Shedding Light on a New Era in Refraction
Shedding Light on a New Era in Refraction
The right diagnostic equipment can give you the knowledge you need to perform better refractions in less time, which should result in more satisfied patients.
By John E. Warren, OD
For many years, refractive error was described with three values: sphere, cylinder and axis, with an add power thrown in for good measure in patients with presbyopia. There were always patients whose vision couldn't be corrected to "normal" or satisfactory levels with spherocylindrical lenses. Some were uncorrectable to a small degree and some to a much larger degree. Most of these patients suffered from irregular astigmatism that was either manmade (from refractive surgery or other anterior segment procedures) or natural in origin (such as keratoconus, marginal corneal degeneration or other degenerations or dystrophies). However, many of these patients suffered from degradation of their optical system due to internal ocular issues, such as tilted, displaced or incorrectly placed IOLs.
Patients with irregular astigmatism, or other forms of ametropia, benefit from being fit with a rigid, gas-permeable (RGP) contact lens, which can neutralize or mask irregular refractive errors due to corneal curvature.
Patients with intraocular issues, such as a decentered or severely tilted IOL, typically weren't aided by an RGP lens, so they were told "sorry, there's nothing else we can do for you." Or, they were given the option to undergo an additional intraocular surgery, in the hopes of reducing their visual issues. But these patients already had one surgery, so pursuing a surgical correction for a surgically induced problem isn't an appealing option.
New advances in diagnostic technology provide us with a much better understanding of the entire ocular system by measuring not only sphere and cylinder but also higher-order aberrations, which can be caused by corneal or lenticular abnormalities. As a result, these devices provide a more complete description of the optical system. This detailed data can explain a patient's complaints and visual problems, and may aid in maximizing the accuracy of the optical correction and bestcorrected visual acuity (BCVA), which helps to prevent unnecessary follow-up surgeries and allows more targeted attempts to correct vision.
As attempted correction patterns become more complex in refractive surgeries, such as LASIK and PRK, knowing more about the "virgin" optical system, and also the postoperative visual system, is critical. Advanced IOLs also demand a better understanding of the preoperative visual system, so that unexpected outcomes can be better understood and corrected if necessary.
The Pupil and Refractive Error
Over the years, vision experts have learned that the pupil has a tremendous effect on the quality of the image that's focused on the retina. Given the fact that refractive error, both spherical and astigmatic powers, changes from the center of the pupil outward to the pupil margin, increasing or decreasing pupil size can greatly affect the patient's quality of vision.
"We all know that changes in pupil diameter dictate changes in vision," says Louis J. Catania, OD, author of Wavefront Science in Vision and Eye Care: Diagnostic and Corrective Applications in Clinical Practice. "We also know that changes in pupil diameter are related to photopic and scotopic vision. Now, thanks to wavefront science and aberrometry, we can quantify the relationships, compare them with electronic phoropters and introduce a whole new dimension of refractive care for our patients."
The change in overall power tends to be toward less hyperopia or more myopia in most individuals. Aspheric spherical contact lenses, such as PureVision (Bausch & Lomb), have a positive effect on visual quality for some patients because of this tendency. It's this phenomenon that causes "night myopia" and reduced contrast sensitivity in most patients — not a decrease in the depth of focus from a larger aperture. In the wavefront map, notice how the optical power moves more toward the red (myopic) powers.
The Optical Path Difference (OPD) map generated by the 3-D Wave (Marco Ophthalmics, Jacksonville, Fla.) provides three refractions for the patient. The first is in the center of the pupil, the second is at the 3-mm zone (Figure 1a) and the third is at the 5-mm zone. You also can see the change in the cylinder power and axis as we move further from the pupil center. The Wavefront Total Map allows you to recalculate the zone to obtain a night prescription based on the patient's mesopic pupil size, out to 6 mm (Figure 1b).
|Figure 1a. The OPD map, generated by the 3-D Wave, provides three refractions for each patient. This image shows the second and third refractions, taken at the 3-mm and 5-mm zones.
||Figure 1b. The Wavefront Total Map allows you to recalculate the zone to obtain a night prescription based on the patient's mesopic pupil size, out to 6 mm.|
After reviewing the maps, it's no wonder this patient complains of poor visual quality in the evening and in low illumination settings. The prescription, which works best for her in normal illumination (-7.50 sphere), isn't appropriate for low light settings (-7.75 × -1.25 × 148). It's not uncommon to see similar refractive changes in patients after refractive surgery, especially if their pupil size approaches the size of their effective treatment zone. In Figure 2, notice how the pupil size in dim illumination is larger than the effective treatment zone, creating a rather large myopic shift and poor night vision. If this patient were examined and refracted in normal illumination conditions, the resulting refraction wouldn't serve them as well as when the light levels are reduced.
Figure 2. A map reveals that the pupil size in dim illumination is larger than the effective treatment zone, creating a large myopic shift and poor night vision.
So, here's the key question: what's the optimum illumination level in which to perform refractions? The answer depends on the illumination conditions the patient expects to function in most often. For patients without much change in their prescription from center to periphery of the pupil, the illumination during the refraction doesn't matter much. But for patients who need to function well in low light or very bright light, especially if they have significant prescription shift to pupil size, the illumination level is critical and should be varied accordingly.
Using a standard phoropter, multiple complete refractions may be necessary. If the refraction is performed using the 3-D Wave and the EPIC or TRS 5100 (Marco), only one refraction is needed. With these machines and the touch of a button, you can show patients the difference between their standard illumination and nighttime refractions, so they'll be able to make an accurate and direct comparison between the two under the appropriate illumination conditions.
"I've had the 3-D Wave for 3 years and I think it's more accurate than any autorefractor I've ever worked with," says April Jasper, OD, FAAO of Advanced Eyecare Specialists of West Palm Beach, Fla. "The wavefront analysis is fabulous. It's so good to know why challenging patients don't refract to 20/20. The 3-D Wave has solved many dilemmas for me and saved my patients from unnecessary referrals. Also, it's helped improve the accuracy of my RGP fits; I find most come out right — even the pellucid and keratoconus patients — the first time."
There are also patients who have nonsymmetrical differences in optical power. Keratoconus patients are a good example of this. In cases of significant asymmetry, an RGP lens (standard or custom parameters) is the best solution for maximizing visual quality. Companies are developing soft lenses to help correct vision in patients with significant higher-order aberrations (HOAs). These lenses are still experimental, but they show promise for future applications.
As you can see, the pupil can have a profound impact not only on the patient's visual function and correction requirements, but also on the refraction results. No longer is it adequate to refract every patient under the same lighting conditions. Many patients would benefit from either a specific-use prescription or a blending of their small- and large-pupil refraction results.
A New Way to Refract
For some time now, we've had access to so-called automated phoropters, which not only improve efficiency and ergonomics, but also provide a more patient- and physician-friendly method for the determining cylinder power and axis. Instead of using the Jackson Cross Cylinder and asking patients to compare one image to a subsequent image, newer devices, such as the TRS, use a bi-prism and present two choices for the patient to compare side-by-side. Based on patient input, the presentation is changed until the two images appear to be equal to the patient. This is done for both the cylinder power and the cylinder axis. This makes the refraction faster and more patient-friendly.
Phoropters, such as the EPIC and TRS 5100, also can show the patient the difference between the subjective refraction and their habitual refraction as measured via lensometry. This greatly improves the patient's ability to determine if they'll benefit from a change in their prescription.
In addition to showing patients the difference between their old and new prescription, the TRS 5100 also shows patients the difference between their subjective refraction and their nighttime prescription, as measured with the 3-D Wave. By decreasing the illumination in the exam room and giving patients a few seconds to adapt, the prescription is changed in just .3 seconds from the subjective prescription to the nighttime prescription using the 3-D Wave. This allows patients to make a direct comparison between their subjective vision with both prescriptions under low-light conditions. If a patient has a moderate to strongly positive reaction to the difference in qualitative vision, a second prescription or a blending of the two prescriptions is indicated for maximum visual efficiency and qualitative vision.
The TRS 5100 also has several different refraction programs that can help ensure a consistent refraction and greatly improve refraction efficiency without cutting corners. This is especially helpful for patients who show a night vision shift, because it ensures the comparison will be made and sets up the chart properly throughout the refraction process, making the exam more ergonomically efficient and faster, too.
By knowing the refractive error in all three zones, the patient can undergo the Marco HD Eye Exam and benefit from having their HOAs considered, not only for daytime refraction but also for nighttime refraction.
Putting HOA Into Action
Even though vision correction technology doesn't allow us to fully correct higher-order aberrations with spectacles or contact lenses, knowing what the patient's HOAs are — and to what degree they are present — is powerful knowledge. Night vision shift can be a significant factor for our patients, but it's equally important to know and understand the overall optical fingerprint for each patient. Patients who have elevated Root Mean Square (RMS) values at the 3-mm or 5-mm zones most likely won't achieve normal visual acuity. Knowing this before you begin the refraction can prevent a 10-minute refraction, which wouldn't result in better vision, regardless of how many times the sphere and cylinder (axis and power) are refined. Conversely, if the RMS values are normal (under .30 at the 3-mm zone and .50 at the 5-mm zone), patients should achieve 20/20 or better vision, unless there's posterior segment pathology.
A patient with a moderate to significant change in astigmatic power or axis from the central to 3-mm zone may benefit from a blending of the two prescriptions to maximize visual performance. In these patients, comparing the auto-refraction to the subjective prescription can tell you a great deal about which prescription would provide them with the best function. Because the EPIC and TRS 5100 have the ability to quickly demonstrate the difference between these two findings, patients can see for themselves which prescription will work best for them in most situations.
Topography and Refractive Error
Because the 3-D Wave captures corneal topography, refraction and aberrometry data in one exam, it's helpful for many patients — not just those who come in for a basic refraction. If the only data available is corneal curvature/power, refraction/auto-refraction and BCVA, it's impossible to know for sure what's causing reduced BCVA. The myriad reports generated by the 3-D Wave allow the user to determine if the issue causing reduced vision is corneal, lenticular or some other cause.
In the map of a post-cataract patient (Figure 3), the corneal topography shows some irregular astigmatism in the superior corneal curvature. This type of topography can cause reduced BCVA, as well as ghosting and other visual disturbances. However, the OPD map shows something else. The arc of blue from 12 o'clock to 7 o'clock shows a considerable change in total optical power with a significant hyperopic shift in that area. In fact, the haptics of the IOL can be seen at 3 o'clock. The internal OPD map on the lower left confirms the real cause of the patient's reduced BCVA, glare and ghosting: temporal displacement of the IOL — not the irregular corneal curvature. The irregular curvature may play a role but correcting the cornea alone, surgically or with an RGP lens, won't come close to solving this patient's significant visual problems.
Figure 3. The corneal topography in this map of a post-cataract patient shows some irregular astigmatism in the superior corneal curvature, which could cause reduced BCVA, ghosting and other visual disturbances, but the OPD map reveals the real cause is temporal replacement of the IOL.
Many other cases exist that are best diagnosed and managed by using corneal topography and OPD information. There are many patients who develop what are commonly referred to as "Milky NS Cataracts." These are patients who have mild nuclear sclerosis, in which the lens doesn't appear to be off color, but simply is more milky in appearance. Many times, these patients don't appear to have lenticular changes that would explain their symptoms, which usually are an overall lack of clarity, moderately reduced BCVA and, in some cases, glare.
Figure 4 shows a patient who has had RK in the past with a hyperopic/astigmatic outcome. She was seeing quite well with a custom, reverse-geometry RGP lens for several years, but then developed complaints of reduced optical quality that couldn't be managed with lens changes. Based on her internal OPD, it's obvious she has lenticular changes, which are contributing to her complaints.
Figure 4. This patient had RK with a hyperopic/astigmatic outcome. She'd been seeing well with a custom, reverse-geometry RGP lens, but then presented with complaints of reduced optical quality that couldn't be managed with lens changes. An internal OPD revealed she had experienced some lenticular changes that were contributing to her new complaints.
Identifying the HOAs, and knowing the exact power distribution of the lower-order aberrations, can be very helpful when planning procedures such as Ortho-K or implanting toric IOLs. Any procedure that modifies the cornea will create HOAs. If a patient already has moderate to significant HOAs, knowing this can prevent performing a procedure that's doomed to failure from the start. If a patient has vague visual complaints after a procedure, being able to compare the pre- and post-treatment HOAs also can be helpful.
Figure 5 is from a patient who was happy overall with her Ortho-K result, but had some issues with glare and halos at night. While the corneal topography appears to have been altered in a manner that provides a large enough treatment zone, the OPD shows that the effective size of the treatment zone isn't quite large enough to prevent the pupil from being larger than the effective treatment zone. Enlarging the treatment zone by .2 mm in each eye resulted in a more acceptable outcome for this patient.
Figure 5. These images were taken in an Ortho-K patient who had issues with glare and halos at night. Enlarging the treatment zone by .2 mm in each eye resulted in a more favorable outcome.
"Although wavefront science is most closely associated with refractive surgery, in the very near future, its applications in vision and eyecare diagnosis and more so, in developing and prescribing customized IOLs, spectacles and contact lenses will be its greatest strengths," Dr. Catania says.
When a patient presents with low RMS values, the 3-D Wave can send the 4-mm Custom Wavefront Refraction results straight to the EPIC or TRS 5100. So the refraction time may be shortened quite a bit, to the benefit of the patient and the doctor.
In cases where you send the 4-mm Wavefront Refraction to the EPIC or TRS 5100, and the RMS values are normal, you can perform the Marco 30-Second Refraction. In these situations, patients are shown a chart that ranges from 20/25 to 20/15. If patients are able to easily read the top two lines, you can perform red/green balance on each eye to refine the sphere power and the result displayed to the patients binocularly. Then, they can compare this refraction to their current prescription and to any potential nighttime refraction, if they have shown a moderate level of shift from the center to the periphery of the pupil.
This refraction will give the patient clear vision while shortening the refraction time. The decision to shorten the refraction time is driven by scientific information, not a desire to spend less time on the exam. And patients don't like lengthy refraction exams, so shorter is better from a subjective point of view, too.
"I purchased the TRS as an afterthought," Dr. Jasper says. "I had trouble with my shoulder as a result of the constant repetitive motion from the older phoropter and needed a quick solution. After implementing the TRS into my practice, the speed of refractions increased dramatically. My patients love the accuracy of the prescriptions I obtain in a reduced amount of time."
Knowledge is Power
As you can tell, learning all we can about the visual systems of our patients is a powerful advantage. Not only can we help explain reductions in BCVA, we also can work toward solutions for our patient's problems, whether the problems are manmade or naturally occurring.
Knowing more about our patient's aberropia also allows us to prevent a refraction process that's tiring — for the patient and the doctor — by allowing us to factor in the HOAs in one or more refractive solutions for our patients. Our patients expect more from us than simply correcting 80% of their visual issues, and there's no reason we can't offer them better than that. This is a great time to be an eyecare practitioner. We can offer our patients diagnostic and therapeutic options that are better than ever before. OM
Dr. Warren is the owner of a group practice that includes several optometrists and an ophthalmologist in Racine, Wis. This practice setting provides a varied patient base that offers many clinical challenges on a daily basis. You can reach him at email@example.com.
Optometric Management, Issue: March 2009