Fluorescein Patterns and Fit Troubleshooting of GP Lenses

What does a fluorescein pattern show on a GP lens?

A fluorescein pattern shows the thickness of the tear layer trapped between the back surface of the lens and the front surface of the cornea.

Sodium fluorescein is instilled, the lens is viewed under cobalt blue light, and the dye fluoresces in proportion to how deep the fluid is beneath it. Where the lens vaults, the layer is thick and the area pools bright green. Where the lens bears on the cornea, the layer is squeezed thin and the area goes dark. That is the whole instrument: a rigid lens is judged by a fluid film you can only see when it is stained.

According to the GPLI, GP Lens Management Guide, the pattern is easier to read with a yellow filter placed over the observation system of the slit lamp, and a topical anesthetic during the fit helps the patient through the initial lens awareness. The pattern is qualitative — it tells you the direction and the region of the mismatch, not a number of microns. The numbers that do get specified, such as edge clearance, are laboratory values and are covered on Peripheral Curves and Edge Lift of GP Contact Lenses.

What do flat, steep, and alignment patterns tell you to change?

A steep pattern is corrected by flattening the fitting relationship, a flat pattern by steepening it, and alignment is the pattern you stop at.

The GPLI guide describes the target as an alignment or “even” pattern, and warns that alignment is not necessarily the same as an on-K base curve, because the cornea is aspheric. It attributes apical clearance to a steep fitting relationship, which can cause peripheral sealoff, poor tear exchange, and possibly adherence, and apical bearing to a flat one, which can end in corneal distortion in the region of bearing. Excessive clearance in any region can trap bubbles between lens and cornea — seen as dimple veiling.

Base curve is the first lever, and it is not the only one. According to Cybersight (Orbis International), Rigid Gas Permeable Lens Assessment and Fitting (Jason Chin, OD, FAAO, New England College of Optometry), a flat lens showing apical bearing can be steepened either by steepening the base curve or by increasing the overall diameter, and a steep lens showing too much central pooling can be flattened either by flattening the base curve or by decreasing the overall diameter or optic zone diameter. That lecture gives the working exchange rate as roughly 1 mm of diameter for 0.1 mm of base curve radius.

A systematic way to land on alignment is bracketing. According to Contact Lens Spectrum, A Toric GP Primer (Annie Chang, OD, FAAO and Dawn Lam, OD, FAAO, December 2013), if the initial lens shows apical touch, steepen the base curve radius until you reach the flattest radius that produces an apical clearance pattern; the alignment radius is approximately 0.25 D flatter than that. The GPLI guide adds the discipline that makes bracketing work: any design change made to affect the fitting relationship should be significant, not a token step.

How does a with-the-rule cornea change the pattern?

On a toric cornea the pattern stops being circular: a spherical lens leaves a dark band along the flatter meridian and pooling along the steeper one.

The Toric GP Primer describes it meridian by meridian. A steep spherical lens on a with-the-rule cornea produces a vertical column of fluorescein flanked by two mid-peripheral areas of bearing. A lens that is too flat on the same cornea produces a horizontal area of central bearing. As the lens steepens toward alignment, the dark band lengthens horizontally until it runs from one edge of the optical zone to the other, appearing as an oval or football shape — and the more toric the cornea, the more oval it looks, transitioning to a thinner, cigar-shaped band. On an against-the-rule cornea the same logic rotates ninety degrees and the dark band runs vertically.

The periphery says the same thing from the outside. That article notes that a spherical lens on a highly with-the-rule cornea shows excessive peripheral clearance at 12 and 6 o’clock and minimum clearance at 3 and 9 o’clock, and that this uneven edge picture is what dictates moving to a toric back surface. Where that threshold sits, and what a bitoric back surface does to the ordered power, is on Bitoric GP Contact Lenses: Design and Power Effect.

What is the difference between apical clearance, apical touch, and three-point touch?

Apical clearance and apical touch describe a spherical lens on a regular cornea. Three-point touch is a fitting philosophy for a cornea that is not regular.

Apical clearance is fluid over the corneal apex; apical touch, or apical bearing, is the lens resting on it. Both are read at the centre of the optic zone, and both are named against the same target — the alignment pattern the GPLI guide describes.

Three-point touch is different in kind. According to Şengör and Aydın Kurna, Update on Contact Lens Treatment of Keratoconus (Turkish Journal of Ophthalmology, 2020), the goal of the three-point-touch approach is that the lens lightly touches the corneal apex while bearing mostly on two separate points in the mid-periphery, 180 degrees from the apex. It is a deliberate distribution of bearing across an irregular surface rather than an attempt at even alignment, and it belongs to specialty fitting on an irregular cornea, which is a different clinical decision from the one this page covers. Ortho-k designs are read the same way but with reverse geometry behind them; the curves that produce that pattern are described on RGP Contact Lens Parameters and Starting Power.

What changes when the lens rides low or decentres?

A decentred lens is corrected with diameter, base curve, peripheral curve, or lid attachment — and until it centres, the fluorescein pattern cannot be read accurately at all.

The Toric GP Primer makes that the precondition: accurate evaluation of the pattern requires appropriate centration, which may mean manipulating the lid to physically centre the lens or re-reading the fit after a blink. It also names the consequences of leaving it — a lens that habitually sits inferiorly creates blur because the patient is not looking through the optical zone, and a lens with excessive movement on each blink creates discomfort.

The following are the sourced moves, and each one is a change to the order rather than to the power:

• Decentred lens. Cybersight: increase the diameter, and steepen the base curve and/or the peripheral curve.
• Excessive movement or lens drop. Cybersight: identify the cause first — too flat, steepen; too steep, flatten; if the lens simply will not settle, change the peripheral curve and edge lift to encourage lid attachment, or increase the diameter for stability.
• Low-riding lens under a high upper lid. Ento Key’s Rigid Corneal Lens Design and Fitting: a relatively high upper lid tends to decentre the lens low, and a larger diameter is often required to encourage the upper lid to grip and hold it — a lid attachment fit. A relatively low upper lid with a narrow palpebral aperture tends to produce upward decentration instead.
• Comfortable lens, poor vision. Cybersight lists the checks in order: confirm the power with an over-refraction, then surface wettability, then whether the lens is positioned so the patient is looking through the optic zone, then flexure or warpage of the lens itself.

Movement, centration, lag, and the push-up test as a general fit-assessment vocabulary are on Contact Lens Fit Assessment: Movement, Centration, and Lag.

Why does a base curve change also change the power?

Because the tear layer you just read is a lens: change the base curve and you change the tear lens, so the ordered power has to move with it.

Steepening the base curve adds plus tear-lens power, so minus is added to the ordered lens — Steeper Add Minus. Flattening does the reverse — Flatter Add Plus. Cybersight gives the chairside step as approximately 0.50 D for every 0.1 mm of base curve radius, and notes that the same correction applies when the diameter or optic zone diameter is changed, because those changes move the effective base curve too. The optics and the worked examples are on Tear Lens and Lacrimal Lens Power of GP Contact Lenses; the arithmetic runs on the RGP Contact Lens Power Calculator.

Order of operations matters. Settle the geometry against the fluorescein pattern first, then measure the over-refraction, then order. Reversing that produces a lens whose power was correct for a fit you no longer have. Residual error after the lens settles is verified on the Over-Refraction Calculator for Contact Lens Parameters.

What a fluorescein pattern on this page does not cover

This page reads the lens-to-cornea fitting relationship only. Fluorescein is also a diagnostic dye, and that use is a different examination.

Grading corneal staining, interpreting epithelial disruption, and identifying any corneal finding are clinical assessments made by a licensed practitioner at the slit lamp. Nothing here interprets them, and nothing here is a reason to keep wearing a lens that hurts. The GPLI guide’s own warnings run in the same direction — sealoff and adherence from a steep relationship, corneal distortion from a flat one — and both are reasons to change the lens with a fitter, not to work around the symptom.

Read Starting Contact Lens Parameters Are Not a Prescription for the bound that applies across this site, and GP Contact Lens Design: Zones, Curves, and Edge Lift for the parameter set every change on this page is made to.

Sources

Clinical claims on this page are attributed to the publications below.

• GPLI, GP Lens Management Guide — Spherical GP Fitting and Problem-Solving— alignment as the target and not necessarily an on-K base curve; apical clearance as a steep relationship with sealoff, poor tear exchange and possible adherence; apical bearing as a flat relationship ending in corneal distortion; dimple veiling from trapped bubbles; yellow filter over the slit lamp observation system; design changes should be significant in nature.
• Contact Lens Spectrum, A Toric GP Primer(Annie Chang, OD, FAAO and Dawn Lam, BSc, MSc, OD, FAAO, December 2013) — steep and flat spherical patterns on with-the-rule and against-the-rule corneas; the oval-to-cigar transition with increasing toricity; uneven peripheral clearance at 12/6 against 3/9; the bracketing method and the alignment radius about 0.25 D flatter; centration as a precondition for reading the pattern.
• Cybersight (Orbis International), Rigid Gas Permeable Lens Assessment and Fitting(Jason Chin, OD, FAAO, New England College of Optometry) — steepening by base curve or by increasing diameter and the reverse; about 1 mm of diameter for 0.1 mm of base curve; decentration, excessive movement and lid attachment moves; SAM-FAP at about 0.50 D per 0.1 mm; the poor-vision checklist of power, wettability, lens position and flexure.
• Ento Key, Rigid Corneal Lens Design and Fitting(Contact Lens Practice) — a high upper lid decentring the lens low and larger diameters encouraging lid attachment; a low upper lid with a narrow palpebral aperture producing upward decentration.
• Şengör T, Aydın Kurna S. Update on Contact Lens Treatment of Keratoconus(Turkish Journal of Ophthalmology, 2020;50(4):234) — the three-point-touch approach as light touch at the corneal apex with bearing on two mid-peripheral points 180 degrees from it.