prescribing for astigmatism
Dealing With Residual Astigmatism
BY PETER D. BERGENSKE, OD, FAAO
When patients complain, or even give up, because of lack of vision quality with contact lenses, the culprit most often is residual astigmatism: either an under-corrected portion of their innate refractive error or an error induced by the lenses. Recognizing the problem is the first step in achieving a solution, and this is easily accomplished by performing a sphero-cylindrical over-refraction with the lenses in situ.
Tricks of the Trade
Here are a few tricks to neutralize problematic residual astigmatism.
1 Use rigidity and flexibility appropriately. When a patient has a good match between his corneal and refractive cylinder, we expect a non-flexing rigid lens to provide full correction of the astigmatism. When cylinders don't match, the lens will either over- or under-correct. When the patient has significant corneal toricity but manifests little or no refractive astigmatism, a soft lens is a logical choice. Should the patient show corneal toricity greater than refractive astigmatism, choose a thin, flexing rigid lens, as it will bend slightly on the eye, decreasing the amount of cylinder corrected while still correcting some. The reverse of this can also occur: if a too thin rigid lens is used when full corneal cylinder needs correcting, the flexing lens will under-correct. The solution is to increase center thickness. An easy way to check for flexure of a rigid lens is to take a K reading with the lens on the eye. A non-flexing lens will yield a spherical measurement, while a flexing lens will show some toricity.
2 Use toric lenses to induce cylinder power effect. With the exception of the spherical power effect bitoric, all toric lenses induce cylinder power effect. For toric soft lenses, this is their obvious application. With rigid lenses there is often some confusion, as we use back surface toricity to achieve a better fit than we might have with a spherical lens, but this also induces some cylinder. This is an advantage when the refractive cylinder is greater than the corneal cylinder (and it is in the same meridian). When we don't want additional cylinder, a front surface cylinder can adjust the amount of induced cylinder.
3 Use the over-refraction if it makes a difference. Once the lenses have settled, a careful over-refraction is called for in cases where acuity is less than desired. The sphero-cylinder over-refraction is really useful only if it improves the vision. The over- refraction can accurately determine a new toric soft lens power in cross-cylinder calculation. With rigid lenses, it can determine the amount of residual astigmatism and can even determine whether the residual astigmatism is due to over-correction (residual cylinder axis opposite to refractive cylinder) or due to under-correction (residual cylinder axis same as refractive cylinder).
4 Try an aspheric lens. For small degrees of residual astigmatism, an aspheric lens can improve the image just enough that you and the patient are satisfied with vision. This is particularly helpful for the low astigmat who has too little cylinder to consider a toric lens but who suffers degraded acuity with conventional optics. Aspheric multifocal lenses also tend to perform better, both in rigid and soft lenses, for patients with residual astigmatism.
5 Don't forget about spectacles. Occasionally a patient with large refractive error is not able to achieve full correction with contact lenses that are also comfortable and affordable. The astigmatic patient may be content with contact lens correction alone a great deal of the time, but benefit from spectacles that correct the residual astigmatism for some activities.
Dr. Bergenske, a Past Chair of the American Academy of Optometry's Section on Cornea and Contact Lenses, has practiced for over 20 years in Wisconsin and now is on the faculty at Pacific University College of Optometry. Email him at: firstname.lastname@example.org.