Field reliability and progression

4. Perimetric Reliability Indices & Global Statistical Metrics

Before interpreting visual field defects, the clinician must confirm the statistical reliability of the examination.

1. Reliability Indices

• Fixation Losses: A stimulus is projected directly into the patient's mapped blind spot (Heijl-Krakau technique). If the patient responds, fixation has drifted. A flagged fixation-loss percentage requires inspection of blind-spot placement, gaze tracking, artefacts and the full test; it is not an absolute validity rule. Real-time video gaze tracking (recording upward/downward gaze excursions) provides superior fidelity.
• False-Positive (FP) Errors: The instrument identifies implausible responses or uses catch trials, depending on strategy; this is not simply an auditory stimulus test. If the patient presses the response button ("trigger-happy" patient), a false positive is logged.
  • The Most Damaging Reliability Error: An FP rate above 15% raises concern, requiring review of the responses and pattern. High false positives artificially elevate the entire threshold surface, producing an abnormally high Mean Deviation, masking genuine scotomas, and generating a classic "white scotoma" (abnormally high sensitivity) on the grey-scale map.
• False-Negative (FN) Errors: A light stimulus significantly brighter than the previously determined threshold is presented at a location where the patient already responded. A failure to respond logs a false negative.
  • Inattention vs. Pathology: In healthy fields, an FN rate $> 25\%$ indicates poor patient concentration. However, in eyes with advanced glaucomatous field loss, damaged receptive fields exhibit profound physiological threshold fluctuations, causing high false-negative rates ($> 30–40\%$) that reflect true disease pathology rather than patient unreliability.

2. Global Statistical Indices

• Mean Deviation (MD): Represents the area-weighted average difference in sensitivity between the patient's results and age-matched normal observers across all test points:
  • Normal values range from $0\text{ to } -2.0\text{ dB}$.
  • Staging of glaucomatous field loss (Hodapp-Parrish-Anderson criteria):
    • Early Defect: $\text{MD} > -6.0\text{ dB}$
    • Moderate Defect: $\text{MD} = -6.0\text{ to } -12.0\text{ dB}$
    • Severe / Advanced Defect: $\text{MD} < -12.0\text{ dB}$
  • Limitation: Highly sensitive to generalized media opacities (nuclear cataracts, posterior capsule opacification, corneal haze) or uncorrected refractive errors.
• Pattern Standard Deviation (PSD): Measures localized, non-uniform depressions across the visual field by mathematically subtracting generalized depression from the total deviation map.
  • Elevated PSD ($p < 5\%, p < 1\%$) is the hallmark statistical indicator of localized early-to-moderate glaucomatous field loss.
  • The Biphasic Paradox: In advanced, end-stage glaucoma, when the entire visual field is uniformly destroyed, PSD paradoxically drops back toward zero because the visual field becomes uniformly flat.
• Visual Field Index (VFI): A summary metric expressing the percentage of remaining normal visual function from $100\%$ (normal field) to $0\%$ (perimetrically blind). Central macular points are weighted far more heavily than peripheral points to reflect functional vision. The VFI is resistant to cataractous media opacities and forms the basis of Guided Progression Analysis (GPA) trend charts, which calculate the linear rate of field loss in $\%\text{ per year}$.
• Glaucoma Hemifield Test (GHT): Evaluates sensitivity differences across the horizontal raphe by comparing five corresponding mirror-image sectors in the superior and inferior hemifields. Outputs include: Outside Normal Limits (superior-inferior difference exceeds $99\%$ of normal population), Borderline, General Reduction of Sensitivity, or Within Normal Limits.

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5. Glaucomatous Visual Field Defects & Topographical Anatomy

Glaucomatous visual field loss directly mirrors the anatomical trajectory of retinal ganglion cell axons entering the optic nerve head:

1. Paracentral Scotoma

A small, circumscribed, dense island of sensitivity loss located within the central $10^\circ–15^\circ$ of fixation, most commonly in the superonasal quadrant. Arises from early focal damage to inferotemporal arcuate fibers.

2. Ronne's Nasal Step

A sharp step-like difference in sensitivity across the nasal horizontal raphe. Because the arcuate axons of the superior and inferior temporal retina meet at an anatomical horizontal raphe without crossing, glaucomatous damage to the inferior arcuate bundle produces a scotoma that terminates sharply at the horizontal meridian in the nasal field, contrasting with the preserved sensitivity of the superior hemifield.

3. Seidel & Arcuate (Bjerrum) Scotomas

• Seidel Scotoma: A sickle-shaped scotoma arising from the superior or inferior pole of the physiological blind spot, arching into the nasal field.
• Bjerrum (Arcuate) Scotoma: A complete arching scotoma sweeping from the blind spot across $10^\circ–20^\circ$ of eccentricity, curving over fixation to terminate abruptly at the nasal horizontal raphe. Formed by progressive loss of an entire arcuate nerve fiber bundle.

4. Ring Scotoma & Altitudinal Defect

• Ring Scotoma: Coalescence of both superior and inferior arcuate scotomas, forming a circumferential ring of visual loss around fixation.
• Altitudinal Defect: Complete loss of an entire superior or inferior visual hemifield respecting the horizontal midline, common in advanced glaucoma or ischemic optic neuropathy.

5. End-Stage Field: Central Island & Temporal Island

In terminal glaucoma, the visual field is constricted down to two surviving areas:

1. Residual Central Macular Island ($5^\circ–10^\circ$): Subtended by papillomacular bundle axons entering the temporal optic disc margin.
2. Peripheral Temporal Island of Vision: Subtended by nasal retinal ganglion cell axons entering the nasal aspect of the optic disc. Residual islands reflect the pattern of axonal loss; their presence is not evidence of universally greater biological resistance of nasal fibres.