HOW IMPORTANT is sodium fluorescein (NaFl) evaluation of orthokeratology (ortho-k) lenses? There is a bit of a conundrum with ortho-k lens fitting patterns on-eye: lenses that show a “textbook” bull’s-eye pattern don’t always perform as well as you think, and vice versa. For this reason, consider this: if the lens looks “acceptable” (minimal movement, good centration, and a defined bull’s-eye), dispense the lens and evaluate its performance via topography changes and visual symptoms. The thinking here is generally that what the lens does when the eyelid is closed overnight is not the same as what it does when the patient is awake and blinking.
Recently presented to me was a case that shows the difficulty in troubleshooting lens fit and topography changes. The patient had baseline myopia of –3.00D in both eyes and the pre-wear topography showed minimal corneal astigmatism. Subsequently, lenses were designed empirically by the lab using the topography data.
Upon evaluation, the lenses had a reasonable bull’s-eye pattern but a bit more movement than ideal. The patient was advised to begin lens wear and return for post-fit evaluations. Day one and day seven revisits showed some treatment decentration on topography, but it was relatively minor, and the patient was advised to continue following the wearing schedule and to return in two weeks for evaluation.
During the two-week follow-up, a lens change was indicated. The patient was close to full correction but had some residual astigmatism not fully correctable to 20/20. Note that the topography subtractive map OD (Figure 1) shows that the lens decentered down and temporal while sleeping.
This usually indicates the sag of the lens is too deep while the diameter may be a little small. The static NaFl pattern (Figure 2) appears a little deep, with a wide midperipheral ring and a relatively small central where there is no NaFl.
However, the lens movement is the real giveaway here. Between blinks, the lens decenters and moves in an arc pattern along the limbus, which is a classic sign of a corneal lens that has insufficient sag depth.
The resting position of the lens is more or less where the topography suggests it should be. It was decided to increase the sag of the lens by steepening the alignment zone, which should improve the treatment centration.
So how important is it to evaluate the lens fit with ortho-k? In my clinical opinion, it’s very important, with the caveat that one needs to consider all aspects of the lens performance—movement, NaFl pattern, and post-wear topography—to get the full picture of how to correct an underperforming treatment.
Ortho-k treatment is usually straightforward, but if it is not, consider working with the lab consultants to determine any parameter changes. The consultants are an invaluable resource providing advice to aid in achieving success for ortho-k and other GP and specialty lens fits. CLS