What approach do you recommend for irregular astigmatism after cataract surgery?
Click Here to Manage Email Alerts
Click here to read the Cover Story, "Surgeons discuss how to address astigmatism during cataract surgery."
Toric IOLs and incisional keratotomies
Irregular astigmatism is a term often used to refer to any type of higher-order aberration that cannot be corrected with spherocylindrical lenses.
In cataract surgery on these patients, it is often possible to improve uncorrected vision using toric IOLs or astigmatic keratotomy. First, we must understand the patient’s desires and expectations and make sure they understand that we cannot achieve a perfect result. If possible, we must also proactively manage the underlying causes of irregular astigmatism before surgery. This means aggressive management of dry eye, removal of Salzmann’s nodules or pterygium, treatment of anterior basement membrane dystrophy or any other pathology amenable to improvement.
Once this is done, we must identify the component of astigmatism that is regular. In cases of mild irregularity, topography often provides useful clues. The prevailing astigmatic shape in the central 2 mm of the axial map generally indicates the axis and magnitude of astigmatism. Newer optical biometry devices such as the Zeiss IOLMaster 700 and the Haag-Streit Lenstar 900 also can provide fairly reliable measurements. In cases of high corneal irregularity that has been stable and long-standing, it is also useful to look at the manifest refraction from before cataract symptoms developed. If it provided meaningful improvement in vision at that time, a similar magnitude and axis of toric correction may work well. This goes against the old rule of relying on corneal measurements exclusively because lenticular astigmatism is likely to play a small role compared with the irregularity of the cornea. This approach is especially helpful in ectatic disease, in which topography can be confusing. Newer toric non-diffractive extended range of vision lenses may also be useful in many of these patients.
As a general rule, it is better to undercorrect rather than overcorrect irregular astigmatism, always alerting patients that corrective lenses may be necessary, despite our best surgical efforts.
John A. Hovanesian, MD, FACS, OSN Cataract Surgery Section Editor, is from Harvard Eye Associates in Laguna Hills, California.
Topo-guided PRK and monofocal IOLs
Patients with low amounts of astigmatism (less than 1 D) will always have some level of irregularity. Because of this, the bad news is that it is harder to nail the axis and amount of astigmatism to treat. The good news is that leaving a small amount of residual astigmatism (less than 0.5 to 0.75 D) usually results in adequate quality uncorrected acuity. In patients desiring multifocal IOLs for spectacle independence, less than 0.5 D of residual astigmatism is desirable. In these patients, choosing a midpoint between the topography and biometry is a reasonable strategy. By operating on axis with the main incision, a surgeon should be able to hit these residual astigmatic goals in a high percentage of patients.
In patients with otherwise normal corneas and less than 1 D of astigmatism, it should be rare to need to perform PRK enhancement solely because of residual astigmatism. I do not start using toric IOLs in with-the-rule patients until they have more than 1 D of astigmatism or more than 0.75 D of against-the-rule astigmatism.
More interesting are the cases with high levels of irregular astigmatism. In patients with irregular astigmatism resulting from previous refractive surgery, for example, there is often significant coma, which contributes to the “best fit” regular astigmatism measured by manifest refraction or by SimK on topography or biometry. In these cases, a “cornea repair” procedure using topography-guided PRK (an off-label use in the U.S.) can be done either before or after cataract surgery with monofocal IOL implantation with great results. Performing topo-guided PRK before cataract surgery allows the surgeon to measure the corneal spherical aberration following stabilization from the PRK. The advantage here is that the surgeon can then choose which monofocal or extended depth of focus IOL to use to leave the patient with a net spherical aberration to best match the goals of the surgery. If the goal is the best quality uncorrected distance vision, a net spherical aberration of zero is best. If the goal is to provide a wider range of uncorrected acuity, then a net negative spherical aberration may be more appropriate.
D. Rex Hamilton, MD, is from Hamilton Eye Institute in Los Angeles.
Collapse
Click Here to Manage Email Alerts