Instrument adds assessment power

January 1, 2011

An age-adjusted retinal nerve fiber layer thickness normative database for a proprietary ocular coherence tomography device gives clinicians a tool for assessing glaucoma risk from a patient's first visit, according to one expert.

FDA clearance of the normative database was announced at Vision Expo West in Las Vegas.

The normative database, combined with the OCT instrument's new posterior pole asymmetry analysis and existing fovea-to-disc (FoDi) alignment software and active eye-tracking capability (via proprietary TruTrack technology), is designed to add to the power and usefulness of the instrument for glaucoma risk assessment and progression management.

The normative database and asymmetry analysis are part of the instrument's Version 5.3 software. The proprietary OCT's platform features fovea-to-disc alignment technology (FoDi) that is designed to improve the accuracy and reproducibility of RNFL measurements by automatically tracking and anatomically aligning circle scans. The alignment technology helps overcome measurement errors due to patients' changing their head or eye position during scanning.

The alignment technology improves the data integrity of the normative database, according to Heidelberg Engineering. Using the instrument's proprietary eye-tracking technology, all scans in the database are aligned along the fovea-to-disc axis, ensuring point-to-point thickness comparisons.

"It's quite a robust normative database," Dr. Asrani said. "In my experience, if there is an abnormality in a patient's eye indicative of possible early glaucomatous changes, the database frequently identifies it."

Along with the normative database, the device's Version 5.3 software release also includes a posterior pole asymmetry analysis capability.

Posterior pole asymmetry analysis

The posterior pole asymmetry analysis maps retinal thickness across the posterior pole and graphs asymmetry both between hemispheres and between eyes. RNFL measurements combined with retinal thickness measurements may help paint a clearer picture of glaucoma.

"Posterior pole asymmetry analysis can help identify early structural damage. Asymmetry is a hallmark of glaucoma, [because] glaucomatous damage is often marked by retinal thinning in the zone surrounding the fovea and extending toward the optic nerve head," Dr. Asrani said. "In glaucoma, the ganglion cells are dying in the macula, and this is a concept that is lost in many people. People start wondering why are we measuring the macula in glaucoma. That's because that's where the action is taking place. It's just that we haven't been able to measure it accurately and in a wide area up until now."

Dr. Asrani said the proprietary OCT unit measures the macular thickness in an 8 × 8 mm area centered on the fovea, with a high density of measurements that are displayed on a macular thickness map in a compressed color scheme.

"The traditional macular thickness maps have three colors: blue, green, and yellow," Dr. Asrani said. "That means you must have a change of 100 µm to be able to see a change in color. It's very difficult to be able to pick out a small amount of changes. The [proprietary OCT device] has a compressed color scale, with many additional colors incorporated into the 0 to 300-µm range to enable us to discern very small changes in thickness."

According to Dr. Asrani, many ophthalmologists already use the proprietary OCT unit for RNFL measurement, but the instrument's Version 5.3 software will allow them to identify and track changes over time, which he called the "holy grail" in glaucoma.

"In the early stages of glaucoma, it's very likely that the changes identifiable with the posterior pole asymmetry analysis and normative database would be not yet be seen on the visual field," Dr. Asrani said. "If we can structurally track changes over time, our dependence on visual fields is reduced.

"However, we must have reliable measurements to be sure that what we are measuring is indeed change, and [not] an error caused by the instrument or user," Dr. Asrani said. "There were already two features in the [OCT device] that helped us with that: its eye-tracking feature, and its alignment software. Now, with the normative database for RNFL measurements and the macular thickness posterior pole map, we are adding two more pieces to help us solve the jigsaw puzzle of glaucoma."

fyiSanjay Asrani, MD
Phone: 919/684-8656
E-mail: asran002@mc.duke.edu

Dr. Asrani receives lecture honoraria from Heidelberg Engineering.