prescribing for astigmatism
Turn Tedious Over-refraction Into a Focusing Friend
BY THOMAS G. QUINN, OD, MS, FAAO
Properly timed and correctly employed, an over-refraction can bring instant clarity to a toric soft lens patient who complains of poor vision. Over-refractions offer the benefit of accounting for all optical components impacting the visual system. This includes not only the contact lens power, but also any lacrimal lens power or contact lens flexural effects. The best visual outcome can be realized by factoring in these additional players.
Have you ever found yourself immersed in the 20-minute OR with no endpoint in sight? Avoid this predicament by first confirming a stable lens fit. Observe the lens on the eye, and query the patient, "Is blur consistent or fluctuating?" If he reports fluctuating vision, change the fit first.
For years, controversy raged over whether or not it is necessary to compensate for lens rotation when combining toric lens power with an over-refraction finding. The answer is yes! Koers and Quinn combined a calculated over-refraction with toric lens power and demonstrated that the correct resultant power was arrived at only after lens rotation was factored into the calculation. So, whatever method of analysis you employ, make sure lens rotation is taken into account.
The Manual Approach
Some practitioners choose to use a trial frame and lensometer to arrive at a new toric lens power. They place loose lenses of the diagnostic toric lens power in the trial frame, with the cylinder power lens rotated in the same direction and to the same degree as the toric contact lens rotates on the eye. On top of this, he inserts lenses equal to the over-refraction finding. Allowing the lens-ometer to calculate the measured power of the trial frame/lens assembly will result in the correct lens strength, but the wrong axis. The correct axis can be arrived at by adjusting the axis using RALS (right add, left subtract).
Caroline and André simplified this approach by adjusting the OR finding using LARS (left add, right subtract) and inserting the contact lens power without modifying the axis.
New Methods of Analysis
Many contact lens manufacturers' web sites now provide on-line calculators to assist you in performing the complex calculation required to combine the cross-cylinders of a toric lens with over-refraction findings. Most will ask you to input the diagnostic lens power, the over-refraction finding and the amount of measured lens rotation. Make sure lens rotation is asked for! Press enter and presto, you new lens power presents itself!
Lindsay et al showed that the rotational position of a toric contact lens on the eye can be deduced by considering the patient's spectacle prescription relative to the over-refraction findings acquired over a toric lens of known power. CooperVision employs this method in its ToriTrack System, which can be downloaded (www.coopervision.com) to a Palm Pilot. This approach overcomes inaccuracy in measuring lens rotation on the eye, relying on accurate baseline refraction.
Programs are also available for a nominal fee (Contact Lens Calculator Plus at www.PalmGear.com) that offer a toric calculator as well as additional features, which can be downloaded to a Palm Pilot for easy access.
With today's quality toric soft lenses (and GP lenses!) and refined methods of analyzing over-refraction results, you are armed to meet the visual needs of the astigmatic patient. Don't compromise by attempting to mask the problem. Correct it!
To receive references via fax, call (800) 239-4684 and request document #92. (Have a fax number ready.)
Dr. Quinn is in group practice in Athens, Ohio, and has served as a faculty member at The Ohio State University College of Optometry.