REFERENCE · DIAGNOSTIC TECHNIQUE

Visual Field Comparisons

Clinician-focused educational reference. These comparisons require specialist training and are not home tests.

Purpose and Suitability
Anatomy and Physiology
Preparation and Safety
Performing the Assessment
Interpreting Findings
Explaining the Result
Evidence and Limitations
Media and Accessibility
Research and Sources

Purpose and Suitability

Use field comparisons to investigate a described area of missing vision and look for a reproducible mismatch within the complete visual examination.

Visual field: The area visible while looking steadily at a central point. Fixation: Keeping the eyes directed at that point.

Ask whether the measured field pattern corresponds to a difficulty the person actually experiences. A field sign is not itself a measure of symptomatic disability.

Anatomy and Physiology

For the same angular field, its physical width on a flat screen grows as viewing distance increases: doubling distance approximately doubles that width. This does not mean the angular field doubles. [2]

Monocular: One eye viewing. Isopter: A boundary joining locations where a particular moving test target is detected.

Field geometry and each eye’s contribution are the basis of these comparisons, not a direct measurement of attention or a universal FND mechanism.

Preparation and Safety

Explain the task, demonstrate the response and check access to the fixation target. Record distance, eye tested, correction, target and method. Support stable, comfortable positioning; agree breaks and a stop signal. These documentation and consent prompts are editorial safeguards.

Never substitute surprise obstacles or threatening objects for field testing. Stop if the person is distressed, fatigued or unable to maintain a usable setup.

Performing the Assessment

Tubular visual fields

A field retaining the same physical width at different viewing distances conflicts with the expected widening of a visual field. It can support functional visual loss, but careful technique, understanding and ocular assessment remain essential. [1]

A trained examiner compares a tangent-screen or appropriate confrontation field at measured distances, maintaining fixation and suitable target conditions. Record physical extent separately from angular extent. [2]

Binocular versus monocular fields

The specialist checks whether binocular vision compensates for a field deficit in the expected way. A reproducible inconsistency may demonstrate preserved function. Interpretation depends on the claimed deficit, fixation and complete eye examination. [2]

Test the relevant monocular fields, then compare with both eyes open. Compensation is expected only where the other eye has usable overlapping vision; a brain-related defect affecting the same side of both eyes is a different situation. [2]

Spiralling, crossing or stacked isopters

Formal field testing may produce contours that change or cross unexpectedly. These patterns can support a functional formulation, but fatigue, poor technique and eye disease can also produce unreliable fields. A cloverleaf pattern alone is insufficient. [1]

Interpreting Findings

Potentially supportive: State the precise repeated discrepancy and explain why the particular field geometry or binocular comparison is informative.

Not demonstrated: An expected field pattern does not alone settle the cause of every visual symptom.

Indeterminate: If fixation, comprehension, fatigue, equipment or other disease prevents a reliable comparison, record the uncertainty. An unreliable plot is not automatically a functional diagnosis.

Preserve the plots and testing conditions so another clinician can understand the finding. The field examination cannot establish whether the person can navigate a busy street or sustain a working day. Ask about the actual environment and support needs.

Explaining the Result

Authored explanation example:

“We compared your field under different viewing conditions. This particular difference helps us understand how vision is functioning during the test. I will show you the comparison and explain what it supports, what it does not explain and whether anything else needs checking.”

Do not describe a plot as proof that a person is pretending.

Evidence and Limitations

These are clinical comparisons described in the cited reviews, not an interchangeable set of validated standalone tests. No universal accuracy estimate is assigned. The inherited warning about cloverleaf patterns and unreliable fields remains explicit. [1][2]

Interpretation requires the complete eye and neurological assessment. Further primary-study appraisal is needed before making test-specific diagnostic-accuracy claims.

Media and Accessibility

A static diagram could show the same angular field at two distances with physical width labelled in separate units. Do not draw two equally sized circles and label them normal expansion. Add a second diagram only if it clearly distinguishes monocular overlap from a defect shared by both eyes. Provide text alternatives and do not rely on colour alone. No media is supplied.

Research and Sources

Citation	Full citation
[1]	Ramsay N, McKee J, Al-Ani G, Stone J. How do I manage functional visual loss. Eye. 2024;38:2257–2266. FND-CIT-0024. https://doi.org/10.1038/s41433-024-03126-w
[2]	Raviskanthan S, Wendt S, Ugoh PM, et al. Functional vision disorders in adults: a paradigm and nomenclature shift for ophthalmology. Survey of ophthalmology. 2022;67(1):8-18. DOI. PMID: 33737039. FND-CIT-0179.