INTRODUCTION

Currently, cataract surgery is considered a refractive surgery, with the ultimate goal of emmetropia and correction of associated presbyopia. The challenge is now much bigger and truly enticing.

At present, the surgeon has at his disposal various types of intraocular lenses for presbyopia correction in lens replacement surgery, namely accommodative and multifocal, refractive, diffractive, extended depth-of-focus, spherical or toric lenses.

The first reports of transparent lens patients undergoing additive presbyopic surgery with implantation of multifocal epi-crystalline lenses are now beginning to appear. The ambitious independence of glasses for near will certainly make presbyopia correction one of the most sought options after refractive surgeries in the near future, as a result of increased life expectancy and the world's population over 50 years of age. There is a marked effort by manufacturers to design more sophisticated multifocal intraocular lenses to reduce the dysphotopsies described by patients with current lenses1-3.

However, there is finally proven neuroadaptation in the first 6 months after their implantation4. New materials are also investigated that may offer greater biocompatibility and eye protection, since the candidates for this type of surgery are increasingly younger and more demanding.

Moreover, the surgical decision can be complex, sometimes very difficult, depending on each clinical case. As is well known, the surgical outcomes result from careful preoperative patient selection, biometric data collection, correct use of various intraocular lens calculation formulas, choice of the best lens to implant according to the type of eye and patient profile, and exclusion of ocular pathology.

Multifocal intraocular lenses may, to a greater or lesser extent, affect the quality of postoperative vision due to the decreased contrast sensitivity with which they are associated5-8. Therefore, it is of paramount importance to assess the visual expectations of patients which are sometimes too high, the time they devote to multiple daily tasks, the importance they attach to each and their psychological profile.

Occasionally, even in cases with good tear film, absence of corneal irregularities due to elevated astigmatism or high-order aberrations, without opacification and/or folding of the posterior lens capsule, subjective visual complaints, that sometimes become disabling, are frequent.

IRREGULAR ASTIGMATISM IN REFRACTIVE CATARACT SURGERY

Multifocal intraocular lens implantation is not at all consensual in eyes where corneal pathologies coexist, namely corneal ectasia (e.g. keratoconus), and in eyes subjected to previous corneal surgery, such as total or anterior lamellar transplantation, LASER refractive surgery, radial keratotomy, among others. These can in themselves lead to decreased quality of vision. All of these clinical situations have irregular astigmatism in common.

Refractive astigmatism is mostly correlated with corneal astigmatism, with the cornea contributing more than 70% of the total refractive power of the eye9, and the crystalline, secondly refractive element, with practically the remainder. It is known that there may be a significant discrepancy between refractive astigmatism and the objective measured by diagnostic auxiliary methods.

Corneal astigmatism, by definition, is the result of non-perpendicularity between the meridians of the cornea, with corneal aberrations leading to asymmetric and irregular corneas.

Coma is the dominant high order aberration in asymmetric corneas, as in keratoconus10,11, and in corneas after asymmetric LASER refractive treatments12, among other clinical situations. In these cases, the morphological alterations are displaced from the optic center of the cornea, originating a wave front of the chromatic type.

Individuals who exhibit elevated irregular corneal astigmatism are accustomed to coexisting with complaints of decreased acuity and visual quality, as well as constant dysphotopsies. When vision is not fully corrected with spherical-cylindrical lenses in patients with no known ocular pathology, these should be studied for the detection and characterization of a possible irregularity of the corneal surface.

There are ancillary diagnostic tests, such as corneal topography, tomography and aberrometry, which can perform a rigorous evaluation of the cornea, both in quantification and characterization of the type of astigmatism, as well as in analyzing low and high order aberrations. Thus, it becomes easier to interpret patients' subjective complaints and the possible orientation for surgical treatments.

Patients with irregular corneal astigmatism with cataract or clear crystalline lens, who seek independence from glasses, through obtaining emmetropia and correction of associated presbyopia, are not usually good candidates for replacement of the lens with multifocal intraocular lenses. The decision to implant these types of intraocular lenses in these patients is controversial in the scientific community due to the unpredictable and discouraging results regarding the quality of the final vision13. Loss of contrast sensitivity will be present whenever corneal aberrations persist.

Multiple studies show that near-perfect visual quality can be achieved after implantation of multifocal intraocular lenses when preoperative corneal astigmatism is less than two to three-fourths of diopter (D) of cylinder14,15, the final spherical equivalent close to plano (0.50 D or less) and regular astigmatism. However, patients with anterior coma, a 3rd order aberration and a form of irregular astigmatism, when greater than 0.32 µm, may have disabling dysphotopsies with multifocal intraocular lenses16. Currently, the evaluation and quantification of high-order aberrations are not covered by refractive surgery guidelines with multifocal intraocular lenses17.

Patients having undergone previously corneal surgery, namely LASER-refractive surgery (LASIK and PRK), radial keratotomy, thermo-keratoplasty, or total or lamellar keratoplasty among others, often exhibit multiple corneal elevations. These can lead to higher order aberrations, sometimes transforming the cornea itself into a multifocal structure17, with marked decrease in contrast sensitivity, as already mentioned, especially in patients with large pupils.

Thus, the implantation of multifocal intraocular lenses in these patients would necessarily result in an additional loss of such contrast sensitivity, a significant decrease in the quality of vision, and possibly a great deal of personal dissatisfaction17.

The biometric calculation of the power of the intraocular lens to be implanted in cases of patients with irregular astigmatism with or without cataract is another enormous challenge, for which some clinical judgment is necessary. In the most varied clinical situations in which the cornea presents anatomical changes with associated irregular astigmatisms, obtaining reliable values ​​of keratometry can be very difficult.

Keratometry measurements performed by different acquisition methods, such as manual, automatic, calculated by partial optical coherence LASER and topographic interferometry, may not coincide with each other. It is the same with the determination of the true axis of the cylinder, making it even more difficult to choose the type of multifocal intraocular lens, toric or not.

Despite the controversy, more and more descriptions of lens surgery with multifocal intraocular lens implantation are emerging in patients with high irregular corneal astigmatism. However, there are very few cases published in the literature and when there are, the sample is extremely low. Ouchi et al18 evaluated the postoperative results of 13 eyes with cataract and concomitant eye disease, including two cases of keratoconus.

In the latter, a refractive multifocal intraocular lens (LENTIS MPlus®, Oculentis GmbH, Berlin, Germany) was implanted leading to improved visual acuity and full independence of glasses for far.

The final quality of vision was classified by the patients themselves as being acceptable. However, the study fails because of the lack of description of the degree of ectasia, the characterization of astigmatism, as well as the evaluation of pre-existing aberrometry.

In cases of keratoconus

In most cases, patients with keratoconus have high, asymmetrical, irregular myopic astigmatism with vertical coma and vision distortion19. In addition, they develop cataracts at younger ages, still with an active working life, when compared to the general population20-22. Thus, it is necessary to evaluate the best time for surgery and what type of intraocular lens to implant.

For a good surgical outcome after implantation of a toric intraocular lens, patients must exhibit certain conditions such as: refractive and topographic stability of keratoconus, good vision corrected with glasses, slight irregular astigmatism (which may be asymmetrical but with highest possible orthogonality), the possibility of axis determination by topography or tomography and, finally, showing a relative enantiomorphism between the two eyes for binocular vision23.

Toric intraocular lenses implanted in patients with grade I and II keratoconus (Amsler-Krumeich classification) and cataract, showed good results in several studies20-26. Montano et al23 published two cases of keratoconus patients submitted to bilateral clear lens surgery with multifocal toric intraocular lens implantation. In one case, SND1T4 multifocal lenses (Alcon Laboratories, Inc., Fort Worth, Texas) were implanted bilaterally in a 50-year-old woman with bilateral keratoconus. In the other case, multifocal lenses AT LISA Toric 909M (Carl Zeiss Meditec) were implanted in a 42-year-old man with bilateral keratoconus and stable refraction for more than one year. In both cases an improvement of the uncorrected and corrected final visual acuity was reported, with the ocular refraction practically plano.

The authors concluded that the implantation of multifocal toric intraocular lenses can provide the correction of visual acuity to satisfactory levels with independence of spectacles23.

However, they admit that patients with good adaptation to rigid gas permeable contact lenses with good visual acuities may not be good candidates for this type of surgery.

IN CASES AFTER CORNEA TRANSPLANTATION

Anterior total or lamellar corneal transplants often give rise to high and often irregular astigmatisms (about 72%)27.

Despite several possible non-surgical approaches, about 8% of these patients require refractive surgery28.

In these cases, surgical hypotheses such as corneal incisions in the donor button, PRK, LASIK, implantation of toric phakic lenses or implantation of pseudophakic toric lenses may be proposed. Nuzzi et al29 suggested cataract surgery with a multifocal toric intraocular lens implant (SOLEKOTM Toric IOL, Rome, Italy) to a 45-year-old patient with full-thickness corneal transplantation at the age of 30 years.

Preoperative corneal astigmatism was 7.50 D and predominantly regular. As a final result, the authors showed improvement of uncorrected visual acuity, from 2/10 in the preoperative period to 6/10 in the postoperative period and a final refractive astigmatism lower than 1.0 D.

In order to calculate the benefit of implantation of multifocal toric IOLs, it is essential in these patients to evaluate the integrity and stability of the donor-receptor corneal wound over time, as well as endothelial cell viability, with consequent determination of the possible longevity of the corneal graft itself.

It was emphasized by Nuzzi et al29 that the implantation of toric intraocular lenses shows good results in patients whose astigmatism is predominantly regular.

IN CASES AFTER RADIAL KERATOTOMY

The eyes submitted to radial keratotomy, a procedure that was very popular in the 1980s for the correction of low myopia (up to about -6 D), usually present refractive changes over time, such as constant hyperopia, irregular and progressive corneal astigmatism, associated to visual acuity fluctuations.

These refractive and subjective disorders result from the biomechanical deformations of the cornea related to multiple radial incisions and may condition future cataract or clear lens surgery.

In the world literature, there are very few studies in which implantation of a multifocal intraocular lens was a possible and well tolerated solution in patients with radial corneal incisions30-32. Nuzzi et al30 presented a clinical case of a 46-year-old man with bilateral cataracts and 16 radial corneal incisions in both eyes, composite hyperopic astigmatism and best-corrected visual acuity of 6/10.

After implantation of a multifocal toric IOL (SOLEKOTM Toric IOL, Rome, Italy) he improved his uncorrected visual acuity to 10/10 with no need of glasses for all focal lengths. Visual acuity remained unchanged at least until the end of the 13-month follow-up.

In the opinion of the authors, this is a complex case in which the decision to implant the multifocal toric intraocular lens was controversial for the following reasons: a large number of radial incisions in the cornea that are likely to give rise to refractive instability over time, possible dysphotopsies generated by the irregular corneal curvature and by the intraocular lens itself, young age of the patient who may have his active professional life conditioned by subjective visual complaints and, finally, the possible difficulty in calculating the intraocular lens.

The preoperative evaluation in these cases is complicated, and it is very difficult to obtain reliable biometric values ​​for keratometry, quantification of astigmatism and determination of its axis, as can be seen in Figure 1, where three corneal tomograms were performed by auxiliary diagnostic methods (Orbscan IIz Bausch & Lomb; Pentacam Oculus; Galilei G4 Ziemer Ophthalmic Systems) in a patient with 8 radial corneal incisions. It is also easy to overestimate refractive corneal power and therefore to underestimate the power of the intraocular lens to be implanted.

There is still no consensus as to which the best method of acquisition is in determining keratometry and which are the best formulas for calculating intraocular lenses to be used.

Topographically calculated keratometry is very inaccurate in flat corneas at its center and in curved ones at its periphery. Keratometry analyzes an annular region around the center of the cornea, overestimating the corneal refractive power33,34.

Thus, it is fundamental that keratometric measurements are carried out at different times and by different methods of measurement and acquisition, and compared with each other, in order to reduce the possible error. There are no ideal intraocular lens calculation formulas for these cases.

The ASCRS online calculator, which uses its own algorithm for calculating the intraocular lens after radial keratotomy, may be an asset, but only for spherical lenses.

These patients, even after cataract surgery, may continue to present a hyperopic shift, with authors suggesting an increase in intraocular pressure as one of the main causes35.