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After cataract surgery, residual astigmatism may need to be surgically corrected
Procedures done at the time of cataract surgery have different calculations from those done in the postoperative period.
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At the time of cataract surgery, many patients elect to undergo refractive surgical procedures in order to provide better vision without glasses. This means honing the lens power calculations in order to minimize myopia and hyperopia but also performing additional treatments for the correction of astigmatism. There are multiple ways to address the astigmatism, including adjustment of the axis of the primary phaco incision, placement of addition corneal relaxing incisions or use of a toric IOL. Some patients will not respond as anticipated and may have residual astigmatism, which degrades their vision in the postop period.
Correcting astigmatism at the time of cataract surgery
While both aim to correct astigmatism, procedures done at the time of cataract surgery have different calculations from those done in the postoperative period. At the time of cataract surgery, we are removing the crystalline lens so its inherent astigmatic properties are not taken into account in our calculations. The most important astigmatic measurement at the time of cataract surgery is the corneal power as measured by keratometry, topography and tomography. Keep in mind that simple keratometry and topography are both measuring the anterior cornea alone and that total corneal power and astigmatism is the sum of the anterior and posterior surfaces. This total corneal power can be measured by tomography or by intraoperative aberrometry done when the eye is aphakic.
In planning our astigmatic treatments for cataract surgery, we typically select toric IOLs for more than 1.5 D of corneal astigmatism, whereas we can use incisional techniques to relax 1.5 D or less of corneal astigmatism. The assumption is that the IOL itself will not induce any astigmatism and that we simply need to counteract the corneal astigmatism.
Correcting residual astigmatism after cataract surgery
As the patient heals after the cataract surgery, multiple factors can influence the refractive result and the degree of any residual astigmatism. While our phaco incisions are thought to cause a predictable degree of flattening, the patient’s age, pachymetry, corneal diameter, healing and other factors can cause this to vary significantly. While non-toric IOLs do not have any inherent astigmatic power when measured on a bench in a lab, when the IOL is placed in an eye, factors such as lens tilt can cause it to induce astigmatism to the eye. A large degree of IOL optic tilt is seen typically in cases with a compromised capsular bag or in asymmetric IOL placement with one haptic in the bag with the other in the sulcus. But even in a perfectly done cataract surgery, the patient’s healing response, scarring and fibrosis can cause an IOL to tilt enough to induce some astigmatism.
Figure 1. If the spherical equivalent of the eye is close to 0, LRIs may work better than laser vision correction, which would attempt to flatten one meridian while steepening the other (left). Laser vision correction works well when the spherical equivalent is minus because it allows correction of both myopia and astigmatism and the excimer laser can ablate a clean cylindrical pattern (right).
Images: Devgan U
Figure 2. In this postop cataract surgery patient, 1.5 D of residual astigmatism needs to be treated. Note that the spherical equivalent of the eye is 0, which is calculated as –0.75 + (1.50/2), so our planned LRIs are a good choice because they will address the astigmatism while not affecting the spherical equivalent. The most critical measurement here is the refraction, and note that the LRI placement is on the steep axis of the refraction.
The key measurement in calculating the correction of residual astigmatism after cataract surgery is the refraction, not the corneal power. This is an important distinction because our goal at this point is just to treat the refraction — the same approach we would use when performing laser vision correction for non-cataract patients. Measuring the refraction should be done on more than one visit to ensure stability and repeatability. The next step is to determine the spherical equivalent of the postop refraction. The spherical equivalent is calculated by taking half of the cylinder measurement on the refraction and adding it to the sphere measurement.
Limbal relaxing incisions vs. laser vision correction
If the postop refraction is plano or close to it, we simply need to perform limbal relaxing incisions in the cornea to address astigmatism. These LRIs, whether performed with diamonds or a femtosecond laser, do not alter the spherical equivalent of the eye. While we could also perform excimer laser vision correction, such as LASIK or PRK, in these eyes, the treatment is mixed astigmatism because the laser needs to steepen one meridian of the cornea while attempting to steepen the other. This tends to be less accurate than myopic astigmatic ablations, which are cylindrical in nature (Figure 1).
With a postop refraction that has a spherical equivalent that is minus powered, laser vision correction is very accurate because the laser simply ablates a cylindrical pattern to flatten the appropriate meridian of the cornea. In addition, the residual astigmatism as well as the residual myopia can be corrected at the same time. This allows us to change the spherical equivalent of the eye back to plano in order to maximize distance vision without glasses.
For our cataract patients who wish to have excellent vision without glasses, we can perform refractive procedures during the original surgery. If they have variability in healing and any residual astigmatism after the surgery, we can still perform additional corrective services to provide excellent vision.
For more information:
Uday Devgan, MD, is in private practice at Devgan Eye Surgery and Chief of Ophthalmology at Olive View UCLA Medical Center. He can be reached at 11600 Wilshire Blvd. #200, Los Angeles, CA 90025; 800-337-1969; email: devgan@gmail.com; website: www.DevganEye.com.Disclosure: Devgan has no relevant financial disclosures.