prescribing for astigmatism
Correcting Astigmatism: More or Less?
BY PETER D. BERGENSKE, OD, FAAO
Although we generally pride ourselves on always providing patients the best possible vision correction, it may be that sometimes this means something other than providing the most precise correction. Since the introduction of soft contact lenses, practitioners have sought to "mask" or ignore astigmatism. Often this works satisfactorily, and for those patients, they have the benefit of simpler, less expensive correction that may also have comfort and physiological benefits. On balance, these patients have the "best" correction for them.
Many patients suffer unacceptable decrement in visual acuity if astigmatism is uncorrected. Fortunately, we have many excellent alternatives for patients needing astigmatic correction, yet situations remain where options are relatively limited. Examples include disposable multifocals and silicone hydrogels, areas in which toric lenses have yet to surface.
When faced with a patient for whom astigmatic correction is not available in his contact lens modality, practitioners can quickly screen for acceptability by performing a "sphere only" refraction. This is particularly advisable prior to attempting multifocal fitting with expected uncorrected cylinder. Too often the compromises inherent in the multifocal lens design are blamed for failure, when in fact the fault lies with the astigmatic condition itself.
Cylinder and Rotation
For toric soft lens fitting, optimal results can be obtained with the least amount of cylinder acceptable to the patient, rather than the full correction of the refraction. This is due to the fact that the negative impact of lens rotation is more or less directly proportional to the magnitude of the cylinder correction: the more cylinder correction we provide, the greater the impact of lens rotation. Stability of vision is nearly as important as clarity. Many patients would prefer constant slight blur to continuously fluctuating sharpness. It makes good sense to correct the least amount of cylinder that is acceptable.
Similar to the "sphere only" refraction used to screen for acceptance of no cylinder, it is likewise possible to perform a "limited cylinder" refraction. After finding the best refraction and acuity, reduce the cylinder in the refraction to the desired, or available, power, and then find a "best sphere" with that amount of cylinder in place. For example, if the patient shows 1.50D cylinder and you wish to see if fitting with a 0.75D cylinder is acceptable, reduce the cylinder power in the refractor to 0.75D and adjust sphere to best acuity.
In theory, practitioners should find this "best sphere" with or without some cylinder, as calculation of spherical equivalent should yield the same result. In practice, however, calculated spherical equivalent may not provide the optimal compromise. The best way to find the answer is to ask the patient for subjective response. What we really need to know is whether the acuity is acceptable, and there is only one way to find that out.
Much of this same logic applies to gas permeable lens fitting as well. We routinely ignore small amounts of residual astigmatism with spherical gas permeable lenses. It is quite common to under-correct refractive cylinder with spherical power effect (SPE) bitoric lenses and over-correct it with back toric designs.
Of course the greatest variable, and one that is difficult to measure objectively, is patient response. Simple tests such as these can help guide the decision process when faced with the challenge of deciding on the best approach for a given patient.
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. E-mail him at: email@example.com.