Prescribing for Astigmatism
Prescribing for Astigmatism
Measuring Toric Lens Rotation
BY THOMAS G. QUINN, OD, MS, FAAO
Lining up the astigmatic correction of a toric soft contact lens with a patient’s astigmatic error is key to successful vision correction. What is the best way to assess rotational position? What if the axis doesn’t line up exactly? How close is close enough?
A common method to determine toric soft lens axis location is to line up a slit lamp beam with the toric markings on a contact lens, then read the slit lamp scale to determine the degree of rotation (Figure 1). This technique can be quite precise. One cautionary note is to look for horizontal displacement of the lens prior to taking a rotational measurement; you may believe the lens is rotated when it is merely displaced laterally (Figure 2). To avoid this pitfall, the slit lamp beam should pass through the toric lens marking and the optical center of the lens, not the center of the pupil.
Figure 1. Using a slit lamp beam to measure toric soft lens rotation.
Figure 2. The upper image shows a lens rotating to the right. The lower image shows a horizontally displaced lens that may appear rotated when it is not.
Some practitioners determine lens rotation by simply observing the toric marking and estimating its position. It’s quick and simple, but if inaccurate, more time may be needed in the fitting process to make adjustments to achieve the desired visual outcome.
One study (Snyder and Daum, 1989) found that experienced clinicians can estimate rotation of a toric lens marking within 8° of the true reading 95% of the time. So, when does being off about 10° become significant?
Consider Toric Power
Every 10° that a lens is off axis induces one-third of the toric power in the lens (Snyder, 1989). So, if a lens with 3.00D of toric power rotates 10° off axis, you would expect 1.00D of residual astigmatism. A lens with 0.75D of toric power that is 10° off axis will induce only 0.25D of residual astigmatism.
There is evidence to support that uncorrected astigmatism of 0.75D or more will have a significantly adverse impact on vision performance (Bayer and Young, 2005; Chin and Moody, 2008; Richdale et al, 2007). In other words, up to about 0.50DC is “OK.” Therefore, the tipping point will be at 1.75D of toric power. A 10° error in axis location will result in 1.75 ÷ 3 = 0.58DC residual error. So, if the toric power is less than 1.75D, estimating rotation should suffice. If it is 1.75D or more, measure it.
Sphere Power Considerations
Be sure to consider spherical refractive error in your decision-making process. The closer a patient’s spherical refractive error is to emmetropia, the more impact residual astigmatism may have on vision performance as it will be a larger part of the total refractive error. So, for those who have low spherical errors, you may need to be a bit pickier on making sure that the astigmatic correction is right on. CLS
For references, please visit www.clspectrum.com/references and click on document #225.
Contact Lens Spectrum, Volume: 29 , Issue: August 2014, page(s): 15