Modification of Preexisting Astigmatism
Residual astigmatism after cataract surgery can impact visual function and patient satisfaction. Approximately 40% of cataract patients have 1.00 D or more of preoperative keratometric astigmatism. Therefore, correction of regular astigmatism during cataract surgery has increasingly become a priority for both patients and surgeons. For additional detailed discussions of astigmatism management, see BCSC Section 3, Clinical Optics, and Section 13, Refractive Surgery.
Refractive astigmatism (eg, as found by manifest refraction) is a combination of total corneal astigmatism and lenticular astigmatism. Lenticular astigmatism, which is contributed by the cataract, is eliminated during cataract surgery. Thus, to manage astigmatism with cataract surgery, the surgeon addresses the total corneal astigmatism.
Total corneal astigmatism comprises both anterior and posterior corneal astigmatism. Anterior corneal astigmatism tends to drift from with-the-rule (steeper vertical meridian) toward against-the-rule (steeper horizontal median) with increasing age. In contrast, posterior corneal astigmatism does not tend to change with age. In over 85% of adults, the posterior cornea is steeper in the vertical meridian. Because the posterior cornea is a minus lens, this creates net plus refractive power horizontally, adding against-the-rule astigmatism to the total corneal astigmatism. The average magnitude of posterior corneal astigmatism is approximately 0.30–0.50 D, but there is considerable variation in the general population. Therefore, anterior corneal measurements alone will often overestimate with-the-rule astigmatism, and underestimate against-the-rule astigmatism, due to the unmeasured against-the-rule effect of the posterior cornea.
Anterior corneal astigmatism can be accurately measured by a variety of methods, including keratometry (manual or automated), topography, Scheimpflug imaging, or optical coherence tomography (OCT). It is best to combine keratometry with other imaging methods, because irregular corneal astigmatism or ectatic disease may not be apparent without the use of topography or tomography. Although accurately measuring posterior corneal astigmatism is difficult, Scheimpflug imaging, OCT, and light-emitting diode (LED)-based devices can be used. (See BCSC Section 3, Clinical Optics, for further discussion of optical instruments.) Many surgeons employ regression formulas, such as the Abulafia-Koch formula, or theoretical formulas, such as the Barrett toric calculator, to account for the unmeasured effect of the posterior cornea.
Although the preoperative refractive cylinder is not a reliable indicator of total corneal astigmatism (due to the potential lenticular astigmatism), the manifest refraction can still provide suggestive information about the magnitude and axis of total corneal astigmatism because it necessarily incorporates the effect of the patient’s posterior cornea.
Another key component to account for in preoperative planning to correct astigmatism is the surgically induced astigmatism (SIA) of the corneal incision(s) used during cataract surgery. A centroid value (ie, vectorial average) for SIA can be input into any of the available online toric calculators. A 2.4-millimeter (mm) temporal clear corneal incision has been shown to have a centroid value of approximately 0.10 D of flattening in the meridian of the incision, though the actual magnitude and meridian of SIA in any individual case can vary dramatically. Alternatively, surgeons may choose to use their personally calculated centroid SIA value, which has been based on a series of cases. If a larger incision is required, placing it across the steeper meridian may reduce preoperative astigmatism.
Abulafia A, Koch DD, Holladay JT, Wang L, Hill W. Pursuing perfection in intraocular lens calculations: IV. Rethinking astigmatism analysis for intraocular lens-based surgery: suggested terminology, analysis, and standards for outcome reports. J Cataract Refract Surg. 2018;44(10):1169–1174.
Abulafia A, Koch DD, Wang L, et al. New regression formula for toric intraocular lens calculations. J Cataract Refract Surg. 2016;42(5):663–671.
Hayashi K, Manabe S, Hirata A, Yoshimura K. Changes in corneal astigmatism during 20 years after cataract surgery. J Cataract Refract Surg. 2017;43(5):615-621.
Koch DD, Ali SF, Weikert MP, Shirayama M, Jenkins R, Wang L. Contribution of posterior corneal astigmatism to total corneal astigmatism. J Cataract Refract Surg. 2012;38(12):2080–2087.
Excerpted from BCSC 2020-2021 series: Section 11 - Lens and Cataract. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.