Field of Dreams: Building a Better Perimeter

Advances allow for more sensitive tracking of glaucoma.

BY RENÉ LUTHE, SENIOR ASSOCIATE EDITOR

The workhorse of glaucoma practices, the automated perimeter, seemed lost in the latest wave of technological advances. Imaging devices such as OCT and nerve fiber layer analyzers represent the most talked about cutting-edge innovations in glaucoma care. However, recent years have seen impressive improvements to the stalwart perimeter, and more are on the way. While new perimeters can be a challenge to cost-justify, especially if you feel your current machine “works just fine,” experts believe newer models can be a worthwhile investment.

The latest generation includes such attractive features as the ability to network with devices from other manufacturers and interface with electronic medical records, and software tools that better enable the clinician to determine if there has been significant glaucoma progression. Here's a rundown of the latest offerings from perimeter manufacturers.

FIGURE 1. Carl Zeiss Meditec's HFA II offers Guided Progression Analysis technology.

Carl Zeiss Meditec

More than 60,000 Humphrey Field Analyzers (HFA) are installed worldwide, according to the company. But CZM continues to develop enhancements that make this platform more advanced — and thus a better value for glaucoma specialists. For instance, one of the HFAs most notable features is its guided progression analysis (GPA), a capability validated in the Early Manifest Glaucoma Trial. GPA software identifies statistically significant changes in visual field threshold sensitivity automatically, according to the company.

CZM has recently improved the HFAs technology, adding several new features. One of those is visual field index (VFI), a global index to determine the percentage of field loss on every visual field. “This is designed to allow us to monitor our glaucoma patients for disease progression across the glaucoma spectrum,” explains HFA user Nathan Radcliffe, MD, of Weill Cornell Medical College and New York Presbyterian Hospital. “The visual field index is a number that gives you the age-adjusted visual field function that is designed to be less sensitive to cataract formation, which can often confound glaucoma diagnosis in perimetry.”¹ Additionally, VFI is center-weighted to reflect a crucial aspect for clinicians — the central visual field.

For every visual field obtained, a VFI is calculated. And because GPA software is “backward compatible,” Dr. Radcliffe points out, previous visual fields performed with the HFA can be printed again and the VFI determined.

“Specifically, if you've been stable on one testing platform, such as SITA standard, for a number of years, you simply rerun the software on all of your previously collected fields,” Dr. Radcliffe says. “Not only do you get the visual field index calculation, you also get to use those previous fields as baselines for determining future progression.” The result, he says, is a regression analysis that indicates the patient's slope of visual function over time. The clinician no longer has to wait for the patient to cross a threshold indicating that he or she has moved into the “abnormal” category.

Dr. Radcliffe also finds that the new GPA software improves patient communication. Patients often ask their physicians what percentage of their vision they have lost, and while an exact number remains difficult to measure, VFI enables doctors to make a close estimation. “You can say to a patient, ‘At the rate you're going now, I think you will lose half your vision in 5 years if we don't intervene,’” says Dr. Radcliffe. Such a conversation can improve a patient's compliance with the prescribed therapy.

The new GPA also allows the clinician to track and store event-based glaucoma progression analysis, Dr. Radcliffe points out, an important advantage for the occasions when the event-based approach is desired.

“In some circumstances, the clinician may prefer event-based analysis over trend-based analysis,” Dr. Radcliffe says. “While the two techniques are very similar in terms of progression detection, event-based analysis is more sensitive, detecting progression with fever visual field tests and over a shorter period of time.” The trend-based approach will offer a higher specificity, he notes, particularly when the physician has a greater duration of follow up and number of visual field exams in the analysis.²

A final benefit of the new CZM platform, according to Dr. Radclifffe, is that it provides ophthalmologists with the “same vocabulary” to discuss structural and functional progression. “We can also talk about guided progression analysis for retinal nerve fiber layer measurements on both OCT of the retinal nerve fiber layer and on retinal nerve fiber layer measurements using GDx,” he explains. “It's nice to see technologies converge. By using a common strategy to identify structural and functional changes, we've seen things get a bit simpler in glaucoma management.”

Other advantages, CZM points out, include new software reporting that offers guidance for severely depressed visual fields, and a new GPA algorithm that allows GPA analysis to be run on more patients immediately by allowing a mix of full-threshold and SITA exams.

For clinicians interested in integrating their diagnostic instruments and having the ability to transfer data to satellite offices or to referring doctors, the HFA can be networked with CZM's image-based data management system, Forum.

Haag Streit

Haag Streit's Octopus 900 perimeter (Figure 2) and its Eyesuite software incorporate several features aimed at improving the field-taking experience and helping clinicans deal with variable testing results.

Figure 2. New software for Haag Streit's Octopus 900 helps physicians spot trends.

The new software helps the clinician identify areas of concern and assess their significance. “It provides analysis by assessing for trends, applying linear regression analysis to clusters, and highlighting significant changes and their severity,” says Jonathan Myers, MD, associate attending surgeon for the glaucoma service at Philadelphia's Wills Eye Hospital. “Every field you add is then added to the analysis.”

Another benefit Dr. Myers notes is the testing algorithm, TOP, for tendency-oriented perimetry, the new software offers. It results in a much quicker test, which, he points out, is an important consideration for some patients and should help to reduce variability. “The key challenges in perimetry continue to be obtaining the best information as quickly and humanely as possible,” he explains. “TOP sacrifices some spatial detail to allow for a much quicker test, and for some patients, this may be a good choice.”

Thanks to TOP, visual field tests are much quicker, usually two and a half minutes per eye, a significant improvement over many visual fields tests done with traditional algorithms. In addition to making the test easier on the patient, the shorter duration leads to happier and more productive technicians.

Eyesuite's eye-tracking software can detect loss of central fixation and halt the test until the patient returns to central gaze. “It also monitors the position of the eye so that if it's not quite centered, the machine's chin-rest will move and re-center the eye,” Dr. Myers notes.

Dr. Myers said that by applying linear regression analysis, the software takes into account a patient's historical fluctuation to determine if variations are statistically significant and to identify trends that may indicate glaucoma progression. The ability to weed out variability in fields from genuine progression makes the Octopus an asset to the physician in determining which patients are in need of more aggressive management to preserve visual function.

Other features of the Octopus include standard white/white perimetry, short-wavelength automated perimetry and flicker for early detection, EMR compatibility, plus automated Goldmann kinetic perimetry. According to Haag Streit, Goldmann kinetic is “ideal for testing outside 30 degrees” as well as for end-stage glaucoma.

To keep clients up to speed with improved technologies, the company says future software upgrades and updates can be accessed through Internet downloads.

Kowa

In the company's AP-5000C ocular fundus perimetry, the visual field is examined with an ocular fundus image displayed on the screen. The system offers the capability to overlap the ocular fundus image and the perimetry result so the clinician can better understand retinal function. The data integrates with the Kowa VK2 Digital imaging system, enabling practices to manage the fundus images and perimetry results, and ensure smooth fundus image perimetry.

The AP-5000C features 25 test programs, including threshold: Center 1 and 2, macula, periphery and meridian; and screening: standard or precision; isopter: Goldmann and automatic perimeter is combined. Stimulus offered is Goldman I, II, III, IV and V. Stimulus color is available in white, red, green and blue. Kowa says the stimulus presentation time is 0.2 seconds selectable. The eye fixation target is RED LED (one point at center, for points for focus exam).

Oculus

Oculus touts the Easyfield perimeter's (Figure 3) combination of portability and software that results in a shorter visual field test. “We wanted to combine fast-threshold examination strategies with an advanced software for interpreting visual field data based on expert systems,” according to a company representative.

Figure 3. The Oculus Easyfield features new software that results in a shorter visual field test.

Oculus has implemented its Continuous Light Increment Perimetry (CLIP) strategy and Glaucoma Staging and Progression (GSP) software on the hand-control unit of the Easyfield. The combination results in shortened threshold examinations during glaucoma screening — exam times averaged two minutes and 37 seconds in a visual field of 20 healthy eyes, according to Oculus, with a standard deviation of 21 seconds. The GSP, the company's advanced visual field data analysis software, enables the possibility to interpret visual field data. No dedicated computer system is required.

In Oculus's study of 20 eyes, researchers compared the values for mean sensitivities obtained using CLIP to the values for mean sensitivities obtained using a 4/2-threshold strategy. The CLIP values were slightly higher, the average difference of the mean sensitivities 0.77 dB with a standard deviation of 0.41 dB. However, the company says, the correlation of the mean sensitivity values obtained with the two different methods underscores the reliability of the threshold measurements performed using CLIP on healthy eyes.

CLIP differs from the 4/2 strategy in that, instead of starting by presenting light stimuli in 4 dB steps in the direction toward the threshold, followed by 2 dB steps in the opposite direction after crossing the threshold with the measurement potentially concluded after the second crossing of the threshold, CLIP makes use of test points with luminance increased at a steady rate. What this means, the company says, is that the test stimulus is always “on,” its luminance being increased in time in smaller steps, typically of 1 dB, until the patient gives a positive answer. A significant reduction in examination time is achieved by measuring the average reaction time of the patient and selecting the appropriate incremental rate of luminance. This shorter test duration is achieved without losing precision or reproducibility, the company says.

Additionally, the Easyfield offers a color LCD display and a fixed-point grid with 79 locations (including 30-2, 24-2, and 10-2) to test full/normal, fast and three different suprathresholds out to 30°.

Paradigm Medical Industries

Paradigm's Dicon LD 400 autoperimeter is unique in three respects, according to Jacob Matthews, marketing manager at Paradigm Medical Industries, Inc. First, while other perimeters only allow patients the use of a button to record their results, the LD 400 also works by voice recognition. Mr. Matthews points out that this feature is a significant advantage to someone whose hands are incapacitated. The perimeter includes voice prompts that are available in 28 languages.

The second feature the LD 400 offers is kinetic perimetry. This method of perimetry reduces patient fatigue, increases patient engagement and enhances reliability and repeatability, the company claims.

“Because the LD 400 utilizes voice prompts and kinetic perimetry, the patient is actively encouraged to maintain proper fixation and test results are more accurate than those from static perimetry. With static perimetry patients are more error prone as they lose interest and become fatigued from the repeated monotony of staring at fixed points until the visual threshold is defined,” explains Mr. Matthews.

Figure 4. Paradigm's Dicon LD 400 is a compact, full-field autoperimeter that features voice recognition.

The third unique feature of the LD400 is its two-button operation providing easy setup and use.

The LD 400 also features screening, quantified missed points, threshold strategies and single- or multiple-stimulus testing capabilities.

The following upgrades are on the horizon: enabling the LD 400 to run off a USB port and syncing the autoperimeter with a wireless printer. These enhancements should be unveiled in time for the October 2010 American Academy of Ophthalmology meeting, says Mr. Matthews. OM

References

1. Bengtsson B, Heijl A. A visual field index for calculation of glaucoma rate of progression. Am J Ophthalmol. 2008 Feb;145:343-353.
2. Casas-Llera P, Rebolleda G, Muñoz-Negrete FJ, Arnalich-Montiel F, Pérez-López M, Fernández-Buenaga R. Visual field index rate and event-based glaucoma progression analysis: comparison in a glaucoma population. Br J Ophthalmol. 2009;93:1576-1579.