THERAPEUTIC INSIGHTS
Glaucoma Tool Review
These instruments can help increase your success diagnosing and managing glaucoma.
By Murray Fingeret, O.D., F.A.A.O., Hewlett, N.Y.

In April's Therapeutic Insights, J. James Thimons, O.D., F.A.A.O., reviewed some of the recently available glaucoma drugs and explained when and how to use them. This month, we're revisiting the topic of glaucoma, but we're straying from the typical therapeutic angle of this column by instead talking about some of the diagnostic instruments available to help us detect glaucoma earlier and manage it better.

In the next few pages, we'll take a look at the new developments in tonometry, assessment of blood flow to the optic nerve, nerve fiber layer and optic nerve evaluation and perimetry.

Tonometry: assessing IOP

Intraocular pressure (IOP) assessment is one of the hallmarks for diagnosing glaucoma. Although elevated IOP no longer defines the condition, it remains an important parameter that helps us understand which particular form of the disease we're dealing with. We rely on a single measurement at a solitary point in time when we measure IOP in the office.

The in-home tonometer. Aside from having a patient stay in the office around-the-clock, there hasn't been a better method to accurately track diurnal IOPs. Until now.

Bausch & Lomb's Proview Eye Pressure Monitor is a new tonometer designed for in-home use. As it stands at present, the full extent of in-home tonometry to manage glaucoma hasn't yet been determined.

However, the Proview is expected to prove helpful in obtaining diurnal curves in those individuals who have normal tension glaucoma or unstable forms of glaucoma that are suspected of progressing despite apparent IOP control.

How it works. The Proview Eye Pressure Monitor measures IOP using the concept of pressure phosphenes. When it directs subtle pressure through the eyelid onto the eye, a pressure phosphene occurs in the field of vision at a certain point.

The Proview Eye Pressure Monitor correlates the force required to generate the image with an IOP measurement in mm Hg. It correlates with Goldmann IOP measurements, but at this point, no independent studies have been published to validate the manufacturer's data.

The Proview is relatively simple to use, and most patients can visualize the phosphene and take IOP readings at home. Patients will use the instrument during the day at home for a defined period and then bring their results, in a log book, back to you for an evaluation. If a patient notices an IOP spike with in-home monitoring, you can re-adjust his dosage schedule.

Economic factors. A Center for Medicare and Medicaid Services (CMS), formerly HCFA, code that reimburses doctors for the professional component of interpreting diurnal curves is expected in the near future.

How often and how long a patient uses the Proview Eye Pressure Monitor will depend on each patient's characteristics. The cost of the instrument to the patient is expected to be around $85, and it's available through pharmacies and doctors' offices without a prescription.

Tonometry: evaluating ocular blood flow

We use tonometry not only to assess IOP, but also to evaluate ocular blood flow. A reduction in ocular blood flow (OBF) is one of the presumed mechanisms for development of open-angle glaucoma. Several research instruments that measure blood flow within the different portions of the ocular vascular network are available, but they're expensive, require technical expertise and aren't typically used in clinical practice.

The operator-friendly OBF tonometer. The Ocular Blood Flow Tonometer by Paradigm-Dicon Instruments, Inc. was recently reintroduced in the United States. It continually measures the IOP during a 10-second period, capturing both the diastolic and systolic measurements. It analyzes the IOP pulse and compares the differences between the minimum and maximum IOPs (diastole and systole). It also measures the pulse amplitude, pulse volume and pulse rate, leading to a calculation of the pulsatile OBF.

A normative database is available for OBF measurements, which vary by gender. In males, the 5% cutoff is 12.0 microliters per second and 13.6 microliters in females. Certain forms of glaucoma may present with reduced OBF. With therapy, if blood flow is affected, the OBF measures should rise. Some systemic conditions are also associated with reduced OBF if circulation is compromised.

Few data have been published as to the instrument's sensitivity and specificity in diagnosing glaucoma. While OBF measurements may turn out to be a valuable parameter in assessing glaucoma, we need additional data before we can clearly define the instrument's role.

Optic nerve imaging

Our dependency on a subjective evaluation of the optic nerve isn't much different from what our predecessors used 50 years ago. While a new generation of fundus lenses has improved our ability to inspect the optic nerve and surrounding retina, it's still a qualitative assessment that varies from examiner to examiner.

Still, the diagnosis of glaucomatous optic neuropathy is capably performed in the hands of a trained observer. An assess-ment of the size and shape of the optic nerve, height of the peripapillary tissue and thickness of the nerve fiber tissue or slope of the walls of the cup will provide additional useful information, complementing the ophthalmoscopic views.

Optic nerve imaging has evolved over the past decade, with four commercially available instruments that analyze the optic nerve, its surrounding peripapillary tissue or nerve fiber layer. We'll review these instruments (the HRT, GDx, OCT and RTA) in the next few sections. The purpose of these instruments is to provide objective, reproducible measurements for the posterior pole and optic nerve structure. They should aid in diagnosis or management, as well as complement perimetric (functional) information.

Optic nerve imaging should also prove helpful in detecting change or progression in the optic nerve. Most practitioners don't perform stereo-optic nerve photography, which is considered the gold standard for optic nerve documentation. Instead, we often rely on monocular photographs, drawings, text descriptions or perimetry to help us detect change.

Because of limitations in photographs and drawings or the variability inherent within the visual system, we need to see significant change before we can detect additional loss. Optic nerve imaging will hopefully allow better detection of change by providing measurements to portions of the optic nerve.

Scanning laser ophthalmoscopy

As you know, we use ophthalmoscopes to examine the interior of the eye. Scanning laser ophthalmoscopes use a scanning camera, rather than a human observer, to view the eye.

The newly released ophthalmoscope. I perform confocal scanning laser ophthal-moscopy with the Heidelberg Retinal Tomograph (HRT) by Heidelberg Engineering. The HRT evolved into the HRT II, which automated the instrument's adjustments to make it easier to obtain images.

How it works. The HRT II focuses a low-power laser beam onto a specific plane of the retina that's 15š x 15š, centered on the optic nerve. Using a beam splitter and photodetector, only light at the plane is registered. More than 1 to 1.5 seconds, the focal plane is adjusted so that a series of optical sections (32 to 64) are taken one over the other. The instrument assembles the sections to form a 3-D image.

Data are computed from the 3-D image to provide optic disc, cup and rim area; cup and rim volume; mean and maximum cup depth; slope of the walls of the cup; linear cup/disc ratio; and mean and cross-sectional area of the retinal nerve fiber layer thickness.

What it tells you. These measurements allow a better understanding of the topography of the optic nerve and surrounding retina, such as the cup's deepest point or its mean depth.

In most normal optic nerves, the deepest spot near the center with sloping margins in the cup's walls.

When the mean cup depth is similar to the maximum cup depth, significant cupping has occurred affecting a greater amount of the optic nerve. Also, the slope of the walls is an important parameter. When the cup's walls become more vertically oriented, damage is usually occurring.

HRT II offerings. The following are some of the features the HRT II has to offer.

• The HRT II offers a movie feature in which a short video plays back, showing the scans of the optic nerve beginning from in front of the optic disc to deep within the optic cup. Cupping becomes more apparent as the deeper images come into view, highlighting subtle changes occurring in the optic nerve.
• The instrument provides a normative data-base within Moorfields regression analysis that compares the rim-to-cup area, taking into account disc size.
  This database cur-rently contains only information from Caucasians, but additional normative information that better reflects the population at large will be available soon.
• A glaucoma change probability program, developed by Balwantray Chauhan, is another option offered with the HRT that requires several images taken over time to conduct an analysis. You need a minimum of three images, preferably four, to look for change.
  You then take a measurement at any area, comparing changes in the depth against the standard deviation for the image. The HRT II calculates and visually displays a change probability to alert you of a suspected significant loss.

In addition to confocal laser scanning ophthalmoscopy, there's also confocal laser scanning polarimetry, which we'll discuss now.

Polarimetry

Confocal laser scanning polarimetry is based on the principle of birefringence, which is caused by the microtubules of the ganglion cell axons. The property of birefringence explains change that occurs as a polarized light passes through a structure, such as the nerve fiber layer (NFL), for a second time.

The confocal laser scanning polarimeter projects a polarized beam of light into the eye to measure the nerve fiber layer's birefringence properties. As this light passes through the nerve fiber layer tissue, it changes and slows. Detectors measure the change and convert them to thickness units. Graphics displaying the findings show the thickest and thinnest areas.

The confocal laser scanning polarimeter GDx by Laser Diagnostic Technologies measures modulation around an ellipse just outside the optic disc, ratios of the thickest points either superiorly or inferiorly to the temporal or nasal regions, as well as average thickness in certain regions. For most individuals, the superior and inferior areas are thickest and the temporal and nasal areas are thinner.

The GDx also provides a neural network analysis using data for many of these measurements to alert you of a problem. The rule of thumb is 0 to 30 normal, 31 to 60 borderline and greater than 60 glaucomatous. A normative database compares measurements against these values and alerts you when values are borderline or outside normal limits.

One concern has been the neutralization of the corneal birefringence component. Software and hardware development are ongoing to deal with this particular problem as elucidated by Greenfield, D.S.; Knighton, R.W.; and Huang, X.R. They showed that the anterior segment compensator for the GDx assumes individuals to have a corneal polarization axis (CPA) of 15š nasally downward.

Their findings showed that corneal polarization varies a-mong individuals just as corneal astigmatism does. Because it's difficult to have one number compensate for all individuals, future versions of the instrument will have some ability to adjust for the patient's CPA.

Retinal imaging

Two other instruments image the retina and are used in glaucoma diagnosis, though both currently have additional uses with retinal conditions.

The OCT. The Optical Coherence Tomography (OCT) by Humphrey Instruments is similar to ultrasonography, but uses light instead of sound to obtain its values. The OCT, using a low coherence interferometer, determines the echo delay time of light backscattered from different layers of the retina. Differentiation of retinal tissue occurs with the creation of a two-dimensional, cross-sectional image of ocular tissue.

A cylindrical scan outside the optic nerve provides nerve fiber values in 12 sectors as well as quadrant measures. The color-coded image illustrates segmental loss in the nerve fiber layer.

The RTA. The Retinal Thickness Analyzer (RTA) by Talia Technology, Ltd. uses a narrow slit of a helium neon laser beam. It places a vertical slit on the edge of the pupil, which is reflected back by the nerve fiber layer and retinal pigment epithelium.

A digital camera placed on the other side of the pupil captures the reflected light in a 3-mm x 3-mm cross section. Sixteen optical sections are imaged, 190 microns apart. The RTA uses optical sections to construct a topographic map that quantifies retinal thickness for the tissues, such as the nerve fiber layer, within the posterior pole.

Forging ahead

We've had several significant developments in the ability to detect or monitor change within the optic nerve and retina, analyze blood flow and measure IOPs. This information will allow us to continue finely tuning our patients' therapeutic regimens to their needs. That's why it's important to keep abreast of all the new advances in technology.

Dr. Fingeret is chief of the Optometry Section at Brooklyn/St. Albans Campus, Department of Veterans Administration New York Harbor Health Care System. He's a member of the board of directors of the Glaucoma Foundation and is chair of the glaucoma diplomate committee for the American Academy of Optometry.

Dr. Christensen has a partnership practice in Midwest City, Okla. He's a diplomate in the Cornea and Contact Lens Section of the American Academy of Optometry. He's also a member of the National Academies of Practice.