What does central pooling in a fluorescein pattern indicate?

Central pooling (bright green fluorescence in the center of the lens) indicates a steep fit. The lens base curve is steeper than the corneal curvature, causing the lens to vault over the central cornea and creating a thick tear layer centrally. The correction is to flatten the base curve (select a longer radius). A steep fit often shows mid-peripheral touch and minimal edge clearance.

What does central touch in a fluorescein pattern mean?

Central touch (a dark area in the center with no fluorescence) indicates a flat fit. The lens base curve is flatter than the corneal curvature, causing the lens to press directly against the corneal apex. The correction is to steepen the base curve (select a shorter radius). A flat fit typically shows mid-peripheral clearance and excessive edge clearance.

What is a dumbbell fluorescein pattern?

The dumbbell pattern is a variant of a flat fit seen on corneas with against-the-rule (ATR) astigmatism. It shows central touch along the steep horizontal meridian with two areas of pooling above and below, resembling a dumbbell shape. It occurs because the spherical lens is flat relative to the steep horizontal meridian. If significant (>2.00 D corneal astigmatism), a back-surface toric GP may be needed.

GP Fluorescein Pattern Interpretation: Alignment, Pooling, and Touch

Reading the Tear Layer

The fluorescein pattern is the most important diagnostic tool for evaluating GP contact lens fit. By instilling sodium fluorescein (NaFl) and observing under cobalt blue illumination, you can visualize the tear layer between the rigid lens and the corneal surface. This pattern reveals the three-dimensional relationship between the lens and the cornea, guiding base curve adjustments for optimal fit.

How to Read Fluorescein Patterns

The basic principles are straightforward:

Bright green fluorescence = Pooling (thick tear layer). The lens is lifted away from the cornea with space filled by tears
Dark areas (no fluorescence) = Touch/Bearing (thin or no tear layer). The lens is in close contact with the cornea
Thin, even green = Alignment (thin, uniform tear layer). The lens curvature closely matches the corneal curvature

Green = space between lens and cornea (clearance). Dark = lens touching or nearly touching the cornea (bearing). The pattern tells you whether the base curve is too steep (vaulting over the center), too flat (touching the center), or aligned (parallel to the corneal surface).

Alignment Fit (Ideal)

An alignment fit is the goal for most standard GP fittings. The pattern shows:

Central zone: Thin, even fluorescence indicating the lens closely parallels the corneal surface
Mid-peripheral zone: Slight, gradual thinning of fluorescence as the lens transitions toward the peripheral curves
Peripheral zone: A bright green ring of edge clearance where tears pool under the edge lift

This pattern indicates:

Good tear exchange (tear flow under the lens during blinking)
Even weight distribution across the corneal surface
Comfortable, stable fit
The base curve appropriately matches the corneal curvature

Steep Fit (Central Pooling)

A steep fit occurs when the base curve is steeper (shorter radius) than the corneal curvature. The pattern shows:

Central pooling: A bright green pool of fluorescein in the center where the lens vaults over the cornea
Mid-peripheral touch: A dark band where the lens bears down on the mid-peripheral cornea
Minimal edge clearance: The tight peripheral fit shows little or no bright ring at the edge

Consequences of a steep fit:

The lens tends to center well but moves minimally
Poor tear exchange because the mid-peripheral bearing acts as a seal
Trapped debris beneath the central vault
Potential central corneal staining from pooled, stagnant tears
Spectacle blur after lens removal (corneal molding from the vaulting pressure)

Correction: Flatten the base curve (select a longer radius) to reduce the central vaulting.

Flat Fit (Central Touch)

A flat fit occurs when the base curve is flatter (longer radius) than the corneal curvature. The pattern shows:

Central touch/bearing: A dark area in the center where the lens presses against the corneal apex
Mid-peripheral clearance: Fluorescein pools in the mid-periphery as the lens lifts away from the cornea beyond the bearing area
Excessive edge clearance: A wide, bright green ring at the edge

Consequences of a flat fit:

Central corneal staining from mechanical bearing on the apex
Excessive lens movement and potential decentration
Uncomfortable rocking sensation
Central corneal distortion (flattening of the central cornea)
Poor visual stability due to excessive movement

Correction: Steepen the base curve (select a shorter radius) to lift the lens off the corneal apex.

Use the mnemonic "SAG" to remember the correction: Steep has Apical clearance and needs to Go flatter. Alternatively: if you see pooling centrally, the lens is pooling because it is vaulting over the apex (steep), so flatten it. If you see touch centrally, the lens is pressing on the apex (flat), so steepen it.

Dumbbell Pattern

The dumbbell pattern is a specific variant of a flat fit that appears on corneas with against-the-rule (ATR) astigmatism:

Central touch along the horizontal meridian (the steepest meridian in ATR)
Two areas of pooling on either side (superior and inferior), creating a dumbbell or figure-eight appearance
The pattern results from the spherical lens base curve being flat relative to the steep horizontal meridian but adequately curved for the flatter vertical meridian

If the dumbbell pattern is significant (more than approximately 2.00 D of corneal astigmatism), a spherical GP lens may not provide an acceptable fit, and a back-surface toric design should be considered.

With-the-Rule Astigmatism Pattern

On corneas with with-the-rule (WTR) astigmatism, a spherical GP lens shows:

Touch at the top and bottom (along the steep vertical meridian)
Pooling at the sides (along the flat horizontal meridian)
The pattern resembles a horizontal band of touch with vertical clearance

Many corneas have enough WTR astigmatism to show this pattern without requiring a toric lens. The tear lens formed between the spherical GP and the astigmatic cornea often corrects the corneal astigmatism adequately.

Using too much or too little fluorescein. Too much fluorescein fills in areas of touch, making a flat fit appear aligned. Too little fluorescein does not reveal the pattern clearly. Use a lightly moistened fluorescein strip touched briefly to the inferior conjunctiva, then have the patient blink several times to distribute the dye before evaluating.

Key Takeaways

Green (bright) = pooling/clearance; dark = touch/bearing; thin green = alignment
Alignment fit: thin, even central fluorescence with edge clearance ring
Steep fit: central pooling, mid-peripheral touch, minimal edge clearance; flatten the BC to correct
Flat fit: central touch, mid-peripheral clearance, excessive edge clearance; steepen the BC to correct
Dumbbell pattern: flat fit variant on ATR astigmatism corneas; may need back-surface toric
Proper fluorescein amount is critical for accurate pattern interpretation