Overrefraction:
Hassle or Help?

BY THOMAS G. QUINN, O.D., M.S.
JAN. 1997

I admit it. When a toric soft lens patient complains of visual blur, I tend to overrefract only as a last resort. It can be so frustrating. But it can also be fruitful. Here's how to make the best use of this sometimes arduous procedure.

FLUCTUATING VISION OR CONSISTENT BLUR

If the overrefraction seems to change with nearly every blink, the lens is probably rotating or flexing. Be attuned to this possibility before you devote too much time trying to pin down an elusive endpoint. In such a case, all you can do is to recognize the instability of the fit and change the lens design. A good patient history will often save you frustration. This patient will probably report fluctuating vision rather than constant blur, and a slit lamp examination of the lens on the eye will provide corroborating evidence.

If the patient complains of fairly consistent blur, this suggests the fit is adequate but the power needs adjustment. Before overrefracting, observe the lens on the eye. Is it coated? Is it rotating as expected? Did you compensate for trial lens rotation correctly when you ordered the lens?

If you're still not sure what's going on, proceed to overrefraction. If you're unable to overrefract to achieve consistent, clear vision, the blur may be caused by a lens that is dirty, optically poor or flexing irregularly, or by something unrelated to the contact lens, such as a retinal problem. If you can achieve a good endpoint, examining the overrefraction results often leads to a quick remedy.

CYLINDER MODIFICATION

If the cylinder component of the overrefraction is nearly aligned with the patient's spectacle cylinder axis (in minus cylinder form), you have undercorrected the astigmatic error. Make a straightforward increase in the patient's contact lens cylinder power. If the cylinder axis of the overrefraction is perpendicular to the patient's spectacle cylinder axis (in minus cylinder form), decrease the contact lens cylinder power by the amount of cylinder power found in the overrefraction.

When the overrefraction cylinder axis is oblique to the patient's cylinder axis, the lens cylinder power is obliquely positioned relative to the eye's astigmatic error. Chris Snyder, O.D., has identified a connection between the magnitude of the residual cylinder found in the overrefraction and the amount of mislocation. For every 10 degrees of axis mislocation, the residual cylinder will be one-third of the astigmatic power in the lens. The residual cylinder will be equal to the lens cylinder power if the lens is mislocated by 30 degrees. Observe the lens on the eye to substantiate the overrefraction results.

Table 1 lists overrefraction results and the indicated lens adjustments. Make sure these adjustments are logical when comparing them to what you know about the patient's spectacle refraction, the toric soft lens power and the lens position on the eye. If a successful toric patient suddenly returns with complaints of blur, always consider the possibility the contact lenses have been switched. An overrefraction can often help you confirm this suspicion.

PREPROGRAMMED CALCULATORS

The preprogrammed calculators incorporate lens flexure and lacrimal lens effects that may alter the contact lens power needed to precisely correct the eye's refractive error. However, there is controversy about whether to account for lens rotation in these calculations. Until there's a definitive answer, be cautious in taking these findings as gospel if the lens is rotating on the eye. CLS

Dr. Quinn has served as a faculty member and research associate at The Ohio State University College of Optometry. He is in group practice in Athens, Ohio.