Getting Started with GP Fitting
Pediatric and Teen CL Care
Getting Started with GP Fitting
By Christine W. Sindt, OD, FAAO
Many practitioners consider fitting GP lenses on pediatric patients not worth the time and effort. Yet GP lenses, arguably, provide the best optics, widest parameter range, and highest safety profile of any contact lens.
Once I established a fitting technique, I found GPs easier to assess than soft or silicone hydrogel lenses; they provide a crisp retinoscopy streak, and alignment is easily evaluated with fluorescein. Best of all, you have complete control over all parameters.
Ironically, complete parameter control is why many practitioners don't fit pediatric GPs, especially on infants. There is confusion on where to start and how to manipulate the lens. This article is geared toward reducing the confusion and providing a place to start.
Selecting the Diameter
Most eye growth happens within the first year of life. It is best to fit pediatric lenses on the large side to prevent sliding and to increase comfort. A good rule of thumb for diameter selection is to fit the lens 1mm smaller than the horizontal visible iris diameter.
Selecting a Base Curve
Measuring keratometry values may be nearly impossible in small children, but a GP lens itself is the best topographer for any patient. If manual, automated or portable keratometers fail, there is nothing wrong with guessing at the base curve and reading the fluorescein pattern. Some practitioners suggest fitting pediatric GP lenses steeper than average K. I generally find it is best to align to the cornea as much as possible. In pediatric patients, a GP lens that is too steep will pop out of the eye. A GP lens that is too flat will slide around on the eye but will generally not pop out.
Fluorescein patterns are most easily read with the slit lamp, but I prefer the cobalt light of the Inova 5 LED flashlight. If the fluorescein reveals a highly toric pattern and the lens won't stay on the cornea, a bitoric lens design may be necessary.
Selecting a Power
GP lenses will provide the best retinoscopy reflex. It may be necessary to dilate a child's eye to make retinoscopy easier. Cyclopege non-aphakes to obtain a true power without fear of accommodation. Set the power based on retinoscopy or manifest refraction, depending on the level of contribution from the child. In general, young pediatric aphakes should be over-plussed (+2.00D to +3.00D) to act as an add until they are able to wear bifocal spectacles (generally between 2 to 3 years old).
Changes in Fit
You'll know when changes in fit are necessary because the lens will start popping out of the eye on a regular basis. This generally means the eye has grown and the fit needs to be flattened or made larger. If the fluorescein pattern looks good but the lens continues to pop out, it may be too thick. The center thickness should be around 0.70mm or less. Adjusting the optic zone may be necessary to achieve this.
Changes in fit are expected at 6 to 8 weeks of age, 4 to 6 months of age, 1 year of age, and 2 to 3 years of age.
As with any GP lens, the base curve flattens as the overall diameter is increased—0.25D for every 0.5mm change in diameter. If the base curve is altered, the power must be changed. There is a 1:1 ratio of power to base curve change, using the pneumonic SAMFAP (steeper add minus, flatter add plus).
Easy Once You Get Going
Children and parents alike can easily adapt to pediatric GPs. I emphasize that it takes 2 weeks to fully adapt, both with comfort and with application and removal. Interestingly, getting started may be the most difficult part of pediatric GP fitting; hopefully, these tips will provide some guidance and reduce the confusion. CLS
Dr. Sindt is a clinical associate professor of ophthalmology and director of the contact lens service at the University of Iowa Department of Ophthalmology and Visual Sciences. She is also the 2010-2011 Chair of the AOA Cornea and Contact Lens Council. She is a consultant or advisor to Alcon, Ciba Vision and Vistakon and has received research funds from Alcon. You can reach her at firstname.lastname@example.org.
Contact Lens Spectrum, Issue: November 2011