When a presbyopic patient in the dysfunctional lens age range (40s to 60s) comes in for a cataract or laser vision correction consult, we always start with the basics, such as corneal topography, manifest refraction and visual acuity. But to choose among the myriad options for this age group (corneal refractive surgery, corneal inlays, lens surgery, and more) I rely on a number of very useful metrics from the advanced diagnostic devices we now have to guide refractive surgical decision-making (Table, this page).
We know, of course, that accommodative ability worsens throughout presbyopia. There is strong evidence that changes in internal higher-order aberrations (HOAs), contrast sensitivity, and forward light-scatter precede lens opacity.1-3 Ray tracing aberrometry, double-pass wavefront, Scheimpflug technology, and femtosecond laser optical coherence tomography (OCT) can help us better characterize these early changes of dysfunctional lens syndrome (DLS), as well as evaluate the size and position of the lens and tear film quality preoperatively. With this information, I can objectively assess the factors affecting visual quality and, therefore, which surgical procedure(s) will be most appropriate to reach the patient’s goals.
These diagnostic tests are also very useful for showing patients what is happening to their eyes and why I make the recommendations I do. For example, let’s say a 50-year-old hyperopic female patient comes in for a LASIK consult because she doesn’t want to wear reading glasses anymore. I may decide that a lens exchange with a premium intraocular lens is a better option if she already has significant lens changes. Being able to show her where the opacities are, or that her internal ocular aberrations far exceed her corneal aberrations, can facilitate patient acceptance of the surgical plan.
Here are the three devices that for me provide the most practical information for evaluating the presbyope:
Preoperative evaluation of the presbyope
- Manifest refraction and visual acuity
- Placido disc topography (with ray tracing or separately)
- Corneal abnormalities
- Scheimpflug device
- Corneal tomography
- Lens density
- Ray tracing aberrometry
- Dysfunctional lens index (DLI) and opacity map
- Chord mu
- Extent and source (corneal or internal) of higher order aberrations
- Depth of focus curves
- Double-pass wavefront
- Objective scatter index (OSI)
- Tear film analysis
- If planning lenticular surgery
- Optical biometry and IOL power calculation
- Femtosecond laser OCT analysis (at time of surgery)
- Lens thickness (LT)
- Lens meridian position (LMP)
- Anterior chamber depth (ACD)
The HD Analyzer (Visiometrics) uses double-pass wavefront aberrometry to assess the degree of scatter as light passes through the tear film, cornea and crystalline lens to the retina. It does this by analyzing information directly from the retinal image point-spread function (PSF). The PSF data are also used to generate other assessments of visual quality, including ocular modulation transfer function (MTF) and Strehl ratio, but the primary metric that many clinicians will use it for is to obtain the objective scatter index (OSI).
The OSI result is displayed as a numerical value (ideally, < 1.0). It is also rated on a color scale, with green indicating excellent visual quality, yellow depicting mild to moderate scatter, and red specifying severe scatter, as one would find with cataract. OSI has been correlated to lens opacity4,5 and has been shown to be a reliable and objective measure of visual impairment, independent of lens opacity.3 The visual degradation of an optotype letter or real-world image that corresponds to different OSI values can be simulated and easily explained to patients. High OSI should rule out a corneal inlay or laser vision correction (Figure 1).
I also like to perform a tear film analysis with this device, which can dynamically measure the PSF over 20 seconds, capturing 40 OSI measurements during that time. This temporal OSI correlates well with clinical tear breakup time as well as patient symptoms and is sensitive enough to detect mild dry eye.6,7 A patient may start with a normal OSI because the lens is not generating any light scatter, but if the tear film quality is poor, the OSI actually worsens between blinks (Figure 2). We need to treat these patients for their dry eye and stabilize the tear film prior to any premium refractive procedure.
When using double-pass wavefront, it is always important to remember that the device measures the entire eye. Scatter could be coming from the tear film, lens, or even from a corneal scar. For this reason, results must be understood in the context of other clinical findings.
There are a number of valuable diagnostic features in the iTrace device (Tracey Technologies). It combines a Placido disc topographer and ray tracing aberrometry.
Ray tracing calculates the ratio of peak focal intensities between the aberrated and ideal PSF.
It provides us with information on aberrations from the whole eye, but can also separate out the aberrations from the cornea and internal aberrations (mostly lenticular).
As shown in Figure 3, the output from this device is very useful for patient education. It shows a numerical dysfunctional lens index (DLI, in this case, of 1.15). DLI takes into account the internal aberrations, MTF or loss of contrast, and compensation for pupil size. It also provides a simulation of the vision chart “E” based on PSF for total aberrations, internal (lenticular) and corneal aberrations. I show this to patients who need to understand they have lenticular issues that cannot be solved by a corneal procedure.
The iTrace also has an opacity map that I find correlates nicely with my slit lamp exam. A cortical cataract, for example, is clearly visible as a white area on the map (Figure 4). This, again, makes it easier for a patient to understand why I might recommend a lens procedure instead of a corneal one. Of course, one can also visualize early lens-density changes with a Scheimpflug densitometry device such as the Pentacam or Galilei.
Another iTrace metric I’ve been using lately is the Visual Strehl of the Optical Transfer Function (VSOTF). This is an optical wavefront error-derived metric that predicts patient visual acuity and objectively determines depth of focus. It is calculated as a function of defocus by creating a through-focus curve. I am conducting a study in which we use VSOTF to evaluate the depth of focus in five different groups: young control subjects; presbyopes who have not had surgery; and pseudophakes with one of three different IOLs — a monofocal lens with negative spherical aberration (Tecnis ZCB00, Johnson & Johnson Vision), an aberration-free lens (Envista MX60, Bausch + Lomb), or an extended depth of focus lens (Symfony ZXR00, Johnson & Johnson Vision). What we are finding is that the Symfony lens offers high quality vision with a good depth of focus. In highly aberrated corneas and posthyperopic LASIK, the EnVista aberration-free monofocal IOL provides extended depth of focus given the preoperative corneal aberrations.
The iTrace device also provides the surgeon with a chord mu (previously often referred to as angle kappa) measurement. Patients with high preoperative HOA (>0.5 µm for a 6-mm pupil diameter) or chord mu exceeding 0.6 mm may not be good candidates for multifocal IOLs.
FEMTOSECOND LASER OCT
Until recently, we have been missing some key pieces of information about the crystalline lens. Preoperative biometry and other diagnostic technologies don’t provide any real-time information about lens meridian position (LMP) or the lens thickness (LT) but we can obtain these anatomical measurements from the OCT on the Catalys femtosecond laser (Johnson & Johnson Vision). In following this metric for routine cases, I have found a very high standard of deviation for both LT and LMP. In normal eyes the lens thickness can range from about 3.1 mm up to 5.8 mm. This huge variability came as a big surprise and one that is somewhat problematic, given that all of our IOLs are essentially one-size-fits-all.
Based on this, I’ve made some changes in my practice. For example, as I do the femtosecond laser portion of the surgery in a toric case, if I discover that the LT is ≥5.2 mm, I will use a capsular tension ring during surgery to reduce the risk of lens rotation in such a large capsular bag. I also may adjust my lens power choice based on LMP and anterior chamber depth. If I have the choice of two powers and expect the lens to seat closer to the nodal point, I will choose the higher power lens; if it will likely seat more anteriorly, I will choose the lower power.
While this is not strictly preoperative diagnostic data, the intraoperative anterior segment biometry certainly gives me advance notice about lens/capsule parameters that would otherwise be unrecognized.
We are fortunate to have such sophisticated tools to guide our decision-making in refractive cataract surgery. Not only are the devices described here helpful to the surgeon, but they provide powerful tools for communicating our findings and recommendations to patients. OM
- Rocha KM, Nosé W, Bottós K, et al. Higher-order aberrations of age-related cataract. J Cataract Refract Surg. 2007;33:1442-1446.
- Kuroda T, Fujikado T, Ninomiya S, et al. Effect of aging on ocular light scatter and higher order aberrations. J Refract Surg. 2002;18:S598-602.
- Zeldin E, Waring GO IV, Rocha KM. Light scatter, ray-tracing aberrometry, and Scheimpflug densitometry as an objective measure of dysfunctional lens syndrome. Poster Presentation at ASCRS, New Orleans, 2016.
- Artal P, Benito A, Pérez GM, et al. An objective scatter index based on double-pass retinal images of a point source to classify cataracts. PLoS ONE. 2011;6:e16823.
- Lim SA, Hwang J, Hwang KY, Chung SH. Objective assessment of nuclear cataract: comparison of double-pass and Scheimpflug systems. J Cataract Refract Surg. 2014;40:716-721.
- Benito A, Pérez GM, Mirabet S, et al. Objective optical assessment of tear-film quality dynamics in normal and mildly symptomatic dry eyes. J Cataract Refract Surg. 2011;37:1481-1487.
- Brundrett A, Crouse M, Waring IV GO, Rocha KM. Dynamic optical quality assessment using a double-pass wavefront system as compared to objective and subjective clinical measures in dry eye disease. Oral Presentation at ASCRS, New Orleans, 2016.