Fitting Contact Lenses Using
A Spectacle Prescription
BY MARY JAMESON, BHS, COA, NCLC, CPOT
As a contact lens assistant, you may be responsible for handling patient questions about eyeglasses vs. contact lenses. Most patients are familiar with eye wear and may be familiar with the adaptation symptoms and improved quality of vision that are part of contact lens wear. Patients may choose contact lenses over eyeglasses for a variety or reasons, but they may not realize that switching from eyeglasses to contact lens wear involves many factors.
Why Contact Lenses?
Patients may choose contact lenses over spectacles:
- to improve cosmetic appearance
- to relieve the uncomfortable weight of eyeglass frames on the nose
- to achieve a wider, more natural field of view
- to change eye color
- to take part in sports and other activities
Optical considerations in deciding whether to fit contact lenses include lens material, lens power and fit. Arriving at the best prescription for contact lens wear is more than a matter of numbers. The fitting process involves both the patient's needs and the practitioner's professional judgment.
Determining Spherical Power
One aspect of lens power relates to vertex distance -- the distance between the back surface of a spectacle lens and the cornea. The spectacle lens vertex distance may range anywhere from 12mm to 16mm. The vertex distance for a contact lens is zero because it rests on the cornea. The "effective power" of a spectacle lens decreases in plus power and increases in minus power as the lens moves closer to the eye.
Because practitioners usually write prescriptions for spectacle lenses following eye examinations, you may need to adjust the power accordingly if you fit the patient with contact lenses. For instance, for powers greater than 4.00D you may need increase plus power or decrease minus power when fitting contact lenses on a patient when using his spectacle prescription.
When fitting GP lenses, the relationship between the curvature of the cornea
(keratometry reading or topography) and the base curve of the GP lens (the back curvature) can create a need for a different power in the lens prescription.
When a patient has astigmatism, the amount of his cylinder correction determines whether he'll needs a toric lens to correct that amount of cylinder. You can correct (or mask) some cylinder with a spherical soft contact lens. Spherical GP contact lenses can "correct" cylinder based on the corneal curvature and base curve relationship.
If a spherical lens won't mask enough of a patient's astigmatism, then you can calculate cylinder correction with an equation called the "spherical equivalent." Take one-half of the cylinder correction in the patient's prescription and add it to the spherical amount. For example, if the prescription is 2.00 1.00 x180, then the spherical equivalent = 2.50.
Check the View
It's important to note that you can't simply translate an eyeglass prescription into a contact lens prescription. In addition to the difference in the vertex distance, the patient may report a subjective difference in vision between viewing through spectacles and viewing through contact lenses.
Ask the patient how his vision is with his contact lenses. If he does experience a subjective difference in vision as compared to his eyeglasses, then make sure you adjust the contact lens prescription accordingly.
Ms. Jameson is laboratory supervisor for the Department of Clinical Sciences at the Pennsylvania College of Optometry and is a past chair of the AOA Paraoptometric
Contact Lens Spectrum, Issue: February 2004