Optic nerve imaging may become first-line method to diagnose glaucoma

Issue: April 1996

MARION, Iowa—Optic nerve imaging may vault to the front of the line as the premier way to detect early optic disk damage in glaucoma, but researchers agree imaging will not replace visual field tests and stereodisk fundus photography in disease management.

mugshot --- Richard F. Noyes, OD

"The value of both visual fields and optic nerve imaging is to better detect changes in the patient," said Richard F. Noyes, OD, in private group practice here. "Visual fields play a major role in established glaucoma, particularly at the level of the glaucoma subspecialist, but imaging is more critical in the hands of optometrists because they want to catch these patients before visual field loss is established."

Noyes said there can be up to a 6-year delay between optic nerve changes and the start of detectable visual field changes in patients with glaucoma. In this amount of time, as much as 50% of the optic nerve fibers can be lost to the disease. The optic nerve fiber, comprised of about 1 million ganglion axons, loses 5,000 axons a year just through normal aging.

Detecting functional changes to the optic disk and nerve fiber layer as early and as accurately as possible means optometrists can initiate therapy sooner, Noyes said. A senior partner in a large glaucoma practice, he sees 12-20 glaucoma patients a day. Noyes finds optic nerve imaging vital to early detection and early treatment.

"This instrument becomes even more valuable to the practitioner who sees less glaucoma because the ability to judge and accurately see change is based on repeated, consistent experience," he said.

Measuring data efficiently

[image] --- Normal nerve fiber layer.

In his practice, Noyes uses optic nerve imaging equipment that takes 64,000 data points from 32 images to measure more than 1 million total data points, all within 0.9 sec. The instrument calculates the area of the neuroretinal rim and measures the vertical and horizontal dimensions and depth of the optic cup.

Gathering this information in a specific, repeatable way, Noyes said, practically eliminates practitioner error in viewing the same optic nerve twice. "Sometimes when they measure cup-to-disk ratio, doctors can read it differently from time to time even when there is no change," he said.

In a multiple-doctor office, the variance can be as much as 0.4 among practitioners evaluating the same eye, representing 30-40% of the nerve fibers and "way too large a margin of error," Noyes said.

He uses optic nerve imaging and visual fields for a two-pronged approach to diagnose and treat glaucoma. Early detection and diagnosis involves optic nerve imaging, while variations in glaucoma therapy are often managed with visual field tests. "At their best, visual fields only measure if a patient is having a good day or a bad day, so multiple visual fields are needed, and they become more important as glaucoma advances," he said.

Reasons to get an image

[image] --- Diffuse thinning of the nerve fiber layer in a glaucoma patient.

Optometrists should perform optic nerve imaging for several reasons: family history, elevated IOP, significant cardiovascular history, trauma to the optic nerve, changes in the optic cup or failure to meet a target pressure.

Elevated IOP, Noyes said, is only "the handle to the drawer" that holds more specific and sensitive tests necessary before clinicians can accurately diagnose glaucoma.

"The number one thing we teach in glaucoma evaluation is baseline, baseline, baseline," Noyes said. "Baseline visual fields and stereoscopic fundus photography are the standard of care, and now we can add baseline optic nerve imaging."

These images can guide glaucoma therapy if they indicate a change in cupping, in addition to the standard reasons ODs might consider altering therapy: signs of a Drance hemorrhage or failure to meet target pressures. "Target pressures are defined as the pressure you believe is going to be a level at which existing damage will not progress or damage will not occur," Noyes said, "but that number changes over the course of a patient's lifetime."

Target pressures are often adjusted for a patient's age and expected longevity, said Noyes. He sets target pressures lower for younger patients, because studies have shown that patients whose IOP is below 16 mm Hg do better over a longer period of time than patients whose pressures are in the upper edge of the normal 12 to 21 mm Hg range.

"When treating glaucoma," Noyes said, "you have to consider the effect of a drug on the patient, the patient's likelihood to continue with the regimen, the side effects and financial cost."

State of the art images

Glaucoma researcher Alan L. Robin, MD, associate professor at Johns Hopkins University, Baltimore, disagrees that imaging has become the preferred way to diagnose and manage glaucoma, and acknowledges the equipment is not something every OD will have.

"The instruments are wonderful but they are expensive, and in a managed care environment that's neither cost- nor patient-efficient," Robin said. "They are still working on validating the software and I wouldn't buy one at this point."

Stereodisk photography is still the preferred way to look at the optic nerve, he said, and it is even more important for clinicians to learn how to look at the nerve fiber layer and document its appearance so new photographs can be compared to old ones. "Optometrists have to develop a systematic way of looking at the optic nerve fiber layer," Robin said. "For example, you compare the superior half to the inferior half, the right eye to the left, then you look at the neuroretinal rim to see if the disk is abnormal."

Optic nerve imaging can help clinicians detect early disk damage, follow progressive disk damage and detect advanced glaucoma damage in a specific, sensitive way.