The State of Glaucoma Diagnostic Technology
New instruments pave the way for earlier detection.
BY DEEPAK GUPTA, O.D.
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IMAGERY BY PHIL HOWE |
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In our roles as primary eyecare providers, nearly all of us are managing glaucoma. In fact, primary open-angle glaucoma (POAG) is the most common cause of irreversible blindness that we see. Fortunately, there have been many significant advancements in detecting or monitoring change within the optic nerve and in measuring intraocular pressure (IOP).
In this article, I'll update you on the latest in glaucoma diagnostic technology.
Measuring IOP
Although glaucoma is no longer defined as a disease of elevated intraocular pressure, IOP remains an important parameter in diagnosing and managing the progression of the disease. Because we frequently measure IOP in the office, we often rely on a single measurement at a solitary point in time to get an idea of a patient's diurnal curve. That idea of what a patient's diurnal curve looks like may get better with the advent of the in-home tonometer.
Bausch & Lomb's Proview Eye Pressure Monitor is a new tonometer designed for in-home use by the patient. The instrument measures IOP using the concept of pressure phosphenes. It correlates the force required to generate the image with an IOP measurement. Patients are to use the instrument during the day at home for a defined period, then bring their results back to you for an evaluation. The numbers may help you better manage the disease. The instrument might prove helpful in obtaining diurnal curves in patients who have normal tension glaucoma or glaucoma that's progressing despite apparent IOP control. Specifically, it might help detect spikes in IOP that were previously undetected because they occurred outside of office hours.
At this time, it's uncertain how the Proview will correlate to the gold standard of IOP measurement, Goldman applanation tonometry. Because compliance is often a problem in glaucoma management, you must emphasize to the patient the importance of returning for proper follow-up care regardless of whether the numbers look good.
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B&L's Proview Eye Pressure Monitor |
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Analyzing the optic nerve
Four commercially available instruments (the OCT, HRT, GDx, and RTA) analyze the optic nerve and its surrounding peripapillary tissue, or nerve fiber layer. The purpose of these instruments is to provide objective, reproducible measurements for the posterior pole and optic nerve structure, making it easier to detect subtle changes that might otherwise go unnoticed. All four work effectively to complement perimetric information and to help us gain a more complete picture for diagnosing and managing glaucoma.
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The Stratus OCT |
Stratus OCT. The Stratus Optical Coherence Tomographer (OCT) by Carl Zeiss Meditec is based on a premise similar to ultrasonography, but uses light instead of sound waves. It uses the echo delay time of light back-scattered from various layers of the retina to create a two-dimensional, cross-sectional image of ocular tissue.
The newest version of this machine, the OCT3, works five times faster and collects 10 times more data than earlier generations of OCT. The OCT3 provides an optic nerve head (ONH) volumetric analysis as well as a determination of retinal nerve fiber thickness. Unlike other machines, the instrument doesn't use polarimetry to measure the retinal nerve fiber layer and can quantify the ONH without reference planes.
The OCT is more versatile than either the HRT or GDx (it's also more expensive) because it can also evaluate the macula, disc and area around the disc. The Stratus OCT has the first glaucoma normative database collected in accordance with FDA protocol.
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Laser Diagnostic Technologies' GDx VCC |
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HRT II. One of the most popular disc topographers is Heidelberg Engineering's Retinal Tomograph II (HRT II), a simplified and user-friendly version of the original HRT. The HRT II uses a confocal scanning laser to measure a patient's optic nerve and nerve fiber layer; it can generate a high-resolution, 3-D image of the ONH. The device displays the depth and shape of the optic nerve cup and provides detailed information about the neuroretinal rim, notching and nerve fiber degeneration.
One of the biggest advantages of the HRT II is that it isn't affected by corneal compensation issues of birefringence and retardation. Thus, it can generate an accurate measurement without having the patient remove his contact lenses and it works well even if a patient has had refractive surgery. The HRT II offers a movie feature in which a short video shows the scans of the optic nerve beginning from in front of the optic disc to deep within the optic cup. This highlights subtle changes in the optic nerve. The HRT also includes a normative database using Moorfields regression analysis that compares the rim-to-cup area.
Currently, the HRT's database contains information about Caucasian patients. The company says that other demographic groups are currently being added and that the expanded database should be ready in a few months.
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The HRT II from Heidelberg |
GDx. One of the more sophisticated ways to diagnose glaucoma early is through changes in the nerve fiber layer (NFL). In some cases, the NFL damage will precede field loss and detectable disc changes by five years.
The GDx, by Laser Diagnostic Technologies, is a confocal scanning laser polarimeter that indirectly measures NFL thickness. It projects a polarized beam of light into the eye. As this light passes through the NFL tissue, it changes and slows; detectors measure the change and convert it into thickness units that are graphically displayed. The GDx measures modulation around an ellipse just outside the optic disc and ratios of the thickest points either superiorly or inferiorly to the temporal or nasal regions.
The GDx detects nerve fiber loss, not vision loss. Numerous studies have reported up to a 50% loss in NFL before we see visual field defects. Using perimetry as our standard measure may be the major reason why we haven't been as successful at early detection and prevention of glaucoma progression as desired.
The latest version, the GDx VCC (Variable Corneal Compensation), features a new custom corneal compensation capability that adjusts for each patient's individual corneal birefringence. If you have an older version, then you can take shots of retinal periphery to gauge qualitatively how much of the scan is impacted by corneal interference.
A new enhanced normative patient database allows for comparison of the patient images to normals. And the GDx VCC offers two features previously available only on the original GDx: a fundus image on the printout for a quick, easy evaluation of image quality and the ONH, and serial analysis capabilities that allow up to four exams on one printout.
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Talia's Retinal Thickness Analyzer |
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The RTA. The Retinal Thickness Analyzer (RTA) by Talia Technology uses a narrow slit of a helium neon laser beam that's positioned at the edge of the pupil. This beam is reflected back by the NFL and retinal pigment epithelium and is captured by a digital camera. Sixteen optical sections are imaged, 190 microns apart. These optical sections are used to construct a topographic map that quantifies retinal thickness.
The RTA's software identifies the NFL and the retinal pigment epithelium, calculating the difference between the two to reflect the retinal thickness, which is graphically displayed in color coded two- or three-dimensional thickness maps.
By overlaying these maps on the images of the fundus, the system can help evaluate deviations in optic disc topography. The RTA also offers a glaucoma index that accounts for the size of the ONH, as well as the area around the neuroretinal rim.
The biggest advantage of the RTA is that it can image large areas of the retina at one time, so you can use it to detect glaucoma and other diseases. It's less widely used than the HRT or GDx, so there's less clinical data available.
The last word
Even with all of the new technologies in glaucoma management, we still don't have one single test to tell us definitively whether a patient has glaucoma. Each of these machines tells you the likelihood of a patient having glaucoma. Only you can look at the patient's entire clinical picture and make the diagnosis.
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In their own words: Optometrists weigh in on diagnostic devices |
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The HRT II: Quick and Accurate The Heidelberg Retina Tomograph II (HRT II) was an important acquisition for our practice, which relies solely on patient referrals from primary care optometrists. After 15 years of stereo photography use, I knew that the interpretation of optic disc changes in photos was highly subjective and time consuming. When we purchased an HRT II instead of a fundus camera, our glaucoma consults immediately increased. The HRT II images the optic nerve quickly and accurately. It's comfortable for most patients and only requires a short learning curve for technicians. The HRT II printouts are an invaluable teaching tool for patients. We travel with the laser, a laptop computer and a printer. Optic nerve imaging (CPT 92135) allows for a technical component as well as an interpretation component. Many optometrists in our network now request HRT II for their patients without interpretation and they interpret the results themselves. Optic nerve imaging has provided me with additional assurance about the effectiveness of treatment, or the stability of the disc on suspects I'm not treating. I wouldn't want to return to my old protocol of managing glaucoma. Daryl Mann, O.D., Chattanooga, Tenn. The RTA: An Indispensable Tool As a diagnostic tool for optometry, the scanning laser ophthalmoscope is an underutilized workhorse. Instruments such as the Talia Retinal Thickness Analyzer (RTA) have a wide range of applications for optic nerve and retinal disease diagnosis and management including glaucoma. The RTA is an invaluable asset for improved patient care and it reduces unnecessary referrals. The RTA provides age-matched norm thickness graphs, deviation probability analysis, and a host of volumetric and geometric indices provide a significant amount of data for the practitioner to evaluate. Separate analysis programs detect thinning vs. thickening or edema of the nerve fiber layer. The operation software and the scanning process are simple to learn. Any doctor or technician who can take fundus photos can learn to acquire excellent scans in a day or two. An experienced technician can scan both eyes in five to seven minutes. The size and sensitivity of the system limits its portability. Scanning laser ophthalmoscopy is billed with CPT code 92135. It's reimbursed by Medicare for more than 20 diagnoses and is routinely reimbursed by other insurance plans. Many patients are willing to pay out of pocket for this test once they see the results and understand its value. The instrument's wide range of application in a primary care optometry practice not only improves the level of care the doctor provides, but makes the RTA a profitable practice builder. Joe W. Deloach, O.D. , Plano, Texas The Stratus OCT: Fast Image Acquisition The Stratus Optical Coherence Tomographer (OCT) provides direct, noninvasive, high resolution (10 microns) cross-sectional imaging of the retina and optic nerve. The device creates color-coded, two-dimensional images of the optic nerve head, nerve fiber layer and macula. Many articles confirm the OCT's emerging role in the management of glaucoma. OCT scans have demonstrated the ability to differentiate between normal and glaucomatous eyes with mild to moderate visual field loss. Another area of interest is measuring the macular thickness of glaucoma patients and evaluating the loss of retinal ganglion cells in glaucoma. Image acquisition is fast, easy and comfortable for the patient. Scans can be obtained through a 3-mm undilated pupil, but better quality scans may be obtained with dilation and on patients without significant cataracts. Each scan takes less than one second and you would typically obtain a few different scan patterns. It takes about five minutes to scan for optic nerve cupping and retinal nerve fiber layer thickness. Studies confirm its accuracy and repeatability. The Stratus OCT scanning unit, its monitor and computer are mounted on a wheelchair-accessible table with an overall size of 48 inches by 34 inches. It is not readily portable. CMS has approved the OCT for reimbursement with the 92135 CPT code; many other commercial insurers will also cover the procedure. The average national reimbursement is $67 per eye. Michael Chaglasian, O.D, Chicago GDx with Variable Corneal Compensation The GDx with variable corneal compensation (VCC) corrects for the corneal birefringence associated with each individual eye versus the old fixed compensation that assumed the same corneal birefringence for all eyes. This allows comparison of the status of your patient's retinal nerve fiber layer (NFL) distribution with distribution of nerve fibers found in healthy eyes from a 750-eye, age- and race-matched normative database. This information is useful in conjunction with other elements of a comprehensive ophthalmic exam, especially with respect to glaucoma. An automated visual field, long the gold standard for glaucoma diagnosis, is a subjective measure of function. The GDx VCC offers a valuable adjunct: an objective test of anatomic structure. The Variable Corneal Compensation technology largely removes artifact occasionally introduced by the cornea, giving an accurate representation of the status of the NFL. The total exam takes less than three minutes and requires no dilation with an acquisition time of less than 0.7 seconds. The GDx VCC is a compact, easy-to-use tabletop unit (24 inches by 14 inches) for space efficiency and portability. The GDx VCC allows for greater ability to accurately track patients' change over time with an exam reproducibility of less than 4 microns test/retest variance. It can make financial sense for any practice capable of performing only two exams per week as it is reimbursable under Medicare CPT code 92135. Joseph Sowka, O.D., Fort Lauderdale, Fla. The Proview for Patient Use The Proview Eye Pressure Monitor is a non-corneal contact, phosphene tonometer that patients can use to measure their diurnal intraocular pressures (IOPs) outside the clinic. Patients place the Proview's small, flat probe tip on their eyelid, near where their nose and eyelid meet. Looking down and away from the Proview tip, they begin to apply slow, gentle pressure until a phosphene is perceived. You need to train patients to recognize a phosphene as a dark circle or arc with a ring of bright light around the edge, akin to a solar eclipse. The company urges patients to check their ability to see a phosphene before they purchase the Proview because some are unable to perceive one and so shouldn't use the monitor. It takes patients about one week to get comfortable with the process of using the device. Once a patient establishes a baseline with the Proview, the instrument will detect changes as long as he takes his IOPs as instructed. The true value of the Proview lies in its ability to detect a change from an individual patient's established baseline IOP. Bernard Fresco, O.D., Toronto
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Dr. Gupta has no financial interests in any of the companies or products mentioned in this
article. You can reach him by e-mail at deegup4919@hotmail.com
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