Corneal Imaging for Evaluations of Patients With Cataracts
J. Bradley Randleman, MD; Marcony R. Santhiago, MD, PhD; and William J. Dupps, MD, PhD
Cataract evaluation and surgery is the cornerstone of many ophthalmic practices; as such, the chapter on corneal imaging for cataract surgery could reasonably be one of the first chapters in the book. The placement as the final chapter, however, is purposeful. Patients with cataracts frequently bring both refractive expectations and varying corneal pathologies that must be taken into account and that have been covered in previous chapters. Thus, in many respects, corneal imaging for cataract surgery represents the culmination of all other considerations for image evaluation and case planning. Patients with cataracts may also be corneal refractive surgery patients if bioptics (combined lens- and corneal-based surgery) is planned.
Figure 10-A. Slit lamp imaging of a patient with cataract. The left image shows peripheral focal opacities consistent with a congenital cataract. The middle image shows the same eye in retroillumination; in this view these peripheral vacuoles are emphasized. The right image shows a slit beam image through the lens, which highlights the nuclear sclerotic component to this cataract.
Optical biometry is the mainstay for cataract evaluation, as it measures axial length, keratometry, and anterior chamber depth in addition to lens thickness, corneal pachymetry, white-to-white values, and retinal thickness measurements. This information provides all the necessary data to determine the appropriate intraocular lens (IOL) calculation power for routine cases. Beyond biometry, some aspects of image evaluation are less emphasized in typical cataract surgery, such as subtle corneal elevation data, corneal thickness, and ectasia screening. On the other hand, certain aspects of imaging are more heavily emphasized, specifically corneal curvature irregularities, regular astigmatism, corneal clarity, and identification of coexisting abnormalities that may impact outcomes.
Figure 10-B. Optical biometry image from the right eye of a patient presenting for cataract evaluation. All of the basic necessary information is contained for IOL power calculations, including axial length (AL), steep (K2) and flat (K1) keratometry, anterior chamber depth (ACD), lens thickness (LT), and white-to-white (WTW) values. The right image displays the steep meridian as measured by the optical biometry device.
Figure 10-C. (A) Placido image of the same right eye. The pattern is indistinct. Keratometry values match up well in magnitude and direction with those measured by optical biometry. (B) Scheimpflug composite image of the same right eye showing axial (upper) and tangential (lower) curvature maps. Keratometry values (upper left) match up relatively well in magnitude and direction with those measured by optical biometry and Placido topography, but there are greater differences in this case between Scheimpflug and other measurements.
Major considerations for cataract surgical planning include routine case evaluations, toric IOL planning, post–refractive surgery optics and IOL calculations, cataract evaluation in patients with radial keratotomy (RK), cataract evaluation in patients with keratoconus or other ectasias, and cataract evaluation in patients with coexisting corneal irregularities and opacities. This chapter will cover all of these situations.
Figure 10-D. Placido imaging from the right eye of a patient who presented for cataract evaluation. All metrics, including keratometry, total astigmatism, asphericity (Q), and individual keratometry ring values appear reasonable. However, the pattern is clearly artifactual and therefore data are unreliable. This scan should be repeated before any data are used for IOL calculations or case planning.
Case note: This image highlights the critical aspect of image evaluation in cataract evaluations so that inaccurate data are not used for case planning. Image review is critical for all patients, but patients presenting for cataract evaluations are particularly prone to having mild tear film abnormalities or other findings that may impact the quality of measurements.
SECTION 1: ROUTINE CATARACT EVALUATIONS
There are a variety of maps available and potentially useful for cataract surgical planning. The specific imaging employed depends in large measure on the refractive outcome desired. Of note, different technologies may provide similar or highly disparate results depending on the eye; in cases where the data differs clinical judgment is needed to determine what data appear most accurate and use that in case planning.
Placido-Based Imaging
Figure 10-1-1. Placido image of the (A) right and (B) left eye from a patient presenting for cataract evaluation. Axial curvature (upper left) shows a relatively homogenous, oval pattern of mild steepening in both eyes, with less than 0.5 diopters (D) of astigmatism in either eye. Corneal wavefront maps (upper right) appear normal and regular in both eyes. Keratometry maps (lower left) are regular in both eyes. Raw ring images (lower right) confirm good centration in both eyes at the time of image capture.
Case note: These images show a normal examination for cataract without significant astigmatism or other corneal optical aberrations that would be impactful to final outcome.
Scheimpflug-Based Imaging
Figure 10-1-2. (A) Scheimpflug composite image of a patient who presented for cataract evaluation. The upper images show axial anterior curvature and lower images show tangential anterior curvature. The patient has regular, symmetric, with-the-rule astigmatism in both eyes, measuring 1.3 D in the right eye and 1.6 D in the left eye. Tangential curvature maps mirror the pattern findings in the axial maps. There is good between-eye symmetry. (B) Scheimpflug refractive display of the same right eye. Anterior curvature (upper left) and pachymetry (lower left) information is the same as the composite image. Anterior and posterior elevation maps (right) are within normal limits and add little to the evaluation process.
Figure 10-1-2. (C) Scheimpflug Holladay equivalent keratometry reading (EKR) report map of the same right eye. The upper left box contains various EKR readings (labeled) at different corneal locations. The upper right graph highlights the comparative change in refractive power (blue line) vs curvature (red and green lines) across the cornea. The lower left graph demonstrates the spread of EKR calculated across a specific zone (4.5 mm is default). In normal corneas this spread is typically low (≈2 D). The lower right graph shows the EKR map across the cornea. (D) Scheimpflug power distribution display of the same right eye. Graphs and maps are similar in gross appearance to the Holladay map but different data points are shown. The upper box contains various keratometry readings (not EKR) in different zones. The lower left graph demonstrates the spread of keratometry (not EKR) calculated across a specific zone (4.5 mm is default). The spread in this eye is similar for keratometry and EKR. The lower right map shows the same axial anterior curvature map as seen in the composite and refractive maps. Figure 10-1-2. (E) Scheimpflug cataract preoperative display from the same right eye. The upper images include axial anterior curvature (upper left), total corneal refractive power (upper middle), and pachymetry (upper right). The lower left image is a raw Scheimpflug image showing the anterior chamber, iris, and anterior lens anatomy. The lower right box displays astigmatism generated from SimK readings as well as total refractive power at 4 mm and total spherical aberrations. (F) Scheimpflug Zernike analysis display of the same right eye. The upper left image shows an overview of total higher-order aberrations (HOAs), while the lower pyramid shows each Zernike variable up to the fourth order (n = 4). In a normal eye spherical aberration (black circle) will have the most impact on IOL selection. In normal unoperated eyes this value is typically close to +0.2 μm (+0.274 μm in this eye). In eyes with previous surgery, scars, or other irregularities, other aberrations, especially coma (marked with asterisks), may be relevant. Figure 10-1-2. (G) Optical biometry image of the right and left eyes from the same patient. All basic data are shown in the upper box and, using that data, an IOL power is generated. In the right eye the biometer is not recommending an available toric IOL. (H) Composite image showing keratometry values for the same right eye as generated by optical biometry (left) and Scheimpflug imaging (right). There is good agreement between devices for flat K (R1 left image, K1 right image) , but steep K (R2, K2) is different by 1 D. The resulting measured magnitude of astigmatism is different by 1 D. The measured steep axis is different by 13 degrees (94 vs 107 degrees).
Patient With Cataract and Epithelial Basement Membrane Dystrophy
Figure 10-1-3. (A) Scheimpflug composite image of the right and left eyes from a patient who presented for cataract evaluation. Anterior axial maps (upper) show an irregularly irregular pattern, with mild central flattening of ≈0.5 D in both eyes. Tangential maps highlight this overall irregular pattern in both eyes. Pachymetry maps are normal and unremarkable in both eyes. There were no abnormal physical findings on examination other than cataract, but the topographic patterns are consistent with mild, subclinical epithelial basement membrane dystrophy in both eyes.
SECTION 2: TORIC INTRAOCULAR LENS EVALUATIONS
Evaluating patients for astigmatism correction at the time of cataract surgery, typically with toric IOLs or incisional surgery, is one of the most challenging aspects of the evaluation process. Astigmatism has both magnitude and directionality, and accurately identifying both components is critical to ensure precise outcomes. A variety of devices are used to measure both the magnitude and direction of corneal astigmatism; in some cases these different technologies provide relatively similar data, while in other cases there is a clinically significant discrepancy that must be resolved.
Figure 10-2-1. (A) Placido image of the right and left eyes from a patient presenting for cataract evaluation. The patient has mildly asymmetric, regular patterns in both eyes, more truncated in the right than left eye. There is no skewing of radial axes in either eye. Simulated keratometry from topography shows 1.27 D of astigmatism oriented at 109 degrees in the right eye and 1.77 D of astigmatism oriented at 63 degrees in the left eye.
Figure 10-2-1. (B) Optical biometry for the right and left eyes from the same patient. SimK from optical biometry shows 1.38 D of astigmatism oriented at 109 degrees in the right eye and 2.11 D of astigmatism oriented at 73 degrees in the left eye. Case note: In this case there is good agreement between topography and optical biometry in terms of both magnitude and direction of astigmatism in the right eye but only moderate agreement in the left eye, with 0.34 D difference in magnitude and 10 degrees difference in direction. Figure 10-2-2. (A) Scheimpflug comparative image of the right and left eyes from a patient presenting for cataract evaluation. Anterior curvature (upper) shows regular, symmetric bowtie patterns oriented obliquely in both eyes, with normal pachymetric distribution in both eyes. Figure 10-2-2. (B) Scheimpflug cataract preoperative display from the same right eye. SimK shows 1.3 D of astigmatism at 36 degrees, while total refractive power shows 1.5 D of astigmatism at 43 degrees. (C) Scheimpflug power distribution display of the same right eye. There is good agreement in K readings throughout the rings, and there is a tight spread of curvature distribution. Figure 10-2-2. (D) Dual Scheimpflug/Placido image of the same right eye. Anterior curvature pattern is analogous to that shown with Scheimpflug imaging. SimK from this map shows 0.9 D of astigmatism at 39 degrees. Figure 10-2-3. (A) Optical biometry of a patient who presented for cataract evaluation. SimK from this device shows 1.04 D of astigmatism at 12 degrees in the right eye and 0.91 D of astigmatism at 14 degrees in the left eye. (B) Placido imaging of the same patient. SimK from this device shows 1.25 D of astigmatism at 34 degrees in the right eye and 0.96 D of astigmatism at 170 degrees in the left eye. Figure 10-2-3. (C) Close up of the Placido image from the same left eye. There is significant skewing of the radial axis (black lines). Case note: This case illustrates an instance where, although when reviewing the SimK numbers only, this patient appears to be an acceptable candidate for a toric IOL, further review indicates some potential issues. In the right eye there is a significant difference in axis between Placido topography and optical biometry. In the left eye there is also a significant difference in axis between devices. Further, the pattern in the left eye is irregular, so it is uncertain how much benefit a toric IOL would have on final visual performance. Figure 10-2-4. Composite image including optical biometry and Scheimpflug imaging of the (A) right and (B) left eyes from a patient presenting for cataract evaluation. There is good agreement between devices in both magnitude and direction of astigmatism (arrow and box). Both eyes exhibit a regular, symmetric bowtie pattern. Figure 10-2-5. Composite image including optical biometry and Scheimpflug imaging of the right eye from a patient with mild keratoconus who presented for cataract evaluation. In this eye with pronounced inferior steepening there is poor agreement between devices (arrow and box). Optical biometry shows 1.97 D of astigmatism at 150 degrees, while Scheimpflug imaging shows 0.4 D of astigmatism at 34 degrees. Case note: This case highlights the importance of corneal imaging beyond optical biometry, especially in cases where toric IOLs are being considered. Implantation of a toric IOL is not warranted in this eye, and placement at the power and orientation prescribed by the optical biometry device would lead to a poor outcome. A patient had RK in both eyes and penetrating keratoplasty (PKP) in the right eye 3 years prior, with a clear graft but significant anisometropia and contact lens intolerance. Patient developed moderate cataracts in both eyes. Figure 10-2-6. (A) Scheimpflug composite examination showing anterior curvature (upper) and pachymetry (lower) for the right and left eyes. Anterior curvature is shown in the Oculus absolute setting. There is significant regular, symmetric steepening centrally in the right (PKP) eye, and significant paracentral flattening in the left (RK) eye. Pachymetry is relatively regular in thickness and distribution in the right eye, and thicker overall in the left eye with a more peripheral thinnest point. Figure 10-2-6. Scheimpflug cataract display of the (B) right and (C) left eyes from the same patient. The graft–host interface is visible in the right eye and an anterior opacity surrounding in RK incision scar is visible in the left eye in the raw Scheimpflug image. There are minimal differences in mean K (Km) and total astigmatism between SimK and total corneal refractive power in the right eye, while Km is significantly different in the left eye. Figure 10-2-6. Scheimpflug power distribution display of the same (D) right and (E) left eyes. There is a relatively tight curvature distribution in the right eye considering this is after PKP, but there is a remarkably wide power distribution in the left eye.
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