Peripapillary RNFL Analysis

The most commonly used OCT measurement in glaucoma is peripapillary retinal nerve fibre layer (RNFL) thickness. A circular scan 3.46 mm in diameter is centred on the optic disc. The OCT machine measures RNFL thickness at each point around the circle and creates a 360-degree thickness profile, displayed as a TSNIT curve (Temporal-Superior-Nasal-Inferior-Temporal — the clockwise sequence starting at the temporal region).

RNFL Measurements and Normative Database

Results are colour-coded based on comparison to an age-matched normative database:

• Green: Within normal limits (average to normal thickness)
• Yellow: Borderline, 5th–1st percentile (mildly reduced thickness)
• Red: Outside normal limits, below 1st percentile (significantly reduced thickness)

A clock-face map displays the same data spatially, allowing easy visualization of which sectors are thin. The ISNT rule (Inferior > Superior > Nasal > Temporal) describes the normal pattern of RNFL thickness — in healthy eyes, the inferior sector is thickest, followed by superior, nasal, then temporal. Violation of the ISNT rule is suggestive of glaucoma.

Global RNFL Measurements

Average RNFL thickness: Overall mean thickness across the entire circle. Used to assess global severity and progression.

Superior and Inferior RNFL thickness: These sectors are typically the first to thin in glaucoma, as the superior and inferior arcuate bundles of nerve fibres are most vulnerable.

Macular Ganglion Cell Complex (GCC) Analysis

While RNFL analysis measures the nerve fibre layer at the optic disc, macular analysis evaluates the ganglion cells and inner plexiform layer (IPL) in the central macula. The GCC includes the retinal ganglion cell layer and the inner plexiform layer where ganglion cell dendrites synapse with bipolar cells. Macular GCC analysis has advantages:

• Complements peripapillary RNFL analysis — adds independent structural measurement
• Particularly useful in eyes with prominent optic disc anomalies, high myopia, or parapapillary atrophy where RNFL interpretation is difficult
• May detect early macular damage earlier than peripapillary RNFL in some cases
• Useful in advanced glaucoma where the central macula is still relatively preserved

Macular IPL (inner plexiform layer) thickness is sometimes measured as an alternative to GCC, as it is less affected by age-related changes in other retinal layers.

Optic Disc OCT Parameters

Beyond RNFL, OCT can measure:

• Cup-to-disc ratio: Diameter of the optic cup divided by optic disc diameter, derived from OCT imaging
• Rim area: The neuroretinal rim area (optic disc area minus cup area); progressive reduction indicates glaucoma
• Disc area: The total optic disc area
• Lamina cribrosa depth: Posterior displacement of the lamina cribrosa (the connective tissue structure through which retinal ganglion cell axons pass) may indicate elevated IOP and is a research biomarker

Early Detection — The Structural-Functional Gap

One of OCT’s major advantages is early detection. Studies show that approximately 20–40% of ganglion cells must be lost before visual field defects become apparent on automated perimetry. Since OCT measures structural loss directly, it can detect the progressive thinning of RNFL and GCC before visual field loss manifests. This structural-functional gap provides a critical window for early intervention — treating patients identified by OCT before irreversible visual field loss occurs.

Progression Analysis Using OCT

Event-Based Analysis

Event-based analysis (such as SuperPixel analysis) identifies a statistically significant change from baseline at a specific location. For example, if the RNFL thickness at a particular location has decreased by >5 dB (decibels, a logarithmic unit used in OCT comparisons) beyond normal test-retest variability on two consecutive tests, this represents a significant change event. Event-based analysis is specific (few false positives) but less sensitive (may miss slow, diffuse thinning).

Trend-Based Analysis

Trend analysis calculates the rate of RNFL thinning (in micrometers per year) using linear regression across multiple OCT scans. A progression rate of greater than 1–2 μm/year is typically considered clinically significant in the context of glaucoma management. Detecting reliable progression requires a minimum of 5–6 scans over 2–3 years. Trend analysis is more sensitive to slow, diffuse thinning but may generate false positives if there is high measurement noise.

Integrated Algorithms

Modern OCT instruments include integrated software (e.g., Heidelberg’s GLAUCOMAP, Zeiss’s OCT Progression Analysis) that combines event and trend analysis, automatically flagging significant changes and calculating progression likelihood. These algorithms improve upon simple trend analysis by reducing noise and improving specificity.

Complementary Use with Visual Fields and Optic Disc Photography

OCT should not be viewed in isolation but as part of a triad of structural and functional assessments:
– OCT RNFL and macular GCC measure structural loss
– Automated visual field testing (24-2, 10-2) measures functional loss
– Optic disc photography documents disc appearance and cup changes

In early glaucoma, OCT may show RNFL thinning while visual fields remain normal. In advanced glaucoma, visual fields better reflect functional impairment. All three are essential for comprehensive glaucoma management.

Limitations and Considerations

Media opacities (cataracts) reduce OCT signal quality and may preclude reliable measurements. Post-refractive surgery eyes and highly myopic eyes present analysis challenges due to anatomical variations. Test-retest variability exists — small variations between scans can occur even without true progression, requiring multiple scans before declaring progression. OCT machines from different manufacturers produce slightly different thickness values; longitudinal comparisons should use the same device when possible.