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Clinical Update: Glaucoma
Optic Disc Evaluation: Back to Basics
By Miriam Karmel, Contributing Writer
 
 

GDx, OCT, HRT—these imaging devices are familiar to ophthalmologists, perhaps too familiar. Some glaucoma specialists argue that ophthalmologists have become so fascinated by high technology that they are overlooking how much information can be gleaned from a brief, routine clinical examination. It’s time, they say, to refocus attention on a good old-fashioned clinical evaluation of the optic nerve.

In the words of Robert D. Fechtner, MD, professor of ophthalmology at the University of Medicine & Dentistry of New Jersey: “We get so enamored of technology in ophthalmology that we can overlook some of the fundamentals.”

The Technologies
Developed over the past 15 years, imaging technologies designed to detect early glaucomatous optic nerve damage and monitor for change include confocal laser scanning tomography (HRT), nerve fiber layer polarimetry (GDx nerve fiber analyzer) and optical coherence tomography (OCT), among others. They permit quantitative measurement of optic disc and retinal nerve fiber layer structure, providing accurate, reproducible information about the optic nerve topography and the surrounding nerve fiber layer.

But they cannot quantify other irregularities of the optic nerve head such as disc hemorrhage, optic disc pallor and peripapillary atrophy, according to David S. Greenfield, MD, associate professor of ophthalmology at Bascom Palmer. What’s more, according to Dr. Fechtner, the imaging devices could miss early signs of glaucoma.

Drs. Fechtner and Greenfield, along with other glaucoma specialists who advocate a return to the basics, contend that high-tech devices are used too often in place of a clinical examination and photography.

A Study Raises Concerns
Their concerns are based on more than casual observations or hunches. Several of them cited a study, headed by Allen M. Fremont, MD, PhD, that assessed conformance to the Academy’s Preferred Practice Pattern for patients with primary open-angle glaucoma. The study, reported in Archives of Ophthalmology, concluded that “care is falling short on several key aspects” of the PPPs.1 According to the study, only 53 percent of 395 working-age patients with POAG received an optic nerve head photograph or drawing.

“It’s data such as these that make one wonder whether we are forgetting to examine and document the appearance of the optic nerve clinically,” Dr. Greenfield said. The data suggest to him that these examinations are not happening.

Technology Is Easy
Why isn’t it happening? Jeffrey M. Liebmann, MD, said that most doctors rely heavily on the various technologies to help make a diagnosis of glaucoma because it is easy to do. Take perimetry, for example. While it may be easy to use, it can overlook early warning signs of glaucoma.

“Glaucoma is a disease of the nerve that leads to visual field loss. But by the time patients experience visual field loss there has already been a fair amount of nerve damage,” explained Dr. Liebmann, clinical professor of ophthalmology at New York University and director of glaucoma services at Manhattan Eye, Ear and Throat Hospital and New York University Medical Center.

“Visual field loss is not early glaucoma,” continued Dr. Liebmann, who warns against depending upon the field alone for diagnosis. “When you have achromatic visual field loss you’ve already had a fair amount of nerve damage. The key is to try to make a diagnosis by looking at the optic nerve before the patient has field loss.”

The range of normal for the optic nerve appearance is huge, Dr. Fechtner notes. “So it’s going to be very difficult for a machine to find the subtly abnormal nerve.”

On the other hand, a clinical evaluation of the area of thinning of the nerve fiber layer or a disc hemorrhage might enable you to detect signs of trouble early, he said.

Refocusing on the Basics
Dr. Fechtner believes ophthalmologists should become less reliant on the imaging technology. “I’m warning not to use those [devices] instead of a careful examination. The technology complements what we can see. It doesn’t replace it,” he said. “What I would like to see happen is for clinicians to re-educate themselves about how to examine the optic nerve.”

Dr. Greenfield agrees. “I have access to them all [imaging devices] and use them all, but I don’t use them as a replacement for optic nerve examination or photography. They are not a replacement for a careful exam of the optic nerve and documentation of the optic nerve appearance with stereoscopic photography,” he said. Rather,
he sees these technologies as a clinical adjunct to the stereoscopic photograph, which he called “the existing structural gold standard.”

None of this is new, Dr. Liebmann said. “It’s refocusing. We need to refocus our attention on the optic nerve.”

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1 Arch Ophthalmol 2003;121(6):777–783.

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Drs. Fechtner, Greenfield and Liebmann have or currently receive research support from Carl Zeiss Meditec. Dr. Greenfield is a consultant to Carl Zeiss Meditec. Dr. Liebmann receives research support from Heidelberg Instruments.

The Two-Step Optic Disc Exam

There are two steps to a good optic disc exam, the glaucoma experts agree. The first step is examination of the optic nerve and nerve fiber layer.

The second step is documentation of the appearance of the nerve with a baseline photo.

“By performing a comprehensive examination of the optic nerve and documenting the appearance of the optic nerve with both a detailed drawing and stereoscopic photograph, clinicians will be able to diagnose glaucoma and monitor glaucomatous change in a more effective manner,” Dr. Greenfield said.

A good optic disc exam can be done in 30 seconds, said Dr. Liebmann, who teaches continuing medical education courses on the subject. He added that while there is no single definitive test, “it’s a question of putting together a series of findings.”

Here is a checklist of things to consider when examining the optic nerve. A thorough two-step exam will include the following:

STEP ONE: THE OPTIC NERVE EXAM

  • Clinically measure the size of the optic disc. You may use the 5-degree aperture of a Welch Allyn direct ophthalmoscope, which is approximately 1.5 millimeters in diameter. A normal optic nerve approximates 1.7 to 1.8 mm in its vertical diameter. Alternatively, a 60-D handheld lens can be used at the slit lamp to directly measure the diameter of the optic nerve without the need to correct for ocular magnification introduced by axial length or refractive error. This information helps to make sense of the cup-to-disc ratio.
  • Assess the shape and contour of the neuroretinal rim.
  • Look for signs of glaucomatous optic nerve damage including: retinal nerve fiber layer atrophy (detectable using red-free illumination), focal damage to the rim, zone beta peripapillary atrophy and the presence of disc hemorrhage.
  • Apply the “ISNT” rule: Inferior, Superior, Nasal, Temporal. In a normal optic disc, the thickest portion of the neuroretinal rim is located inferiorly at 6 o’clock, followed next by the superior, nasal and temporal portions of the rim. The glaucomatous eye may violate the ISNT rule. Thinning will occur at the inferior neuroretinal rim. This violation suggests an increased risk of glaucoma, or the likelihood that glaucoma is present.

STEP TWO: THE PHOTOGRAPH

  • Document the appearance of the optic disc. “While technologies may change over time” said Dr. Greenfield, “optic disc photography has remained the gold standard throughout these changes and will continue to be so moving forward.” As Dr. Liebmann put it: “Every patient deserves at least one stereo disc photograph at the time of diagnosis.”