Article Date: 5/1/2004

imaging for AMD
RETINAL IMAGING and Your AMD Patient Base
Know the different facets involved in using retinal imaging to diagnose and manage AMD.
BY RICHARD HOM, O.D., F.A.A.O., San Mateo, Calif.

According to the American Optometric Association, age-related macular degeneration (AMD) is the leading cause of blindness in America. While there's no known treatment for the dry form, if we can diagnose and treat the less common wet form early enough, then we can make a huge difference in our patients' lives. New retinal imaging technology can help us accomplish this. In fact, authors, lecturers and manufacturers are espousing retinal imagers as being "essential" in evaluating, detecting and diagnosing the various forms of AMD. Proponents of retinal imaging suggest that the "near histological" detail of their images can reveal subtle or occult changes of the macula common in AMD. Often, subtle tissue changes precede actual damage. Traditional techniques such as fundus contact lens and fluorescein angiography may require obvious tissue changes to be visible to a clinician. Only with experience could clinicians adequately detect AMD and perhaps not as early as with these new technologies.

Patients have high expectations for care and some will leave a practice for another that has the "more advanced instrumentation." It's perception! Appearances count. Patients in the United States are accustomed to the "best medicine" and the "best diagnostic instrumentation." You should plan on showcasing retinal imaging to highlight the progressive nature of your practice. It's good marketing!

Overall, these instruments aren't new. Most have been on the market for at least three to five years and have already established themselves among the first wave of early optometric adopters. But the majority of O.D.s and M.D.s aren't yet owners of these instruments. For those of you who are still on the sidelines, doubts remain whether this technology is right for you and your practice and whether you can do without.

Discussing the standard

Much of this technology isn't the standard of care -- yet. According to Scott Righthand, a San Francisco litigator who specializes in medical negligence and personal injury law, ". . . the definition of standard of care is the degree of carefulness that a similar specialist would have . . . ."

But legal issues aside, retinal imaging can also mitigate the damage of nonexudative AMD. Staging AMD is crucial. The Age-Related Eye Disease Study (AREDS) recommends nutritional supplements in the intermediate stages of AMD in one or both eyes. Accurate staging of AMD in combination with nutritional therapy may prolong or improve prognosis.

Weighing the risks

So if you think about it, waiting has risks. For one, a jury could consider you falling below the standard of care if an expert witness doctor testifies that using high technology might have created a different result for the patient and "is the standard of care." Although not a perfect defense, high technology imaging and documentation may make it more difficult for patients who initiate litigation to win in court.

Another way that waiting could bring risk is that patients may exist already who may have been treated differently had their eyecare practitioner instituted retinal imaging earlier. If a doctor doesn't perform dilation routinely on either "at risk" or on all of his patients, then it's likely he could miss a pathology. Similarly, in the case of an exudative AMD, it's possible for a doctor to miss or misjudge a lesion as normal when in fact it's exudative or newly vascularized.

Evaluate the choices

Which retinal imaging technology is the best? How do you choose? Use an evaluation model by modality or by disease entity. (That means, does the instrument measure the patient's ability to function or does it measure the structure of the retina? Does it merely document structure or does it also analyze it? Will it detect AMD earlier?)

Instruments that are structural measure and quantify some anatomical feature of the retina whether it's the thickness of the retina, the retinal nerve fiber layer (RNFL) or the optic nerve head (ONH) or retinal lesion. Each instrument also measures and analyzes the retina differently.

The Stratus Optical Coherence Tomographer (OCT 3), by Carl Zeiss Meditec, directly measures the retina and its various layers much like a histology slide. The OCT uses low-coherence superluminescent diodes to pass near-infrared frequency light through the eye to the retina. The retina's reflectance of this light is measured with interferometry. This is much like ultrasound, but with much higher resolution, as the near infrared wavelength is much shorter than the ultrasound.

It's a secondary diagnostic instrument that provides a tissue cross-section evaluation of the retinal tissue. From this data, representation and measurements of the actual layers of the retina are possible. Compare RNFL and retinal thickness results with normal values to help with diagnosis.

Much like the OCT, but with lower axial resolution, the Retinal Thickness Analyzer (RTA), by Talia Technology, takes 80 slit images. It uses these slit images to produce an image of retinal thickness or a topographical map.

Optomap (Optos) is a straightforward two-laser retinal imaging system that functions like a "super" ophthalmoscope. Its 200-degree field provides a broad view of the retina without dilation. The digital imaging capability of the Optomap is advantageous because it allows the examiner to view more retina in one image than any other technology. The Optomap is an instrument for primary examination of the retina and provides a high resolution (four mega pixels), digital image of the retinal surface and optic nerve. The ability to view images from the Optomap's independent lasers allows the practitioner to identify the depth of lesions in the retinal tissue.

The Heidelberg Retina Tomograph (HRT) II (Heidelberg Engineering), although more widely used in glaucoma than in AMD, also images the retina. The HRT scans the retina in 15-degree fields of view with a laser that scans side-to-side and can take up to 64 horizontal cross sections down to a depth of up to 4mm. Each cross-section requires only a refocusing of the laser to scan a deeper layer of the retina. This method results in a 3-D topographical image of more than 247,000 data points. The HRT's new retinal module collects the laser light reflectance from the retina's internal limiting membrane and can evaluate differences in density or hydration by analyzing the differences in rates of reflectance.

Functional versus structural

Instruments that are functional measure the ability of the eye to respond to a stimulus. The most common is the visual field tester. Still the traditional standard for evaluating the current state of the retina and the optic nerve, this kind of instrument won't go away any time soon.

To a prospective purchaser, structural instruments might not seem that much better than the "trusty" threshold visual field tester, a functional instrument. But to those of you who do a lot of visual field testing, patients can do poorly on this test and it's not infrequent for doctors to repeat visual field tests to verify the earlier result. In fact, the Ocular Hypertension Treatment Study (OHTS) suggests that, ". . . 86% of initial, positive field findings were not found upon retesting . . . ." Structural instruments aren't psychophysical and aren't subject to this variability. With such freedom, structural instruments may, therefore, be more suited to discerning early trends or detecting small changes in the retina.

Decisions, decisions

Can you own one type of instrument and not the other? Yes and no. Yes, because functional instruments are still the standard of care and do a good job for doctors. The OHTS study may suggest otherwise, but visual fields are still accepted by ophthalmology and neurology for moderate to advanced visual defects that may not have glaucoma as an etiology. But no, because dependence on functional instruments alone might not be sufficient for patients who can't reliably complete psychophysical tests.

Choosing which instrument(s) to buy isn't simple, but several paradigms might help.

1. Use the instrument to detect, diagnose, manage or document a specific disease or set of disease entities. All of the structural instruments can make you feel more comfortable in detecting and managing this disease in your office. One or more of these instruments help doctors "see" the histopathology of either clinical or occult eye problems and can provide that additional evidence necessary for appropriate further medical or surgical management by a subspecialist.

2. Think about using the instrument beyond your initial intention or its scope. For example, use the Optomap for routine screening and for disease detection and subsequent surveillance.

3. Most of these retinal imaging products have or will soon have telemedicine capabilities, which is a natural environment for these instruments. Patient comfort and convenience is still an integral component of patient loyalty. If you could share a suspected finding with a consultant and render a clinical decision while the patient is in the chair, then the possibilities are endless.

Keep the benefits in mind

Early adopters and larger practices have already acquired one or more retinal imaging products. For many O.D.s who are unaccustomed to third-party reimbursement for this procedure, the cost seems formidable. And not all third-party plans will cover this procedure (although Medicare will). CPT code 92135 ("Scanning Computerized Ophthalmic Diagnostic Imaging" or SCODI) is already the single largest cost for an ocular diagnostic test for Medicare (Note: this code doesn't apply to the Optomap).

A perfect compliment

To many doctors, retinal imaging is a "must-have" item that greatly facilitates and simplifies the management of AMD. Unfortunately, there still isn't a single instrument or test that can make clinical decision making absolutely simple. These instruments can't replace sound clinical judgment and they can't give an absolute "yes" or "no" answer to a problem.

Doctors who can understand the place that these technologies have in their practices will undoubtedly make themselves better doctors to their patients and to other doctors.

Dr. Hom is the coordinator of primary care optometry for the San Mateo Medical Center. E-mail him at rhom@co.sanmateo.ca.us.

 

 



Optometric Management, Issue: May 2004