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Astigmatism management vital to successful refractive surgery practice
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Both lower-order aberrations, such as myopia, hyperopia and astigmatism, as well as higher-order aberrations, such as spherical aberration, coma, trefoil and secondary astigmatism, degrade the quality of uncorrected vision. Although small residual refractive errors — as much as 0.50 D — and mild higher- order aberrations — as much as 0.50 root mean square — are tolerated, they nevertheless reduce quality of vision.
Many of us refract in 0.25-D increments, or, for a few highly demanding patients, 0.125-D increments. Most of us begin prescribing glasses, perhaps just for night for driving, at −0.50 D, and all correct 0.25 D of astigmatism when prescribing glasses for someone with a meaningful spherical error. The ideal refractive state is plano sphere with no higher-order aberrations, as repeatedly taught by my friend, Jack T. Holladay, MD, MSEE, FACS. A little astigmatism or spherical aberration can increase depth of focus, but only with a measurable loss in quality of uncorrected distance vision and contrast sensitivity, especially in a mesopic environment.
Aspheric IOL studies have shown that reducing spherical aberration by as little as 0.20 root mean square can improve reaction time and reduce breaking distance to a full stop when confronted with a hazard while driving in a mesopic environment, thereby enhancing safety.
Thresholds
The threshold for treatment of defocus, astigmatism and higher-order aberrations is a topic of much debate among refractive corneal and cataract surgeons when confronted with a patient who desires reduced dependence on glasses. But, unless attempting a compromise with increased depth of focus in the presbyopic patient, the ideal outcome is plano sphere with no higher-order aberrations.
When I do LASIK or PRK in the under-40, pre-presbyopic patient, my goal is always plano or slight hyperopia with zero astigmatism and the least possible residual higher-order aberrations, if significant accommodative amplitude remains.
My threshold to treat defocus is 0.50 D, and when treating myopia or hyperopia, my threshold is 0.01 D for associated astigmatism and 0.01 root mean square for higher-order aberrations. I program my astigmatism and higher-order aberration correction with wavefront-driven excimer laser treatments in 0.01-D increments.
Refractive cataract patients
So, what about refractive cataract surgery? For the patient desirous of reduced dependence on glasses, my astigmatism nomogram begins at 0.25 D. In the U.S., I have IOLs that can induce +0.20 D, zero, −0.17 and −0.27 root mean square spherical aberration, allowing several options to treat higher-order aberrations.
Astigmatism is especially impactful on quality of vision and spectacle independence when using a presbyopia- correcting IOL. In addition to experience with glasses, contact lenses and corneal refractive surgery, those who implant a significant volume of multifocal IOLs know that as little as 0.25 D of residual astigmatism has the potential to reduce patient satisfaction, and anything greater than 0.50 D of astigmatism deserves strong consideration for an enhancement treatment. Some patients may accept more residual refractive error, but patient satisfaction when measured at 1 year is reduced, impacting a patient’s willingness to recommend advanced-technology lenses to their friends as a good value proposition.
For me, the surgical incision in cataract surgery is a form of limbal relaxing incision and, depending on the length, design and meridian placed, can correct 0.25 D to 0.50 D of astigmatism. In the cataract patient, I always target a little with-the-rule astigmatism because the eye tends to drift slowly against the rule with age, and with the lids serving as stenopeic slits, with-the-rule astigmatism can be reduced by squinting.
Treatment
My astigmatism treatment begins with a willingness to place the incision on the steeper meridian. Having operated at all meridians, and being comfortable with temporal, oblique, superior and even nasal incisions, this works well for me; however, many surgeons are uncomfortable with anything other than an incision on the horizontal meridian.
If surgically induced astigmatism can be estimated or, preferably, measured along with posterior astigmatism after lens removal in the aphakic state using intraoperative aberrometry, a treatment plan can then be formulated using either limbal or corneal relaxing incisions and/or a toric IOL. This, however, is dependent upon the actual measured residual astigmatism of the entire optical system after the primary and secondary incisions are made, and that the natural lens, which also includes the posterior corneal astigmatism, is removed in an accurate fashion.
I have found the WaveTec ORA very useful in measuring the amount of astigmatism to treat. I find myself commonly changing the spherical and astigmatism power of the toric IOL recommended by the preoperative planner, as well as the axis of placement. When utilizing a diamond knife, I can customize the length, optical zone, depth and axis of the corneal or limbal relaxing incisions based on a measurement in the aphakic state, enhancing accuracy.
When using a femtosecond laser, I place the corneal or limbal relaxing incisions before removing the cataract, but I can titrate the opening of the relaxing incisions or extend them with a diamond knife based on my residual astigmatism measurement post- cataract removal. Utilizing these tools in a series of 25 patients, I have been able to achieve an outcome of 96% within 0.50 D of emmetropia. Of course, astigmatism can be expected to change with time, but my experience during the course of 30 years of performing relaxing incisions is that their effect tends to increase with time.
Taking this factor, as well as the eyes’ expected slow drift toward against-the-rule astigmatism, into account allows me to create a customized treatment plan for each patient that can be expected to maximize their outcomes over a lifetime.
When available, I prefer using a toric IOL beginning at 1 D of astigmatism. Toric IOLs, primary and secondary incision placement, and relaxing incisions can be used together as experience is gained to make increasingly sophisticated, titratable, customized astigmatism management plans. We now have an excellent toric accommodating IOL, the Bausch + Lomb Trulign. Unfortunately, we are still waiting for approval of a toric multifocal.
When using these IOLs, my astigmatism correcting nomogram with corneal relaxing incisions at an 8-mm optical zone extends up to 3 D. But, at the higher levels of astigmatism, relaxing incisions are less accurate and less stable, increasing the number of patients who require excimer laser enhancements.
Astigmatism management is critical to a successful refractive surgery practice, whether it be corneal- or lens-based. We are experiencing a period of rapidly expanding knowledge and technology adjuncts. The younger ophthalmologist who wishes to have a busy cataract surgery practice in the year 2020 must acquire the knowledge, skills and technology required to generate refractive outcomes within 0.50 D of target to be competitive in their marketplace.
It is equally a very exciting and very challenging time to be an ophthalmic surgeon. Those who embrace change and become expert at reproducibly and accurately generating the refractive outcomes their patients desire will provide a great service to their patients and prosper while doing so — a laudable outcome for patient and surgeon.