Test Parameters and Interpretation

Global Indices

Mean Deviation (MD): The overall average sensitivity compared to age-matched normal values. Expressed in decibels (dB). Normal MD is greater than -2 dB. In glaucoma: mild loss MD -2 to -6 dB, moderate -6 to -12 dB, severe <-12 dB.

Pattern Standard Deviation (PSD): The localised variation from the pattern expected for normal subjects of the same age. It highlights non-uniformity of visual field loss characteristic of glaucoma. Higher PSD indicates more irregular (non-random) loss, typical of glaucomatous defects.

Visual Field Index (VFI): Expressed as a percentage, represents how much normal visual field remains. VFI 100% = completely normal; VFI 50% = half of normal visual field retained. Useful for patient communication about severity.

Glaucoma Hemifield Test (GHT): Compares clusters of visual field points in the superior and inferior hemifields. The test categorises results as “Outside Normal Limits,” “Borderline,” or “Within Normal Limits.” It is sensitive for detecting early glaucomatous defects.

Glaucomatous Visual Field Defects

The pattern of visual field loss helps identify glaucoma and assess its severity:

Arcuate (Bjerrum) Scotoma

A dense area of reduced sensitivity in the superior or inferior nasal region, curving around fixation like a boomerang or “sickle” shape. This is the classic glaucomatous pattern, corresponding to optic nerve fiber bundle loss along the arcuate pattern of retinal nerve fibres. Often the first detectable visual field defect in glaucoma.

Nasal Step

A horizontal step-like depression that respects the horizontal meridian — the superior or inferior nasal region is depressed relative to the temporal region. This also reflects nerve fiber bundle loss and is relatively specific for glaucoma.

Altitudinal Defect

Loss of the entire superior or inferior half of the visual field (more common in non-arteritic ischemic optic neuropathy than glaucoma, but can occur in advanced glaucoma).

Paracentral Scotoma

A discrete area of loss within 5 degrees of fixation. Often an early sign of glaucoma, detected before the classic arcuate defect develops. Corresponds to early damage of the papillomacular bundle (nerve fibres running from the optic disc to the macula).

Progression Analysis

Detecting progression requires serial visual field tests over time. Two main approaches are used:

Event-Based Analysis

Compares each test to the baseline test and flags statistically significant changes at specific locations using methods such as SuperPixel (significant change is 3+ adjacent points showing >5 dB loss at <0.05 significance level, confirmed on two consecutive tests). Event-based analysis is more specific but less sensitive than trend analysis.

Trend-Based Analysis

Calculates the rate of MD decline (in dB per year) using linear regression across multiple tests. A statistically significant negative slope (faster decline than expected) indicates progression. Rate of progression is usually expressed as dB per year; a loss of -1 dB/year or steeper is considered clinically significant progression that may warrant treatment intensification. Requires at least 5–6 tests over 2–3 years for reliable rate calculation.

Guided Progression Analysis (GPA) is a sophisticated algorithm within the Humphrey software that combines event and trend analysis, integrating all tests and providing a summary of likely progression status — improved compared to event or trend analysis alone.

Reliability and Limitations

Test reliability is critical. Unreliable tests due to high fixation losses, false positives, or false negatives should be repeated. Media opacities (cataract), pupil size, refractive errors, and patient fatigue can all affect results. Patients with advanced glaucoma may have difficulty performing the test due to severely restricted visual fields. Cataracts can diffusely reduce sensitivities and simulate glaucomatous loss — cataract extraction followed by repeat testing is sometimes needed to clarify the diagnosis.