New methods of diagnosing glaucoma still need validation

Key Points

New Orleans-Advances in structural and functional tests are promising earlier diagnosis of glaucoma. Well-designed studies demonstrating the usefulness of incorporating these new testing methods in clinical practice still are lacking for most of these instruments, however, said Felipe A. Medeiros, MD, PhD, at the American Academy of Ophthalmology annual meeting.

Soon, more studies will compare these new technologies, said Dr. Medeiros, associate professor of ophthalmology, University of California, San Diego (UCSD). "We need longitudinal studies that show us how best to incorporate these technologies into our clinical practice. These technologies evolve very fast, and we need time to evaluate them."

Dr. Medeiros described developments in imaging technologies for structural evaluation in glaucoma.

"However, the relatively slow acquisition time with this technology made possible acquisition of only a limited number of scans. For example, for assessment of the nerve fiber layer, we usually acquired only about 500 points around the optic nerve. We also had to rely on interpolation for structural assessment of the macula and the optic nerve head."

The recently developed technology of spectral-domain OCT (SD-OCT) addresses those shortcomings, Dr. Medeiros said. In SD-OCT, a spectrometer in the detection arm of the interferometer measures the spectrum and converts it into depth information by Fourier transformation.

"Now we can get a more complete profile of the nerve fiber layer and much more complete structural information," he said. Dr. Medeiros predicted strong competition among the several commercially available instruments, "which I believe will be good for us clinicians."

Scanning laser polarimetry is another imaging technology that has evolved over the years, he said. One of the latest advances is a new software-based method of corneal compensation known as enhanced corneal compensation (GDxECC, Carl Zeiss Meditec). The software modifications aim to eliminate or reduce the so-called atypical retardation patterns observed in some patients with variable corneal compensation technology (GDxVCC, Carl Zeiss Meditec).

In a recent study, he and Robert N. Weinreb, MD, Distinguished Professor of Ophthalmology, Hamilton Glaucoma Center, UCSD, and colleagues showed that the newer corneal compensation technology performed significantly better than the previous corneal compensation technology for diagnosing glaucoma in patients with more severe atypical patterns of retardation and at earlier stages of the disease.

Improvements also have occurred in confocal scanning laser ophthalmoscopy (SLO), which has been limited by the need for an examiner to approximate the optic disk margin with a contour line to calculate the stereometric parameters and the Moorfields regression analysis. This requirement added an unwanted element of subjectivity to the assessment, Dr. Medeiros said. Accurate tracing of the contour line necessitated simultaneous visualization of the optic nerve photographs, which decreased the value of this technology as a replacement method for stereophotographs in assessment and documentation of the optic disk, he added.

A new version of confocal SLO (Heidelberg Retina Tomograph 3, Heidelberg Engineering) can identify the probability of structural abnormality (Glaucoma Probability Score, Heidelberg Engineering) using an automated analysis based on the three-dimensional shape of the optic disk and the retinal nerve fiber layer (see also Page 22). Dr. Medeiros and colleagues have investigated this parameter, following for about 6 years a group of 250 patients suspected to have glaucoma.

"The [analysis] was able to predict which patients would develop visual field loss over time," he said. "Patients who had more abnormal results on the [analysis] had a higher chance of developing visual field loss over time, and, more important than that, the ability of the [analysis] to predict which of these patients developed visual field loss was similar to that of subjective assessment of stereophotographs by glaucoma experts, even after adjusting for all the known risk factors."

Functional technologies also have been updated. The addition of software (SITA SWAP, Carl Zeiss Meditec) to proprietary perimetry technology (Humphrey Field Analyzer, Carl Zeiss Meditec) has reduced testing time and variability, he said, addressing limitations of short-wavelength automated perimetry (SWAP) alone; SWAP's superiority over standard automated perimetry (SAP) already was established before the addition of the software, Dr. Medeiros said.

Complete testing with the improved perimeter can be obtained in less than 5 minutes per eye, compared with approximately 15 minutes per eye with full-threshold perimeter testing, he continued.