How is the optic nerve head (ONH) and retinal nerve fiber layer (RNFL) assessed in patients at risk of glaucoma, particularly those with a family history?

Assessment of Optic Nerve Head (ONH) and Retinal Nerve Fiber Layer (RNFL)

ONH and RNFL assessment requires both clinical examination techniques (ophthalmoscopy, slit-lamp biomicroscopy with stereoscopic viewing) and objective imaging modalities (optical coherence tomography, confocal scanning laser ophthalmoscopy, scanning laser polarimetry), with documentation through stereoscopic photography being essential for longitudinal monitoring. 1

Clinical Examination Techniques

Direct Ophthalmoscopic and Slit-Lamp Assessment

• Ophthalmoscopy and optic disc photography are fundamental but highly subjective methods with poor interobserver agreement and high variation between examiners. 1
• Stereoscopic examination through dilated pupils using slit-lamp biomicroscopy with high-magnification lenses (78D, 90D, or contact lenses) allows three-dimensional assessment of the ONH. 1
• Look specifically for vertical elongation of the optic cup, decreased neuroretinal rim width, cup enlargement, and diffuse or focal narrowing of the neuroretinal rim. 2
• Assess for beta-zone parapapillary atrophy adjacent to the disc margin, which indicates RNFL damage. 2

Key Structural Features to Document

• Cup-to-disc ratio (both vertical and horizontal) through direct visualization. 1
• Neuroretinal rim thickness and symmetry, particularly noting any focal notching or thinning. 2
• RNFL defects appearing as wedge-shaped dark areas in the peripapillary region, best seen with red-free (green) light. 1
• Disc hemorrhages at the margin of the optic disc, which indicate active glaucomatous damage. 1

Objective Imaging Modalities

Optical Coherence Tomography (OCT) - Primary Recommendation

• OCT is the preferred objective imaging method, providing quantitative measurements of RNFL thickness, ONH parameters, and macular ganglion cell complex. 2, 3, 4
• Spectral-domain OCT offers superior resolution (approximately 3-6 micrometers axial resolution) compared to older time-domain systems. 4
• RNFL thickness measurements show the best discriminating performance, with inferior RNFL thickness having the largest area under the ROC curve (0.91) for glaucoma detection. 3
• Combined RNFL and ONH parameters achieve the highest diagnostic accuracy (AUC = 0.97) when used together. 3
• OCT can measure lamina cribrosa depth, which may show deformation before detectable RNFL thinning occurs. 5

Reference Plane Positioning for OCT Analysis

• Position the reference plane at 150 micrometers above the retinal pigment epithelium for optimal glaucoma detection and correlation with visual function. 6
• This specific offset provides the largest area under the ROC curve (0.966) for early glaucoma detection. 6
• Measurements at this level show the strongest correlation with visual field mean deviation (r = 0.793). 6

Alternative Imaging Technologies

• Confocal scanning laser ophthalmoscopy (CSLO) using Heidelberg Retinal Tomography provides topographic maps of ONH surface height. 5, 7
• Scanning laser polarimetry (SLP) measures RNFL retardance as a surrogate for thickness. 5
• These modalities have poor to moderate diagnostic precision for population-based screening but are useful for longitudinal monitoring in diagnosed patients. 1

Documentation and Baseline Establishment

Stereoscopic Photography

• Obtain stereoscopic optic disc photographs at baseline for all patients at risk or suspected of having glaucoma. 2
• This provides the gold standard for detecting progressive ONH changes over time. 1

Quantitative Parameters to Track

• ONH measurements: disc area, cup area, cup-to-disc area ratio, rim area, rim volume, cup depth, and cup shape. 7, 6
• RNFL thickness: global average, quadrant measurements (superior, inferior, nasal, temporal), and clock-hour sectors. 3, 6
• Macular ganglion cell complex thickness: inferior outer macular thickness shows good discriminating ability (AUC = 0.81). 3

Baseline Structural Predictors of Progression

Certain baseline structural characteristics predict faster functional progression:

• Larger baseline cup area and cup-to-disc ratio associate with shorter time to visual field progression. 7
• Thinner baseline RNFL thickness (particularly mean RNFL and RNFL cross-sectional area) predicts faster progression. 7
• Smaller rim area and rim volume at baseline indicate higher risk of functional deterioration. 7
• Thinner central corneal thickness is an independent risk factor for progression from preperimetric to perimetric glaucoma. 2

Integration with Functional Testing

• Structural changes often precede detectable visual field defects, making ONH and RNFL assessment critical for early detection. 5
• ONH and lamina cribrosa deformation can be observed before RNFL thinning and functional loss, providing a therapeutic window for intervention. 5
• RNFL thickness correlates better with visual function than ONH measurements alone and serves as a superior indicator for monitoring disease progression. 6

Common Pitfalls to Avoid

• Do not rely solely on IOP measurement for glaucoma assessment, as half of glaucoma patients have IOP below 22 mmHg. 1, 8
• Avoid single-method assessment—combining clinical examination with objective imaging provides superior diagnostic accuracy. 3
• Account for corneal thickness when interpreting IOP, as LASIK or PRK causes falsely low readings. 1
• Recognize that subjective clinical methods alone (ophthalmoscopy, disc photography) have high interobserver variation and should be supplemented with objective imaging. 1