It's in the Details
prescribing for astigmatism
It's in the Details
BY CHRISTOPHER SNYDER, OD, MS
The markings on toric hydrogels do not indicate the actual position of the cylinder axis, but offer a reference location of vertical (90 degree meridian) for locating meridians of the lens. In any
approach to determining final lens power an important assumption is made that the ordered lens will orient on the eye in the same way in which the diagnostic lens oriented. If this is not the case, then further adjustments to lens power(s) and axis are necessary at a dispensing visit. (By the way, lens orientation/markings may be anywhere, as long as they are consistent in their position.)
Why bother to perform a sphero-cylindrical overrefraction (SCOR) with a toric soft contact lens (TSCL)? Don't bother if you measure an "unrefined" good
visual acuity . Do bother if you find "poor" TSCL visual acuity, and a spherical overrefraction does not improve to spectacle acuity level. In this case, if you do not look for cylinder in the overrefraction, you will neither know why the acuity is poor nor what to do next.
A spherocylindrical overrefraction is of value in that your confidence may be boosted by a resulting good visual acuity, regardless of the actual (potentially baffling) numbers of the over-refraction. The following can be helpful in removing some confusion about the SCOR results: if the SCOR axis = the refraction axis, consider increasing the lens cylinder power (if reasonable); if the SCOR axis = 90 degrees away from the refraction axis, decrease the lens cylinder power; if the SCOR axis is oblique, you have a crossed-cylinder effect. (The numbers appear to be confusing and yet good vision is achieved, so they must have accomplished the corneal plane refraction!).
A Case Example
When a pure crossed-cylinder effect is present, the spherical equivalent of the overrefraction is plano. How this appears to you is a baffling overrefraction of oblique axis associated with a resultant cylinder twice the spherical component (and of opposite sign). Occasionally, this shows up in a pure form as represented by this patient whom I saw recently: A 48-year-old with a spectacle prescription of -5.25 -2.50 x 090 (20/20) was fitted with a prism-ballast design TSCL with an axis of 090 (20/30-). Overrefraction was +0.75 -1.50 x 140 (20/20); the SCOR resembled the results one might expect from a pure crossed-cylinder effect. Evaluation revealed that the lens rotated approximately 20 degrees to the left. And here is a good tip: manual rotation of the lens on the eye (with the fingertip) to adjust for this rotation temporarily cleared the chart for the patient. I concluded that the lens was likely of appropriate powers but simply not aligning well on the eye. The lens was reordered with a different axis (110) which aligned 20 degrees left, as hoped and expected, and gave good vision (20/20). (This example combined the use of the SCOR and the LARS technique to make the adjustment).
The TSCL markings can be anywhere they want to be, as long as they are consistently in that position. When reordering a TSCL after making an adjustment for cylinder position (axis), it is hoped and expected that the contact lens rotation evidenced by the position of the TSCL markings is the same as with the previous contact lens.
When malalignment is suspected as the primary cause of poor vision, manually rotating the lens on the eye while the patient views the eye chart can be revealing. There is value in performing spherocylindrical overrefractions with
Dr. Snyder is a professor of optometry and serves as chief of contact lens patient care at the School of Optometry at the University of Alabama at
Contact Lens Spectrum, Issue: April 2000