This article will discuss the key surgical options available for surgical treatment of regular astigmatism, with emphasis on the advantages and disadvantages of each option.
Astigmatism is incredibly common, affecting 32% of the adult European population.1 With rare exception, any astigmatism greater than or equal to 0.75D will negatively impact on the quality of vision and require some form of correction for best vision. Whereas glasses provide a solution for low magnitudes of astigmatism, for moderate or high astigmatism they are a suboptimal solution for many patients. Any slight tilt on the way the glasses sit on a patient’s face will affect visual quality. Soft spherical contact lenses can correct up to 1D of astigmatism; however, greater amounts of astigmatism require either soft toric lenses or rigid gas permeable (RGP) contact lenses. Soft toric lenses move on the cornea and any rotational movement will again albeit transiently degrade the image quality. Visually, rigid gas permeable lenses will undoubtedly provide the best quality of vision of all visual aids; however, getting patients to comfortably wear them can sometimes be a challenge for contact lens practitioners. For these reasons, many patients will seek out surgical options to eliminate their astigmatism and will source guidance on the options available from their optometrist. It is, therefore, critical for practitioners to be up to date with surgical options available with modern technology and to know and understand what the various advantages and limitations of such options might be for patients.
The surgical management of patients with astigmatism presents both diagnostic and surgical challenges. Pure low myopia can sometimes allow presbyopic patients to read unaided. Pure hyperopia in young patients can often be overcome with accommodation and still allow excellent unaided vision. Astigmatism, on the other hand, confers no such advantage. It is, therefore, uniquely rewarding as well as visually advantageous for refractive surgeons to be able to accurately diagnose and eliminate all but the lowest magnitude of astigmatism.
Astigmatism can be defined in several ways and it is worth exploring each of these in turn as the correct diagnosis depending on subtype has implications on selecting the appropriate treatment option.
Refractive astigmatism refers to the cylindrical error that can be predominantly corrected with glasses. It can be caused by either corneal astigmatism, lenticular astigmatism or a combination of the two. The significance of this is that without further diagnostic devices/testing to properly quantify and qualify the presence and nature of corneal astigmatism it can be impossible to know if a patient with refractive astigmatism also has corneal astigmatism or not. Likewise sometimes lenticular astigmatism can cancel out corneal astigmatism such that, despite a patient having significant corneal astigmatism, there is little cylindrical correction needed in the glasses prescription. For laser refractive surgery the necessity to make a distinction between the two is less important as the treatment is predominantly targeted on the refraction. However, for patients having refractive lens exchange or cataract surgery, both the refractive astigmatism and lenticular astigmatism are irrelevant as all treatment must be targeted on the corneal astigmatism.
The topographic features can also define astigmatism. When the astigmatism looks like a ‘bow-tie’ on topography it is said to be regular. If it does not look like a ‘bow-tie’ then it is irregular. There are some ‘in between’ options such as asymmetric bow tie (more steepness on one side than the other) or skew deviation (where the two ‘halves’ of the bow-tie do not perfectly line up). However, if in doubt, anything other than a perfect bow-tie should be assumed to be irregular.
The location of the bow-tie is significant such that if predominately vertical in orientation, that is to say the cornea is steepest at 90 degrees, then it is said to be ‘with-the-rule astigmatism.’ If opposite to this, where the bow-tie is horizontal in orientation and the cornea steepest at 180 degrees, then it is said to be ‘against-the-rule astigmatism.’ In between the two is oblique astigmatism.
The mainstay of diagnosis is carried out using corneal topography. Contact lens wear can induce corneal warpage and, therefore, a period of one week out of soft contact lenses and one month out of RGP contact lenses prior to refractive surgery evaluation is recommended.
Until techniques such as slit-scanning and Scheimpflug imaging appeared, the field of corneal imaging was restricted to the analysis of the shape and optical quality of the cornea’s anterior surface using placido imaging. The main limitation of placido disc based imaging is that the first component imaged is actually the pre-corneal tear film, rather than the cornea, and as such this technique is very sensitive to the creation of artefact in the presence of ocular surface disease. New anterior segment imaging technologies are capable of reconstructing the three dimensional structure of the cornea from two-dimensional optical cross-sections which greatly enhances the ability to investigate corneal properties. The current gold standard is considered to be elevation-based topography. The earliest device was a scanning-slit device (Orbscan) and most clinics will now use Scheimpflug based devices such as either the Pentacam or Galilei devices. Scheimpflug imaging can achieve a wide depth of focus, which provides images that include information from the anterior corneal surface through to the posterior crystalline capsule (see Figure 1: Regular with-the-rule corneal astigmatism on elevation based Scheimpflug topography)
Despite these advances in technology, a poor tear film can still interfere significantly with the accuracy of measurements. It is essential therefore that this is taken into account and ideally optimised with medical dry eye therapy prior to surgical planning.
General surgical principles
Precise surgical treatment of astigmatism requires not only reducing or eliminating the magnitude of the corneal astigmatism but also accurately targeting the correct axis of astigmatism. There is always a small contributing component from posterior corneal astigmatism, which tends to be against-the-rule in most patients. Furthermore, over several decades, with-the-rule corneal astigmatism tends to reduce slightly in magnitude whereas against-the rule astigmatism tends to increase.
For this reason it is wise to be a bit more surgically proactive in treating against-the-rule astigmatism compared to with-the-rule astigmatism. Furthermore, patients do not generally like the axis of their astigmatism to be rotated by 90°, referred to as ‘swinging’ of their astigmatism. With this in mind, it is useful to try not to overcorrect the astigmatism but rather to err on the side of a slight under correction.
Cyclotorsion of the eye (rotation) is very common when patients lie down and as a result it is important for surgeons to have accurate ways of marking the eye so that the correct axis of astigmatism is targeted surgically when the patient invariably is lying down for the procedure. Modern lasers, with the exception of small incision lenticule extraction (SMILE), have powerful torsional multidimensional tracking systems to overcome this phenomenon.
Small amounts of rotational misalignment can have a significant impact on the efficacy of the magnitude of astigmatic correction, such that a 1° misalignment will result in a 3% reduction in the magnitude of astigmatic correction. If, for example, a toric intraocular lens (IOL) is misaligned by 10°, the effective reduction in the amount of astigmatism corrected is 30%. Furthermore, significant toric misalignment can induce higher order aberrations (normally coma), which can be difficult to correct with a laser eye surgery enhancement alone.
Laser refractive surgery
Laser refractive surgery remains the treatment of choice in patients with astigmatism of less than 6D who are not yet presbyopic, providing patients fulfil the standard safety criteria for laser vision correction such as a normal corneal topography (regular astigmatism) and an adequate tear film. The LASIK flap makes wound healing and regression of effect less of an issue here than with surface laser treatments.
Astigmatism excimer laser treatments require more laser pulses in the periphery of the cornea than standard myopic spherical treatments; therefore, many of the considerations to laser vision correction of hyperopia discussed in the author’s last CET article also apply to excimer laser treatment of astigmatism.
There are now several excellent studies showing good predictability and accuracy of excimer laser vision correction for astigmatism. For example long-term findings from one study found that for all types of astigmatism (myopic, compound hyperopic, and mixed), uncorrected visual acuity was 20/25 or better in over 90% of eyes at three years.2
As discussed earlier, SMILE laser eye surgery is limited in accurately treating astigmatism as there is no cyclotorsional tracking. Furthermore, in significant astigmatism treatments, like with hyperopic laser, the effective optical zone is reduced and, therefore, the absence of pupil tracking with SMILE is also potentially problematic as a decentered lenticule is possible and can be difficult to treat.
Incisional techniques work by inducing flattening of the cornea in the meridian in which they are placed. They have mostly been superseded by alternative options discussed elsewhere within this review. However, they do occasionally have a place. The limitation with incisional techniques is that they involve corneal wound healing and the extent to which this occurs is inconstant and inevitably results in some variation of effect. In general terms, more effect is achieved when treating with-the-rule astigmatism and when treating older patients. They should not be carried out on patients with significant ocular surface disease as they can induce temporary exacerbation of dry eyes due to cutting peripheral corneal nerves. They can be carried out either manually with a calibrated guarded blade or else with OCT guidance and femtosecond laser.
They are useful in cases of low astigmatism (less than 1D on the cornea) when a toric IOL may not be needed. An example would be when fitting a multifocal IOL during cataract surgery into a patient with 0.9D of corneal astigmatism where a limbal relaxing incision can be used at the time of surgery to reduce the astigmatism and increase the tolerance of the multifocal lens.
Refractive lens exchange/cataract surgery
Toric IOLs represent an excellent option for presbyopic patients undergoing refractive lens exchange or cataract surgery. In a randomised controlled trial, Holland et al reported less than 1D postoperative residual refractive astigmatism in 88% of toric IOL eyes versus 48% in the monofocal group; 60% of the toric IOL group reported spectacle independence compared with 36% in the monofocal group.3 Toric IOLs can be used to achieve emmetropia or myopia (normally in targeted monovision or mini-monovision) for patients who have 0.75D or more of corneal astigmatism (see Figure 2: Toric IOL alignment calculation and surgical plan). Most modern toric IOLs are very rotationally stable and rotational instability (>5°) is uncommon. If the lenses do rotate off axis in the postoperative period, re-rotation is advisable if diagnosed early (within one to two weeks of primary surgery), before the capsular bag has fibrosed. If diagnosed late or else rare late rotational instability occurs then a laser eye surgery enhancement is preferred.
Multifocal/trifocal toric IOLs
These are an option for patients who are motivated to be completely independent of glasses and who also have corneal astigmatism. Patients with multifocal vision are exquisitely sensitive to even low amounts of astigmatism and any surgeons using multifocal or trifocal IOL technology must be willing and able to address this either pre- or post-operatively. Although combined multifocal torics represent a modality to correct both presbyopia and astigmatism, caution does need to be applied for levels of more than 3D of corneal astigmatism. Some small degree of toric misalignment is relatively common and, whereas monofocal toric IOL patients will be unlikely to be bothered by small amounts of residual or consecutive astigmatism, multifocal IOL patients most likely will be.
Extended depth of focus toric IOLs
The considerations here are very similar to those above (multifocal/trifocal toric); however, these lenses tend to be slightly more forgiving with respect to residual astigmatic error.
Small aperture – IC8 IOLs
These lenses do not actually correct astigmatism; however, they do increase the depth of focus (similar to a pinhole effect) and can be used to eliminate symptoms from astigmatism of up to 2D.
For patients who are not yet presbyopic or only very early presbyopes (normally under the age of 50) and who do not fulfil the criteria for safe laser eye surgery, toric phakic IOLs represent another alternative. The caveat here is that rotational alignment of these lenses either fixated to the iris or in the ciliary sulcus is not quite as stable as an ‘in the bag’ toric IOL as used in cataract surgery or refractive lens exchange surgery – the reason for this is that fibrosis of the lens capsule tends to ‘lock’ the in-the-bag IOL in place in the weeks that follow surgery.
The main evidence-based surgical options for astigmatism correction have been reviewed. There are several excellent options, with the optimal choice of procedure being dependent on the prescription being treated, the age of the patient and the unique anatomical factors of the individual.
About the author
Allon Barsam MB, BS, MA, FRCOphth is a consultant ophthalmic surgeon at the Luton and Dunstable University Hospital, UCL partners and honorary consultant at the Western Eye Hospital, Imperial College Healthcare NHS Trust. His NHS practice is a tertiary centre of excellence in the treatment of complex cornea and anterior segment disease and his private practice is almost exclusively laser eye surgery and lens based refractive surgery including cataract surgery with premium lenses.
- Wolfram C, Ho¨hn R, Kottler U, et al (2014) Prevalence of refractive errors in the European adult population: the Gutenberg Health Study (GHS). Br J Ophthalmol Jul;98(7):857-61
- Roszkowska AM, De Grazia L, Meduri A, et al (2013) Long-term results of excimer laser procedure to correct astigmatic refractive errors. Med Sci Monit 19:927–933.
- Holland E, Lane S, Horn JD, et al (2010) The AcrySof Toric intraocular lens in subjects
with cataracts and corneal astigmatism: a randomized, subject-masked, parallelgroup, 1-year study. Ophthalmology 117:2104–2111
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- Katz T, Wagenfeld L, Galambos P, et al. LASIK versus photorefractive keratectomy for high myopic (>3 diopter) astigmatism. J Refract Surg 2013; 29:824–831
- Hirnschall N, Gangwani V, Crnej A, et al. Correction of moderate corneal astigmatism during cataract surgery: toric intraocular lens versus peripheral corneal relaxing incisions. J Cataract Refract Surg 2014; 40:354–361.