Prescribing for Astigmatism

Tips for Fitting Daily Disposable Lenses for Astigmatism

BY THOMAS G. QUINN, OD, MS, FAAO

Rapidly expanding arrays of daily disposable options are now being offered to our astigmatic population (see Table 1 for a full list of available options). This is welcome news, given the well known safety and convenience benefits of daily disposable lenses. However, be aware of some limitations, especially when fitting higher levels of astigmatism.

Expanding Cylinder Power

Daily disposable lenses to correct astigmatism are now available in toric powers up to –2.25D. However, lenses offering –2.25DC are available in only a limited number of cylinder axes. Even if an available axis matches a patient’s spectacle axis, proceed with caution.

Here’s why. If a lens is 30º off axis, it will result in residual astigmatism equal to the cylinder power in the contact lens (Snyder, 1989; Quinn, 2009). For example, a lens that has –2.25D of astigmatic correction that rests 30º off of the spectacle axis will result in 2.25D of residual astigmatism. Of course, a lens that rotates 30º may not be a stable fit and may be abandoned from the start.

What about the more common scenario in which a lens is 10º off axis? This is one-third of the 30º off-axis example above, and this will result in residual astigmatism that is one-third of the toric lens cylinder power. In the case of a –2.25D cylinder power lens, the residual astigmatism would be 2.25D/3 = 0.75D.

It has been demonstrated that 0.75D of residual astigmatism is clinically significant, resulting in a loss of about one line of acuity (Richdale, 2007). So, if we are fitting a patient who has a spectacle astigmatic axis of 180º with a –2.25D cylinder power contact lens that has an axis of 180º and rotates 10º to the right, it will, as stated above, result in 0.75D of residual astigmatism.

The ideal solution, assuming the lens position is consistent and stable, is to change the axis of the contact lens to 170º. The 10º to the right lens rotation would then align the contact lens axis perfectly with the spectacle astigmatic axis of 180º. The problem? Current daily disposable lenses offered in a –2.25D cylinder power are available only in axes of 20º, 90º, 160º, and 180º. The closest option would be to select the 160º axis, which would then rotate right to 170º, again resulting in 0.75D of residual astigmatism.

Possible Problem Solvers

Currently, the –2.25DC daily disposable option is available only in minus powers. When fitting a myopic patient who has high cylinder, don’t forget that vertexing the power back to the corneal plane will often result in a lower cylinder power needed in the contact lens. For example, a patient who has a spectacle refractive error of –7.00 –2.25 x 180 will require a contact lens power, after vertex adjustment, of –6.50 –1.75 x 180. Now a –1.75D toric power can be utilized, expanding the axes options.

Even in a case in which –2.25D is calculated to be the ideal power in the corneal plane, often patients can do quite well if the lens cylinder power is biased down. Decreasing cylinder power in the phoropter following determination of a patient’s best spectacle prescription can provide a good indication of how the patient might respond to this approach (Bergenske, 2003). Remember that you may need to add a small amount of minus to the sphere power to compensate for reducing the cylinder power. CLS

For references, please visit www.clspectrum.com/references and click on document #237.

Dr. Quinn is in group practice in Athens, Ohio. He is an advisor to the GP Lens Institute and an area manager for Vision Source. He is an advisor or consultant to Alcon and B+L, has received research funding from Alcon, AMO, Allergan, and B+L, and has received lecture or authorship honoraria from Alcon, B+L, CooperVision, GPLI, SynergEyes, and STAPLE program. You can reach him at tgquinn5@gmail.com.