over the observation system enhances fluorescein visibility by blocking the blue excitation light and allowing only the green-yellow fluorescence to pass through. This significantly improves contrast, making subtle tear layer differences easier to detect.
Evaluating Soft Contact Lenses
Soft contact lens evaluation does not typically use sodium fluorescein because the dye can absorb into hydrogel lens materials, staining them. Instead, soft lens fit is assessed using white light illumination.
Assessment Criteria
- Centration: The lens should be centered over the cornea or slightly decentered superiorly. Significant decentration can cause visual disturbances and discomfort
- Coverage: The lens edge should extend at least 1mm beyond the limbus in all directions
- Movement: On blink, the lens should move approximately 0.25 to 0.50mm. This movement ensures tear exchange beneath the lens. Excessive movement causes visual fluctuation; insufficient movement restricts tear exchange and may cause binding
- Push-up test: Gently push the lens upward with the lower lid. A well-fitting lens should move easily and return to its centered position smoothly. A tight lens resists movement and snaps back quickly. A loose lens moves excessively and may not recenter
High-Molecular-Weight Fluorescein
When fluorescein assessment is needed for soft lenses (such as evaluating corneal staining after lens removal), high-molecular-weight fluorescein can be used. This larger molecule does not absorb into the lens matrix as readily as standard NaFl, though it is less commonly available in clinical practice.
Assessing Complications
The slit lamp is also used to detect contact lens-related complications:
- Corneal staining: Viewed with fluorescein and cobalt blue after lens removal. Patterns include punctate staining (dry eye or solution toxicity), arcuate staining (tight lens edge), and central staining (flat fit or dryness)
- Neovascularization: New blood vessel growth from the limbus, visible with white light or red-free filter
- Papillae on tarsal conjunctiva: Upper lid eversion reveals papillary response, especially in GPC
- Microcysts and vacuoles: Epithelial changes visible with retroillumination, indicating chronic hypoxia
Key Takeaways
- RGP lenses are evaluated with sodium fluorescein under cobalt blue illumination
- Green fluorescence = tear pooling (clearance); dark areas = bearing (touch)
- An ideal RGP fit shows alignment centrally with edge clearance peripherally
- Soft lenses are evaluated with white light; do not use standard NaFl on soft lenses
- Soft lens fit assessment includes centration, coverage, movement (0.25-0.50mm), and push-up test
- A Wratten yellow filter enhances fluorescein pattern visibility for RGP evaluation