New perimetry device targets different pathway

September 1, 2008

A new perimetry device (Heidelberg Edge Perimeter, Heidelberg Engineering) shows to be very useful in the precise evaluation of patients with glaucoma and suspect glaucoma. This unique instrument may very well replace standard automated perimetry devices.

Key Points

Fort Lauderdale, FL-A new perimetry device [Heidelberg Edge Perimeter (HEP), Heidelberg Engineering], used for visual field testing, may prove to be more precise in the evaluation of patients with glaucoma and suspected glaucoma than standard automated perimetry devices that are currently being used, said John G. Flanagan, PhD, MCOptom, FAAO, developer of the technology, at the annual meeting of the Association for Research in Vision and Ophthalmology.

The Humphrey field analyzer, considered the international standard, is one automated perimetry device that uses a bowl onto which light is projected. The traditional targets used (white targets on a white background) offer a broadband stimulus so that all aspects of vision are stimulated to some extent and evaluated.

"The [new perimetry device] is different from the standard automated perimetry devices currently being used because it not only tests standard targets but also uses a new stimulus concept called the flicker defined form, which targets the magnocellular (M) pathway. The M pathway is believed to be sensitive to glaucomatous damage, and can therefore have the potential of detecting glaucoma earlier," said Dr. Flanagan, professor, Department of Ophthalmology & Visual Sciences, at the University of Toronto and the School of Optometry at the University of Waterloo, and director of the Glaucoma Research Unit at the Toronto Western Research Institute.

Studies have shown that the performance of the new perimetry device improved with higher contrast, a greater random dot density, greater background element organization, and mid-peripheral viewing. According to Dr. Flanagan, the new perimetry device demonstrates good test-retest characteristics, an ability to detect glaucoma early, as well as showing deeper and larger defects compared with standard automated perimetry.

Dr. Flanagan explained that the new perimetry device is different in so far as it is not a projection system but a monitor-based system, which uses stimuli that are on a monitor, making the device more compact than most standard automated perimetry devices. This approach attempts to preferentially stimulate the M cell pathway, a subset of the visual system, in the hope of finding the damage that occurs in glaucoma-early.

"Most of the perimetry devices are really driven by the glaucoma market, in part because glaucoma is a leading cause of worldwide blindness but is also an asymptomatic disease. One of the major ways of picking up glaucoma early is to measure visual function. Anything that can identify a defect sooner in the disease process or can detect disease progression earlier or with more confidence-that is better for patient management and is an advantage to the patient," Dr. Flanagan said.

The new perimetry device is a subjective test that evaluates important aspects of a patient's vision. Early glaucoma also can be detected using the retina tomograph and optical coherence tomography (OCT) devices, because those instruments image the structures of the eye, such as the optic nerve and retinal nerve fiber.

For clinical setting

Dr. Flanagan said the new perimetry device is designed to be used in any clinical setting. It is built on the manufacturer's Eye Explorer operating system, which enables the linking of perimetry data directly to the retina tomograph's optic disc analysis, to give a combined structure-function analysis. Dr. Flanagan said that the manufacturer will be producing structure-function maps in which both structural and functional aspects will be combined, fashioning a more complete diagnostic picture of the level of glaucoma disease and its progression.

Tools such as the new perimetry device map out the visual field and attempt to establish the area of vision and not simply the quality of central vision. This evaluation can then be linked to the structural deficits that can be imaged with diagnostic tools such as the retina tomograph and OCT instruments.

"These are all very useful diagnostic tools and provide pieces to a complex puzzle that are needed to reach the precise diagnosis of glaucoma and the severity of the disease, as well as the progression of the disease. Once the disease has been diagnosed, we need to know whether the treatment that the patient is receiving is stabilizing the condition well enough or whether the disease is progressing. This accurate evaluation will help physicians make better decisions on changes in therapy, which may ultimately affect prognosis," Dr. Flanagan said.