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Indian Journal of Ophthalmology logoLink to Indian Journal of Ophthalmology
. 2024 Feb 23;72(4):508–519. doi: 10.4103/IJO.IJO_1241_23

Current concepts in the management of cataract with keratoconus

Maneck Nicholson 1,, Vivek M Singh 2, Somasheila Murthy 1,2, Damien Gatinel 3, Savio Pereira 4, Aditya Pradhan 5, Shail Vasavada 6, Prajakta Dandekar 1, Mekhla Naik 1, Supriya Sharma 1
PMCID: PMC11149527  PMID: 38389251

Abstract

This review analyzed all pertinent articles on keratoconus (KCN) and cataract surgery. It covers preoperative planning, intraoperative considerations, and postoperative management, with the aim of providing a simplified overview of treating such patients. Preoperatively, the use of corneal cross-linking, intrastromal corneal ring segments, and topo-guided corneal treatments can help stabilize the cornea and improve the accuracy of biometric measurements. It is important to consider the advantages and disadvantages of traditional techniques such as penetrating keratoplasty and deep anterior lamellar keratoplasty, as well as newer stromal augmentation techniques, to choose the most appropriate surgical approach. Obtaining reliable measurements can be difficult, especially in the advanced stages of the disease. The choice between toric and monofocal intraocular lenses (IOLs) should be carefully evaluated. Monofocal IOLs are a better choice in patients with advanced disease, and toric lenses can be used in mild and stable KCN. Intraoperatively, the use of a rigid gas permeable (RGP) lens can overcome the challenge of image distortion and loss of visual perspective. Postoperatively, patients may need updated RGP or scleral lenses to correct the corneal irregular astigmatism. A thorough preoperative planning is crucial for good surgical outcomes, and patients need to be informed regarding potential postoperative surprises. In conclusion, managing cataracts in KCN patients presents a range of challenges, and a comprehensive approach is essential to achieve favorable surgical outcomes.

Keywords: Astigmatism, cataract, custom toric IOL, IOL formulas, IOL power calculation, keratoconus, phacoemulsification, RGP contact lens, toric IOL


Keratoconus (KCN) is a non-inflammatory corneal condition with onset commonly in early adulthood, which gradually progresses and often stabilizes later in life. This entity’s clinical signs and symptoms have been well described in the literature.[1] The unique corneal changes in KCN are seen in topography and tomography. The corneal astigmatism in KCN adds to further loss of vision when one develops a cataract. People with KCN are known to develop a cataract earlier than people without.[2] Most of these patients are myopic, which increases their risk for cataract formation.[3]

A visually significant cataract in a KCN patient can pose a challenge. Irregular astigmatism and steep corneal topography confound the ocular biometry and interfere with the accurate calculation of the intraocular lens (IOL). Depending on the stage of KCN and the location of the cone, corneal regularizing procedures (such as the placement of intracorneal ring segments (ICRS)) or lamellar and penetrating keratoplasty can be combined either sequentially or simultaneously to address the problem of the irregular cornea as this helps obtain satisfactory results. Choosing an appropriate IOL and precise calculation of the IOL power in eyes with variable and inconsistent keratometry (K) and axial length (AL) readings are the main challenges.[4] Therefore, a tailored rather than a one-size-fits-all approach might be more suited in these cases.

This review describes various approaches to cataract surgery in a patient with KCN to provide solutions to this clinical challenge.

Diagnosis

According to an extensive meta-analysis conducted by Hashemi et al.,[5] the current global prevalence of KCN stands at 0.14%. However, it is worth noting that in certain population studies, the estimated prevalence has been as high as 8.9%.[6] A study conducted to analyze the prevalence of subclinical KCN among adults undergoing routine, uncomplicated age-related cataract extraction showed that subclinical KCN was present in 27.5% of eyes and was more prevalent in females and non-white ethnicities, suggesting that preoperative evaluation should include tomography.[7]

Lens spokes are common in KCN patients, and these are usually not visually significant. The most commonly identified type is the nuclear sclerosis type.[8] However, many young patients with KCN may have steroid-induced cataracts with posterior subcapsular opacification due to the chronic use of steroids for ocular allergy. The onset of cataract in KCN has also been reported to occur earlier than normal.[2] Most of these patients are myopic, which further increases the risk of cataract formation.[3]

One must maintain a high index of suspicion while performing routine K in cases with high astigmatism. It is advisable to inquire about whether the patient had suboptimal vision since childhood or youth and to check for a high cylinder in their glasses. In cataract patients who are unaware of the prior existence of KCN, topography is key. Topographical indices for the diagnosis of KCN are outside the scope of this review; it suffices to say that an elevation-based topography as against video-based topography has been advocated over the past decade. The slit-scanning technique is used in Orbscan II (Bausch and Lomb Surgical, Inc., Rochester, NY, USA); several instruments use the Scheimpflug technique (Pentacam, Oculus, Arlington, WA, USA; Galilei; Ziemer Ophthalmic Systems AG, Port, Switzerland; Precisio, Ligi Tecnologie Medicali, Taranto, Italy; and Sirius, CSO Ophthalmic, Instrument Company, Auckland, New Zealand) or HR OCT technology (Anterion Heidelberg, Casia Tomey).

Recognizing early vision loss in patients already diagnosed with KCN can be challenging. Therefore, meticulous evaluation is necessary to distinguish between vision loss resulting from corneal or lenticular changes. Performing cataract surgery in a patient with KCN progression and minor lenticular changes based on the assumption that cataract is the cause of vision loss would be erroneous. A high index of suspicion and scrupulous clinical and topographical examinations would help to characterize the impact of lens opacities on vision. History should include personal habits, lifestyles, and comorbidities. The clinical examination of a patient with KCN begins with a comprehensive medical history and a slit-lamp examination. Previous topography must be reviewed for stability. For a KCN patient with stable topography and declining BCVA, the physician should assess the lens for lenticular opacities and rule out retinitis pigmentosa or glaucoma secondary to steroid use. For those with access to the iTrace aberrometer (Tracey Technologies), the dysfunctional lens index can serve as a useful tool to evaluate cataract progression in patients with stable KCN.

It is important to elicit a history of eye rubbing in individuals with ocular allergy as it can trigger repeated biochemical stress on a genetically fragile cornea, potentially leading to disease progression and irregularities on the corneal surface, resulting in unreliable K readings and postoperative poor healing with the possible increased risk of infection.[9]

There are many ocular associations[10] of KCN, and Fuchs corneal dystrophy is one of these.[11] Cremona et al.[12] reported 27 cases of Fuchs dystrophy with KCN, and three patients were combined with anterior membrane dystrophy. Clinicians should be wary of the possibility of corneal thinning caused by KCN and false normalization due to an increase in corneal thickness caused by Fuchs dystrophy. Preoperative topography and specular microscopy would help identify this entity.[13]

Late-onset progression of KCN is an important entity that must be kept in mind while planning cataract surgery in these people, despite the traditional teaching that regardless of treatment, its progression is limited or does not progress after the age of 30 years due to naturally occurring cross-links in the cornea.[10] The Collaborative Longitudinal Evaluation of KCN (CLEK) study reported that fewer patients progressed to transplantation later in life (12%–20% aged 10–40 years vs. 8%–3% aged >40 years) and that older age at baseline was protective against requiring transplantation (OR: 0.72).[14] However, certain eyes with KCN do progress, the natural mechanism of which is not yet determined but could be linked to the pursued eye rubbing.[15] This is further compounded by the fact that many patients wear rigid gas permeable (RGP) contact lenses, which mold the cornea, compromising its measurements’ quality.[16] Many causes of pseudo progression have been documented in the literature, such as measurement errors, superficial punctate keratopathy, corneal stromal scarring, and even vernal or atopic keratoconjunctivitis. In those patients, progression was transient, and the topographic deterioration was fully reversible.

Most people diagnosed with KCN are chronic contact lens users, especially RGP lenses. However, the impact of long-term contact lens use cannot be ignored, and its history is pertinent to this discussion. History of swimming or sleeping with the contact lens on, using tap water to clean and store the contact lens, and non-adherence to the replacement schedule must be asked for.[17] Hypoxic damage caused by long-term RGP wear can affect the morphological and biological characteristics of the cornea, including pleomorphic changes in endothelial cells.[18] These damage the normal physiological function of tears and aggravate dry eyes.[19] Therefore, a comprehensive assessment of every KCN patient’s contact lens use is a must prior to surgery. As cataract is an elective procedure, one can wait to document a stable cornea by confirming a repeatable K after abstaining from contact lens wear. The RGP contact lens wear should be discontinued for at least 1–2 weeks to achieve a stable corneal topography.[20,21]

Guidelines for Cataract Surgery in KCN

The approach to a patient with cataract and KCN is summarized in Fig. 1.

Figure 1.

Figure 1

Flowchart of the approach to a patient with KCN and cataract

Corneal Procedures Prior to Cataract Surgery

KCN may be managed with scleral lenses, particularly when other lens modalities fail. Should the contact lens fitting fail, surgical management should be considered. These include the following (a detailed description is beyond the scope of this article) [Fig. 1]:

  • 4.1 Corneal collagen cross-linking

  • 4.2 Refractive surgery

  • 4.3 Intracorneal ring segments

  • 4.4 Corneal transplantation (PK/DALK)

  • 4.5 Combination of two or more procedures mentioned above

  • 4.6 Stromal augmentation techniques.

Corneal collagen cross-linking

Less than 10% of eyes with KCN were found to progress after the age of 30 years.[22] This finding is of considerable importance when considering cataract surgery in older patients with KCN. Notably, corneal collagen cross-linking (CXL) has been shown to be a safe treatment option for older patients, with reported complication rates of 2.6% in patients over the age of 35 years as compared to 3.9% in patients younger than 35 years. CXL attempts to increase the biomechanical stability and rigidity of the cornea in an attempt to prevent KCN progression. CXL has also been used successfully in combination with corneal ring segments and other surgical techniques.[23,24,25,26,27,28] Not much is written on CXL in people with KCN and cataract. Spadea et al.[29] evaluated the visual and topographic outcomes of a two-stage approach treatment for progressive KCN associated with high myopia and cataract. It involved CXL followed by phacoemulsification with IOL implantation, with at least a 6-month inter-procedure interval. The authors reported an improvement in uncorrected and corrected distance vision, corneal topography, and better tolerance of spectacles. There were no complications. A multicentric study[30] has shown that after CXL, refraction stability is achieved at 6 months, with no significant changes from 6 to 12 months. Hence, it is recommended to calculate the IOL power at least 6 months after CXL to obtain optimal corneal curvature data.

Intrastromal corneal ring segments

They were developed and FDA-approved in 1999 to treat low myopia; however, laser refractive procedures quickly overshadowed it. Colin et al.[31] reported their usefulness in treating mild-to-moderate KCN. ICRS implantation aims to improve spectacle-corrected vision or contact lens tolerance, thus delaying or eliminating the need for a corneal transplant.[31] The added advantage is that the procedure is reversible, and the segments are highly biocompatible.[32] ICRS in eyes with KCN reportedly improves visual quality.[33,34,35,36,37,38,39,40,41,42,43,44,45] Patients with moderate KCN and poor CDVA present a challenge. Corneal regularizing procedures such as ICRS may be beneficial prior to cataract surgery in such patients. Benefits of sequential ICRS and cataract surgery have been reported. Lee et al.[46] performed a femtosecond-assisted asymmetric Kerarings implantation, followed by cataract surgery with PCIOL implantation 3 months later. Following the Kerarings implantation, the central K reduced from 60 D to 55 D, and after cataract surgery, the best corrected spectacle-aided visual acuity improved to 20/25. Alfonso et al.[47] evaluated sequential Ferrara ICRS and IOL implantation. The interval between ICRS and cataract surgery was 6 months, and follow-up was up to 5 years. The procedure was safe, and 47 of 70 eyes regained good vision.

Topography-guided treatment

A new treatment algorithm developed by a private practice in Sydney, Australia aimed at optimizing refractive outcomes for patients with KCN and cataract describes cataract extraction with small-aperture IOL insertion, followed by topography-guided photorefractive keratectomy with simultaneous corneal cross-linking (CXL). The results of four eyes are reported, all achieving RGP contact lens independence, improved visual acuity, and regularization of corneal curvature with cone reduction.[48]

Penetrating keratoplasty (PK)

PK has been the surgery of choice for advanced KCN for over seven decades.[49] Around 10%–20% of patients with KCN eventually need a PK.[14,50,51] Currently, elective PK surgery is reserved for those with coexisting endothelial dysfunction (such as Fuchs endothelial corneal dystrophy/posterior polymorphous dystrophy) or when deep corneal scarring severely affects the visual axis up to the Descemet membrane layer. The PK triple procedure is an effective surgical procedure for patients with corneal diseases coexisting with cataracts.[52] The PK triple procedure has the advantages of a single procedure, rapid visual rehabilitation, and no additional endothelial trauma. Another choice is a sequential procedure, wherein cataract surgery is performed after keratoplasty. It allows us to accurately estimate IOL power; however, there are some drawbacks, such as multiple procedures, slow visual rehabilitation, and endothelial damage during cataract surgery.[53] Certain modifications have been suggested to improve the safety of PK triple procedures. These include the use of chandelier illumination for performing continuous curvilinear capsulorrhexis[54] and core vitrectomy before the PK triple in high-risk patients.[55]

Deep anterior lamellar keratoplasty (DALK)

DALK may be preferred in patients with KCN because of the reduced risk of endothelial rejection, earlier tapering of topical steroids, decreased risk of secondary glaucoma, and increased wound strength.[56] DALK should be offered before cataract surgery if the corneal pathology does not allow adequate surgical view for safe cataract surgery or if the patient needs improved refractive outcome.[57] It was reported that when a type-1 big bubble is achieved, simultaneous DALK and phacoemulsification can be safely performed. Dua’s layer allows a clear view for performing phacoemulsification with the added benefit of its toughness, which maintains a stable anterior chamber for cataract surgery. Phacoemulsification must not be attempted when a type-2 big bubble is achieved.[58]

Stromal augmentation techniques

The literature describes several techniques for the treatment of corneal disorders, including Bowman’s layer transplantation (BLT), stromal lenticule addition keratoplasty (SLAK), and intrastromal implantation of regenerated stromal lamina. The BLT technique involves harvesting a donor cornea and isolating the Bowman’s layer (BL), which is then transplanted onto the recipient cornea. The rationale behind this technique is that replacing the ruptured BL with the harvested BL will provide additional strength to the weak cornea, flatten the anterior corneal surface, improve visual acuity, and halt disease progression.[59] The SLAK technique involves implanting a negative meniscus-shaped lenticule, which is thinner in the center and thicker in the periphery, onto the recipient cornea. This surgical approach aims to augment corneal thickness and achieve corneal flattening, similar to the effect achieved after intrastromal corneal ring segment implantation.[60] These techniques are still in their nascent stages, and incorporating them into the management of KCN with cataract would require further studies.

IOL Choices

The primary goal of cataract surgery with IOL is visual rehabilitation by restoring ocular transparency and reducing the total corneal cylinder to make the patients comfortable with spectacles and contact lenses. There are successful reports of several varieties of IOL in people with cataract and KCN. Some of these are mentioned below [Fig. 1].

Monofocal single piece non-aspheric IOL

Thebpatiphat et al.[2] reported a reduction of −1.44 ± 1.69 D in mean spherical equivalent in eyes with mild KCN and a reduction of −5.85 ± 3.94 D in eyes with moderate KCN with implantation of non-aspheric monofocal IOL after phacoemulsification. In a study by Watson et al.,[61] the mean spherical equivalent was less than 1 D in 60% of eyes with mild KCN and in 41.9% of moderate KCN.

Toric IOL

In 2003, Sauder et al.[62] were the first to report successful implantation of silicon toric IOLs in two eyes with advanced KCN with good postoperative outcomes. In 2009, Navas and Suarez reported successful refractive lens exchange (RLE) with hydrophobic acrylic toric IOL implantation in forme fruste KCN eyes.[63] In another study, RLE with toric IOL implantation in patients intolerant to routine RGP lenses has been shown to reduce overall astigmatism without inducing any higher-order aberrations. In addition, the corneal incisions did not result in any significant changes in corneal astigmatism in these eyes.[64]

Subsequently, toric IOLs have been used widely for non-progressive KCN with documented improvement in the uncorrected distance visual acuity, corrected distance visual acuity, reduction in the spherical equivalent, and reduction in the preoperative cylinder.[65,66,67,68,69,70,71] However, these results are limited to the mild and moderate stages of KCN. Advanced KCN has shown variable results owing to the large amount of irregular corneal astigmatism in these eyes.[61,72] Therefore, toric IOLs are recommended only for mild-to-moderate stages of KCN. Patients with advanced KCN where keratoplasty is anticipated in the future are not ideal candidates for toric IOL implantation.[73]

Toric IOLs are good for eyes with high preoperative regular corneal astigmatism.[74] Eyes with KCN could have significant regular or irregular astigmatism. The magnitude of astigmatism in KCN eyes can be very high. Neutralizing such a high cylinder at the corneal plane requires an IOL with high toricity for in-the-bag implantation. Commonly available toric IOLs correct up to 4 D of astigmatism at the corneal plane.[74] Correction of higher astigmatism will require customized toric IOLs. Reddy et al.[75] reported a significant reduction in the cylinder by using customized toric IOLs in four KCN eyes [Video 1]. Toric IOLs should be used with caution as they are at risk of misalignment owing to the long AL and large capsular bag.[76] However, re-alignment of these IOLs has been shown to restore the visual acuity even on late presentation [Figs. 2 and 3].[77]

Figure 2.

Figure 2

(a) Printout photograph of the Pentacam of the left eye with cataract and high astigmatism due to pellucid marginal degeneration. (b) Corresponding printout photograph of IOLMaster showing a higher cylinder; however, the magnitude of astigmatism is slightly lesser than seen on the topography. (c) Postoperative slit-lamp photograph in retroillumination showing the in-the-bag implantation of a customized toric intraocular lens. (d) The iTrace aberrometer shows a slight misalignment that can happen in patients with an irregular cornea. Postoperatively, however, the patient improved significantly and could read 20/50 unaided improving to 20/25 with −1.50 Dcyl

Figure 3.

Figure 3

(a) Preoperative Pentacam image of the right eye of a patient with severe keratoconus with the presence of a posterior subcapsular cataract, showing an astigmatism of approximately 8 D. The patient underwent cataract surgery with implantation of hydrophobic customized intraocular lens. (b and c) Comparative pre and postoperative iTrace images of the same patients showing a marked reduction in the cylinder from −10.0 Dcyl to −3.5 Dcyl, respectively, with a best corrected visual acuity improving to 20/25

Multifocal IOL

Good visual outcomes have been reported with a bifocal toric IOL in eyes with forme furste and non-progressive KCN,[78] with diffractive trifocal toric IOL in mild-to-moderate KCN.[79]

Piggyback IOL

Piggyback IOLs play a dual role in correcting the full dioptric error in highly ametropic patients, as well as correcting any residual, undesirable spherical, and toric refractive error post cataract surgery. The incidence of interlenticular opacification[80] has been reduced by placing one lens in the posterior capsular bag and the other in the sulcus, rather than both lenses in the bag. However, research on their use in KCN patients is limited, with only one case report demonstrating success in correcting residual spherical refractive error in a highly myopic patient. Recently, toric implantable collamer lenses (ICLs) have also been used off-label to correct residual refractive error post-cataract surgery in these patients.

Corneal asphericity

Corneal asphericity is another important factor to consider while choosing an IOL in these eyes. Savini et al.[81] have shown a trend of myopic surprise in prolate corneas, whereas oblate corneas may result in hyperopic surprises. KCN patients have hyperprolate corneas with a large negative asphericity (Q). An IOL with a negative asphericity might add to the existing negative asphericity of the KCN eye. Therefore either a non-aspheric IOL or an IOL that has positive asphericity is likely to result in more favorable outcomes.[82]

Newer devices

A mention must be made here about the Xtrafocus pinhole device, which is an alternative for the treatment of cases of irregular corneal astigmatism caused by KCN and PK. In a study conducted by Trinidade et al., a marked improvement in visual function was seen with high patient satisfaction. Despite the expected reduction in the light entrance, the apparent brightness perceived by the patients was not proportionally impaired. However, its clinical role needs further evaluation.[48]

IOL Power Calculation

IOL power calculation is considerably difficult in eyes with cataracts and KCN as it requires a few assumptions thrown askew by their keratometric changes. If the patient is using contact lenses, it is advisable to request that they discontinue wearing them for at least 1 month before biometry and subsequent cataract surgery.

We make certain assumptions about a normal cornea for IOL power calculations. The standard refractive index of the cornea is 1.3375. However, using a standard K index can lead to an overestimated corneal power and underestimated IOL power, thus leading to a hyperopic refractive error.[8]

Manual and automated K (e.g., Placido disk-based corneal topography) are widely used worldwide. Hashemi et al.[83] examined the repeatability of five devices based on five different measurement techniques, which included the Pentacam, EyeSys (EyeSys vision, Houston, Texas, USA), Orbscan, IOLMaster (Carl Zeiss Meditec AG, Jena, Germany), and Javal manual keratometer. They found that the Pentacam had better repeatability, followed by the Javal manual keratometer in eyes with a Kmax of 55 D or less. However, there was weak reliability in all imaging systems when the Kmax was greater than 55 D. Using Orbscan II (Bausch and Lomb, Rochester, NY), OPD-Scan III (Nidek, Gamagori, Japan), and iTrace (Tracey Technologies), Guilbert et al.[84] also showed that topographies performed in keratoconic eyes are less repeatable than those performed in normal eyes. Yagci et al.[76] found the repeatability of the flat K and steep K values measured by a biometry device combining optical interference and Scheimpflug principle (AL-scan, Nidek Co., Ltd.) was high in both normal and KCN eyes. However, the steep K had less reproducibility, a finding consistent with Hashemi et al.’s study. Newer machines, such as IOLMaster 700 with TK (based on a swept source OCT), and elevation-based devices, such as the Pentacam and Galilei, measure the posterior conceal curvature and are increasingly used in clinical practice. There is no agreement on the specific corneal location where this measurement should be made; some authors propose a central location, which has more correspondence with the visual axis.

Watson et al.[61] found that using actual K values in eyes with severe KCN resulted in less predictable refractive outcomes than standard K (43.25 D). They suggested using standard Ks for severe KCN and the patient’s actual Ks for mild-to-moderate disease. Thebpatiphat et al.[2] reported no advantage of topography-derived Ks over standard Ks in mild cases and found a myopic error in advanced cases.

In a multicentric study, Kamiya et al.[85] evaluated two devices to measure the refractive power of the cornea. In IOLMaster 500, the keratometric readings within a ring located approximately 2.4 mm from the center and using the standardized anterior/posterior corneal ratio (1.3375) showed a high tendency toward hyperopia in advanced KCN. When the keratometric value of choice was total corneal power, as measured with the Pentacam topographer within the 15° central ring (equivalent to the 3.0-mm ring), the postoperative refractive error showed a myopic trend. They concluded that using a keratometric index of 1.3375 results in an overestimated corneal power and induces an error in IOL power calculation. The difficulties encountered in IOL power calculation in KCN patients have been summarized in Table 1. The various formulas and their role in IOL power calculation in such patients are summarized in Table 2.

Table 1.

Difficulties in IOL power calculation

Difficulty Cause Solution
Overestimated corneal power and underestimated IOL power, leading to hyperopic refractive error[8] Using standard K index for IOL power calculations Use actual K values for mild-to-moderate keratoconus and standard K values for severe keratoconus
Weak reliability in all imaging systems when Kmax is greater than 55 D[83] Use of topography in keratoconic eyes Use newer machines such as IOLMaster 700 with TK and elevation-based devices such as Pentacam and Galilei, which measure posterior corneal curvature
Less predictable refractive outcomes in severe keratoconus[61] Use of actual K values Use standard K values for severe keratoconus and actual K values for mild-to-moderate disease
Overestimated corneal power and induced error in IOL power calculation[85] Using keratometric index of 1.3375 on IOLMaster Use total corneal power measured with Pentacam topographer within the 15° central ring for keratometric readings

IOL: Intraocular lens; K: Keratometry; Kmax: Maximum Keratometry; TK: Total Keratometry

Table 2.

Studies evaluating formulas for IOL power calculation in eyes with cataract and KCN

Study IOL formulas compared Findings
Thebpatiphat et al.[2] SRK, SRK II, and SRK T SRK II had the least prediction error in people with keratoconus.
Mehrjerdi et al.[86] SRK II Most reliable and ideal formula for IOL power calculation in patients with various stages of keratoconus irrespective of the axial length, although it was less reliable in less severe stages.
Hashemi et al.[87] Hoffer Q (<22.0 mm), SRK II (22.0–24.5 mm), Holladay 1 (24.5–26.0 mm), and SRK T (>26.0 mm) K derived from the 3-mm central zone in the axial map of corneal topography and the use of the SRK T formula results in the least error in IOL power calculation.
Savini et al.[88] Barrett Universal II, Haigis, Hoffer Q, Holladay 1, and SRK T SRK T was more accurate than others, although the results were worse in the advanced stages of keratoconus.
Zhang et al.[89] SRK T, Haigis, Hoffer Q, Holladay, and Barrett Universal II Barrett Universal II formula was more accurate in predicting the IOL power.
Alio et al.[69] SRK T and Hoffer Q SRK T formula tended to under-correct myopes, and Hoffer Q formula tended to overcorrect hyperopes.
Ton et al.[90] Barrett True-K formula for keratoconus with measured or predicted posterior corneal power, Barrett Universal II formula, Kane formula, Kane formula for keratoconus, SRK T formula, Haigis formula, Holladay 1 formula, and Hoffer Q Barrett True-K formula with measured posterior corneal power, SRK T formula, and Kane formula for keratoconus resulted in a prediction error within ±0.50 D of 87.5%, 59.4%, and 53.1%, respectively. Keratoconus-specific formulas had a lower mean error in predicted refraction than conventionally calculated formulas.
Kane et al.[91] Haigis, Barrett Universal 2, Holladay 1 and 2, Hoffer Q, SRK/T, and Kane) compared to IOL formulas which are made exclusively for keratoconus patients (Kane keratoconus formula and Holladay 2 with keratoconus adjustment The Kane Keratoconus formula performed the best in all stages of keratoconus, followed by SRK/T

SRK: Sanders–Retzlaff–Kraff; IOL: Intraocular lens; D: Diopter

IOL power calculation post ICL surgery

The accuracy of IOL power calculations in patients having undergone ICL implantation prior to cataract surgery is a subject of interest. Various advanced optical biometry instruments are utilized, such as IOLMaster 700, IOLMaster 500, Pentacam AXL, and Sirius, each with distinct measurement techniques.[92,93,94] However, the impact of ICL presence on these measurements remains unclear, with limited research available. Some studies suggest minimal AL deviations post ICL surgery, leading to negligible IOL power errors. Similarly, deviations in ACD and lens thickness (LT) generally have minor effects on IOL calculations.[95] Most deviations fall within acceptable clinical limits, and the primary influencers of IOL power are K and AL, rather than ACD and LT. One study suggests that misidentification of the ACD with IOLMaster 700 could potentially influence power calculations when using the Haigis and Barrett Universal II formulas. Conversely, the SRK/T formula appears to be unaffected. To address this issue, it is recommended to substitute AS-OCT derived ACD and lens thickness (LT) values for accurate measurements in cases utilizing the Barretts Universal II and Haigis formulas.[96]

Cataract Surgery

Patients with KCN tend to develop lens opacification earlier on in life.[2] Modern phacoemulsification surgery can be technically challenging in such patients. The technical difficulties depend on the degree of KCN. Early KCN can be managed like a routine phacoemulsification; however, severe KCN with steep meridians and possible corneal scars can be troublesome. The following guidelines are worthy of consideration while performing phacoemulsification in these eyes.

Patient expectations

The patients must be informed about potential postoperative visual symptoms and dry eye, as well as the possibility that the surgeon may not be able to achieve the intended refractive target. Despite the team’s best efforts, it is highly likely that patients will still require spectacles for both distance and near-vision correction. Therefore, it is crucial not to make unrealistic promises to patients regarding complete freedom from glasses after surgery. Under-promising and over-delivering can ensure patient satisfaction. In addition, patients should be advised about changes in the power of their contact lenses and the need for a new pair.

Main incision

In corneas with abnormal structural properties, as in KCN eyes, the postoperative K values and corneal shape can change unpredictably, unlike in normal corneas, where the main incision produces a negligible astigmatic error. Hence, the peripheral corneal thickness should be considered to decide the site of the main incision in the KCN eyes instead of preoperative K values. In the case of an inferotemporal cone, a superior or superotemporal approach should be preferred; in superior steepening, an inferotemporal or temporal incision should be chosen. Scarring is usually seen in advanced KCN, and it should be taken into account as well. In case of inferior scarring, a superior approach is not advisable as most maneuvers would be performed inferiorly, and adequate clear visualization would not be possible. On a general note, it is better to choose the main incision at a site 90° away from the site of scarring.[97] The KCN corneas are thinner and more pliable than normal corneas; hence, a smaller 2.2-mm incision is better and should be sutured at the end of the procedure. The maneuverability of the phacoemulsification probe is important; one must not undersize the incision at the cost of injuring it or making it unstable. This is more crucial in eyes with advanced KCN with central thinning/hydrops/scarring where the biomechanical stability of the cornea is largely compromised. A 3-step sclerocorneal approach offers better postoperative stability than a clear corneal wound. It must also be borne in mind that corneal incisions can weaken the cornea, and all attempts must be made to secure the wound and keep a close watch over it post-surgery.

Paracentesis

No significant change in the site and structure of paracentesis is warranted in KCN.

Ophthalmic viscosurgical device (OVD)

A dispersive OVD that protects the endothelium in the eyes with KCN.

Intraoperative visualisation

In cases of diffuse scarring, the surgeon should expect a worse view during surgery than assessed on a slit lamp. This is because the light scattering is greater under an operating microscope with central diffuse illumination than with slit-lamp examination with oblique illumination. In addition, the KCN with intracorneal ring segments (ICRS) would have an irregular corneal surface hampering visualization due to a parallax error. This can be overcome by coating the cornea with a viscoelastic during the entire procedure for a better intraoperative view. Incidentally, it also offers mild magnification.[98] An alternative is to use a sterile RGP contact lens (7.8-mm base curve, 8.8-mm diameter, 0-D power) over an OVD spread on the cornea.[99] The contact lens is sterilized at a low temperature (55°C for 210 min) by using ethylene oxide gas, after which it is placed on the corneal surface, and a cohesive viscosurgical device is used to fill the eye–contact lens interface. A significant improvement in visibility has been reported [Fig. 4].[97]

Figure 4.

Figure 4

(a) Slit-lamp photograph of the right eye with advanced central ectasia and a distorted view of the anterior segment and the underlying cataract. (b) Topography map of the same eye showing a steep localized central cone. (c) Intraoperative image of the capsulorrhexis being performed through an RGP CL. The stained capsule helps enhance the view of the capsulorrhexis margin. (d and e) Intraoperative image of phacoemulsification being performed through a rigid gas permeable contact lens with minimal distortion and significant improvement in the surgical view

Continuous curvilinear capsulorrhexis (CCC)

In spite of good red reflex, it is recommended to use capsular staining dye for better visualization and a more predictable CCC.

Femtosecond laser-assisted cataract surgery (FLACS)

Docking patients with advanced KCN can be challenging due to the irregular corneal shape and higher chances of suction loss. It is never advised to opt for corneal incisions and arcuate keratotomy incisions despite accurate intraoperative OCT scans; the postoperative results are uncertain due to the unpredictable biomechanics of the cornea. Corneal scars, corneal distortions (compression zones), and corneal segments hamper accurate laser delivery. CCC and nucleotomy should be performed only in the absence of scarring and ICRS. Care should be taken while dealing with eyes with an ICL as the femtosecond machine OCT will account for the anterior surface of the ICL as the anterior surface of the anatomical lens for the CCC by default. Manual adjustment is mandatory in these cases so that the femtosecond laser machine achieves a complete CCC.

Surgery post ICL implantation

ICL explantation can be done at the time of cataract surgery in the same sitting. It is done prior to making the primary incision, from the side port. The AC is filled with viscoelastic, ensuring viscoelastic is injected under the ICL. Micrograsping forceps are then used to prolapse the haptics into the anterior chamber, and the ICL is brought into the AC. The main incision is then made and the ICL is removed in toto or cut into two halves and explanted from the eye.

Postoperative Visual Rehabilitation

Timing

It is important that post cataract surgery, the wound has completely healed and the topography indices are stable before refraction. At the second follow-up visit, which is usually at 4–6 weeks, the refraction stabilizes and glasses can be prescribed. If the patient is still on a tapering dose of topical steroids, it is recommended to commence a contact lens trial once the course of medication is complete. If the wounds were sutured, refraction should be done 2 weeks after the sutures are removed.

Glasses/CL

Glasses not only provide visual acuity correction but also offer protection to these fragile corneas. The recommended glass materials are polycarbonate and Trivex, which have a high refractive index, are lightweight,[100] provide 100% UV protection from UV-A and UV-B rays, and stay strong and impact resistant. However, the patient must understand that with spectacles, the best possible visual function after surgery will be the same as the best visual function with glasses prior to the cataract. Post cataract surgery, monofocal IOLs will correct the spherical error, and toric IOLs will correct some components of regular astigmatism. Residual astigmatism (both regular and irregular) can be corrected by contact lenses, and if lenses have been worn prior to surgery, their continued use would be beneficial to maximize visual outcomes. Calculation of IOL power depending on the preoperative contact lens power is not recommended as one must try to correct the maximum refraction with the IOL itself. Patients should be made aware of this fact as the cost of a new contact lens after surgery must be factored in, though the type of contact lens may remain the same.

Residual error

Early large refractive errors can be corrected by performing a monofocal IOL exchange. In a study comprising 34 eyes diagnosed with cataract and stage 1 and 2 KCN, it was found that 32% of the cases required an IOL exchange due to inaccurate calculation of IOL power. In patients with a large error and old fibrose capsular bags, piggyback IOL or phakic ICL implantation may be better choices. One study analyzed the outcomes of their cases in which a toric ICL was implanted as a secondary procedure (toric IOL was not implanted when the manifest and topography astigmatism axis did not correlate) and found good outcomes.[101]

Corneal procedures post cataract surgery

The available literature does not provide a definitive answer regarding the efficacy of ICRS and topography-guided treatment (TPRK) for correcting residual refractive errors in such patients. However, it can be stated that TPRK may offer more accurate results than ICRS, which may be less predictable.

Special Situations

Down syndrome with KC

The association between Down syndrome and KC has been established in many studies,[102] although the precise underlying mechanism remains elusive. Various investigations have reported KCN frequencies ranging from 0% to 71% among individuals with Down syndrome; however, many of these studies have been constrained by limited sample sizes and biased patient selection.[103,104,105] Consequently, there is a compelling rationale to consider the implementation of screening protocols for KCN in individuals with Down syndrome. To address the continuous inciting factor, eye rubbing in these children, CXL is recommended for all cases and is performed under general anesthesia. Both eyes can be cross-linked simultaneously. The Dresden protocol is considered ideal due to its proven effectiveness as it can be challenging to bring these children to the hospital.[106] However, there is a higher risk of infection,[107] especially when there is associated vernal keratoconjunctivitis (VKC). To strike a balance and reduce this risk, other protocols have been explored. For instance, the “Epithelium on” protocol reduces the possibility of infection. Another protocol, known as “Sub 400,” is used for thin corneas, with a safety margin of 70 microns from the endothelium.[108]

Early cataract occurrence has been documented in children aged up to 17 years with Down syndrome, with reported rates of up to 50%.[109] Therefore, it may not be uncommon to encounter such patients with Down syndrome and KCN requiring cataract surgery in clinical practice, and general ophthalmologists must be prepared to treat them. Surgical intervention in these individuals may be delayed due to systemic issues related to the cardiac or endocrine systems. This issue gains particular relevance as many patients initially present with white cataracts and impaired vision upon their first visit to our hospital. Simultaneous bilateral cataract surgery is usually performed in cases with bilateral white cataracts, primarily to mitigate the risk of a second exposure to general anesthesia and expedite visual rehabilitation.[109] Postoperative visual rehabilitation usually involves the use of glasses. Evaluating visual function and acuity in these patients may be challenging due to subnormal mental intelligence, which makes assessing visual acuity difficult in these patients. Moreover, suboptimal visual outcomes must be anticipated not only due to the presence of KC but also due to associated ocular comorbidities, such as myopic maculopathy, glaucoma, nystagmus, and corneal scars. These conditions must be actively searched for and appropriate counseling provided before surgery.

Vernal keratoconjunctivitis

KC has long been associated with VKC.[110] Although both diseases are self-limited, their progression primarily occurs in early adulthood. The coexistence can lead to visual deterioration due to visually significant corneal scars; in addition, prolonged steroid use can result in significant cataracts.[111] Chronic steroid usage causes posterior subcapsular cataracts, and in most cases, these steroid-responsive individuals also experience elevated intraocular pressure (IOP).

Egrilmez et al.[111] demonstrated that the group with both KC and VKC had a higher incidence of steroid-induced cataracts (P = 0.038) than the group with KC alone. Furthermore, these cases may be complicated due to constant ocular surface inflammation, resulting in secondary vascularization and a tendency toward trophic epithelial defects.

Surgeons encounter several challenges when operating on such patients. Corneal scars and shield ulcers can impede biometry and intraoperative visualization. Managing elevated IOPs is crucial through medical management or sequential/simultaneous glaucoma surgical procedures. In addition, limbal stem cell deficiency is a known association in such patients who may experience pre- and postoperative exacerbated dryness.

In cases of extensive scarring in conjunction with VKC and KC, biometry becomes challenging. The average K value obtained from corneal topography over the corneal center is utilized for IOL power calculation. In cases with significant distortion in keratometric values, standard K can assist in IOL calculation. The use of premium IOLs is generally not recommended.

Successful cataract surgery depends on the careful management of inflammation and modifiable risk factors such as eye rubbing, which can impact prognosis and outcomes. Discontinuing eye rubbing[112] and controlling inflammation[113] can potentially halt or slow down KC progression.

Before proceeding with cataract surgery, it is essential to ensure that the ocular surface is free of inflammation. This can be achieved through various topical treatment options. Mast cell stabilizers are safe with minimal side effects but are less effective than topical corticosteroids and immunomodulators, which are the most potent anti-inflammatory agents. During surgery, the surgeon may consider performing clear corneal phacoemulsification (although SICS and ECCE can be performed when indicated). It is best to avoid areas of corneal thinning and be mindful of the corneal epithelium. A low threshold for suturing wounds is to be maintained. It is preferable to avoid NSAIDs in the postoperative period and opt for preservative-free lubricants and antibiotics instead.

Conclusion

The management of cataracts in patients with KCN requires extensive preoperative intraoperative and postoperative planning to achieve optimal visual outcomes. Patient counseling is paramount for them to have realistic expectations in light of the somewhat unpredictable outcomes. It would be beneficial for ophthalmologists to familiarize themselves with the patient’s history, measurement devices, and IOL power calculation and formulas before operating such challenging albeit rewarding cases.

Financial support and sponsorship:

Shantilal Shanghvi Foundation (SSF).

Conflicts of interest:

There are no conflicts of interest.

Video available on: https://journals.lww.com/ijo

Download video file (27MB, mp4)

Acknowledgement

We would like to express our gratitude to Dr. Taraprasad Das for his expert review and insightful suggestions.

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