KEY CONCEPTS
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Cataract surgery in the keratoconus patient is challenging. Difficulties in determining accurate measurements for intraocular lens calculations in advanced cases can lead to inaccurate and unpredictable results.
Introduction
Keratoconus (KC) has been classically regarded as a noninflammatory condition characterized by progressive corneal thinning and steepening that results in high irregular astigmatism, myopia, and eventually visual impairment. , Recent studies have suggested that inflammatory markers are overexpressed in KC patients, even at a subclinical stage. The prevalence of KC estimated from population-based studies ranges from 120 to 3300 per 100,000 individuals. It may be possible that KC patients are at higher risk of developing cataract sooner when compared with non-KC patients. A plausible explanation, for example might be the frequent use of steroids for allergic conjunctivitis. , From the standpoint of management of cataract in the KC patient, calculation of the intraocular lens (IOL) power is challenging due to irregular high astigmatism and atypical biometric characteristics (greater axial length, anterior chamber depth, and white-to-white measurements). In this chapter we will review preoperative assessment, IOL power calculation, selection of IOL, considerations during surgery, and special situations encountered in cataract surgery in KC.
Preoperative Assessment
Patients with clinically significant cataract may often complain of glare and loss of contrast. These symptoms are usually present in patients with KC without lens opacification. Therefore, it is crucial to perform a complete and thorough examination to distinguish the precise origin of the visual complaint. The conjunctiva should be carefully examined and both tarsi explored.
Allergic conjunctivitis is commonly present and should be treated and stabilized before surgery is planned. Corneal epithelial irregularities are common in long-standing contact lens users and should be treated with intense lubrication and/or therapeutic contact lens. Corneal subepithelial fibrosis is also common, both as a component of KC and from chronic contact lens touch by apical rigid gas permeable lenses.
Salzmann nodular degeneration may also be present. The decision to treat this condition before surgery is recommended to achieve a proper IOL calculation. Treatment of Salzman´s nodular degeneration before cataract surgery is recommened in order to properly calculate IOL power and alignment. Although outside the scope of this chapter, this can be treated with excimer laser phototherapeutic keratectomy (PTK) ablation, surgical nodulectomy, and anterior lamellar transplantation. The stroma should be examined for anterior scarring or evidence of previous corneal hydrops present as posterior corneal fibrosis. Although KC is largely considered an anterior disease, the endothelium should also be carefully evaluated. Although infrequent, KC may coincide with Fuchs endothelial dystrophy or posterior polymorphous dystrophy. In the first case, slit-lamp examination by specular reflection may show cornea guttata in the central corneal endothelium or edema localized to the posterior stroma. In the latter, biomicroscopy may reveal posterior corneal vesicles and opacities in linear bands and other polymorphous configurations. In patients with abnormal endothelium, specular microscopy should be mandatory.
A complete ophthalmological assessment should be performed with corneal topography/tomography, aberrometry, interferometry, and endothelial cell count. The axial length and the keratometry are the two parameters required by all currently available formulas. The stability, severity, and location of the KC have a great influence on preoperative biometry, resulting in an overestimation of the corneal power and underestimation of the IOL power with consequent postoperative hyperopia.
The irregular corneal shape in eyes with KC makes reliable keratometry difficult to obtain and use because of K values from the apex that differ from the rest of the cornea, displaced visual axis, fixation difficulties during measurements, and variability in K readings between and within devices. The Pentacam TM Scheimpflug pachymeter and the Sirius TM system appear to have better repeatability than other devices for K values less than 55 diopter (D). , Cross-linking or intracorneal ring segments (ICRSs) prior to cataract surgery may improve biometry measurement and final visual outcomes (see Algorithm 35.1 , and refer to Combined Procedures section later).
Calculating the corneal power with the standard keratometric index ( n = 1.3375) may result in an overestimation of 0.5 to 2.5 D. This index is used to calculate the refractive power of the entire cornea from the anterior corneal curvature, assuming a normal ratio between the anterior and posterior corneal curvature. This assumption is likely disrupted in the corneal ectasias. Camps et al. developed an optimized algorithm that minimizes the error for IOL power calculation in KC.
Intraocular Lens Power Calculation
Formulas for IOL power calculation are the same as those used for patients without KC. The best method for IOL calculation in KC is controversial, Studies have evaluated the predictability of various formulas, including Haigis, Holladay 1, Holladay 2, Hoffer Q, SRK II, SRK/T, Barrett Universal, and Kane formulas, in patients with KC, with variable results. , , , A study by Leccisotti obtained good visual results by calculating IOL power for early-stage KC with the Holladay 2 formula, targeting a residual myopic astigmatism using the steepest meridian over the central 3 mm of the cornea as the K1 and the flattest meridian as the K2. Each meridian power was calculated by averaging the values of its two meridians. Other authors used the Hoffer Q for lower axial lengths and SRK/T for standard axial lengths targeting for emmetropia, resulting in undercorrection of the SRK/T group in contrast with overcorrection of the Hoffer Q. Another study evaluated SRK, SRK II, and SRK/T formulas, and concluded that SRK II was the most accurate formula in mild KC. The predictability of IOL power calculation in eyes with stage 3 and 4 KC is poor. Patients with K values >55 D have more predictable results when using a standard K value (43.25 D) instead of the actual K values. , Recently, Wang et al. demonstrated Barrett Universal as a predictable formula for different grades of KC. Kane et al. presented good results with the Barrett formula but proposed Kane original and Kane modified (Kane keratoconus) formulas for all stages of KC, showing less mean absolute prediction error compared with other formulas ( Table 35.1 ).
| Mean Absolute Prediction Error (D) | |||
|---|---|---|---|
| Formula | Stage 1 K m ≤ 48 D | Stage 2 K m > 48 D≤ 53 D | Stage 3 K m > 53 D |
| Kane keratoconus | 0.49 | 0.53 | 1.44 |
| Kane (original) | 0.49 | 1.00 | 2.64 |
| Barrett | 0.54 | 0.89 | 2.45 |
| SRK/T | 0.56 | 0.51 | 2.32 |
| Holladay 1 | 0.56 | 1.12 | 3.07 |
| Hoffer Q | 0.57 | 1.47 | 3.36 |
| Haigis | 0.58 | 1.34 | 2.88 |
| Holladay 2 (original) | 0.62 | 1.05 | 3.01 |
| Holladay 2 (keratoconus adjustment) | 0.64 | 1.41 | 3.19 |
a Median absolute predicted error of the different formulas divided into keratoconus stage. Adapted from Kane JX, Connell B, Yip H, et al. Accuracy of intraocular lens power formulas modified for patients with keratoconus. Ophthalmology . 2020;127(8):1037–1042.
Some toric calculators use data from predicted lens position and spherical power of the IOL, as well as posterior corneal astigmatism, to derive the suggested lens power.
A review by Gupta and Caty suggested the Barrett calculator, the new Alcon toric calculator (a derivation of the Barrett calculator), and the Holladay toric calculator (integrating the Abulafia-Koch regression formula) as the most accurate methods to calculate toric lens.
Some of the available toric IOLs are Acrysof toric, Rayner toric, Tecnis toric, and Zeiss toric, among others, each with its own online toric IOL calculator ( www.acrysoftoriccalculator.com , www.raynertoriccalculator.com , www.tecnisiol.com , www.zcalc.meditec.zeiss.com ).
Intraocular Lens Selection
Deciding between monofocal, spheric, aspheric, toric, multifocal, or trifocal IOLs requires a thorough evaluation. The contour of the KC cornea renders it multifocal, but the visual axis is not at the apex of the cornea, which commonly generates asymmetric astigmatism, a challenge for refractive surgeons. The goal of implanting toric IOLs is to provide less spectacle dependence, better corrected vision, and improved quality of vision. This type of IOL is a suitable option for KC patients with the following characteristics , , , :
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Grades 1–2 KC of the Amsler-Krumeich classification.
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Nonprogressive corneal ectasia, at least in the previous 12 months.
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Low possibility of future keratoplasty or postoperative use of rigid gas permeable or scleral lenses.
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Good historical visual acuity (20/40 or better) not dependent on rigid contact lens use.
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Central 4 mm of the cornea without significant irregular astigmatism.
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Orthogonality or a proper axis to align the toric IOL, which is mandatory ( Fig. 35.1 ).
Fig. 35.1
High toric intraocular lens (IOL) implantation in both eyes of a keratoconus patient, despite extreme astigmatism on corneal topography (A, C). There was an identifiable axis to properly align the toric IOLs. Notice the linear IOL marks coinciding with topographic astigmatism (B, D).
Patients with long axial length (>25 mm) or large white to white (>12.5 mm) have an increased risk of IOL rotation, so these factors should be considered during IOL selection. If a toric IOL is chosen for these patients, a capsular tension ring placement is recommended to avoid IOL rotation ( Fig. 35.2 ).
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