Array of devices available for preoperative cataract testing

Karolinne Maia Rocha, MD, PhD, and Jonathan D. Solomon, MD, share their expertise on how to utilize these instruments.

Issue: September 25, 2017

ByKarolinne Maia Rocha, MD, PhD

ByJonathan D. Solomon, MD

Welcome to another edition of CEDARS/ASPENS Debates. CEDARS/ASPENS is a joint society of cornea, cataract and refractive surgery specialists, here to discuss some of the latest hot topics in ophthalmology.

As our cataract surgery precision continues to improve, patient expectations continue to escalate. There are now many new technologies available to aid in our cataract testing to help optimize outcomes, but interpretation of the results from these devices can be overwhelming to surgeons. This month, Karolinne Maia Rocha, MD, PhD, and Jonathan D. Solomon, MD, two leading experts in these technologies, discuss how they use these devices for cataract surgery. We understand that most surgeons do not have access to all of these devices, but this column should help explain the benefits of each. We hope you enjoy the discussion.

Kenneth A. Beckman, MD, FACS
OSN CEDARS/ASPENS Debates Editor

Advanced diagnostics helpful, but remember the basics

Important considerations should be taken into account when evaluating patients for presbyopia-correcting IOLs. Patient expectations, ocular surface optimization, anatomical considerations, refractive error, pupil size and topographic findings should be considered when selecting patients for premium IOLs.

I start my preoperative evaluation with a comprehensive ocular examination to rule out diseases that affect visual outcomes. Patients with macular pathologies, moderate to severe glaucoma, corneal scars and irregular astigmatism are not good candidates for presbyopia-correcting IOLs. Also, the preoperative discussion is crucial to explain the limitations of various available technologies. Understanding a patient’s occupational needs and lifestyle will guide the surgeon to select the best option for that specific patient.

Residual refractive error, dry eye syndrome and posterior capsule opacification are the leading causes of dissatisfaction among patients with presbyopia-correcting IOLs. Residual astigmatism of as little as 0.5 D to 0.75 D can significantly degrade visual quality; dry eyes can affect contrast sensitivity by increasing higher-order aberrations and light scatter; and mild levels of posterior capsule opacification can degrade visual quality by increasing light scatter.

Accurate and customized biometry is key to achieving the best possible outcomes. Correcting refractive error, especially astigmatism, ensures high-quality full range of vision for our patients. Corneal topography/tomography and optical biometry are the two most helpful devices. As most surgeons have noticed, keratometric readings and optical biometry measurements can vary significantly in patients with dry eyes and ocular surface disease in general. For this reason, I always repeat my Placido and Scheimpflug maps. If the measurements are not reproducible or the topography is irregular, I treat the ocular surface before taking my final measurements. I use optical biometry routinely and double-check the measurements with immersion A-scan biometry in special cases (dense cataracts, posterior staphyloma, vitreous opacities, etc). Newer-generation formulas have shown effective outcomes. I use the SRK-T formula and Barrett and Hill-RBF calculators. For calculating toric IOLs, I use the Baylor nomogram and Barrett toric calculator.

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In addition to performing standard evaluations for cataract patients, I rely on advanced diagnostic technologies to guide my surgical decision-making in presbyopic patients in the dysfunctional lens age range. Dysfunctional lens syndrome comprises progressive loss of accommodation, increase in higher-order aberrations and light scatter, decreased contrast sensitivity and finally clinically significant lens opacities with decreased functional vision (cataract). Scheimpflug lens densitometry, double-pass wavefront and ray-tracing aberrometry can objectively characterize the factors affecting visual quality.

Lens density

Scheimpflug imaging (Pentacam HR, Oculus, and Galilei, Ziemer) applies a blue light-emitting diode and a rotating camera to capture slit images and elevation data points, and to generate a 3-D representation of the anterior segment. The 3-D plot provides an image of the lens from which density can be obtained at a specific point or area. Lens density measurements are analyzed utilizing the area densitometry function offered by the native Pentacam software. A 180° slice can be used to draw a region of interest around the lens and to record the average density and standard deviation values on the image. This tool is helpful in showing patients the changes in their crystalline lens and early signs of dysfunctional lens syndrome.

Figure 1. Dynamic measurement of the PSF over a 20-second interval in normal eyes (control group) and a patient with dry eye syndrome.

Figure 2. OSI results in a patient with a clear lens (top) vs. a patient with a cataract. OSI is an objective optical method that quantifies the degree of dysfunctional lens and cataracts.

Figure 3. DLI software shows increased lenticular aberrations. This is a helpful tool to show the patient a recommendation of a lens-based procedure instead of a corneal procedure.

Ocular scatter

Optical quality of retinal image is affected primarily by ocular aberrations and light scatter. Ocular scatter is a property of light as it travels through optical media in the eye, resulting in localized deviations of light from its path due to a combination of diffraction, reflection and refraction. Forward scattering refers to ocular scatter as light travels from the front of the eye toward the retina. Increase in forward ocular scatter indicates optical effects due to spatial variations in refractive index such as lens opacities, or microscopic inhomogeneity such as inflammatory particles in the cornea or irregularities in the tear film.

Ocular scatter is measured by the HD Analyzer (Visiometrics) using double-pass wavefront to determine the point spread function (PSF). The Objective Scatter Index (OSI) calculation is achieved by assessing the ratio of light intensity between the central 1 minute and the periphery of the PSF between 12 and 20 minutes. The greater the aberrations and intraocular scatter, the greater the spread of the PSF and greater the OSI.

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I use the tear film analysis software to assess preoperatively the tear film stability and retinal image quality in patient candidates for premium IOLs. The HD Analyzer provides dynamic measurements of the point spread function while the patient blinks normally over a 20-second interval. The OSI in a normal eye will be low and stable, but it fluctuates and worsens during the testing period in patients with dry eye syndrome (Figure 1). Additionally, OSI is an objective measurement of the effect of changes in crystalline lens transparency on incident light in otherwise healthy eyes. This is observed by a strong correlation of OSI with Scheimpflug lens density and subjective grading based on LOCS III (Figure 2).

Ray tracing

The iTrace device (Tracey Technologies) combines a Placido disc topographer and ray tracing aberrometry, which gives information on aberrations from the whole eye. Corneal aberrations are calculated based on the Placido topography, and internal aberrations (mostly lenticular) are calculated by subtracting total aberrations from corneal aberrations. The Dysfunctional Lens Index (DLI) is a useful tool for educating patients and for guiding surgeons’ decisions. DLI takes into account internal higher-order aberrations and modulation transfer function, and compensates for pupil size. Figure 3 shows the simulation of the vision chart “E” based on PSF for corneal, internal and total aberrations. In this case, lenticular higher-order aberrations cannot be corrected by a corneal procedure. The iTrace device also provides the surgeon with a chord µ measurement (previously called angle kappa). Patients with high preoperative higher-order aberrations (> 0.5 µm for a 6-mm pupil diameter) or chord µ exceeding 0.6 mm may not be good candidates for presbyopia-correcting IOLs.

Looking ahead, the use of in vivo simulation with adaptive optics devices to allow patients to compare different IOL designs will become an important and useful tool of the preoperative evaluation. In summary, advanced diagnostic technologies are helpful for making recommendations to patients and for surgical planning, but they do not replace the most important preoperative assessments: comprehensive eye exam, corneal topography and optical biometry.

References:
Artal P, et al. PLoS One. 2011;doi:10.1371/journal.pone.0016823.
Brundrett A, et al. Dynamic optical quality assessment using a double-pass wavefront system as compared to objective and subjective clinical measures in dry eye disease. American Society of Cataract and Refractive Surgery meeting; May 6-10, 2016; New Orleans.
Lim SA, et al. J Cataract Refract Surg. 2014;doi:10.1016/j.jcrs.2013.10.032.
Magalhães FP, et al. Arq Bras Oftalmol. 2011;doi:10.1590/S0004-27492011000200008.
McMillin JC, et al. Objective evaluation of visual quality before and after Nd:YAG laser capsulotomy in patients with visually significant posterior capsular opacification. American Society of Cataract and Refractive Surgery meeting; May 6-10, 2016; New Orleans.
Woodward MA, et al. J Cataract Refract Surg. 2009;doi:10.1016/j.jcrs.2009.01.031.

For more information:
Karolinne M. Rocha MD, PhD, can be reached at karolinnemaia@gmail.com.

Disclosure: Rocha reports no relevant financial disclosures.

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Remain consistent and review raw data

When considering an individual for cataract surgery, I prefer more information than what is offered by traditional biometry. Fortunately, my IOLMaster 700 (Carl Zeiss Meditec) provides much more than axial length and keratometry measurements and more than the lens thickness or white-to-white measurements necessary for some of the newer IOL algorithms. Of particular interest is the confirmation of the distance between pupil center and coaxial light reflex upon fixation (line of sight). Furthermore, telecentric keratometry reduces the chances for technical error during data collection, and evaluation of the individual light reflexes off of the corneal surface allows for a confirmation of quality of the measurements. This is particularly useful when there are concerns for ocular surface disease. At the same time, a high-quality infrared image is captured on the anterior ocular surface for intraoperative registration with the Callisto system (Zeiss), which we have found to be reliable for toric IOL placement. Finally, swept-source OCT through the fovea provides a level of reassurance during the acquisition process for fixation confirmation, as well as a level of comfort with the absences of anatomic irregularity.

The next stop for any surgical candidate is the Nidek OPD-III, which is a double-pass automatic retinoscope, wavefront aberrometer and Placido-based topographer. Corneal higher-order aberrations are calculated, along with a measurement of angle kappa, that can significantly affect the performance of aspheric, toric and multifocal IOLs with as little as 0.5 mm misalignment, should it effectively result in decentration of the optic. Another advantage provided by the OPD-III is an opportunity to provide intraoperative registration and guidance for delivery of astigmatic keratotomy or axis identification as part of the IntelliAxis software with the Lensar laser when placing toric IOLs. This opportunity for integration provides a distinct advantage to me as a surgeon, my clinical team and the patient by reducing transcription error and minimizing the need for reference marks in the perioperative area along with reducing alignment error that would otherwise be placed with ink marks.

Subsequently, I ask that each patient be measured with the Cassini (i-Optics), which uses specular reflections of colored LEDs to construct topographic maps of the anterior corneal surface and the specular reflections to measure the curvature of the posterior surface. Using forward ray tracing, it will calculate total corneal astigmatism (TCA) at the 3-mm zone, which has been shown to be more accurate than traditional Placido-based topographers. Once more, the infrared image of the non-mydriatic anterior segment is provided to the Lensar laser as part of a wireless Streamline iris registration process when the patient’s dilated eye is docked for femtosecond laser-assisted cataract surgery.

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Simple guidelines are provided to the clinical team when the biometric data is being gathered. Fatigue is real; as a patient transitions from one device to the next, the ocular surface can change, and we all have a limit to our attention. When there is evidence of weariness, the staff will encourage gentle blinking and redirect fixation, and when necessary, we will apply aerosolized sterile water to each eye. Before and after images of the corneal surface are helpful to illustrate the impact of dry eye disease. It is also important to remember each device measures corneal power differently, although typically within the central 3 ± 0.5 mm zone. Therefore, subtle difference can be expected in power and orientation of the steep axis even under ideal conditions.

I rely on the corneal measurement from the IOLMaster 700 for the IOL power selection because it provides the smallest zone at 2.5 mm and the help of the Barrett True-K formula when dealing with post-refractive eyes. For astigmatic correction, the ocular wavefront profile from the OPD-III is helpful to review the degree of cylinder in the preoperative eye and to reduce the impact of line of sight misalignment to be sure there is less than 0.4 mm of angle kappa, but TCA from the Cassini has been an extremely valuable addition when it comes to preparing for surgical management of astigmatism and individualizing treatment beyond the Baylor nomogram. Currently, I am getting good results with a method that includes “big data” analysis with the Ladas Super Formula for IOL power selection.

Regardless of your biometric device of choice, it is important to remain consistent for purposes of continued refinement of your personal surgeon factor for each lens, or A-constant. It is always important to remember that a computer just processes what it is given. “Garbage in, garbage out” refers to the tendency to put unwarranted faith in the accuracy of generated data. So, as a final thought, I am always reminded to review the raw data and the images acquired from each of the devices.

References:
Ladas JG, et al. JAMA Ophthalmol. 2015;doi:10.1001/jamaophthalmol.2015.3832.
Snellenburg JJ, et al. Opt Express. 2010;doi:10.1364/OE.18.019324.
Solomon JD, et al. J Cataract Refract Surg. 2017;doi:10.1016/j.jcrs.2017.01.012.
Wang L, et al. Arch Ophthalmol. 2005;doi:10.1001/archopht.123.9.1226.
Yeu E. Comparison of toric IOL results of the Barrett toric IOL calculator, Baylor toric nomogram and an LED topographer. American Society of Cataract and Refractive Surgery meeting; May 6-10, 2016; New Orleans.

For more information:
Jonathan D. Solomon, MD, can be reached at jdsolomon@hotmail.com.

Disclosure: Solomon reports he is a consultant to Carl Zeiss Meditec, Lensar Laser and i-Optics.