Optic Nerve Analysis With Cirrus HD-OCT

Automatic disc evaluation and progression detection are breakthroughs in glaucoma care.

"The introduction into clinical practice of SD-OCT is revolutionizing the diagnosis and management of glaucoma. The result is improved patient outcomes, specifically reduced functional impairment and blindness." — Robert N. Weinreb, MD

Compared with its time domain predecessor, spectral domain OCT can provide earlier and more precise detection of glaucoma. "The introduction into clinical practice of SD-OCT is revolutionizing the diagnosis and management of glaucoma," says Robert N. Weinreb, MD, Distinguished Professor of Ophthalmology and Director of the Hamilton Glaucoma Center at the University of California, San Diego. "The result is improved patient outcomes, specifically reduced functional impairment and blindness."

This article summarizes the capabilities of the newest version of software for the Cirrus HD-OCT. Version 5.0 gives physicians powerful new tools including optic disc analysis, quantitative assessment of progression and reports that provide structural information for diagnosing and managing glaucoma patients.

Optic Disc Analysis

Glaucoma specialists have been using new technology to assess the retinal nerve fiber layer (RNFL). However, what is unique about the new Cirrus software is that it applies the same highresolution data cube used for RNFL analysis to the optic disc. Therefore, it is possible to assess not only the RNFL but also the optic disc, which has been the standard for glaucoma diagnosis and treatment for many years. In addition, optic disc analysis can be performed retrospectively on images obtained before the new software was created.

The new software allows, for the first time, automatic evaluation of several disc parameters:

• rim area
• disc area
• average cup/disc ratio
• vertical cup/disc ratio
• cup volume.

Disc area allows the clinician to ascertain the significance of the rim area. With the new Cirrus software, the significance limits for the optic disc parameters are used to provide the standard colorcoded scheme for comparison to normals. They are derived from the same normals used in the system's RNFL Normative Database. This comparison to normative data should be available for the Cirrus later this year.

One of the challenges in measuring the optic disc is determining its margins. The Cirrus HD-OCT 5.0 software accomplishes this by defining the edge of the disc as the termination of Bruch's membrane.¹ This is done at multiple points around the nerve head. The resulting optic disc outline, which is used for computing the disc parameters, is displayed for evaluation. TSNIT analyses of the RNFL are taken from a circle that is centered on the optic disc. Neuroretinal rim width around the circumference of the optic disc is then determined in the plane of the disc.^2,3

According to Dr. Weinreb, measuring rim area in the same plane as the optic disc may be helpful for examination of tilted discs. "In cases where the nerve exit is oblique, the disc is viewed at an angle by the clinician, foreshortening the image," he says. "Therefore, areas visualized via ophthalmoscopy, photographs or other imaging modalities will be reduced. Because the measurements taken by Cirrus HD-OCT are in the same plane, it is likely that results more accurately represent the true anatomy."

Structure and Function Data in One Report

For decades, those with an interest in glaucoma diagnosis and assessment of progression have sought the ability to put together structural and functional information. Soon, using data from Cirrus and HFA, the Zeiss eyecare data management system, Forum, will create a Combined Report. This convenient summary of structure and function data simplifies patient management and also can be useful for patient education.

"Structural and functional information are not interdependent," Dr. Weinreb says. "If a change in the visual field is observed, there is not necessarily a change in the optic disc." Last year, Dr. Weinreb and colleagues published a paper showing how structure can predict function in many patients.⁴ "However, that is not always the case," he says. "In some patients, function can predict structure. Structural and functional testing are complementary."

SD-OCT Case Studies

The following cases of patients with glaucoma illustrate how Cirrus HD-OCT enhances the clinician's diagnostic and monitoring capabilities.

Case 1 - Figure 1

The optic disc photograph of this patient with glaucoma shows thinning of the inferior rim and an area of peripapillary atrophy. The visual field has a supranasal step. The SD-OCT analysis of RNFL thickness clearly reveals a wedge-shaped defect inferiorly. Following along the calculation circle centered on the optic disc, an infratemporal depression into the abnormal zone is visible. The depression corresponds to the RNFL defect.

Case 2 - Figure 2

The eye of this patient with glaucoma has almost no neuroretinal rim. Notable in this case is how clearly SD-OCT shows the presence of a broad, wedge-shaped defect that corresponds to a change in the typical neuro-double-hump pattern, with loss of RNFL tissue. It would be difficult, if not impossible, to visualize the RNFL with a handheld lens using any of the methods suggested previously, including red-free or green light or high-resolution, monochromatic photographs.

Case 3 - Figure 3

This glaucoma patient has inferior thinning of the rim. Note how SD-OCT reveals the broad-shaped defect in the images as well as in the description of the RNFL.

Figures 1-3. In these 3 cases, Cirrus HD-OCT optic disc and retinal nerve fiber layer analyses reveal defects that would be difficult to detect by other means.

Progression Analysis

Cirrus HD-OCT is also equipped with GPA software for detection of glaucoma progression. Clinicians can quantitatively assess images and look for disease progression in the RNFL and � in the future — in the optic disc. The following two cases show detection of disease progression that occurred in a relatively short period.

Case 4 - Figure 4

In this patient, baseline testing shows a possible small change in the neuroretinal rim superiorly and inferiorly. The visual field shows a small infranasal step. The patient was followed at 3- to 6-month intervals over less than 2 years. During that time, neither the visual field nor the results of clinical examination of the optic disc changed. However, upon analysis with the GPA software, an RNFL change became apparent superiorly, with RNFL thinning corresponding to RNFL loss in the supratemporal region.

Case 5 - Figure 5

In this patient, baseline visual field results were not changing, but Cirrus GPA software revealed repeatable RNFL changes in the infratemporal region.

Figures 4 and 5. Using new Cirrus HD-OCT software for detection of glaucoma progression, clinicians can quantitatively assess images and progression in the RNFL and optic disc. In these cases, changes in the RNFL and optic disc are visible before changes in other parameters.

Major Impact in Glaucoma Care

Dr. Weinreb predicts that the latest improvements in how SD-OCT technology can be used to diagnose and follow glaucoma will have a major impact in ophthalmology. "They will allow the individualization of therapies to a greater extent than is possible today," he says, "which will further reduce the burden of glaucoma-related functional impairment and blindness."

REFERENCES

1. Strouthidis NG, Yang H, Fortune B, Downs JC, Burgoyne CF. Detection of optic nerve head neural canal opening within histomorphometric and spectral domain optical coherence tomography data sets. Invest Ophthalmol Vis Sci 2009;50:214-223.
2. Strouthidis NG, White ET, Owen VM, Ho TA, Garway-Heath DF. Improving the repeatability of Heidelberg retina tomograph and Heidelberg retina tomograph II rim area measurements. Br J Ophthalmol 2005;89:1433-1437.
3. Tan JC, White E, Poinoosawmy D, Hitchings RA. Validity of rim area measurements by different reference planes. J Glaucoma 2004;13:245-250.
4. Medeiros FA, Alencar LM, Zangwill LM, Bowd C, Sample PA, Weinreb RN. Prediction of functional loss in glaucoma from progressive optic disc damage. Arch Ophthalmol 2009;127:1250-1256.