the contact lens exam
BY JEFF SCHAFER, OD, MS, FAAO
To deliver the best visual outcome, we can customize our exam, particularly the over-refraction, for each patient. Common decisions we often face are whether to perform a spherical over-refraction (SOR) or a spherocylinder over-refraction (SCOR), and whether to use the phoropter.
Fast and Convenient
When a patient is happy with his vision and efficiency is important, many clinicians prefer an autorefraction through contact lenses. A major advantage is that technicians can easily perform this test before the practitioner examines the patient.
Disadvantages include expense, as well as resulting prescriptions that may not agree with a careful manifest refraction, partly because autorefractors may induce a small amount of proximal accommodation. Also, use caution with GP lenses that may move excessively or decenter.
When Accuracy is Key
When a patient has visual complaints and accuracy is the important factor, consider a careful SOR behind the phoropter. An SCOR, while providing more accurate refractive data, can be time consuming and isn't necessary for every patient. If visual acuity is less than expected, an SCOR may help determine if uncorrected astigmatism is the culprit. Over-retinoscopy can quickly reveal any uncorrected residual power.
Toric Lens Over-refraction
When patients present with toric lenses, I'm sure We're all guilty of crossing our fingers and hoping we won't need to change anything. If a patient presents happy with good visual acuity, an over-refraction may not be necessary or a simple SOR may suffice.
Evaluate the lens fit before attempting an over-refraction. Assess the lens for rotation and stability. Check for deposits on less frequently replaced lenses. I proceed with an over-refraction only if the lens fit is acceptable. If the lens is unstable, an over-refraction wouldn't provide a reliable, repeatable endpoint and you should refit the patient.
A toric patient who experiences constant blur with a stable lens may require only a spherical power change to improve vision, so start with an SOR to find the best-corrected spherical equivalent. If vision hasn't adequately improved, then proceed to an SCOR and turn to LARS or a cross-cylinder calculation to determine the new power.
Multifocal Lens Over-refraction
When patients present wearing multifocals, I again rely on their visual complaints to direct how I begin over-refracting. If a patient has visual complaints, try to determine if the problem is more with distance activities or with near tasks. If the patient is unsatisfied with both, re-evaluate his motivation and consider a different design or approach.
If you can determine that a patient is happy with distance vision but would like more near correction, start over-refracting at near. If the opposite is true, then start with distance over-refraction. In both cases, and with all simultaneous multifocal designs, avoid the phoropter altogether. Binocular testing with loose trial lenses or flippers allows for better results in a more natural environment.
Begin over-refracting at the distance that needs the most help and introduce lenses in 0.25D steps, in plus powers to improve near, minus to improve distance. Once you achieve satisfactory vision, check acuity through the diagnostic lenses at the opposite distance. The patient must accept the new power at both distance and near to be successful.
This technique allows patients to see the "give and take" nature of multifocal lenses. I've found that patients quickly understand what effects a power change, even as little as 0.25D, can have on their vision. It's much less time consuming to demonstrate these effects than to describe them. Patients can generally decide which power they prefer by taking into account their visual needs. CLS
Dr. Schafer is a clinical assistant professor and chief of the contact lens service at The Ohio State University College of Optometry.
Contact Lens Spectrum, Issue: June 2007