Tracking Structural and Functional Glaucomatous Change Using Guided Progression Analysis for the Humphrey Perimeter and the Cirrus HD-OCT

By Vincent Michael Patella, Vice President, Professional Affairs at Carl Zeiss Meditec, Inc.

For years, the rate of a patient’s visual field change was determined using mean deviation — the average change relative to age-corrected normal sensitivity. Today, the Humphrey^® Visual Field Index (VFI™) is the number we prefer, because it’s less affected by cataract and changes in pupil size. Doctors are looking at the same visual field results they’ve always used, but they’re assessing rate of change in a different way.

VFI estimates the overall percentage of loss on each visual field. You can retrospectively go back and calculate VFI on any Humphrey 24-2 or 30-2 threshold visual field ever done since the introduction of the HFA™ in 1984. VFI is weighted to emphasize the central visual field more heavily than the peripheral field and it’s adjusted to at least partially correct for the effects of cataract. VFI was designed to be as reflective as possible of ganglion cell damage. We know there is about 8 or 9 times the density of ganglion cells in the center of the retina as in the periphery, and therefore VFI was designed to give considerably more weight to the central field. Central field functionality also is more important to the patient, so it makes sense from that point of view as well. The standard HFA Guided Progression Analysis™ (GPA™) automatically presents VFI information for all available visual field tests. A linear regression analysis is performed as soon as the patient has completed five tests.

With Cirrus, we’re dealing with a million times as much data as with visual fields. We capture a 3D image of the whole area around the optic nerve and store it all as a data cube. This is a detailed 3D image of the optic nerve and the surrounding retina. From that, we can determine the thickness of the RNFL, as well as a number of different tilt-compensated measurements of the optic nerve head. Our optic nerve analysis uses the termination of Bruch’s membrane to automatically define optic nerve size, exactly in line with current thinking. If there are multiple optic nerve data cubes collected over time, we use Cirrus™ GPA to align the cubes and then look for change that exceeds typical test-retest variability— an analysis that is conceptually very similar to that of the Humphrey GPA. Standard macular data cubes can now be analyzed to estimate ganglion cell density in the central retina.

We know that many doctors have branch offices, multiple perimeters and archival needs. So, we’ve also created a product called FORUM^® that can combine data from multiple perimeters into a single database and store it on a central server. As long as all instruments are connected to the same server, each HFA has access to all tests. You can similarly combine data from multiple Cirrus HD-OCT instruments into a single centralized database.

FORUM also allows us to combine test results from the HFA and the Cirrus HD-OCT into a single report. Doctors have told us they want to see not only if the perimetry is stable, but also what the structure looks like — and how well the two measurements correlate. We believe that this streamlined process will help doctors concentrate more on what they do best — delivering high quality healthcare.

GPA analyses of Humphrey perimetry and Cirrus HD-OCT data both can now be efficiently used to help doctors assess rate of progression as well as progression events. This represents a major change in the way we are now suggesting that doctors think about glaucoma management. In my view, it’s no longer a question of “did the patient get worse?” We know that most glaucoma patients progress, even if ever so slowly. Today, it also is a matter of how fast, and GPA can help us assess both structural and functional change more effectively. ■

	RNFL Assessment CIRRUS SmartCube™ analysis reports RNFL thickness over the entire peripapillary area for a complete picture of differences from normal values.
	Optic Nerve Head Assessment CIRRUS SmartCube anatomy-based, Bruch’s membrane oriented, ONH analyses account for complex disc morphology including tilted discs, atrophy and other challenging pathologies.

Cirrus HD-OCT GPA Report

With Guided Progression Analysis (GPA), Cirrus HD-OCT can perform event analysis and trend analysis of RNFL thickness and ONH parameters (e.g. Average Cup-to-Disc ratio). Event analysis assesses change from baseline compared to expected variability. If change is outside the range of expected variability, it is identified as progression. Trend analysis looks at the rate of change over time, using linear regression to determine rate of change.

RNFL Thickness Map Progression (best for focal change)
RNFL Thickness Profiles Progression (best for broader focal change)
Average RNFL Thickness Progression (best for diffuse change)
Average Cup-to-Disc Progression (best for global change)