Optical coherence tomography provides new tools for glaucoma assessment

Approved recently by the FDA are the age-adjusted retinal nerve fiber layer (RNFL) thickness normative database and the PosteriorPole Asymmetry Analysis software (developed with Sanjay Asrani, MD). Both programs give glaucoma specialists and general ophthalmologists tools for objectively measuring changes in patients. Both features were launched at the annual meeting of the American Academy of Ophthalmology.

The RNFL thickness database was developed using the OCT system's trademarked fovea-to-disc (FoDi) alignment software. The software ensures that the circle scans in the database all are properly aligned at the same start-and-stop point even if a patient moves his or her head or eye during the scan.

"There can be significant rotational error in a normative database that is not FoDi-aligned," McCallum said. "True tissue loss can be hidden by the error that's inherent in the database, or loss might be falsely indicated because the alignment was different. FoDi offers greater precision when comparing circle scans."

Asymmetry analysis

The second tool, asymmetry analysis, was developed in conjunction with Dr. Asrani, associate professor of ophthalmology and head of the Glaucoma OCT Reading Center, Duke University Eye Center, Durham, NC. Dr. Asrani said he has worked about 15 years on better ways to measure the macula but was frustrated by instruments and tests that lacked precision.

"We have been measuring visual fields for ages, but we have not been able to get an exact anatomic correlate in the eye," he said. "Until now, we have focused the objective parameters by measuring the optic nerve, but we have never taken full advantage of the macular region, which is where the ganglion cells have died."

Although physicians have long suspected that asymmetry could indicate glaucoma, they had imprecise tools to detect such differences. However, the new software scans and maps the posterior pole with a precision that was not previously available. While some older devices limited physicians to six radial lines, this OCT platform can measure up to 100 lines, although Dr. Asrani said 60 or 61 lines in a 10 mm × 10 mm block are adequate. Six lines of data meant physicians were forced to assume data between those lines.

"That was not real measurement, just a sampling," Dr. Asrani said. "That is not good enough to give us a diagnosis, and it is not at all good enough for monitoring."

After conducting the scan, the device creates a rectangular block of data that is color-coded to indicate the macula's thickness and help physicians quickly determine differences.

The device then performs an asymmetry analysis, providing a black, white, and gray grid over the block of data and highlighting areas of asymmetry compared with the patient's other eye and with the other hemisphere of the same eye. Darker segments indicate greater dissimilarities.

"Even if you can only scan one eye, you can do a hemisphere analysis and check for symmetry," McCallum said. "Having this objective measurement of asymmetry is a helpful tool, and physicians are very excited about it."

The software is based on Dr. Asrani's finding that there is tremendous value in evaluating the symmetry between the two eyes of an individual and between the upper and lower half of the same eye.

"In many cases we were able to pick out the very subtle losses that were present in one eye when compared with the other eye," Dr. Asrani said.

"In patients who didn't have another eye, or [where the two eyes were very symmetrical], we found the upper half could be subtracted from the lower half of the retina," he added.

The use of such anatomic facts may be helpful for earlier detection of glaucoma, he said. These measurements should be used in conjunction with other evaluations, including nerve fiber layer measurements, IOP readings, and visual field assessment.

"In patients with very advanced glaucoma, both the nerve fiber layer and the visual fields are almost extinguished, so very little is left to follow patients with," Dr. Asrani said. "But if you have retinal thickness measurements, they are still present and change can be found there."

He has begun longitudinally following his patients with glaucoma that was first measured 1 year ago with the system. Patients will be re-examined annually to detect any changes, and determine how those changes correlate with changes in the visual field tests. A normative database of macula thicknesses across ages and races is being developed.

"It's a useful tool to be used in conjunction with already existing measures of glaucoma, and it may prove superior to the existing measures of glaucoma," Dr. Asrani concluded.