Essentials of Biometry (Part 3): What are common refractive surprises?
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Title: Essentials of Biometry (Part 3): What are common refractive surprises?
Authors: Courtney Goodman, MSIV at University of Miami Miller School of Medicine, Mark Mifflin, MD
Date: 11/14/2022
Keywords/Main Subjects: biometry; cataract surgery; refractive surprise; axial length; central corneal power
Review:
Accurate biometry is critical for successful preoperative intraocular lens (IOL) power calculations, especially when measuring axial length (AL) and central corneal power (K). Despite advances in biometry which have undoubtedly reduced the incidence of error in measurement, refractive surprises after cataract surgery are still relatively common. In this article, we will explain some reasons why such errors occur.
Refractive Surprise due to Error in Axial Length Measurements
As discussed in Part 1 of this series, AL is the biometric measurement with the most impact on IOL power calculations. For reference, the adult eye has an average AL of approximately 24 mm. For every 0.33 mm error in AL, there is approximately 1 diopter of postoperative refractive error. As can be imagined, ocular conditions that increase the risk of inaccurate AL measurements potentially cause the most dramatic refractive surprises.1 Let’s discuss some of these conditions that cause pitfalls in AL measurements, which are listed in Table 1.
High myopia is associated with an increased risk of postoperative hyperopic error.2 One explanation is that posterior staphyloma often afflicts highly myopic eyes, which is present in about one in two or three cases, and with over 70% located in the macular region (Figure 1).3-6 Macular posterior staphylomas are associated with erroneously longer measurements of AL if the depth of the staphyloma is measured rather than the fovea. This risk of error is greater with ultrasonic biometry than laser interferometry, as a tilted macula can deflect the ultrasound beam.2 Apart from posterior staphyloma, the intrinsically long AL in myopia also leads to inaccuracies in determining effective lens position (ELP) and other parameters used for calculations.7 Though various formulas and constants were introduced to optimize IOL power calculations in highly myopic eyes, the outcomes continue to be unsatisfactory, especially for extreme cases with AL greater than 28 mm.8
Figure 1: Posterior Staphyloma. A posterior staphyloma is a protrusion of the eye wall and its layers, including the uveal tissue and the retina. High myopia is associated with posterior staphyloma and most commonly affects the macular region, depicted here. AL can be erroneously longer if the depth of the staphyloma is measured rather than the fovea, often due to inaccurate fixation or alignment during measurement. In ultrasonic biometry, a tilted macula due to the staphyloma can also cause deflection of the ultrasound beam, further contributing to error. Created with Biorender.
Inaccurate AL measurement is also prominent in silicone-filled eyes. Silicone oil has a higher refractive index than vitreous, which alters the velocity of both light and ultrasound, leading to erroneously longer AL measurements with modern biometry methods.9 Laser interferometers can be configured to adjust for this difference in refractive index, but still face risks of error such as if the silicone oil is underfilled in the cavity, or if oil droplet artifacts skew measurements.10 Since the presence of silicone oil often leads to cataract development, combined silicone oil removal and cataract extraction is a relatively common procedure and thus optimizing biometry in these cases is an important area of research.
Conditions which cause a lack of fixation, such as oculomotor dysfunction or nystagmus, can lead to errors in AL especially when using laser interferometry. In these patients, immersion A-scan ultrasound is preferred to better stabilize the eye.
Finally, human error is always a potential source of refractive surprise when obtaining AL measurements, especially with ultrasonic biometry (see our discussion in Part 2). Laser interferometry introduced automation which has significantly reduced the incidence of human error, but can still suffer from lack of calibration, incorrect settings, or unclean optical surfaces.11 AL discrepancies between eyes, such as a difference greater than 0.7 mm, should prompt repeat measurements.12 Biometric data that does not fall within the average, such as extreme hyperopia and myopia, also benefits from repetition or confirmation with an alternate modality.
Refractive Surprise due to Errors in Central Corneal Power Measurements
Next, let’s discuss errors that can arise when obtaining K, the second most important biometric measurement in calculating IOL power. For reference, a keratometric error of 1 diopter leads to postoperative refractive error of the same amount, 1 diopter. Sources of refractive error due to inaccurate K measurements are listed in Table 2.
Dry eye disease (DED) is one of the most common culprits of inaccurate biometry in older adults, with studies demonstrating that an unstable tear film produces variable keratometry readings.13,14 Other conditions that also lead to irregular astigmatism can similarly impact keratometry, including pterygium, keratoconus, and Salzmann nodular degeneration. As such, optimizing the treatment of such conditions prior to biometric measurements prevents inaccuracies. For example, the accuracy and repeatability of keratometry readings has been shown to increase when DED is adequately managed.15 Epithelial basement membrane dystrophy (EBMD) and Salzmann nodular degeneration can be treated with superficial keratectomy (SK) or phototherapeutic keratectomy (PTK) to similarly improve postoperative refractive outcomes.16
Corneal edema is another important source of inaccurate K measurements that fluctuate with disease severity.17 Specifically, stromal edema tends to flatten the posterior corneal curvature and induce a myopic shift. If a patient undergoes combined endothelial keratoplasty (EK) and IOL implantation with power derived from biometry of the edematous cornea, there is risk of postoperative hyperopic error after improvement of the edema and re-steepening of the posterior curvature.18 Therefore, if delayed cataract surgery is tolerated, surgeons often recommend that patients with corneal edema be treated with EK first and allowed to fully recover months prior to the biometry appointment.
Next, calculating the IOL power in patients with a history of laser vision correction (LVC), specifically laser in situ keratomileusis (LASIK) and photorefractive keratectomy (PRK), poses a unique set of challenges. The flatter, post-refractive anterior cornea alters the anterior to posterior curvature ratio, which can confound calculations of the true radius of the curvature and ultimately K (Figure 2).19 If standard formulas are used, patients who were originally corrected for myopia can erroneously end up hyperopic, and those corrected for hyperopia can end up myopic. To prevent this, anterior and posterior surfaces should be separately measured, such as using Scheimpflug imaging, and special formulas adjusting the curvature ratio must be applied. However, it is important to note that the formulas appropriate for post-hyperopic versus post-myopic LASIK surgery differ.20 A potential source of error in surgical planning is incorrect or unavailable documentation of this distinction in the medical record. Corneal topography can be useful in distinguishing the type performed.
Figure 2: Corneal changes following laser vision correction (LVC). Refractive surgery, such as that involving LASIK or PRK, changes the curvature of the anterior corneal surface with a laser while preserving the posterior corneal surface. This change can confound calculations of the true radius of the corneal curvature and ultimately K. A) The normal cornea before LVC is compared to B) after myopic refractive surgery which has uniform laser application of the anterior corneal surface, and C) after hyperopic refractive surgery which involves laser application to the midperiphery to produce a steeper center. In rare cases, refractive surgery can induce D) corneal ectasia, where the corneal stroma progressively thins and both anterior and posterior corneal surfaces steepen. Created with Biorender.
Choosing the optimal IOL power in radial keratotomy (RK) can be even more challenging compared to LVC surgery. Let’s discuss three reasons. 1) Post-RK eyes tend to undergo hyperopic shift, where the optical zone of the cornea may progressively flatten over years, or even decades.21 This flattening occurs at a variable rate and with variable predictability. As such, clinical judgement is needed in deciding whether to implant an IOL that is intentionally more myopic to account for hyperopic shift. 2) Another challenge to biometry is that K and ACD values tend to fluctuate diurnally, as post-RK corneas tend to flatten overnight, and steepen as the day progresses.22 Therefore, it is sometimes reasonable to perform comparative biometry at different times of the day. 3) Radial incisions flatten both the anterior and posterior corneal surfaces, and usually more extremely in the centermost region. Since standard keratometry and placido-based imaging average measurements over a fixed distance in the center, overestimation of K can occur with these methods (Figure 3).19 With these three points, it is clear that RK is a special case in which minimizing final refractive error requires careful planning.
Figure 3: Corneal changes following radial keratotomy (RK). The RK procedure creates incisions radially along the mid-peripheral cornea. These weakened areas bulge, causing adjacent flattening of the central optical zone of the cornea and affecting both anterior and posterior surfaces equally, as seen comparing A) before RK and B) after RK. Created with Biorender.
Corneal manipulation should be minimized prior to biometry.23 This includes routine workup such as applanation or tonometry. Soft contact lens wearers should stop one week prior to the biometry appointment. Rigid gas-permeable contact lens wearers, on the other hand, should stop contact lens use a minimum of three weeks prior, and undergo serial biometric measurements to ensure corneal stability.24 Some hard contact lens wearers do not achieve stability for months or years following discontinuation, and therefore should be counseled as needed.
Finally, as with measuring AL, steps should be taken to minimize human error when conducting keratometry measurements. Appropriate calibration and correct documentation should be ensured. A discrepancy of 0.9 D in K between eyes or extreme results (< 40 D or > 48 D) warrants repeat measurements.12
Conclusion
Overall, postoperative refractive error after cataract surgery can be prevented by recognizing and adjusting for conditions that can confound biometry. AL and K are the most important biometric measurements where accuracy must be ensured to prevent refractive surprise. Selecting appropriate formulas, monitoring contact lens use prior to measurements, treating conditions when appropriate, and recognizing discrepencies in calculations are all important actions to consider, among others.
Determining IOL power is not a perfect process, and refractive surprises still sometimes occur even when the utmost care is taken in biometry and preparing the patient for surgery. As such, patient expectations should be actively managed during the informed consent process.25 Patients should always be counseled on the possibility of needing prescription glasses or contact lenses after cataract surgery, with LASIK, PRK, or IOL exchange also being potential options to treat unexpected results in select cases. Nonetheless, refractive outcomes following cataract surgery have improved considerably over the years with the advancements in biometry. We hope this article increases awareness of and helps prevent the most common causes of refractive outcome surprise following cataract surgery.
References:
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Identifier: Moran_CORE_126609
Faculty Approval by: Dr. Mark Mifflin
Copyright statement: Copyright Courtney Goodman, ©2022. For further information regarding the rights to this collection, please visit: http://morancore.utah.edu/terms-of-use/
