glaucoma instruments
Wielding Technology Against Glaucoma
Here's the secret to success in glaucoma therapy
BY J. JAMES THIMONS, O.D., F.A.A.O., Fairfield, Conn.

The last decade has brought us countless technologies designed to assist us in diagnosing and managing glaucoma. Unfortunately, the application of these advances isn't always as clearly defined or as easily implemented as we'd like. This article will explain how to maximize money you've already invested in equipment as well as how to spend your money wisely for an important upgrade.

How do you decide

Deciding what best serves the needs of your practice isn't always easy. For most of us, the categories are:

► Optic nerve/nerve fiber layer analyzers

► Visual field systems (new or upgrade)

Base your decision to buy on two factors:

1. Your desire to improve the level of care you offer to patients

2. The practice's ability to support the cost of the system with adequate patient flow.

Here's the formula

The best way to decide whether a technology is right for you and your practice is to take an inventory of your current glaucoma patient volume. Then look at that segment so that you can predict future needs as well as current use. In many practices, this analysis provides useful information regarding which new technologies are most likely to reward the practitioner and benefit the patient.

A simple formula to determine patient activity in glaucoma is to look at the number of threshold fields you bill in a 12-month period. Assuming that most patients undergo two visual fields each year and that the vast majority of fields are for some type of glaucoma, this gives you a realistic assessment of the number of ocular hypertensives, glaucoma suspects and chronic open-angle glaucoma patients you're currently treating or monitoring. From here you can decide whether that visual field upgrade will pay for itself or whether the optic nerve analyzer or nerve fiber layer analysis system is justified.

The rule of thumb is that it takes about 15 new glaucoma patients each month to underwrite the cost of a nerve fiber analyzer or optic nerve scanner. To support a new visual field system, you'll need approximately 10 patients each month; an upgrade such as a chromatic perimetry typically requires five patients each month.

Comparing analyzers

Let's compare optic nerve analyzers and nerve fiber layer analyzers. The debate over these two devices has raged over the past five years. The bottom line is that both have significant merit but approach the assessment from completely different directions.

The optic nerve scanning systems, such as the Heidelberg Retina Tomograph II (HRT II, Heidelberg Engineering) and the OCT III, examine the physical characteristics of the nerve -- the total area of the optic nerve head (ONH), the cup-to-disc (c/d) ratio, neuroretinal rim area and cup volume. These data points give us an excellent mechanism of initial analysis, but are even more useful in the assessment of progression over time, the critical factor in glaucoma therapy. Combined with visual field examination, they're a powerful tool for the practitioner to use in improving glaucoma care.

Analyzing pearls

Here are a few pearls regarding the use of optic nerve analyzers and nerve fiber analyzers that make them especially helpful in selecting patients:

Optic nerve analysis. The ability to analyze ONH square area and to quantify optic nerve size has been extremely important in two patient categories we commonly see in practice:

1. The patient who has assymetric c/d ratios

2. The patient who has large c/d ratios and normal visual fields.

In many of these cases, the answer is the ONH size and the fact that the assymetry of the c/d ratio is commonly an assymetry of the ONH size and not that of the c/d ratio, therefore reducing the risk for glaucoma. Equally, we can differentiate the patient who has large c/d ratios and normal fields by the overall size of the ONH. Individuals who have ONH square areas that are greater than one or more standard deviations from the norm will have larger c/d ratios without visual field loss than the same c/d ratio in a patient who has normal ONH square area.

Another critical factor in your consideration of which system is superior is the ability of the systems to provide progression analysis for patients measured over time. Both the HRT II and the OCT III have standardized systems that allow for critical analysis of subtle change, which is essential in the long-term care of the glaucoma patient.

The limitations of both systems are that while you can examine almost all optic nerves, certain nerve types are consistently misinterpreted -- for example, individuals who have hypoplastic discs, tilted nerves and other anomalies of construction. Also, patients who have high astigmatism require correction for accurate readings.

A factor that has received little discussion is the ability to use this type of system in the examination of post-refractive surgery patients. Both units (the HRT and the OCT) bypass the corneal reshaping issues that affect the accuracy of many other instruments (automated refractors, nerve fiber analyzers, etc.).

Nerve fiber analysis. Instruments such as the GDx (Laser Diagnostic Instruments) have long been around and have experienced some growing pains as it matured. The new GDx Variable Cornea Compensator (VCC) system measures both the corneal and retinal birefringence and compensates by subtracting one from the other to give an accurate measure of the level of retinal nerve fiber layer thickness.

The goal of nerve fiber analysis has traditionally been to produce early diagnosis of glaucoma and to give practitioners a perspective on ocular hypertensives and glaucoma suspects. In this area, in its new design, it has proven itself a very accurate tool and an adjunct to visual fields and the physical exam. Recently, experts have addressed the use of the technology to monitor long-term change in nerve fiber layer thickness and the predictive value of that information in developing treatment plans.

GDx VCC developed a change probability program that's similar to those in the HRT II and OCT III in its ability to identify subtle changes in the NFL over time. Again, experts haven't yet established the accuracy of birefringence in the post-refractive surgery patient. Considering the wave of these patients that will enter the pool of glaucoma diagnoses over the next two decades, it's critical that these issues be addressed.

Made up your mind yet?

So is one of these systems right for you? While the use of either doesn't constitute a standard of care at this time, it's clear that they are a significant addition to your glaucoma armamentarium; both technologies are effective and proven. Many practitioners base their choice on issues such as ease, size of the footplate and verstatility of the system.

While the GDx VCC is the most portable, (allowing you to use it in a multi-practice setting), it's also the least flexible in that it only examines the nerve fiber layer. The OCT III and the HRT II provide not only glaucoma analysis but retinal examination, which some find a complement to their glaucoma practice. Further, the expanded use provides a larger billing base to support the cost of the instrument.

Does anyone still use visual fields?

Does anyone still use visual fields? Yes! Although the optic nerve analyzers and nerve fiber layer systems have been getting most of the press, quantified perimetry is still the backbone of glaucoma practice. So what's new and how does it work?

Three systems are worth considering for the practice intertesed in expanding its glaucoma technology. The first of these is short wavelength automated perimetry (SWAP), also called chromatic perimetry. Because the original work by Sample and Johnson demonstrated an increased sensitivity, the scientific base of support for this type of perimetry has grown steadily. It uses the principle that blue/yellow analysis is sensitive to glaucomatous loss at a much earlier point in the damage of the ganglion cells than is standard perimetry.

While the technique is limited by nuclear sclerosis, age-related performance, time of testing and greater sensitivity in patients with existent loss, it's a powerful tool at a reasonable price for identifying pre-white-on-white visual field loss. For the individual who doesn't have the financial resources to look at the optic nerve head systems, this is an ideal upgrade that will provide excellent information in evaluating the ocular hypertensive and glaucoma suspect. While the current system doesn't have an accelerated analysis system such as "SITA," manufacturers are developing this. It will bring the test time to reasonable levels that will add even greater accuracy to the results.

Frequency doubling technology (FDT) is another of the developments that have shown tremendous applications in general practice. The system uses a combination of low spatial and high temporal frequency to create a stimulus that's specifically sensitive to the stimulation of the magnocellular pathways. Researchers have demonstrated these to be the earliest of the ganglion cells damaged in glaucomatous loss. The attractive aspect of this technology is both its cost and portability (it weighs less than 15 lbs.). Initially, clinicians questioned its accuracy relative to traditional quantified white-on-white systems, but the literature has given it excellent reviews over the last several years. A recent version of this system is the "Matrix," which uses FDT technology and produces a threshold outcome that's more typical of traditional perimetry.

Another new technology in this area is that of the Humphrey Glaucoma Progression Analysis program. This algorithm highlights changes from the baseline that are larger than typical clinical variability. It also accounts for media effects more efficiently and measures the likelihood and rate of progression, a critical aspect in long-term care of the glaucoma patient.

The weakness of visual fields has always been the ability to document progression. This system is the first of its kind and is a significant step forward. In a busy glaucoma practice, this system is a significant addition in that it assists the clinician in the decision process as regards the need to adjust therapy over the course of the disease.

Make an educated choice

The world of high technology in the field of glaucoma can prove difficult to navigate. But do your homework and base your choice on your practice's and your patients' best interests, and it can also be rewarding and provide you with real long-term growth and enhancement of care.

Dr. Thimons is a nationally and internationally acclaimed speaker and serves as medical director at Ophthalmic Consultants. He was awarded Optometry's Top Educator in 1999.