the contact lens exam
Choosing a Refraction Method for Contact Lens
BY JEFFREY J. WALLINE, OD, PHD
In an ideal world where time isn't of the essence, we'd perform a
sphero-cylindrical manifest refraction over every contact lens we fit to determine if any residual refractive error exists. Time constraints
constraints and patient endurance typically don't allow us to do this, so we must choose the best available alternative. Several options exist:
autorefraction, retinoscopy and spherical over-refraction. I'll discuss the benefits and disadvantages of each method to provide you with a menu of options for determining what contact lens power to prescribe.
When You Need Speed
Technicians can perform autorefraction before practitioners even enter the exam room. Autorefraction requires no input from the patient and little time to collect, so technicians can perform it on young children and non-communicative adults.
Autorefractors can take several measurements within a few seconds, which improves the repeatability of the measurement. This helps us detect small refractive differences over long periods of time. However, it doesn't necessarily provide prescriptions that agree with the gold standard manifest refraction for the spherical equivalent or measures of astigmatism.
Many autorefractors are available, and each has its advantages and disadvantages. Most autorefractors are stand-alone machines that require a patient to move from the exam chair, and they can take up space in an already crowded exam room. They're also fairly expensive.
All autorefractors can take readings through contact lenses, but GP lenses that move greatly may elicit invalid readings (which are easy to identify). Most autorefractors use a fogging lens to control accommodation, but they usually induce at least a small amount of proximal accommodation. Hand-held autorefractors require less space and you can use them while the patient is in the exam chair, but they may yield inappropriate readings resulting from patient accommodative response (especially in children) or poor positioning.
When choosing an
autorefractor, ask the manufacturer if you can use its model in your office for a short period of time. Use it in every possible condition to help you determine whether it's right for your office.
When Accuracy Counts
autorefraction, retinoscopy allows you to determine an over-refraction for a patient who has difficulty communicating. Technicians often can't perform
retinoscopy, which means it requires "doctor time," but you can perform it relatively quickly. Prescriptions from retinoscopy typically agree with a manifest refraction, but they're not as repeatable as
One or Two?
Probably the most common method of checking whether the power of a contact lens is appropriate is the spherical over-refraction. This time-tested, gold standard method is both accurate and quick, and technicians can perform it. However, the spherical over-refraction is subjective, so you can't use it for non-communicative patients or for infants or young children. You should also use it in conjunction with a
sphero-cylindrical over-refraction if the visual acuity doesn't meet expected levels. And don't forget to perform a balance to accommodation test.
The Choice is Yours
Several options are available to help us determine the appropriate contact lens power for our patients. If you gain knowledge of all of these methods, then you'll be able to choose the best method for each patient.
Dr. Walline is a research scientist at the Ohio State University College of Optometry and he is the principal investigator of the Children's Overnight Orthokeratology
Contact Lens Spectrum, Issue: March 2004