Astigmatism is a refractive error caused by differences in the curvature of the cornea or lens along different meridians. It can occur alone or in combination with myopia or hyperopia.
Total ocular astigmatism (total refractive astigmatism) is the sum of corneal astigmatism and lenticular astigmatism. Because lenticular astigmatism disappears after cataract extraction (removal of the cloudy lens), postoperative astigmatism is essentially only corneal astigmatism. Therefore, preoperative refractive astigmatism is not used as a reference value for astigmatism correction planning.
Prevalence and Clinical Significance of Astigmatism
A systematic review of patients undergoing cataract surgery revealed that 47% of eyes have pre-existing corneal astigmatism of 1.0 D or more. Global data show the prevalence of mild astigmatism (<1.5 D) is 74.6–89.6%, moderate astigmatism (1.5–2.5 D) is 8.1–14.9%, and high astigmatism (>2.5 D) is 2–6.8%.
Uncorrected astigmatism of 0.75 D can reduce visual acuity to 20/25 (approximately 0.8), and 1.5 D can reduce it to 20/40 (approximately 0.5). Generally, the goal of astigmatism correction during cataract surgery is to achieve a postoperative residual astigmatism of 0.5 D or less.
Cataract surgery also provides an opportunity to insert an intraocular lens (IOL), and astigmatism can be corrected without additional invasiveness by selecting a toric IOL or modifying the corneal incision. Uncorrected astigmatism leads to postoperative spectacle dependence, affecting the patient’s quality of life and financial burden.
Visual symptoms due to astigmatism vary depending on the degree.
Classification of Astigmatism
Corneal astigmatism is classified based on the relationship between the two principal meridians.
Regular Astigmatism
With-the-rule (WTR): The steep meridian is vertical (60–120 degrees). Common in young people.
Against-the-rule (ATR): The steep meridian is horizontal (0–30 degrees or 150–180 degrees). Increases with age.
Oblique astigmatism: The steep meridian is more than 30 degrees away from both horizontal and vertical (31–59 degrees or 121–149 degrees).
Irregular Astigmatism
Irregular pattern: Astigmatism where the two principal meridians are not perpendicular (not 90 degrees apart).
Causes: Keratoconus, corneal scarring, pterygium, post-corneal surgery, ocular surface diseases, etc.
Characteristics: Cannot be fully corrected with cylindrical lenses. Not suitable for toric IOLs.
Most corneal astigmatism is due to congenital asymmetry of the corneal shape. About one-third of patients have corneal astigmatism of 1.0 D or more before cataract surgery.
Age-related changes in astigmatism
The direction of astigmatism changes with age. WTR astigmatism is common in young to middle-aged individuals, but with aging, a shift toward ATR astigmatism occurs2). Therefore, elderly patients undergoing cataract surgery tend to have relatively more ATR astigmatism.
Causes of Postoperative Astigmatism
Accurate measurement of corneal astigmatism is the foundation for selecting the appropriate correction method and achieving good postoperative outcomes.
Keratometry
Optical biometers such as autoref keratometer, IOLMaster 700 (Zeiss), and Lenstar are standardly used. They measure the amount and axis of corneal astigmatism and are used for IOL power calculation.
Creates a two-dimensional shape map of the anterior corneal surface. It is used to verify corneal astigmatism data obtained from biometers and to differentiate between regular and irregular astigmatism. Since toric IOLs correct only regular astigmatism, this is an important test to check for irregular astigmatism.
Corneal Tomography (Scheimpflug Imaging)
Three-dimensional analysis of the anterior and posterior corneal surfaces is possible, providing actual measurements of posterior corneal astigmatism (PCA). For cases with high PCA (0.5 D or more), toric IOL calculation using TCA is recommended 2).
| Measurement Device | Measurement Target | Notes |
|---|---|---|
| Autorefkeratometer | Anterior corneal astigmatism | Screening |
| Optical biometer | Anterior corneal astigmatism, axial length | Standard device for IOL calculation |
| Scheimpflug imaging | Anterior and posterior corneal astigmatism (TCA) | Recommended for high PCA cases |
Axis Marking
Preoperative marking is essential for accurate alignment of the toric IOL.
When lying supine, the eye rotates several to over ten degrees (cyclotorsion). If marking is not done in the seated position, the toric IOL axis alignment during surgery in the supine position will be inaccurate, increasing residual astigmatism.
There are several methods for correcting astigmatism during cataract surgery, chosen based on preoperative astigmatism amount, type of astigmatism, eye condition, surgeon experience, and cost.
Principle and Indications
The mechanism aligns the steep meridian of the cornea with the flat meridian of the IOL to cancel corneal astigmatism. The cylinder power at the IOL plane corresponds to 1.5–6.0 D, correcting 0.75–4.75 D of corneal astigmatism.
Toric IOL should be considered for regular astigmatism of 1.0 D or more, with strong evidence supporting its use for >2.0 D (ESCRS guidelines, GRADE ++).
A meta-analysis (13 trials) showed that toric IOLs significantly improve postoperative uncorrected distance visual acuity (UDVA, logMAR) compared to non-toric IOLs (with or without incisions) (mean difference -0.07, 95% CI -0.10 to -0.04) and reduce the risk of not achieving 20/25 or better. A 2016 systematic review and meta-analysis also showed that toric IOLs reduce residual astigmatism the most compared to corneal relaxing incisions 4).
As of 2022, four types of toric IOLs are available in Japan (excluding multifocal toric): Vivinex Toric, Tecnis® Toric II, AcrySof® IQ Toric, and Clareon™ Toric.
Effect of Axis Misalignment on Correction
For each 1 degree of axis misalignment, the corrective effect decreases by approximately 3.3%. At 30 degrees of misalignment, the corrective effect is almost lost, and with greater misalignment, postoperative visual function may be worse than with a non-toric IOL. IOL rotation tends to occur early, from 1 hour to the day after surgery, so stable fixation within the capsular bag at the end of surgery is important.
Risk factors for axis misalignment
In long axial length eyes, simultaneous insertion of a capsular tension ring (CTR) has been reported to be effective in preventing IOL rotation.
Contraindications
Irregular astigmatism (keratoconus, corneal scar, corneal ectasia), zonular weakness or rupture, posterior capsule rupture, poor pupil dilation, severe dry eye, and history of vitreoretinal surgery with buckling or glaucoma implant are all relative contraindications.
Principle and indications
Limbal relaxing incision (LRI) is a method of correcting astigmatism by making relaxing incisions along the steep meridian of the cornea to flatten it. It can correct up to 3.0 D, but predictability is highest up to 1.5 D. It has the advantage of being usable when toric IOL is contraindicated (e.g., posterior capsule rupture, zonular instability).
Opposite clear corneal incision (OCCI) is a cost-effective option for astigmatism of 0.75 D or less.
A systematic review of LRI and OCCI showed a mean correction index of 0.77 ± 0.18 (range 0.39–1.0), indicating a tendency toward undercorrection. The mean correction index was 0.82 ± 0.13 in the LRI group and 0.69 ± 0.22 in the OCCI group, with no statistically significant difference between groups (p = 0.17). Mean corneal astigmatism decreased significantly from 1.86 ± 0.53 D preoperatively to 1.04 ± 0.48 D postoperatively, and mean refractive astigmatism decreased from 1.96 ± 0.62 D to 0.98 ± 0.36 D (p < 0.01).
Surgical Planning
The correction effect is determined by the location (optical zone), length, and depth of the incision. Settings are determined using various nomograms (e.g., Johnson & Johnson LRI calculator) or dedicated web software. It is recommended to perform the incision at 90% of corneal thickness.
Contraindications
Corneal ectasia, peripheral thinning, and advanced dry eye are relative contraindications.
Principles and Features
This method uses a femtosecond laser to perform arcuate keratotomy (AK). Because the laser irradiation design is based on anterior segment OCT measurements, incision accuracy and depth uniformity are higher than with manual diamond knife incisions.
A systematic review reported that manual arcuate keratotomy and femtosecond laser arcuate keratotomy show equivalent visual and refractive outcomes in terms of safety and efficacy, with refractive stability achieved by 3 months postoperatively for both methods.
Pham et al. (2025) conducted a cohort study of 34 eyes with 5-year follow-up, showing that simultaneous femtosecond laser arcuate keratotomy and cataract surgery significantly reduced mean corneal astigmatism from 1.63 ± 0.886 D preoperatively to 0.53 ± 0.628 D at 3 months postoperatively (p = 0.001), and remained stable over 5 years (0.55 ± 0.624 D at 5 years, p > 0.05)3).
In that study, 67.6% of eyes achieved UDVA of 20/25 or better at 3 months postoperatively, with no change over 5 years (p>0.05). The proportion of eyes with MRSE within ±0.50 D reached 91.2% at 5 years 3). Mean induced astigmatism was 1.09±0.413 D, with a correction index of 0.67 (undercorrection), but no complications related to arcuate incisions (such as displacement, perforation, or fibrosis) were recorded 3).
Cost
Femtosecond laser equipment and maintenance are expensive, increasing facility costs.
When residual astigmatism remains after cataract surgery, secondary correction with an excimer laser is effective. LASIK (laser in situ keratomileusis) can simultaneously correct spherical refractive error and provides relatively stable vision early after surgery. PRK (photorefractive keratectomy; PAK: photoastigmatic keratectomy) is chosen for cases with corneal opacity or after radial keratotomy (RK).
Wavefront-guided LASIK (laser ablation based on higher-order aberration measurement) is also useful for irregular astigmatism cases that could not be corrected by Acanthamoeba keratitis or LRI.
Note: Facilities with laser equipment are limited, and there are constraints on residual corneal thickness.
Selection criteria for treatment
| Astigmatism amount | Recommended method |
|---|---|
| ≤0.75 D | OCCI / steep axis incision |
| 0.75 to 1.0 D or more | Consider toric IOL |
| 1.0 to 2.0 D or more | Toric IOL (moderate evidence) |
| More than 2.0 D | Toric IOL (strong evidence) |
For moderate astigmatism of 1.0 to 2.0 D, multiple studies have shown that toric IOL results in less residual astigmatism and higher predictability than incisional methods (LRI, OCCI). Toric IOL also offers better long-term stability. However, the choice should consider cost, contraindications, and surgeon experience.
In a spherical cornea, the curvature is equal in all meridians, and incident light converges to a single point (emmetropia or uniform spherical refractive error). In astigmatism, the curvature differs between the two principal meridians, causing incident light to form two focal lines, resulting in a blurred image (Sturm’s conoid).
The posterior cornea has negative refractive power (negative refractive surface), and its steep meridian is often vertically (or near-vertically) oriented in many eyes. Therefore, PCA often acts in the opposite direction to anterior corneal astigmatism.
Jin et al. (2023) conducted a retrospective study of 62 eyes with high PCA (≥0.5 D) and reported that when toric IOL calculation was performed using TCA, postoperative overcorrection occurred in both the against-the-rule (ATR) and with-the-rule (WTR) groups (correction index: ATR group 1.14±0.29, WTR group 1.25±0.18) 2).
In the same study, the mean error (ME) was 0.22±0.52 D (p=0.03) in the ATR group and 0.65±0.60 D (p=0.00) in the WTR group, both significantly shifted toward overcorrection 2). When overcorrection occurs in WTR eyes, the axis flips, resulting in postoperative against-the-rule residual astigmatism, which has a greater impact on visual function. Therefore, the authors conclude that TCA-based toric IOL calculation is recommended for ATR eyes but requires caution in WTR eyes 2).
Cases with PCA exceeding 0.5 D account for 9–14% of all cases. In this group, conventional formulas using only anterior corneal curvature have reduced accuracy, so it is desirable to use actual PCA measurements or formulas incorporating them (e.g., Barrett Toric Calculator).
In younger individuals, with-the-rule (WTR) astigmatism is common, but with aging it shifts to against-the-rule (ATR) astigmatism. This is thought to be due to age-related hardening of the peripheral lens and the effect of eyelid pressure, which changes the corneal meridian direction. This shift should be considered in toric IOL power calculation and LRI planning.
In cases with high PCA (≥0.5 D), calculating toric IOL power using only anterior corneal curvature tends to cause overcorrection. Overcorrection in WTR eyes in particular can induce postoperative against-the-rule astigmatism, significantly affecting visual function. It is recommended to use devices that can measure posterior corneal astigmatism, such as Scheimpflug imaging, and formulas that account for TCA 2).
This lens allows postoperative fine-tuning of IOL refractive power through ultraviolet light irradiation. By confirming residual refraction after surgery and then adjusting the power with light exposure, improved accuracy of astigmatism correction is expected 1).
This system measures the refractive state of the aphakic or phakic eye in real time during surgery, guiding optimal IOL power and axis.
Measurements using intraoperative aberrometry have been reported to result in residual astigmatism less than 0.5 D in 75% of eyes, compared to 53% with preoperative calculation methods such as the Barrett Toric Calculator, suggesting superiority.
Real-time correction is expected to reduce the risk of reoperation, but it requires dedicated equipment and additional time 1).
Development of personalized IOLs based on detailed preoperative corneal imaging and patient-specific refractive profiles is underway. Furthermore, approaches to personalized medicine that elucidate the genetic background of refractive errors and establish treatment plans according to individual eye characteristics are being explored 1).
High-resolution anterior segment evaluation using swept-source OCT is expected to further refine surgical strategies in complex cases 1).
Mallareddy V, Daigavane S. Innovations and Outcomes in Astigmatism Correction During Cataract Surgery: A Comprehensive Review. Cureus. 2024;16(8):e67828. doi:10.7759/cureus.67828
Jin T, Yu L, Li J, Zhou Y. Refractive outcomes of toric intra-ocular lens implantation in cases of high posterior corneal astigmatism. Indian J Ophthalmol. 2023;71(8):2967-71. doi:10.4103/IJO.IJO_3385_22
Pham TMK, Nguyen XH, Pham TTT, Hoang TT. Five Years Follow-Up Outcomes of Femtosecond Laser-Assisted Cataract Surgery on Patients with Preexisting Corneal Astigmatism. Int Med Case Rep J. 2025;18:373-379. doi:10.2147/IMCRJ.S506198
American Academy of Ophthalmology. Cataract in the Adult Eye Preferred Practice Pattern. Ophthalmology. 2022;129(1):P1-P126. (PIIS0161642021007508)
