One of the things that makes treating glaucoma unique—and one reason I enjoy doing this for a living—is that helping a patient is like solving a puzzle. Thanks to multiple diagnostic technologies, we can assemble different pieces of information and draw conclusions that help us decide how to most effectively treat a patient. Sometimes one piece of the puzzle won't fit—but that's what makes managing glaucoma a challenge.
Here, I'd like to share some of the strategies we use to ensure that we get the most accurate "puzzle pieces," so our patients receive the best treatment possible.
Maximizing Every Test
With so many tools at our disposal, there's always something new to learn. Here are some strategies that we've developed in our clinic for using different technologies.
• Pachymetry. Today, pachymetry is standard testing for any glaucoma suspect or patient. However, pachymetry will only produce an accurate measurement if the cornea is well-hydrated (and the tip is held perpendicular to the corneal apex). In one study in which right eyes were measured first, pachymetry always found the left eye's cornea to be thinner. The reason? The technicians were not making patients blink between measurements, so the left eye's cornea was drying out. (Make sure your technician doesn't make this mistake.)
Pachymetry is reimbursed once in a lifetime, but if old records are not available to you, you may be able to appeal a denial.
• Tonometry. A Goldmann tonometer is still the standard technique for checking IOP. However, if a patient is very young or has corneal scarring, a pneumotonometer is more likely to be accurate. Newer-generation devices, such as the Pascal Dynamic Contour Tonometer, are probably not ready for prime-time until we have more studies that help us understand the accuracy of the measurements.
• Gonioscopy. This is required for every patient who has glaucoma or is a suspect. The Preferred Practice Pattern suggests that gonioscopy be repeated in phakic patients every one to five years, or if the patient's IOP suddenly becomes uncontrolled, or if an angle closure component is suspected.
Normally, you can get reimbursed for gonioscopy if it's accompanied by a glaucoma diagnosis code and it's medically necessary. Some payers have recently been denying payment for gonioscopy because they consider it to be included in the exam, but the CPT guidelines clearly state that it's a separate service that should be paid in addition to the exam. When we encounter a denial, we appeal it with the carrier. If that fails, we talk to the plan's medical director, who often has a better understanding of our perspective.
• Ultrasound biomicroscopy. This can serve as an adjunct to gonioscopy when, for example, a patient has narrow angles for unexplained reasons (especially a unilateral angle closure), or a focal area of narrowing that could be an iridociliary cyst or tumor. This device is also very useful when corneal opacity precludes gonioscopy.
Ultrasound biomicroscopy can be particularly helpful when a patient has had a laser iridectomy but the angle doesn't open satisfactorily. These patients may have plateau iris syndrome, which UBM can detect.
Generally only available in larger glaucoma centers, UBM is coded as 76513—anterior segment ultrasound.
• Visual field testing. At the outset, you may need to do several visual fields in a fairly short period of time to get the patient past any learning curve so you have a reliable baseline test.
Visual field testing by ophthalmologists in the United States (as opposed to optometrists) is usually done with the Humphrey Visual Field Analyzer, using a variety of testing programs. White-on-white 24-2 SITA (Swedish Interactive Threshold Algorithm)-Standard is the most efficient, accurate and reproducible program.
If patients do well and have a significant scotoma, then it's appropriate to continue to follow them with SITA-Standard. SITA-Fast is only a minute quicker per eye, and because the program makes more assumptions, you lose some accuracy. Also, SITA-Fast can't be used with the Glaucoma Progression Analysis statistical package.
If you find no visible scotoma using SITA, yet you're very suspicious that the patient may have glaucoma, it may be worth switching to the blue/yellow or SWAP (Short-Wavelength Automated Perimetry) test—as long as the patient is a good test taker, is alert enough to tolerate the blue/yellow stimuli, and has no significant cataract or media opacity. SWAP has been shown to detect damage up to five years earlier than standard white-on-white perimetry. (SWAP-SITA is coming soon and may make it easier to use the SWAP strategy.)
In most states visual fields are reimbursable once or twice a year, as long as medical necessity is demonstrated. (If the patient needs to have a third test during the year, you can still submit for reimbursement, but you may need to send a letter explaining why you felt you had to do the test more frequently than is usually allowed.)
• Frequency doubling technology. Many practices have an FDT; it's very useful for screening. Also, some patients who can't do standard perimetry have no problem doing the FDT. If you've done a standard white-on-white a number of times and the evidence suggests that the patient isn't very good at taking the test, it might be worth switching to the FDT or Matrix to see if you get a better response. (The Matrix, a more advanced version of this technology that tests more areas within the visual field, appears to share many of the early-detection advantages of SWAP testing, but it's not widely available yet.)
Imaging the Optic Nerve
Most practices are increasingly using modalities such as the HRT2 (Heidelberg Retinal Tomograph, from Heidelberg Engineering), Stratus OCT (Optical Coherence Tomographer, from Zeiss Humphrey) or GDx-VCC Nerve Fiber Analyzer (from Laser Diagnostic Technologies) to image the optic disc and/or retinal nerve fiber layer. Each of these instruments has strengths and weaknesses.
• The HRT. The HRT is extremely good at showing the topography of the optic nerve, and an HRT scan is particularly useful to confirm your impression of a smaller than average optic nerve or a very large diameter optic disc. However, its accuracy depends on the technician or physician drawing a contour line that accurately traces the outer edge of the optic nerve.
In addition to measuring the optic disc and evaluating cupping, the HRT does a good job of distinguishing between patients who are progressing and those who are not, by detecting differences in the optic nerve contour. Even in the Ocular Hypertension Treatment Study, initial abnormal HRT parameters were likely to predict which patients went on to develop glaucoma. The weakness of the HRT is that it's probably not the best instrument for measuring the thickness of the RNFL.
In order to use the HRT effectively to monitor for progression, it's imperative that the physician evaluating the test look at the optic nerve and check the printout to be sure the line is drawn in the proper location. It's especially important to get that line right on the first test, because later tests will use that same contour line automatically for change analysis.
• The OCT. Optical coherence tomography is an accurate way to directly measure the thickness of the retinal nerve fiber layer; it correlates highly with disc damage and visual field loss. The current model, the Stratus OCT, is highly reproducible, and very useful for confirming a diagnosis. When evaluating the optic disc and cup size, however, the automatically drawn disc contour line is based on the edge of the RPE reflectance, which is sometimes inaccurate. This can cause the instrument to delineate the disc edge incorrectly.
The OCT RNFL data is reproducible and correlates highly with clinical findings of disc damage and visual field loss, but it's still not possible to definitively determine progression using the OCT. There is a progression program that overlays the RNFL contour lines from different dates, which is useful; the problem is that we don't yet know how much change from visit to visit is clinically significant.
• The GDx with variable corneal compensator. The GDx with VCC, which uses scanning laser polarimetry to measure the RNFL, is very reliable. However, it's important to avoid using the old model GDx that does not include compensation for corneal birefringence. The birefringence problem reportedly caused about 15 percent of the older scans to be inaccurate—and there was no way to know which scans were affected! However, the GDx with VCC has solved this problem. You may still find a patient who has an unusual birefringence pattern, but you can tell right away if it will affect the accuracy of the data.
Note: If a patient has a large area of peripapillary atrophy, both the OCT and GDx can have trouble getting accurate readings of the RNFL because backscatter from the sclera interferes with measurements. In this situation, it makes more sense to use the HRT to measure the optic disc.
Imaging is reimbursable at least once per year by most carriers, but the rules vary widely and you should check the requirements for payment in your area.
The most important thing to remember about any of these imaging devices is that they need to be used within a larger clinical context—not in isolation.
• Stereo Disc Photos. Photographs are important to have in the chart because photography is a consistent modality; regardless of changes in technology, you can always compare photographs. I still have plenty of cases where I need one more piece of information to verify that there's either stability of progression. I can get that confirmation by comparing optic nerve photographs, looking for a shift in a blood vessel, or an area of thinning on the neuroretinal rim.
If you do this kind of comparison regularly, you'll become good at picking up subtle change. (I like to get an optic nerve photograph every few years, especially if I suspect a change, so I'll have a new baseline photo for comparison.)
You can get reimbursed for disc photos, but not on the same date of service as imaging. However, you only need to take a disc photo every two to four years, so it becomes a matter of careful scheduling to make sure all the information is gathered in a timely manner.
With so many tools to monitor patients, it can be difficult to keep track of what's been done and what needs to be done. To manage this, we've created a simple grid that goes in the patient's chart; either I or the technician fill in the date as the fields, scans or photographs are taken. At a glance I can see when the patient last had a visual field, for example, and whether he's due for another.
Hopefully, when electronic medical records become more standardized in practices, it will be easy to track this information onscreen. But for those of us still using paper charts, a grid like this is very helpful. REVIEW
Dr. Mattox is vice chair of the department of ophthalmology at Tufts-New England Medical Center, and director of the New England Eye Center Glaucoma & Cataract Service.