Catching the First Signs

Spectral domain OCT delivers precision RNFL analysis.

By Iqbal K. Ahmed, MD, FRCSC

A variety of imaging devices for glaucoma have debuted in the last 15 years. Optic disc topographical analysis, for example, provided by the Heidelberg Retinal Tomograph (HRT, Heidelberg Engineering Inc.), has longevity, low variability, good repeatability and mature progression software, although it has limited screening value. Optical coherence Tomography (OCT), GDx and possibly HRT offer retinal nerve fiber layer (RNFL) imaging, representing a more useful screening strategy. However, earlier generation OCT devices give us a number of false positives and some variability, plus early progression software remains immature.

Glaucoma specialists look at these options in the context of their most pressing goal: to preserve vision by detecting and treating the earliest signs of the disease.

There's some debate about whether these signs occur first in the RNFL or in the optic disc, particularly in preperimetric glaucoma. One key study¹ showed that the RNFL might be a better place to look for early glaucomatous structural changes. Researchers showed the GDxVCC had a 0.83 correlation with preperimetric glaucoma, with documented optic disc progression on clinical exam, compared to a 0.70 correlation for the HRT II.

Clearly, because RNFL changes likely may be the first signs of glaucoma, there's a clinical advantage to getting the best RNFL imagery possible. Today, you can obtain these images with spectral domain optical coherence tomography (SD-OCT).

Advantages of SD-OCT

What are the advantages of SD-OCT for glaucoma?

• Improved resolution and precision, compared to time domain OCT

• A large area of not only axial resolution, but also excellent transverse resolution, allowing you to obtain optic disc, RNFL and macular assessments

• Better repeatability than time domain OCT, which means lower variability

• Vastly improved serial overlay to scan repeatedly and ensure we're measuring the same points over time

• Improved ease of use and speed.

With Cirrus HD-OCT, an SD-OCT device, the automated registration and ease of use go hand in hand. In the classic scan, which is similar to macular scanning, the device gives us a 6-mm cube, 200 A-scans × 200 B-scans. But unlike other instruments, technicians don't have to place the circle exactly where they want it to be around the disc, creating another source of error.

Technicians use the exquisite fundus image of the line scanning ophthalmoscope to visualize the optic nerve head during scan acquisition. Then Cirrus HD-OCT automatically identifies the center of the disc and performs an RNFL analysis with a 3.4-mm diameter. Then, the software compares the TSNIT graph to a normative database of about 300 patients.

Cirrus HD-OCT also gives us a beautiful display and printout of the RNFL thickness analysis so we can make efficient use of the data (Figure 1). The printout displays peripapillary RNFL thickness and a comparison with the normative data in graphic format. Also, the RNFL thickness map for the full 6-mm × 6-mm area shows the patterns and thickness of the RNFL, and it aids in the detection of pattern defects while the RNFL deviation map is overlaid on the OCT fundus image to illustrate precisely where RNFL thickness deviates from a normal range.

Figure 1. This printout displays RNFL analysis and comparison to normative data for the full 6-mm × 6-mm cube of data, as well as the 3.46-mm diameter circle that's automatically centered on the disc.

Case Study: Closer Look at the RNFL

A 54-year-old African American man presented with IOPs of 23 mmHg and 22 mmHg and central corneal thicknesses of 540 microns and 545 microns. His right eye showed some early nasal changes in the visual field, and the left eye was essentially normal.

When you look at his optic discs, you can see some asymmetry between the right eye and left eye. The right eye showed some inferior thinning and possibly some superior thinning (Figure 2).

Figure 2. This patient has some asymmetry, as well as inferior thinning and superior thinning in the right eye. There also appears to be some superior loss and possible early inferotemporal RNFL bundle thinning in the left eye. Mapping and data charts spell out the patient's RNFL thickness deviation and its comparison to normative data.

The RNFL thickness analysis of Cirrus HD-OCT offered additional data:

• In glaucoma, we expect to see strong thickness superiorly and inferiorly, and this patient's map clearly shows RNFL loss in the right eye superiorly more than inferiorly, with perhaps some superior loss in the left eye.

• The global, quadrant and sector analyses are like looking at a mean deviation on a visual field. The average thickness is indicative of the overall health of the peripapillary RNFL, while the quadrant and clock hour analyses reveal more specific areas and symmetry data. In this case, we saw significant changes superiorly and inferiorly in the right eye. In the left eye, there were some changes superiorly. A subtle inferotemporal area that's beyond the circle around the optic nerve may have had some early changes.

• The TSNIT plot is similar to what we've seen with GDx and time domain OCT. We saw a depressed superior pole with some inferior thinning in the right eye, and the left eye showed some subtle superior loss with perhaps some inferotemporal changes.

• The deviation map showed these changes more clearly. Overall, the results for this patient's right eye were consistent, although the results for the left eye possibly were deceiving. The left eye had some superior changes, and perhaps some inferotemporal changes that were beyond the disc margin and the peripapillary RNFL.

Assessing Progression

The ability to assess progression has been lacking in devices that provide RNFL analysis. And this is why we're excited about Cirrus HD-OCT because it has an excellent ability to register serial images. Because progression can occur in several ways, this software, which is under development, analyzes:

• RNFL thickness map progression, which detects focal defects

• The RNFL thickness profile, which pinpoints more shallow or broad defects

• Average RNFL thickness progression, which identifies more diffuse changes.

The software also provides an overall assessment of possible RNFL loss in terms of these three calculations.

This software will be the next natural and essential step in HD-OCT technology. We've improved resolution and axial measurements. One scan can quickly capture the disc and the peripapillary RNFL and reduce the subjectivity of placing the circle around the disc. The singular report is enhanced, giving you not only an RNFL profile around the peripapillary region, but also around the entire optic nerve. And, most importantly, the registration reproducibility gives you the power to move forward in looking at RNFL progression analysis. The addition of progression analysis software will help pull all of these capabilities together. OM

Dr. Ahmed is assistant professor of ophthalmology at the University of Toronto, Ontario, and clinical assistant professor at the University of Utah in Salt Lake City.