Retinal nerve fiber layer loss precedes visual field loss in glaucoma

Issue: November 1995

PANAMA CITY BEACH, Fla.—To improve your ability to detect glaucoma damage early, incorporate retinal nerve fiber layer evaluations into your clinical practice, said Anthony B. Litwak, OD.

mugshot --- Anthony Litwak

In a presentation at the 12th Annual Bay Point Anterior Segment Symposium here, Litwak, editor of Clinical Eye and Vision Care and director of Residency Programs at the Baltimore/Fort Howard VA Medical Centers, said these evaluations do more than back up visual field tests.

The correlation between visual fields and nerve fiber layer (NFL) evaluations is important because visual fields alone are not reliable, Litwak said. Objective evidence of early glaucoma damage lies within the optic disc and retinal nerve fiber layer. "All you need is a bright light source, a dilated pupil, a clear media and a green light," he said. "The technique is quick, inexpensive and non-invasive."

Check light pattern

Litwak advises optometrists to evaluate the nerve fiber layer with a clear 78 D lens at the slit lamp. "Although you do not need stereopsis to examine the NFL, the 78 D lens provides excellent magnification and stereopsis for optic nerve evaluation," he said.

After examing the optic nerve, Litwak said, "Simply click your red-free filter in your slit lamp, turn your light source to the maximum setting and look for that bright-dimmer-bright pattern."

It is important to use a clear 78 D lens because the yellow lenses will wash out the NFL appearance. The characteristic bright-dimmer-bright appearance, Litwak said, indicates a normal nerve fiber layer.

In a normal situation, retinal ganglion cell bodies send out axons that travel across the retina to the optic nerve. At the optic nerve, the axons take a 90-degree turn as they exit through the scleral foramen. It is the death of these axons that causes thinning of the neuro-retinal tissue.

The NFL evaluation, Litwak said, involves simply examining the axons before they make that turn into the optic disc. The ganglion cell axons are grouped into bundles as they transverse the retina. Light directed into the eye reflects off the axons to give the NFL a fine, white, linear, striated pattern.

"The NFL will get brighter as you approach the optic disc and less bright as you travel further from the disc," Litwak said. "Because the NFL lies in the most superficial layers of the retina, it will cast a white haze over the underlying retinal structures. Smaller tertiary retinal blood vessels will not be visible where the NFL is prominent."

Study normal NFL appearance

--- Normal NFL appearance.

Optometrists must become familiar with the clinical variation of the normal NFL appearance, Litwak said, since the visibility of the NFL appearance will vary depending on the patient's age, media clarity and amount of fundus pigmentation. "In general, the NFL will be most prominent in a young patient with a clear media and heavy fundus pigmentation," he said. "The NFL will be arduous to evaluate in a patient with a media opacity of 20/40 or greater if the patient has a blonde fundus."

The take-home message, he continued, is to know what normal looks like before determing that the NFL is abnormal. "Evaluate the NFL in every patient you examine to gain experience."

Specifically, he said, scrutinize the superior and inferior arcuate zones of the NFL for glaucomatous damage, because these zones are where glaucoma damage typically occurs.

--- Wedge defect.

There are two patterns of NFL loss in glaucoma, Litwak said: focal and diffuse defects. Slit or wedge defects represent focal loss of ganglion cell axons.

"A slit defect should be larger than an arteriole width in size and travel all the way back to the optic nerve," he said. A slit defect that does not meet these two criteria is called a pseudo-slit. It is observed in about 10% of normal patients and is not considered a true NFL defect.

Conversely, the wedge defect will get more narrow towards the disc and broader as you travel away from the disc. "It is the easier type of NFL defect to identify, although it is not the most common," Litwak said.

Atrophy is common defect

--- Diffuse defect.

The most common nerve fiber layer defect in glaucoma is diffuse atrophy or thinning of the NFL in the superior and inferior arcuate bundles. The NFL will show more widely spaced striations, and smaller tertiary retinal vessels will become more visible.

"Diffuse loss is the most difficult pattern to identify," Litwak said. "It is helpful to compare between the superior and inferior arcuate zones and also to compare between the two eyes. Asymmetry of the NFL brightness between these zones may indicate diffuse NFL loss."

NFL evaluation is objective

Since a significant number of ganglion cell axons (20%-40%) are lost before a reproducible visual field defect appears, optometrists can use the NFL evaluation in suspected glaucoma patients who do not exhibit visual field loss. "Remember, the visual field is a subjective test, while the disc and NFL evaluation are objective tests," Litwak said.

NFL evaluation, he continued, is presently the best clinical test to diagnose glaucoma early. "The technique does require some degree of experience in interpretation skills, but this can be obtained by first learning how a normal NFL appears, then looking at obvious glaucomatous damaged eyes, and finally by using the technique in your glaucoma suspect patients," he said.

Five Clinical Pearls to Better Diagnose Glaucoma:

Elevated IOP does not define the diagnosis of glaucoma; it is only one risk factor.
A normal visual field test does not exclude the diagnosis of glaucoma.
Always evaluate the optic nerve in stereo.
Never interpret the visual field test without concurrently evaluating the optic disc.
Every time you look at the optic disc with a clear 78-D lens, click in your green filter, turn the light source to its maximum setting and evaluate the nerve fiber layer.