Prescribing for Astigmatism
Surgical Correction of Astigmatism, Part 2
BY JARED JAYNES, OD, FAAO, & TIMOTHY B. EDRINGTON, OD, MS, FAAO
In our previous column, we discussed the use of LASIK and incisional keratotomy surgical procedures for correcting astigmatism. Now let's focus on toric intraocular lenses (IOLs).
Whether your patients are candidates for cataract surgery or refractive lens exchange, consider recommending toric IOLs if they have ≥1.00D of corneal astigmatism. While patients may have lenticular astigmatism contributing to the cylinder power in their manifest refraction (MR), after their crystalline lenses are removed, it is purely their corneal astigmatism that is targeted for correction.
Although surgeons make the final recommendations on appropriate IOL implants for your patients, bear in mind a few important factors. Current technology allows a maximum of 6.00D of astigmatism correction to be incorporated into toric IOLs. However, it is essential to recognize that a product's specific published toric values may be based on correction at the IOL plane, with the actual correction at the corneal plane being somewhat less.
For example, the Tecnis Toric Aspheric IOL (Abbott Medical Optics) ZCT300 model corrects up to 3.00D of astigmatism at the IOL plane, but 2.06D at the corneal plane.
Additionally, surgically induced astigmatism from incision location may decrease or increase patients' corneal astigmatism by approximately 0.50D. Typically, the incision is located temporally at 180 degrees, flattening the 180-degree meridian by 0.50D (increasing with-the-rule or decreasing against-the-rule corneal astigmatism).
Irregular Astigmatism and IOLs
If a patient's astigmatism is irregular and secondary to a corneal ectatic disorder such as keratoconus, it is imperative to understand that the optics of a toric IOL will not correct irregular astigmatism.
Surgeons and patients should discuss the available options. If patients are satisfied with their vision through MR or spectacles, consider correcting the MR cylinder. If your patients need to wear GP lenses after IOL surgery to obtain adequate vision, consider spherical IOLs or toric IOLs that correct the residual cylinder found in the over-refraction through a GP lens.
Cross-Cylinders and IOLs
As with fitting toric soft contact lenses, the concept of resultant cross-cylinders comes into play with toric IOLs. If the implanted IOL misaligns with its intended orientation, unwanted refractive cylinder is induced. The greater the amount of cylinder in the IOL, the more optically important it is for the correcting cylinder axis to be in proper alignment.
Clues about misalignment or rotation of a toric IOL can be found in a patient's refraction axis pre- and postoperatively, topography data, and the alignment markings on the toric IOL upon dilation. If you find that the toric IOL is misaligned 10 degrees or more from the original intended axis, consider returning the patient to the surgeon for IOL rotation. If the IOL needs to be rotated surgically, early identification is key, prior to the IOL haptics scarring into place in the capsular bag.
Regardless of the method chosen to correct patients' astigmatism, it benefits you and your patients to consider their vision needs and lifestyle profile when recommending a certain treatment for astigmatism. CLS
Jared Jaynes, OD, is practicing at the Cavanaugh Eye Center in Overland Park, Kansas, specializing in corneal and anterior segment care including refractive and cataract surgeries. Dr. Edrington is a professor at the Southern California College of Optometry at Marshall B. Ketchum University. You can reach him at firstname.lastname@example.org.