Article Date: 5/1/2002

High-Tech Glaucoma Management
Here's a round-up of glaucoma diagnostic technologies for you to choose from.
BY JERRY SHERMAN, O.D., F.A.A.O.

We consider glaucoma to be the most common cause of irreversible blindness. Because this blindness is preventable when we diagnose and treat glaucoma promptly, technologies that promise earlier diagnosis have been developed. They can be grouped into four categories:

With so many high-tech glaucoma devices available, it's obvious why clinicians in the market for a new instrument are confused. Here, I'll discuss glaucoma diagnostic technologies and try to clear up some confusion.

A UBM of a patient with a localized narrow angle. The image clearly shows a ciliary body cyst displacing the iris anteriorly towards the filtration angle. A similar narrow angle could have been caused by a solid mass, such as a ciliary body malignant melanoma.

Roll call

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. Instead of obtaining three individual scans of the optic nerve head surface and then obtaining an average, this equipment requires only one scan per eye, which takes about 4.5 seconds.

Since the cup-to-disc ratio in a normal population varies from 0.0 to 0.9, a single scan (or a single fundus photo) at one point in time is rarely sufficient to establish a diagnosis except in the most obvious cases. Rather, a change over time is the hallmark finding to confirm a diagnosis of glaucoma. Since the HRT II quantifies numerous disc parameters, a change over time is easier to document than with disc photos or drawings.

In addition to assessing the disc, the HRT II yields information about the retinal nerve fiber layer (NFL). But it can't do true thickness measurements because it assesses only the surface of the NFL and assigns a baseline value so that it can compare the topographical findings. However, dilation is generally not required and the exam can typically be accomplished in about 5 minutes.

GDx Access nerve fiber analyzer. The GDx Access nerve fiber analyzer is available through Laser Diagnostic Technologies. It's the portable and user-friendly version of the original GDx. The entire system is one unit weighing about 40 pounds. It fits into a padded carrier with wheels, so the same system can be used at multiple sites. Rather than measure disc parameters, the GDx completely ignores the disc and measures the thickness of the NFL 1.75 disc diameters away from the center of the disc.

The GDx is a scanning laser polarimeter that measures NFL thickness indirectly by assessing its birefringence. Because the thickness of the NFL varies considerably less than the cup-to-disc ratio in a normal population, and an age- and race-related normative database has been established, a diagnosis of normal or glaucoma is often possible on the first visit. Like the HRT, the GDx can yield a serial analysis capable of detecting change over time.

The GDx has a patented, fixed corneal compensator to neutralize the unwanted birefringence of the cornea. But not all corneas have the same axis and magnitude of birefringence. Hence, some patients (5% to 10%) have significant persistent birefringence that complicates the analysis.

The new GDx Access has a variable corneal compensator. It scans each eye without dilation in 0.7 sec, and the exam for both eyes takes about 5 minutes.

The OCT (Ocular Coherence Tomographer) III. This instrument will be available through Humphrey Instruments in May and is the newest and most simplified version of the OCT. Although the OCT III has recently received FDA approval, the normative database won't be available for about 1 year.

OCT images are all cross-sectional and resemble those in an ocular anatomy textbook. The resolution is far greater than ophthalmic ultrasound's, but the OCT is similar to ultrasound in that it obtains cross-sectional cuts. Rather than using sound, the OCT uses light to obtain the slices. The OCT is more versatile than either the HRT or GDx because it can take slices of the macula, disc and area around the disc. This system can assess the NFL, but comparisons with the GDx NFL measurements haven't been performed yet.

Although the OCT III is more versatile, it costs just around $50,000.00 -- about $20,000.00 more than the HRT II and $10,000.00 more than the GDx Access. It's quite stylish in design, but the system and table have a large footprint and can't be considered portable.

The OCT III holds considerable promise for retinal and glaucoma specialists because of its ability to yield high-resolution cross sections of any area of the fundus, including the optic nerve head. But a normative and glaucoma database must be incorporated to allow you to make an immediate diagnosis. Serial scans should be able to detect change even without the normative database, but you need to assess resolution and reliability.

An OCT through the fovea and surrounding tissue. Note the various retinal layers, including the retinal nerve fiber layer, which you can measure separately for glaucoma detection.

The RTA (Retinal Thickness Analyzer). Talia Technology, Ltd.'s system measures the thickness of the entire retina but, unlike the OCT, it can't separate the various retinal layers. Think of the RTA as a slit lamp and fundus camera combined.

Glaucoma involves only two layers of the retina: the ganglion cell layer and the NFL. Hence, most of the retina isn't affected and early loss of the ganglion cells and NFL may not be detectable because the thickness of the other layers is normal.

The RTA easily detects thick retinas, as occur in clinically significant diabetic macular edema. It appears to be better at detecting thickness than most clinicians. It generally requires a dilation to about 6 mm and performs poorly in the presence of intraocular lenses, especially multifocal intraocular lenses such as the Array lens.

Unlike the OCT, the RTA can image large areas of the retina at once. The OCT, while allowing separation of the various retinal layers, yields single cross-sectional views, and you need numerous cross sections to analyze large areas of the retina.

Completely normal GDx of a normal 80-year-old white male without retinal or optic nerve disease.

The UBM. Available through Paradigm-Dicon, the Ultrasound BioMicroscope system resembles standard ophthalmic ultrasound in that it uses high-frequency sound waves to obtain cross-sectional images.

In ultrasound, there's a trade-off between resolution and penetration. With a 50 MHz transducer frequency, the UBM yields high resolution of the anterior segment but doesn't penetrate to the rear of the eye. In contrast, standard ophthalmic ultrasound uses a 10 MHz frequency and yields much poorer resolution but does penetrate to the retina, optic nerve and orbit.

The UBM is helpful in various disorders, including angle closure, narrow angle, plateau iris, ciliary body cysts and ciliary body tumors. No other ophthalmic system yields such high resolution of anterior segment structures.

Changes in anterior segment anatomy, such as those that follow laser iridotomy, are easily documented and particularly informative.

The HRTF (Heidelberg Retinal Flowmeter). For those interested in blood flow devices, this system is available alone or in combination with the original HRT. Few private practices have obtained this system, although some important studies with it have come from research facilities.

A normal HRTII from a normal patient. All parameters fall within the normative data base.

A less expensive approach to blood flow studies in glaucoma is to obtain the Paradigm-Dicon Pulsatile Ocular Blood Flow device (POBF). Several European studies have dem-onstrated the relationship between glaucomatous field loss and poor ocular blood flow as measured with the POBF. The POBF system applanates the cornea in the same way as the Goldmann tonometer probe but also documents the change in IOP with the cardiac cycle by measuring the IOP hundreds of times within seconds.

Points to ponder

What should you consider when buying any of these instruments?

First, consider what will help you most in your clinical care. Is it disc topography, NFL thickness or blood flow?

Second, evaluate ease of operation and price.

Finally, think about reimbursement and fees. The HRT II, GDx Access, OCT III and RTA all use the same 92135 code. Although each provides somewhat different information, you'd be hard pressed to use all of them because of the difficulty in getting paid for services rendered.

However, a wider range of diagnosis codes (not just glaucoma) will support instruments that can evaluate more areas of the retina.

One thing is certain: These new technologies offer valuable tools to help us accurately diagnose glaucoma.

Dr. Sherman practices at the Eye Institute and Laser Center in Manhattan and is a distinguished teaching professor at the State University of New York College of Optometry.

 


Optometric Management, Issue: May 2002