As you know, early detection of glaucoma is the best defense against permanent loss of vision. Fortunately, it's now possible to detect evidence that glaucoma may be imminent -- abnormal changes in the optic nerve head and nerve fiber layer (NFL) -- sooner than ever before.

One of the most promising of these technologies is retinal tomography, pioneered by Heidelberg Engineering. This technology uses a confocal scanning laser ophthalmoscope to create an accurate, detailed 3-D "picture" of the optic nerve head and surrounding tissues. This data can then be compared to a normative database or to previous images of the same optic nerve head.

Inside the HRT II

The Heidelberg Retina Tomo-graph II, or HRT II, has three characteristics that make it an especially powerful tool for detection of early glaucomatous changes:

�        First, it provides information about the two areas most likely to reveal glaucoma in its early stages: the optic nerve head (by direct measurement) and the nerve fiber layer (by indirect calculation). The HRT II creates a detailed, 3-D topographic map of the optic nerve head, including information on the cup and rim. (Because nerve fiber bundles are concentrated at the optic nerve head, this is a logical place to look for early signs of developing glaucoma.) The HRT II also calculates the NFL thickness, making it possible to monitor changes there as well. (Studies have shown that the thickness calculated by the HRT II correlates well with the actual number of optic nerve fibers.)

�        Second, it can compare the topographic data to a normative database and graphically display any data that deviate significantly from that database. (Color coding shows the extent of deviation.)

�        Third, it can compare data recorded from the same patient during consecutive visits. It graphically displays all changes, color-coded to show how extensive the change at each point has been.

Measuring the optic nerve

To analyze the topography of the optic nerve head, the instrument uses CAT-scan-like technology to record three sequences of cross-sectional images during a 4-second period; it then assembles these into a 3-D topographic map. (The final 3-D map is based upon more than 148,000 bits of measured information.)

Data obtained by the HRT II also has been shown to correlate very well with the results of visual field testing. (The HRT II, of course, may detect a change long before any defect in the visual field is apparent.) According to Heidelberg, more than 200 peer-reviewed papers have attested to the accuracy and reproducibility of the data collected using this technology.

It's worth noting that this technology is entirely different from scanning laser polarimetry (SLP). SLP-based instruments measure changes in the polarization of light reflected off the NFL and use that information to calculate the thickness of the NFL. However, the cornea can also alter the polarization of the reflected light, potentially altering the data. The HRT II's measurements are not affected by this factor. (Also, scanning laser polarimetry only provides information about the NFL; it can't be used to measure the optic nerve head itself.)

Comparing to a normative database

The HRT II analyzes and classifies a first-time measurement using the regression analysis technique developed by Moorfields Eye Hospital in London. This technique estimates the likelihood of a problem by analyzing the relationship between neuroretinal rim area and optic disc area.

After taking the measurement, the HRT II divides the image of the optic nerve and surrounding region into six sectors. Each sector is evaluated and graded separately. Differently colored on-screen marks indicate whether each sector falls within normal, borderline or abnormal parameters.

Measuring changes over time

The HRT II can compare data captured at consecutive exams and display color-coded maps that clearly identify any significant changes. (See picture, above.) The colors indicate how extensive the changes have been. You can montior changes over the course of either two or three follow-up exams, and you can set a new baseline for comparison at any time.

According to the company, these maps make it possible to detect glaucomatous progression as much as 3 to 4 years earlier than would be possible by monitoring visual field changes.

Comments from the field

Mark Latina, M.D., of Reading Mass. -- known for his invention of Selective Laser Trabeculotherapy -- uses this technology in his practice daily.

"I've found the data to be very reproducible," he says. "Also, I can check what the machine is telling me against what I see through the ophthalmoscope, giving me confirmational feedback that the machine is providing accurate data. This isn't possible with scanning laser polarimetry, which only provides information about the nerve fiber layer.

"I think nerve layer thickness and changes in optic nerve topography are both important to monitoring glaucoma, and this technology provides information about both . . . I believe the HRT II is a worthwhile investment for any doctor who treats a substantial amount of glaucoma."

Other features and benefits

Other practical advantages of the current design include:

�        The measurement is quick (only 4 seconds), non-contact and usually requires no dilation or bright light in the patient's eye.

�        The HRT II requires minimal training and input by the operator; it features single-button operation. Image acquisition, axial scan depth and sensitivity adjustment are automatic. The instrument is lightweight and portable.

�        The color-coded maps make changes or differences obvious. They can also be used to help patients understand the problem, increasing compliance.

�        The HRT II automatically rejects poorly focused images or images blurred by patient movement. It also provides a standard deviation figure for each image, indicating how accurate the measurement was.

�        Information can easily be transmitted to other locations. The system also has a notebook computer option.

Investing in the HRT II

The HRT II system includes the tomograph, a Pentium III computer system with a 17" monitor and optical disc drive, software, a color printer, and a specially designed table for the system. It's currently priced at about $33,000, and clinical use is reimbursable. Special financing options are also available.

For more information, contact Heidelberg Engineering by calling (800) 931-2230 or by visiting their website at www.heidelbergengineering.com.

Are you aware of new products or technology that have made (or are likely to make) a
significant difference in practice? Contact Christopher Kent at kentcx@boucher1.com to find out about possible coverage in a future issue.