Prescribing for Astigmatism
Prescribing for Astigmatism
GP Residual Astigmatism and What to Do with It
BY THOMAS G. QUINN, OD, MS, FAAO
Gas permeable (GP) contact lenses can provide exceptional visual correction, but this can be thwarted by the presence of uncorrected astigmatism. A few steps can help you identify the source of residual astigmatism and provide insights on how to fix it.
Three Case Studies
CASE #1: Corneal toricity is less than refractive astigmatism. Keratometry findings: 43.00 @ 180, 43.50 @ 090. Spectacle Rx: –3.00 –1.50 x 180.
There is more refractive astigmatism than corneal toricity, thus the tear lens created under a non-flexing spherical GP lens (43.50D – 43.00D = 0.50D) will not be enough to correct the refractive astigmatic error (1.50D), leaving a full 1.00D of residual astigmatism. In such cases, it may be best to simply fit a toric soft lens. However, when a spherical GP lens works perfectly on the other eye, a front-surface toric, prism-ballasted lens may be used to correct the residual astigmatism.
CASE #2: Corneal toricity is equal to refractive astigmatism. Keratometry findings: 42.00 @ 180, 43.50 @ 090. Spectacle Rx: –3.00 –1.50 x 180
When corneal toricity matches the vertexed refractive astigmatism, the tear lens created by a non-flexing spherical GP lens should precisely correct the refractive astigmatic error.
The most common cause of residual astigmatism in a case like this is lens flexure. Flexure may occur when a spherical GP lens is fit on a cornea with 1.00D or more of corneal toricity (Harris and Chu, 1972). Flexure can be confirmed easily by performing keratometry or topography over the contact lens while on the eye.
Lens flexure can be reduced by increasing lens center thickness, flattening the base curve (Herman, 1983), or choosing a more flexure-resistant material.
CASE #3: Corneal toricity is greater than refractive astigmatism. Keratometry findings: 42.00 @ 180, 43.50 @ 090. Spectacle Rx: –3.00 –0.75 x 180.
In this case, we would expect a tear lens under a non-flexing spherical GP lens to provide 1.50D of toric power, which is twice as much as needed to properly correct the refractive error. A soft lens, likely a toric soft lens, is a reasonable option in this case. If the opposite eye is particularly well suited for a non-flexing spherical GP lens, or if the patient is already adapted to a GP lens and may be disappointed in the optics of a soft lens, consider a GP lens that is designed to flex on the eye. Promote flexure of a spherical GP lens by thinning the lens, steepening the base curve, or selecting a material that is more prone to flex.
Sometimes keratometry readings will not accurately reflect true corneal shape along the visual axis. This is most likely to be the case with chronically decentered GP lenses or with corneal conditions such as keratoconus. The best way to explore the presence of unanticipated residual astigmatism is to perform a sphero-cylindrical over-refraction and keratometry or topography over the contact lens while worn on the eye and compare the results.
Don’t forget that vertexing spectacle powers, especially in cases of high refractive error, can change the astigmatic component. Astigmatic error will decrease when myopic refractive errors are vertexed back to the corneal plane, and it will increase with hyperopic refractive errors. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #219.
Contact Lens Spectrum, Volume: 29 , Issue: February 2014, page(s): 15