What do nerve fiber defects look like?

Retinal Nerve Fiber Layer Defects in Glaucoma

Retinal nerve fiber layer (RNFL) defects in glaucoma typically appear as diffuse or localized thinning, most commonly at the inferior and superior poles of the optic disc, and can be detected before visual field changes become apparent. 1

Characteristic Appearance of RNFL Defects

Localized (Wedge-Shaped) Defects

• Appear as dark, wedge-shaped areas extending from the optic disc margin to the peripheral retina
• Most commonly located in the inferior temporal and superior temporal fundus regions 2
• Have a three-dimensional nature, appearing as "grooves" or depressions in the retinal surface 3
• Blood vessels crossing these defects show characteristic dipping into the defect area 3
• Highly specific for indicating optic nerve damage 2
• More common in normal-tension glaucoma than in primary open-angle glaucoma 2
• Often associated with disc hemorrhages 2

Diffuse RNFL Defects

• Present as generalized thinning of the nerve fiber layer
• Result in reduced visibility of the normal striations of the RNFL
• More difficult to detect on clinical examination than localized defects
• Often require imaging technologies like OCT for accurate detection

Distribution Patterns of RNFL Defects

• RNFL normally follows the ISNT rule: thickest Inferiorly, then Superiorly, Nasally, and thinnest Temporally 4
• Defects affecting inferior RNFL (causing superior visual field loss) tend to be:
  • Wider than superior defects
  • Located closer to the horizontal meridian of the optic disc 5
• Superior RNFL defects (causing inferior visual field loss) are typically narrower 5

Clinical Detection Methods

Direct Visualization

• Stereoscopic examination through dilated pupils using biomicroscopy with slit lamp (preferred method) 4
• Red-free photography enhances contrast and improves detection of RNFL defects
• Indirect ophthalmoscopy for stereoscopic magnified visualization 4

Advanced Imaging

• Optical Coherence Tomography (OCT) provides quantitative measurements of RNFL thickness 4
• Enhanced depth imaging OCT (EDI-OCT) allows three-dimensional visualization of the lamina cribrosa 4
• Stereoscopic color photography for documenting qualitative appearance of the optic nerve 4

Correlation with Visual Field Defects

• RNFL defects typically precede detectable visual field loss 6
• A large number of nerve fibers can be damaged before functional loss is detected 7
• Typical visual field defects corresponding to RNFL damage include:
  • Nasal steps
  • Arcuate field defects
  • Paracentral depression in clusters of test sites 1
• In early/moderate cases, visual field loss across the horizontal midline in one hemifield often exceeds loss in the opposite hemifield 1

Special Considerations

• In approximately 9% of cases, visual field defects may be present with an apparently normal RNFL 7
• Early glaucomatous damage may be missed without RNFL photography, especially when confined to the central 10° of fixation 6
• Patients with high myopia may have diffusely thin RNFL without glaucoma 4
• Regular monitoring of RNFL thickness (every 6-12 months) is recommended, with a thinning rate >2.0 μm/year indicating significantly increased risk of glaucoma progression 4

Clinical Pearls

• RNFL defects are often more easily detected in younger patients due to better contrast against the retinal pigment epithelium
• The presence of RNFL defects in a glaucoma suspect warrants close monitoring even with normal visual fields
• Training in identifying the three-dimensional nature of RNFL defects significantly improves detection rates 3
• RNFL defects are more common in early rather than advanced glaucoma, making them valuable for early diagnosis 2