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BEYOND TRADITIONAL VISION MEASURES
Newer diagnostic devices provide better insights into patients overall optical qualities
BY DANIEL S. DURRIE, M.D.

With multiple optical elements to the eye, diagnosing an individual patient's ocular distortion is a daily challenge ophthalmologists must face. As more of the emphasis historically has been about treatment options rather than locating the distortion, ophthalmic diagnostic technology has lagged behind our surgical capabilities.

The phoropter, for example, has served ophthalmologists well for years, but we now know it has limitations. With the advent of wavefront, we discovered most patients had asymmetrical errors and higher-order aberrations, such as coma, and we began to question whether we were performing the right diagnostic tests to do the right surgery.

Fortunately, we are in a time when we have more comprehensive diagnostic devices available, and I believe a greater emphasis is going to be placed on achieving an accurate and complete measure for

quality of vision. We are entering an era where ophthalmologists will utilize technology to locate distortions, thus providing insights leading to more accurate diagnoses and the development of treatment plans to better serve our patients.

The Effects of LASIK By Daniel S. Durrie, M.D.

During a recent grand rounds at the Kansas University ophthalmology department, I examined two residents who had previous refractive surgery. One (patient A) had a conventional laser correction procedure and a mechanical microkeratome created flap in 1999; the other had a wavefront-guided procedure with an IntraLase femtosecond laser flap. Both were plano and 20/15 without correction, but the resident who had the earlier procedure had significant halo and glare symptoms, which I was able to confirm with wavefront analysis (Figure). A wavefront-guided upgrade is planned to relieve the symptoms. Figure. I was able to utilize wavefront analysis to diagnose a former conventional LASIK patient (patient A) with high order aberrations. Without modern diagnostic technology, I would not have been able to find the problem.

The Camera

In first examining this issue of quality of vision, we need to ask, "how do we look at the optics of the whole eye and obtain better diagnostic information for our refractive and cataract patients?" As such, we occasionally have to remind ourselves that the eye is a two-lens camera with the cornea and the crystalline lens working in tandem. We spend a great deal of time using diagnostic tests looking at the cornea but we also need to examine the crystalline lens.

In middle-aged patients, examining for potential nuclear sclerosis or asymmetrical errors is essential. Both issues can distort visual acuity, and asymmetry in the lens optics may cause an imbalance between the cornea and the lens.

This imbalance may account for a large portion of patients' symptomatic problems, but we must look at the lens and use the diagnostic equipment that identifies the location of the distortions. For a patient in his or her 20s, this may not be as significant a problem due to the natural balance between the young lens and the young cornea. The young lens has negative spherical aberration that balances out the positive spherical aberration in the cornea. As the lens ages, we not only develop presbyopia, we also lose this optical balance as the lens gets thicker and larger. Eventually, the lens has positive spherical aberration and the optical system loses depth of focus. I document the lens by taking a slit lamp photo to obtain an image of the lens density using the Pentacam (Oculus, Lynnwood, Wash.). These images can serve as objective measurements for comparison in the future.

Bundling Diagnostics

Traditionally, we have had individual diagnostic instruments that provide good optical data about patients' vision all the way through the eye, but we have not had good information about where the actual problem lies. If we do a manifest refraction, for example, it gives us the measurement of the optical system back to the retina, including the cornea and the lens.

As a standalone device, topographers are very accurate in measuring defects of the anterior cornea, but you can have the same topography on a patient who is a -10 as you have on a +10, and this diagnostic tool does not provide you with the overall quality of vision.

As such, the ophthalmic industry is starting to condense diagnostic tests and functions by bundling them into single devices. The OPD-Scan, (Nidek, Fremont, Calif.) for example, combines wavefront, topography, autorefraction and autokeratometry into one product, and I have started to integrate some of these hybrid machines in my practice.

Advanced Ocular Analysis

For my patients who wish to have their ocular conditions diagnosed and evaluated to see if they are good candidates for refractive surgery, I do an advanced ocular analysis.

I begin by doing a manifest refraction, topographical exam then a wavefront evaluation looking for higher and lower order aberrations in each eye. I then do an analysis of the lens using digital slit lamp photography with the BX 900 (Haag Streit, Koeniz, Switzerland). I record the photos digitally and send them directly into our EMR. I feel this is a more effective and efficient way to keep a record of a patient who may have a 2+ nuclear sclerosis, rather than just writing it in the chart manually. As an early adopter of technology, I try to integrate not only the latest diagnostic instruments, but I also use tools that can help me at every level of my practice, including EMR.

Next, I will use the Pentacam to measure the anterior curvature of the cornea, posterior curvature, and pachymetry using its Scheimpflug technology. Scheimpflug images can be captured from various positions or in 3D scans. This is done with the eye dilated because it also allows me to evaluate the density of the lens and the degree of cataract that exists.

Combining a slit lamp photo with a Scheimpflug image allows me to not only use the information clinically, but provides tangible evidence patients can see. I am able to go back and tell a patient postoperatively, "here is where your eye was and here is where it is now." Having this photographic evidence is good from a risk management standpoint as well. Some patients who have had LASIK develop a cataract one year or 2 postop, and they have a tendency to blame the LASIK procedure for it. This type of issue can be mitigated or eliminated by utilizing these images preoperatively and discussing the condition of their lenses.

Detecting and measuring scatter is another diagnostic feature we have found useful. When using typical wavefront utilizing Hartmann-Shack technology, the returning ray from the retina is broken up into tiny little dots, and the computer analyzes and measures both the lower and higher aberrations that were focused on the CCD camera. However, it does not give an indication to the strength of the returning signal. The signal as it comes back through the lens can be disrupted by nuclear sclerosis and other changes in the lens.

One new device, the OQAS (Visometrics, Barcelona, Spain), uses a double-pass technique that measures the loss of that returning signal. The OQAS measures light scatter and higher-order aberrations by measuring the modulation transfer function (MTF). MTF provides an indication whether some of the signal was absorbed in the crystalline lens. We have several ongoing studies using the OQAS to evaluate if it can identify cataracts, if the light scattering that occurs is visually significant, if the scatter is from their higher-order aberrations, and to measure the overall effects of multifocal IOLs on optical quality. In our clinical studies, we are doing contrast sensitivity testing using the 6500 Optec Vision Tester (Stereo Optical, Chicago, Ill.) It is advantageous for clinical studies because we can have the same illumination and testing device in multiple locations so we will have no variability from site to site.

Additional Benefits

Not only does securing more information on the optical quality of patients aid physicians from a clinical standpoint, it also helps them to foster better relationships with patients. Over the long term, patients can witness the changes in their eyes, and physicians informing patients of these changes can assist in setting up realistic expectations for patients and steer them in a direction that they can plan for accordingly. For example, if a former radial keratometry patient who is significantly hyperopic is developing a cataract and is going to need surgery, we may discover his or her corneal optics are so badly diminished it will be very difficult to pick an IOL. In this situation, we may suggest the patient have his or her corneal optics repaired through a surface ablation procedure, so we can ready the eye for a future cataract procedure.While it would seem that using more diagnostic tests would lead to many more devices scattered around the office, the development of these multifunctional devices can minimize the need for extra office space in your practice. These products also allow you to keep patients in one room or area and not have them move frequently going from test to test. Lastly, with some of the well-known ophthalmic companies starting to develop these multifunctional devices, these manufacturers can offer a good service record when maintenance or upgrading equipment becomes necessary.

The Future is Here

We need to look at the value of investing in contemporary diagnostic devices and tools to see what helps us in our practice. While technology will continue to change how we practice medicine, and we must adapt to it, ultimately, it will aid us. Investing in diagnostic technology can help measure the true quality of vision and provide a better assessment of the eye so as to help decide what procedure or course of action is best for the patient. As a result, you will strengthen and build your ongoing patient-physician relationships.

Daniel S. Durrie, M.D., is principal of Durrie Vision, Overland Park, Kan., and clinical associate professor at University of Kansas Medical Center. He can be e-mailed at
ddurrie@durrievision.com.