What is the recommended protocol for using Optical Coherence Tomography (OCT) to measure Retinal Nerve Fiber Layer (RNFL) thickness?

OCT Protocol for RNFL Thickness Measurement

Use spectral-domain OCT with high-resolution scanning (512 scan points) centered on a 3.4 mm diameter circle around the optic disc, measuring both the peripapillary circle and inner ring of the ETDRS grid.

Scanning Protocol Specifications

Resolution Requirements

• Employ 512 scan point resolution rather than 256 scan points for optimal diagnostic accuracy 1
• Higher resolution scanning provides superior diagnostic sensitivity (AUROC 0.912) and stronger correlation with visual function (R = 0.75 vs R = 0.56) 1
• The fast RNFL scan protocol (256 points) systematically overestimates thickness values and should be avoided for diagnostic purposes 1

Measurement Parameters

• Center the scan on a 3.4 mm diameter circle around the optic disc 2, 1
• Measure RNFL thickness in the following regions:
  • 6 standard peripapillary segments (superior, superior-temporal, temporal, inferior-temporal, inferior, nasal) 2
  • Inner ring of the ETDRS grid surrounding the optic disc border 2
  • 8 areas corresponding to the ETDRS grid for comprehensive mapping 2

Key Measurement Areas

• The inner ring surrounding the optic disc border provides the highest diagnostic accuracy (AUROC = 0.940) for detecting early glaucomatous damage 2
• ETDRS grid area 4 also demonstrates excellent discrimination (AUROC = 0.903) 2
• Focus particularly on inferior and superior quadrants, as these show the earliest detectable changes in glaucoma 3

Critical Technical Considerations

Machine-Specific Differences

• RNFL measurements are NOT interchangeable between different OCT devices 4
• Topcon OCT systematically measures 10 μm thicker than Cirrus OCT in normal eyes and 13 μm thicker in glaucomatous eyes 4
• When monitoring patients longitudinally, always use the same OCT instrument to avoid spurious changes 4

Normal Reference Values

• Normal mean RNFL thickness: approximately 91-93 μm for the peripapillary circle 2
• Normal inner ring thickness: approximately 91 μm 2
• Values below the 95% confidence interval warrant clinical concern, particularly in inferior and superior quadrants 3

Clinical Interpretation Pitfalls

Early Disease Detection

• A 35% reduction in inner ring RNFL thickness can occur in preperimetric glaucoma before visual field defects appear on standard automated perimetry 2
• Patients with normal standard perimetry but abnormal frequency doubling technology results frequently show decreased RNFL thickness on OCT, indicating early structural damage 3

Quadrant-Specific Sensitivity

• Superior-temporal and temporal sectors are less sensitive for detecting early glaucomatous changes 2
• Inferior quadrant measurements provide the most reliable early detection (significant at 99% CI) 3

Non-Glaucomatous Applications

• For papilledema assessment, expect diffuse RNFL thickening in all quadrants, with greatest increases in inferior and superior regions 5
• RNFL thickness correlates positively with papilledema severity grading 5