When to Modify the Fit
BY THOMAS G. QUINN, OD, MS, FAAO
You did everything right. You evaluated lid position and aperture size, then selected a diameter. Based on the diameter, you adjusted the base curve selection (bias flatter for larger lens, steeper for smaller lens). You calculated the power needed by vertexing the spectacle sphere and combining it with the tear lens power. But...the lens on the eye doesn't have the textbook fit you envisioned. Now what? Do you jump in and adjust the lens design? When is it "good enough?"
It's okay to say "good enough." You may feel a bit uneasy, like you're giving in and not rising to the standards you've set for yourself. But the reality is, like many things in this world, not all fits will be perfect. Often the imperfection is due to factors over which you have little or no control, such as lid tension, astigmatic orientation or refractive error.
The Low-Riding Lens
A lens commonly drops more than expected. Should you modify the lens to help it ride up better? Let your goals guide you in your decision-making process. What goals have you set for yourself in fitting a patient with a gas permeable lens?
My goals are:
- Provide good vision.
- Keep the patient comfortable.
- Do not harm the eye.
Change the lens design if the low-riding lens compromises vision by inducing flare or fluctuating vision, if it hits the lower lid and arouses complaints of discomfort or if it sits on the lower lid during a blink, inducing heavy 3 o'clock and 9 o'clock staining and injection.
If the low-riding lens provides good, stable vision, if the patient reports that the lens is comfortable or if it rides low but moves well during a blink cycle and you observe no evidence of physiological compromise with the slit lamp, don't change the lens.
The Time Factor
Assess the fit after the patient has worn the lenses for a number of days. A lens that drops or otherwise "misbehaves" at dispensing may position just fine after adaptation. Also, extended wearing experience ensures that the patient has adequate opportunity to assess his/her vision and comfort response.
The follow-up visit should take place after the patient has worn the lenses for at least four hours on the day of the visit. Physiological compromises such as 3 o'clock to 9 o'clock staining, injection and corneal edema may not be evident until the patient has worn the lenses for at least a few hours.
3 O'clock to 9 O'clock Staining
The most common adverse physiological effect resulting from gas permeable lens wear is 3 o'clock to 9 o'clock staining. In an ideal world, we would not accept this staining. But in reality, small amounts can be present without the need to institute remedial action.
If 3 o'clock to 9 o'clock staining is light and diffuse, if there is no observable associated conjunctival injection and the patient is comfortable, you are not required to change the fit.
If the staining is heavy and coalesced, if there is associated injection or if the patient develops increasing discomfort as lenses are worn, change the lens.
The causes of 3 o'clock to 9 o'clock staining can include a low-riding immobile lens, excessive edge lift (creating a tear gap at the lens edge), inadequate edge lift (causing chafing of the epithelium, resulting in vascularized limbal keratitis), a dry eye and an inadequate blink. Explore each of these possibilities to determine what course of action you need to solve the problem. Make the change only if one of your goals is not met.
Dr. Quinn is in group practice in Athens, Ohio, and has served as a faculty member at The Ohio State University College of Optometry.