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Fitting Techniques
The techniques the contact lens professional utilizes to fit an adult with a GP lens must be altered to fit an infant or small child. The ability to capture a reliable image with a topographer or accurate keratometric readings is often impossible to obtain in small children. Keratometric readings obtained at the time of surgery or an exam under anesthesia should only be considered as a starting point or a guide to the initial diagnostic lens. The application and evaluation of a diagnostic lens is the best method to obtain an appropriate fit in small children. I utilize diagnostic lenses that do not have a UV filter.
I find these lenses allow me to better interpret the fluorescein pattern when using a handheld burton lamp or LED cobalt flashlight. Once the appropriate fit has been determined, the lens is remade incorporating a material that provides an ultraviolet filter. I find it easier to determine the approximate corneal shape and curvature initially with a relatively flat fitting lens on the eye. If the diagnostic lens being evaluated vaults the anterior corneal surface, the interpretation and extrapolation of corneal curvature is difficult if not impossible. As in any GP fitting, the goal is to equally distribute lens mass and provide peripheral fulcrums to maintain stability and a central position. This central position of the lens is especially important in higher powers to minimize spherical aberrations. In recent years, I have found myself fitting looser and larger GP diameters. A general rule to follow with small children and GP lenses is that a tight lens will tend to dislodge from the eye and a loose fitting lens will tend to displace off the cornea onto to bulbar conjunctiva.
As with GP fitting on small children, soft lens fitting techniques are also a bit different. In order to determine appropriate movement of a soft lens on a small child, the “spring back” test may be helpful. With the soft lens on the eye, digitally displace the lens off center. If the lens immediately “springs back” into place on the cornea, the lens may fit too tightly on the ocular surface. If the lens stays off center while manually closing the lids to mimic a blink, the lens may be fitted too loosely on the ocular surface. In addition, retinoscopy over the soft lens to determine if the reflex maintains clarity during the blink is a finding seen with a well fitting lens. If the reflex is clearer with a blink, the fit may be too steep. If the reflex is worse with a blink, the fit may be too flat. The reflex seen with a well-fitted soft lens will maintain the same clarity before, during and after a blink. The retinoscope is not only used to determine the final lens power with any type of lens but also an important instrument to guide you to the best cornea lens relationship. Pediatric fitters of contact lens should be proficient with a retinoscope.
Little Lenses for Little People?
The pediatric contact lens professional is not limited to “off the rack” products. In addition to custom GP lenses, there are many lens manufacturers of custom made soft and silicone hydrogel contact lenses that allow us the opportunity to provide any child any parameter. In addition, liberal exchange policies implemented by these manufacturers of custom products allow us to provide these products to the patients who require them in a fair and effective manner. However, the delay in time to deliver the product to a pediatric patient in an urgent situation is a potential problem. Any delay in optical correction and visual rehabilitation with a young pediatric patient may result in permanent loss in vision.
Silicone Hydrogel Custom Products
After many years of anticipation, in 2010 Contamac received FDA approval for Definitive, a latheable silicone hydrogel material. The Definitive material can be manufactured by a limited number of laboratories in the U.S. in virtually any group of parameters. This inherently wettable, high water content and low modules material has a DK of 60. While 60 DK is not as high as other “off the rack” silicone hydrogel materials, the effective DK in many of the parameters utilized in pediatric fitting is higher than the same parameters made in HEMA-GMA materials. While this material is a welcome addition to our armamentarium of contact lens options in our practice, my clinical experience specific to pediatric indications and this material has led me to two conclusions. The application of a lens to the eye of a small child manufactured in the Definitive material is more difficult than HEMA-GMA materials and the time delay of up to ten days is often too long in an urgent case common to the pediatric patient. In time, both of these concerns can be overcome with practice and improved efficiency on the part of the patient, the practitioner and the laboratory.
New News About an Old Lens
Silicone Elastomer (Silsoft) has a long and well-documented history of being the lens of choice for the majority of pediatric professionals to manage small children following cataract surgery. The truth is that there would be many “blind” children if not for this particular lens. Silsoft Super Plus contact lenses for pediatric aphakia (>20 diopters) are available with the following parameters: diameter, 11.3mm, base curves: 7.5 mm (45.00D), 7.7mm (43.75D), and 7.9mm (42.75D), optic zone of 7.0mm and powers ranging from +23.00D to +32.00D in 3D steps. The Silsoft material has an oxygen permeability (Dk) value of 340, with oxygen transmissibility (Dk/t) of 58 at 0.61mm. One of the concerns about Silsoft has been the limited availability of parameters. As a result of the tireless efforts of Joe Barr, O.D., B+L may ultimately decide to expand the parameters of their Silsoft Super Plus product. While this announcement is far from official at the time of this article, I would like to applaud Joe and encourage you to do the same. Whether you are a proponent or opponent of Silsoft, any improved technology to provide children with the opportunity to safely develop better vision is worthy of the efforts. On behalf of the industry, the children and their families, thank you Joe.
Conclusion
As contact lens professionals, we have the responsibility, opportunity and privilege to provide products and service to young patients and their families. These products and associated services are necessary to maintain and or develop possibly the most important gift one may ever possess, the gift of sight. Again I ask you, are you a “healer of children”?
Buddy Russell, FCLSA, COMT
Associate, Specialty Contact Lens Service
Emory University Eye Center