Prescribing for Astigmatism
Correcting Higher Levels of Regular Astigmatism
BY HEIDI G. MILLER, OD; TIFFANY GATES, OD; & TIMOTHY B. EDRINGTON, OD, MS, FAAO
Several soft toric contact lens options are available in expanded parameters, allowing you to provide clear, stable vision to more astigmatic patients. Advances in toric lens technology and manufacturing provide improved rotational stability, higher oxygen transmissibility, and more replacement schedule options than in the past. Patients’ visual needs and lifestyle, as well as lens parameter availability, should be considered when prescribing a soft toric lens.
Oxygen Transmissibility with Extended Toric Parameters
As you know, toric contact lens designs utilize technology (e.g., prism ballast) that is intended to improve rotational stability. These designs are often associated with different thickness profiles for toric lenses that may reduce the oxygen transmissibility of the contact lens.
As such, the lens material selected is very important to ensure that oxygen is available to the cornea. As the oxygen permeability is generally highest in silicone hydrogel materials, that makes this class of material a prime one when considering options for toric lens wearers. However, to our knowledge, there is only one non-custom silicone hydrogel toric contact lens that is available in a cylinder power of –2.75D, and most daily disposable toric lens options are limited to cylinder powers up to –2.25D.
Of course, there are plenty of custom toric lens options, and several laboratories provide custom soft toric contact lenses with up to –10.00D of cylinder correction. However, many of these lenses are manufactured in materials that have Dk values of less than 20, and they tend to have thick lens profiles due to these higher powers. Regular monitoring of corneal health is imperative when fitting a low-Dk contact lens that has a thicker lens profile. The Definitive (Contamac) silicone hydrogel lens material (efrofilcon A), with a Dk value of 60, can be ordered through select laboratories for their custom contact lens products.
Higher Cylinder and Rotational Stability
With any soft toric contact lens, the precision of the cylinder axis alignment and the rotational stability is essential to achieve optimal vision correction. The amount of rotation becomes more critical as cylinder powers approach –2.00D and above.
Rotational amount and stability may also vary among toric lens designs; therefore, switching the contact lens brand may be necessary to avoid induced cross cylinder effects if excessive axis rotation is noted. According to Snyder’s rule, for every 30º of axis rotation, the full amount of cylinder will appear in the over-refraction (Snyder, 1989).
If Soft Torics Don’t Work
As always, GP corneal and scleral contact lenses, as well as hybrid lens designs, may be considered as alternative contact lens options for patients who present with significant amounts of astigmatism. In fact, these lens options can also serve as good first choices in cases of high astigmatism. CLS
For references, please visit www.clspectrum.com/references and click on document #235.
Dr. Miller and Dr. Gates are the cornea and contact lens residents at the Southern California College of Optometry (SCCO) at Marshall B. Ketchum University (MBKU). Dr. Edrington is the cornea and contact lens residency coordinator at SCCO. He is also a Fellow of the American Academy of Optometry and a Diplomate in its Cornea, Contact Lenses, and Refractive Technologies Section.