contact lens primer
Prescribing for Astigmatism: Rigid or Soft Toric Lenses?
BY TIMOTHY B. EDRINGTON, OD, MS, FAAO, & JOSEPH T. BARR, OD, MS, FAAO
A patient presents to your office enthusiastically desiring contact lenses. This particular patient has no preconceived impressions about lens modality. The manifest refraction reveals astigmatism at a level that would unsatisfactorily degrade vision if you prescribe spherical soft contact lenses. How do you determine the most appropriate lens type and design for this patient?
Recommend the Most Appropriate Treatment
OK, so we stretched the truth about the patient not having any preconceived notions regarding his care. But, as an eyecare practitioner, your recommendation still has tremendous influence.
There are many factors to weigh in making this decision. Amount and orientation of the astigmatism, amount of the astigmatism relative to the sphere, the amount of the astigmatism that is corneal versus internal, and vocation and avocations of the patient are all important factors in prescribing.
The dioptric amount of the sphere and cylinder influence the decision to prescribe soft toric versus rigid lens designs. If the sphere component of the Rx is outside the range of +4.00D to 6.00D, the patient might benefit from the additional oxygen afforded by today's rigid lenses. These values are only guidelines. A patient may be prescribed a +6.00D soft toric contact lens on a part-time or even a full-time wearing schedule if no signs of corneal edema (such as striae or microcysts) are noted at follow-up visits.
Patients differ in their demand for precise vision. Determine their expectations during the fitting evaluation. Patients with refractive cylinder of 0.75D to 3.00D are good candidates for soft toric contact lenses. Also, the amount of the cylinder relative to the sphere might contribute to the patient's happiness with his vision. If the cylinder amount is equal to or greater than the spherical component of the prescription, patient satisfaction can be more challenging to achieve. Again, these are only guidelines. We all have patients who are exceptions to these "rules."
Rigid contact lenses may provide the crispest vision correction for the majority of your patients, especially during low light tasks. The exception is patients whose refractive astigmatism is primarily internal (not corneal), and their corneal toricity is not sufficient to stabilize rotational alignment with a back surface toric rigid lens design. We suggest a minimum of 1.50D of corneal toricity when prescribing a rigid back surface toric.
Lens position is a factor in prescribing rigid contact lenses for the astigmatic patient. Consider bitoric RGPs for patients with 1.50D or more corneal tori-city. Even though a spherical RGP might provide good comfort and vision, corneal molding and/or induced corneal distortion tends to be minimized with a bitoric design. Thus, the patient is less likely to experience spectacle blur.
The recent improvement in toric soft contact lens reproduci-bility and the increasing availability of frequent lens replacement options have enhanced practitioner excitement in prescribing them for almost all astigmats. However, there remains a substantial number of astigmatic patients who show increased benefit from wearing rigid lens designs.
Dr. Edrington is a professor and chief of contact lens services at the Southern California College of Optometry. E-mail him at firstname.lastname@example.org.
Dr. Barr is editor of Contact Lens Spectrum and assistant dean of Clinical Affairs at The Ohio State University College of Optometry.