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Residual Astigmatism Correction — Why and How
Top tips for dealing with post-cataract surgery astigmatism. Dr Soosan Jacob reports
Every cataract surgeon will sometimes face residual astigmatism after cataract surgery, even with astigmatism correcting options such as toric IOLs. Dr Basak Bostanci gave an informative talk on residual astigmatism correction at a recent conference.
She explained that with-the-rule astigmatism, even up to 1 D, is often easily tolerated by many patients and every patient with residual astigmatism need not be treated. However, surgeons should investigate sources of error to improve surgical technique and future results.
Post-cataract astigmatism has many causes. These include measurement, calculation or transcription errors, and posterior corneal astigmatism. Surgical causes include surgically induced astigmatism; sub-optimal rhexis shape, size, or centration; IOL tilt; subluxation; toric IOL misalignment/ rotation; and capsular bag fibrosis. Wound healing and remodelling and Nd:YAG laser posterior capsulotomy can also play a role.
Using two or more keratometric sources to avoid variability and false measurements and properly marking reference points for toric IOLs are important to prevent postoperative errors. IOL rotation can occur because of retained viscoelastic in the capsular bag, hypotonic or overinflated anterior chamber, or incomplete unfolding of IOL haptics. A ten-degree misalignment leads to a 30% loss of effective power. This is especially important in highpowered IOLs. A 30% rotation leads to 100% loss of effective power. A capsular tension ring is helpful when dealing with a late rotation or in high myopes.
“Even though digital imaging modalities and intraoperative aberrometers are available, only 10% of surgeons use these for toric IOL intraoperative alignment,” she noted.
Dr Bostanci suggested the surgeon should start by treating factors contributing to irregular cornea and astigmatism, such as poor ocular surface, dry eye, and even PCO. Nonsurgical approaches include spectacles and contact lenses. Toric IOL patients may be more demanding, and older, infirm patients may find contact lens use challenging.
Lens- or cornea-based surgery may need to be considered. The first step is to calculate spherical equivalent (SE). If the SE is zero or at targeted refraction, rotation of toric IOL or corneal relaxing incisions (CRI) are effective. If not, IOL exchange, laser vision correction (LVC), or even piggyback IOLs may be required. Online calculators www.astigmatismfix.com and LRICalculator. com are helpful in making decisions.
IOL rotation should be delayed until refractive error is stable but is preferred within one to two weeks of surgery before fibrotic adhesions develop. Arcuate keratotomy or limbal relaxing incisions can correct up to 2.5 D of astigmatism but are less predictable and are dependent on age, pachymetry, intraocular pressure (IOP), and corneal rigidity.
Considering risks of capsular-zonular damage, vitreous prolapse, CME, infection, etc., IOL exchange is limited to patients with high SE errors, tilt, decentration, or damage to the IOL. If possible, avoid IOL exchange in patients with Nd:YAG capsulotomy. Newer toric piggyback IOLs have lower risk of pigment dispersion, interlenticular opacity, and increased IOP, but rotational stability still needs assessment.
Small refractive errors not correctable by IOL rotation and eyes with Nd:YAG capsulotomy are best fine-tuned by LVC. Procedures such as LASIK, PRK, and SMILE have proven safety, efficacy, predictability, and stability. Refraction, incisions, and IOL position should be stable before considering LVC. This usually takes three months, though co-existing ocular conditions may delay it further.
“These patients are in a different age group, often two decades older, and may have tear film abnormalities, a higher refractive index, and lower stromal hydration, all decreasing predictability of LVC. It is also important to remember that LVC creates three competing axes at different locations—axes of refraction, topography, and toric IOL,” Dr Bostanci said.
Dr Bostanci made her presentation at the ESCRS Virtual Winter Meeting 2022.
Basak Bostanci MD is an Associate Professor and eye surgeon practicing at Medicana International, Istanbul, Turkey. drbbostanci@gmail.com Dr Soosan Jacob is Director and Chief of Dr Agarwal’s Refractive and Cornea Foundation at Dr Agarwal’s Eye Hospital, Chennai, India, and can be reached at dr_soosanj@hotmail.com.
