4. VF testing: Indices of VF loss

What do the reliability indices on Humphrey visual field testing measure, and what is the ideal result?

They assess how trustworthy the test is:

• Fixation losses = poor fixation during testing

• False positives = patient responds when no stimulus is presented

• False negatives = patient misses a stimulus they should likely see

What do the threshold values on Humphrey visual field testing represent, and what unit is used?

Threshold values are the measured retinal sensitivities at each tested location in the visual field.

• They are reported in decibels (dB)

• Higher dB = greater sensitivity

• dB is a logarithmic scale

• 1 log unit = 10 dB

How should you think about the grayscale and numeric formats together on Humphrey visual field testing?

• The numeric threshold values give the exact retinal sensitivity at each point

• The grayscale gives a quick visual summary of defect location, size, and depth

• Together, they help you identify where the defect is and how severe it is

What is a false positive error on Humphrey visual field testing, and what kind of patient does it suggest?

A false positive occurs when the patient responds even though no stimulus was presented.
This suggests a “trigger-happy” patient who is over-responding. Greater than 33% false positives is considered unreliable/abnormal.

How are false positive errors estimated in SITA perimetry, and how can they be detected in projection perimetry?

• In SITA testing, false positives are estimated using reaction time. If the patient clicks faster than a true visual response would realistically allow, it suggests a false positive.

• In projection perimetry, the machine may simulate presenting a stimulus (e.g., projector movement/noise) without actually showing one; a response to that is recorded as a false positive.

What is a false negative error on visual field testing, and what does it usually indicate?

A false negative occurs when a stimulus brighter than the patient’s measured threshold is presented, but the patient does not respond. Greater than 33% false negatives is considered abnormal.
This may indicate:

• Inattention

• Fatigue

• Poor test-taking reliability

Why are false negative errors often higher in glaucoma patients or in eyes with field loss?

False negatives are often increased in glaucomatous or damaged visual fields because these eyes show greater threshold variability from point to point and from test to test.
So a missed suprathreshold stimulus may reflect true fluctuation in damaged areas, not just poor patient reliability.

How should false positive and false negative errors be interpreted clinically?

• False positives mainly suggest patient over-response / unreliable performance

• False negatives may suggest inattention, but in glaucoma they may also reflect advanced field loss and threshold variability

What does a fixation loss indicate on Humphrey visual field testing, and how is it measured?

A fixation loss suggests the patient is not maintaining gaze on the central fixation target.
It is commonly measured using the Heijl-Krakau method: if the patient responds to a stimulus presented in the physiologic blind spot, a fixation loss is recorded.

What fixation loss rate suggests unreliable visual field data?

>20% fixation losses suggests the test may be unreliable.

What are the reliability indices in automated perimetry, and what does each one assess?

• False positives → patient responds when no stimulus is presented (trigger-happy)

• False negatives → patient misses a brighter-than-threshold stimulus (inattention, fatigue, or damaged field)

• Fixation losses → patient is not steadily fixating on the target

Why should visual field results not be discarded solely because reliability indices are abnormal?

Reliability indices are helpful but imperfect, and may themselves be less reliable than hoped. So visual field tests should not usually be rejected based only on reliability indices; interpretation should also consider the overall pattern, clinical context, and repeatability.

What do upward vs downward deflections on the gaze tracking trace mean?

• Upward deflections = degree of gaze error during stimulus presentation

• Downward deflections = unsuccessful gaze tracking at that moment, often from blinks or lid interference

How should abnormal downward deflections on gaze tracking be interpreted clinically?

Frequent downward deflections usually indicate loss of tracking signal, commonly due to:

• Blinks

• Eyelid interference

• Other interruptions of eye monitoring

This does not necessarily mean true fixation loss, but it can reduce confidence in test quality.

How does fixation monitoring differ between Humphrey Field Analyzer (HFA) and Octopus perimetry?

• HFA reports fixation losses (via the Heijl-Krakau method) and provides a gaze tracker trace.

• Octopus typically does not report fixation losses or a gaze tracker in the same way; instead, it uses Fixation Control, which pauses/interruption the test whenever adequate fixation is not maintained.

What is the main principle of Octopus Fixation Control?

Octopus uses real-time fixation control to prevent fixation losses rather than simply recording them.
The test is paused when conditions are not adequate and resumes when fixation is acceptable again.

What mechanisms are included in Octopus Fixation Control, and what does each prevent?

• Blink control → pauses testing when the eye is closed; prevents missed stimuli from blinking

• Contact control → ensures proper head/chinrest contact; prevents loss of alignment and helps reduce lens rim artifact

• Pupil position control → detects an off-centered pupil; ensures proper gaze direction

• Dart control → detects rapid eye movements/searching behavior; prevents fixation loss from unstable fixation

• Automated Eye Tracking (AET) → automatically re-centers/adjusts eye position to maintain optimal alignment

What are the clinical advantages of Octopus fixation control?

It improves test reliability by:

• Preventing missed stimuli during blinks

• Allowing blinking more normally → better comfort / less dryness

• Ensuring proper head position and pupil centration

• Reducing artifacts from off-centered eye position or trial lens rim

• Maintaining fixation even with slight patient movement

What does the Total Deviation Plot on Humphrey visual field testing represent?

The Total Deviation Plot shows, in dB, the difference between the patient’s measured sensitivity and the age-matched normal value at each test point.

• Negative values = worse than normal sensitivity

• Probability symbols indicate how likely that point is to be normal

What does the Pattern Deviation Plot represent, and how is it different from the Total Deviation Plot?

The Pattern Deviation Plot shows deviations from age-matched normal after correcting for any generalized change in overall sensitivity.

• The software adjusts the whole “island of vision” up or down

• This removes diffuse depression or generalized elevation

• It highlights localized defects more clearly than the Total Deviation Plot

Why is the Pattern Deviation Plot often considered the most useful analysis on a single Humphrey field printout?

Because it helps detect subtle local defects that might otherwise be hidden by generalized depression of the field.
This makes it especially useful for distinguishing:

• Diffuse/global loss (e.g., cataract) from

• Localized loss (e.g., glaucoma)

Why is the Pattern Deviation Plot often considered the most useful analysis on a single Humphrey field printout?

Because it helps detect subtle local defects that might otherwise be hidden by generalized depression of the field.
This makes it especially useful for distinguishing:

• Diffuse/global loss (e.g., cataract) from

• Localized loss (e.g., glaucoma)

What does Mean Deviation (MD) measure on a Humphrey visual field, and what kind of loss does it reflect best?

Mean Deviation (MD) is the center-weighted average of the total deviation values across the visual field. It reflects overall/generalized depression of sensitivity rather than focal defects.

What MD value is classically associated with severe visual field loss or legal blindness on a 30-2 field?

On a Humphrey 30-2, an MD around -22 dB is classically associated with legal blindness.

What does Pattern Standard Deviation (PSD) measure, and what type of defect does it highlight?

PSD reflects the extent of localized irregularity in the visual field: i.e., how much the shape of the field departs from normal. It highlights localized defects rather than generalized depression.

How does PSD behave in normal fields, moderate localized loss, and near-total blindness?

PSD is:

• Near zero in normal fields

• Highest in moderate localized field loss

• Near zero again in end-stage/generalized loss or near blindness

Why can’t PSD be used reliably as a staging or progression index?

Because PSD does not increase linearly with worsening disease:

• It rises when localized defects develop

• But then may fall again when the field becomes diffusely depressed in advanced disease

What is the Octopus equivalent of Humphrey Pattern Standard Deviation (PSD)?

The Octopus equivalent is Loss Variance (LV), and in many modern Octopus/EyeSuite analyses it is commonly represented as sLV (square root of loss variance).

What does the Glaucoma Hemifield Test (GHT) evaluate on a Humphrey visual field?

The GHT compares pattern deviation probability scores in 5 zones of the superior field with their mirror-image zones in the inferior field. It is designed to detect the superior–inferior asymmetry that is typical of glaucoma.

What are the possible Glaucoma Hemifield Test (GHT) results, and which result is least useful clinically?

GHT can report:

• Within normal limits

• Outside normal limits

• Borderline

• Overall reduction in sensitivity

• Abnormally high sensitivity

Why is the GHT useful in glaucoma, and why is it less affected by cataract than raw global depression measures?

GHT is useful because glaucoma often causes localized asymmetric defects between the superior and inferior hemifields. Because GHT uses pattern deviation information, it is less sensitive to diffuse depression from cataract or generalized media opacity than measures based mainly on overall sensitivity loss.

What is the Visual Field Index (VFI), and how is it interpreted?

VFI is a staging/progression index expressed as a percentage:

• ~100% = normal visual field

• ~0% = blind field

It is center-weighted, so central points contribute more heavily.

How does VFI differ from Mean Deviation (MD), and why is VFI often preferred for progression analysis?

• MD reflects overall/generalized depression of the field

• VFI is also a staging index, but it uses pattern deviation plot information and is less affected by cataract and generalized visual field loss

What visual field metric is used for trend analysis in the Humphrey GPA report?

VFI is the index used for trend analysis in the GPA (Guided Progression Analysis) report.
This is because it better reflects glaucomatous functional change while being less influenced by diffuse media opacity than MD.

What is Guided Progression Analysis (GPA) on Humphrey visual field testing?

GPA is Humphrey software used to detect visual field change over time.
It uses:

• Event analysis = asks whether the field is significantly worse than baseline

• Trend analysis = quantifies the rate of change over time

What is the difference between event analysis and trend analysis in GPA?

• Event analysis asks: Has this field worsened compared with baseline?

• Trend analysis asks: How fast is the field changing over time?

What does “Possible Progression” mean on Humphrey GPA event analysis?

Possible Progression means 3 or more test points show significant deterioration compared with baseline on 2 consecutive exams.

What does “Likely Progression” mean on Humphrey GPA event analysis?

Likely Progression means 3 or more test points show significant deterioration compared with baseline on at least 3 consecutive exams.

Exam pearl:

• 2 consecutive tests = possible progression

• 3 consecutive tests = likely progression

What do the GPA event analysis symbols mean on the Glaucoma Change Probability Map?

• Open/light triangle = P < 5% deterioration at that location on one test

• Half-filled triangle = P < 5% deterioration on 2 consecutive tests

• Filled black triangle = P < 5% deterioration on 3+ consecutive tests

• X = out of range / cannot be analyzed for significant change

Why do GPA event analysis symbols become progressively darker/more filled over time?

Because worsening at the same point becomes more repeatable across consecutive tests, making true progression more likely rather than random fluctuation.

Before calling visual field loss or progression real, how many reliable fields are usually needed?

You need more than one reliable, repeatable visual field showing the same finding before confirming true loss or progression.

What is the best general approach to evaluating progression of visual field loss?

Do not rely on a single field or only the most recent test. Instead:

• Review all available fields over time

• Consider all indices/plots together

• Correlate with other testing if needed (e.g., OCT)

What is the first step in systematically interpreting a visual field?

First ask: Is the field reliable?
Check:

• Fixation losses

• False positives

• False negatives

• Other reliability clues (e.g., gaze tracking)

After confirming reliability, how should you systematically approach visual field interpretation?

A practical sequence is:

1. Assess reliability

2. Compare both eyes together (OS left, OD right): normal or abnormal?

3. Review global indices such as MD, PSD, VFI

4. Decide whether loss is in one eye or both

5. Determine the location and shape of the defect

6. Assess for progression using multiple tests (e.g., GPA)

Why are laterality and defect shape important in visual field interpretation?

They help localize pathology:

• One eye only → suggests a pre-chiasmal problem

• Both eyes / congruous defects → raises concern for a post-chiasmal lesion

• The location and shape of the defect provide clues to the site of disease

When should you pursue more testing or consultation during visual field interpretation?

If the field is uncertain, unreliable, non-repeatable, or does not fit the clinical picture, consider:

• Repeating VF testing

• Adding other tests such as OCT

• Seeking consultation when lesion localization or progression remains unclear