Perimetry Viva: Reading a Field Printout Out Loud, Correctly

The safest visual-field viva answer has seven moves: identify the test, judge whether it is interpretable, locate the loss, compare total with pattern deviation, describe the meridian relationship, state the global indices, then correlate it with structure and the rest of the examination. A grey scale is an overview, not a diagnosis. A cluster of probability symbols is evidence to explain, not a licence to announce glaucoma or a neurological lesion from one printout.

This is postgraduate examination education for doctors. It is not a patient-specific diagnostic or management algorithm. Automated perimetry programmes, printout layout and reliability flags vary by perimeter and software version; name what is printed in front of you.

The one-minute order: do not begin at the grey scale

When an examiner places a Humphrey-style field in front of you, start at the header. The ordered approach below is close to the sequence set out in a peer-reviewed visual-field teaching paper, but the wording is designed for an oral station rather than silent pattern recognition. AAO EyeWiki’s automated-perimetry reference is also useful because it labels the usual components of a standard printout.

The point of this order is not to sound ceremonial. It stops two common errors: describing the field of the wrong eye and calling an unreliable or diffusely depressed test a classic localised defect. The European Glaucoma Society guideline also notes a learning effect across early tests, which is a reason to be restrained about a first unfamiliar field.

Step 1: identify what was actually tested

Before interpreting a defect, say enough about the programme to make your answer falsifiable. A 24-2 and a 10-2 do not sample the same distribution of points; a screening test and a threshold test do not carry the same type of information; and a right-eye blind spot sits on the opposite side of the page from a left-eye blind spot. Do not manufacture details that are not printed.

Useful header questions are:

• Which eye is shown? Is the blind spot in the expected temporal field position?
• What programme is printed: 24-2, 30-2, 10-2 or another pattern? Is it a threshold or screening result?
• Which strategy and stimulus size are named? Read the label rather than assuming a default.
• Is a near correction, pupil value, foveal threshold, or test duration supplied? These are context, not decorative metadata.
• Is this one field, or is there a serial overview? A progression claim needs comparable tests, not a single isolated sheet.

The NCBI/StatPearls Humphrey visual-field chapter describes the different test patterns and printout zones. In a viva, it is enough to say that programme choice changes what has been sampled. Do not turn that into an unsupported claim that one programme rules out all central or peripheral loss.

Step 2: reliability is a judgement, not a ritual percentage

Reliability indices tell you how much confidence to place in the response behaviour during that test. They do not tell you whether the eye has glaucoma, optic neuropathy or a retinal disorder.

Fixation losses

On many Humphrey-style reports, fixation losses are based on responses to stimuli presented at the previously mapped blind spot. A high value can reflect unstable fixation, but it can also be misleading if the blind spot was not located well or if pathology has changed its expected appearance. Check that the blind spot is plausible before treating this number as a verdict. A printed fixation-loss flag should lower confidence; it should not make you ignore every other feature automatically.

False positives

False-positive responses occur when a response is recorded without a stimulus presentation. They deserve particular attention because trigger-happy responding can create improbably high sensitivity and distort the field’s shape. In an oral exam, say what you see: “The false-positive rate is elevated, so apparent focal preservation or unusually high sensitivities need caution.” Do not offer a universal pass/fail percentage when the printout has its own software context.

False negatives

A false negative is recorded when a patient fails to respond to a brighter re-presentation at a location previously detected at a lower intensity. Fatigue, fluctuating response behaviour and established field loss can contribute. It is therefore less useful to say “false negatives equal unreliable” than: “This value is [finding], and I will interpret it with disease severity, test duration and the remainder of the printout.”

The AAO reference lists fixation losses, false positives, false negatives and test time as the standard reliability information. Its important practical message is that high rates can cloud interpretation, not that every perimeter or candidate must use a single magic cut-off. If the report looks inconsistent with the indices, say so. The examiner is testing whether you can weigh evidence, not recite a number.

Step 3: use the grey scale as a map, then prove it with plots

The grey scale converts threshold sensitivity into a visual map: darker regions represent lower measured sensitivity. It quickly shows whether loss is central, arcuate-shaped, hemifield-predominant, peripheral or diffuse. It is also vulnerable to artefact, generalised depression and the visual drama of a dark patch. Use it to form a question, then check the numerical and probability displays.

The threshold/numerical display tells you the measured sensitivity at each tested location. The total-deviation display compares each threshold with an age-matched normative database. A negative total-deviation value means sensitivity was below the expected reference value at that point; the probability plot adds how unusual that result was in the database. A statistical abnormality is not, by itself, a disease label.

Pattern deviation is particularly valuable when a broadly depressed field may be obscuring local loss. The adjustment is intended to reveal local variation after a generalised reduction in sensitivity, such as one associated with media opacity, miosis or inadequate refractive correction. It can also become less informative when loss is very extensive. The accurate viva line is “pattern deviation helps me describe localised loss after allowing for generalised depression,” not “pattern deviation proves glaucoma.”

Step 4: describe the pattern before naming the mechanism

Most field-viva marks sit in the description. Give laterality, location, depth and meridian relationship before you volunteer a differential. Three questions organise the page.

Does it follow the horizontal meridian?

Retinal nerve fibre layer patterns and many glaucomatous defects tend to respect the horizontal meridian. A nasal step is a difference in sensitivity across the horizontal meridian on the nasal side. An arcuate defect follows the course of nerve fibre bundles and may extend from the blind-spot region towards the nasal field. A paracentral defect lies near fixation. In an exam, do not call every superior or inferior cluster “arcuate”: state whether its curve, blind-spot relationship and horizontal-meridian behaviour actually support that description.

For a glaucoma-shaped printout, a disciplined statement is: “There is a localised [superior/inferior] arcuate-pattern defect, extending [from/towards] [location], with [nasal-step/paracentral] involvement if visible, and it respects the horizontal meridian.” Then say that disc, OCT and repeat-field correlation are required. The glaucoma MCQs and study guide is the relevant broader revision page; it is not evidence that every arcuate-looking field is glaucomatous.

Does it respect the vertical meridian?

Defects that respect the vertical meridian raise a localisation question rather than a final diagnosis. A homonymous pattern requires corresponding loss in the same side of visual space across both eyes; a bitemporal pattern requires temporal loss in both eyes. You cannot honestly declare either from one eye’s field. Put both printouts side by side, confirm the eye labels and use the blind spots for orientation.

A good neuro-ophthalmology viva sentence is: “Across the two fields there is [or is not] a congruent defect respecting the vertical meridian; if confirmed and reliable, that pattern would warrant correlation with the neuro-ophthalmic examination and the clinical context.” For associated terminology and broader study, use the neuro-ophthalmology study guide. Avoid the leap from “vertical” to a named lesion when the station has supplied only a partial or unreliable field.

Is the reduction diffuse?

Diffuse depression can make the total-deviation plot look globally abnormal. Possible explanations include test performance, pupil size, refractive correction and media opacity, alongside true widespread functional loss. A pattern-deviation plot that is comparatively quiet after adjustment is useful descriptive evidence for generalised rather than focal change. It is not a licence to dismiss the result. State the observation and the need for correlation.

Step 5: global indices summarise; they do not replace the maps

Global indices make comparison faster, especially in a series, but they compress a complex field into a few values. Read the map first and quote the values exactly as printed.

MD reflects overall depression; PSD reflects irregularity. That pair is more useful than either number alone. A field with diffuse depression may have an abnormal MD without a striking local pattern. A localised defect can make PSD abnormal even when overall average loss is limited. In advanced loss, the remaining field may no longer create the kind of irregularity PSD is designed to express. The StatPearls and AAO sources explain these indices in the context of the entire report.

VFI is most useful when the printout itself presents a series or trend, but a trend line still depends on comparable, sufficiently reliable tests and clinical correlation. In a first-look viva, do not pretend that one VFI percentage measures a patient’s future course.

Three model presentations for a printout station

Use the supplied values in the brackets. The templates keep you from filling gaps with diagnoses.

1. Localised glaucoma-shaped pattern

“This is the [right/left] eye tested with [programme/strategy]. Reliability is [finding], so my confidence is [finding]. The grey scale and total deviation show a [superior/inferior] localised defect. On pattern deviation, the cluster remains [location], with a configuration consistent with an arcuate/nasal-step/paracentral pattern and respect for the horizontal meridian. MD is [value], PSD is [value], and GHT/VFI is [value if printed]. I would correlate this pattern with the optic nerve, OCT and serial fields; the printout alone does not establish the diagnosis.”

2. Vertical-meridian pattern across two eyes

“These are paired [programme] fields. I first confirm eye labels and reliability. The defects are [right/left] [homonymous/bitemporal only if supported] and respect the vertical meridian. The probability plots support [location] loss, while the global indices are [values]. This is a neuro-ophthalmic localisation pattern to correlate with acuity, pupils, colour vision, fundus and the wider clinical assessment; I would not name a lesion from the field alone.”

3. Generalised depression with uncertain local detail

“The test is [reliability finding]. Total deviation shows broad depression. Pattern deviation [largely normal/retains this localised cluster], so I would describe [generalised depression with/without a superimposed local defect]. MD is [value] and PSD is [value]. I would check the printed test conditions and correlate with ocular examination before assigning a mechanism or comparing progression.”

These answer forms earn marks because they make uncertainty visible. The examiner can see that you know where observation ends and inference begins.

Five examiner follow-ups worth rehearsing

Why is the blind spot important? It is a normal scotoma corresponding to the optic disc and helps confirm field orientation. On a standard field printout, its expected temporal position is also a quick check before you rely on fixation-loss calculations.

Why not diagnose glaucoma from a dark grey scale? The grey scale is a visual representation of measured sensitivity. It can show a useful pattern, but reliability, total/pattern deviation, disc/OCT correlation and repeatability affect its meaning.

Total deviation versus pattern deviation? Total deviation compares every tested location with age-matched reference data. Pattern deviation adjusts for generalised depression to highlight local irregularity. Neither plot independently identifies the cause.

MD versus PSD? MD summarises average depression across the field; PSD summarises irregularity of the hill of vision. Quote both in the context of the map and disease stage.

What would you do with a suspicious first field? In an examination answer: verify test quality, compare with the clinical and structural findings supplied, and seek repeatable evidence in the relevant clinical setting. That is an interpretation principle, not patient-specific medical advice.

A 20-minute printout drill

Repeat the drill with a field that looks superficially similar but has a different meridian relationship or reliability profile. That is better viva training than memorising one “classic” picture.

Errors to remove before the practical

• Reading the grey scale before confirming the eye, programme and blind-spot orientation.
• Saying a field is reliable or unreliable solely because one number crosses a remembered threshold.
• Treating false positives as a trivial detail when the sensitivities look implausibly high.
• Calling all dark regions glaucomatous without checking total and pattern deviation.
• Calling a unilateral defect homonymous or bitemporal.
• Quoting MD, PSD or VFI without first describing where the loss lies.
• Claiming progression from one printout rather than a comparable series.
• Giving patient-specific treatment or imaging instructions when the station asks only for field interpretation.

For planned report-reading revision, the OphthaMCQ report-and-test revision resource is the closest product page. Practical station rehearsal belongs with the OSCE, Practical & Viva Voce Ready bundle. These are revision resources; they do not replace supervised perimetry practice or clinical judgement.

Sources

1. AAO EyeWiki: Standard Automated Perimetry — reliability indices, printout components, total/pattern deviation and global-index context; checked 18 August 2026.
2. StatPearls: Humphrey Visual Field — programme/printout context, reliability and global indices; checked 18 August 2026.
3. European Glaucoma Society Guidelines, 5th edition — learning effect and longitudinal visual-field context; checked 18 August 2026.
4. Visual field interpretation for ophthalmologists — structured printout-reading sequence and pattern-language teaching; checked 18 August 2026.