REFERENCE · DIAGNOSIS

Functional Visual Symptoms

Refers to:

assessed functional visual loss or blindness;
blurred or reduced acuity, field or tunnel-type loss, and double-vision presentations; and
functional photophobia or difficulty accessing vision when supported by specialist assessment.

Scope boundary: Light sensitivity from migraine, eye disease or another cause is not automatically a functional visual symptom. The techniques below require ophthalmic or neuro-ophthalmic context and do not apply to generic dizziness or sensory overload.

Featured technique: Positive demonstration of preserved visual function during a complete ophthalmic or neuro-ophthalmic assessment.
Diagnostic method: Compare reported vision with objective visual behaviours or responses, such as optokinetic nystagmus, while also looking for eye and brain disease.
Media needed: An orthoptist-, ophthalmologist- or neuro-ophthalmologist-led close-up demonstration.

Diagnostic techniques at a glance

These brief entries describe signs, observations, criteria and investigations clinicians may consider. They are not a checklist of tests everyone needs. Evidence and limitations differ for each entry; a positive sign must fit the whole clinical picture, including possible coexisting disease.

The longer explanations already on this page remain below. Individual technique pages will be developed and reviewed separately.

Visual acuity at different distances

A specialist compares reading performance with the change expected when distance changes. A reproducible optical inconsistency may demonstrate better vision than the person experiences. This is a clinical demonstration with limited test-specific accuracy evidence, not a test of honesty. [1]

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]

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]

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]

Optokinetic response

A moving visual target can elicit an automatic tracking response, demonstrating some preserved visual function. It does not establish normal conscious vision or explain every deficit. Avoid intolerable moving patterns; this is a specialist observation with limited standalone validation. [1]

Mirror tracking

Following a reflected image may demonstrate visual capacity during apparent severe loss. This is a described bedside observation rather than a broadly validated diagnostic-accuracy test. Preserved tracking must be interpreted with neurological and ophthalmic findings. [2]

Fogging

The clinician blurs the better-seeing eye with lenses and assesses what can still be read. Reading beyond that eye’s available vision can demonstrate function in the affected eye. Explain the finding openly; this is not a covert test of truthfulness. [1]

Prism dissociation

Prisms separate images so reported double vision or corresponding eye movements can demonstrate input from both eyes. Interpretation depends on alignment and the specific protocol. Clinical descriptions support its use, but universal sensitivity and specificity are not established. [1]

Prism fusion response

A prism-induced eye adjustment can demonstrate binocular visual function even when double vision is not reported. This requires specialist interpretation and differs from the subjective prism-dissociation comparison. It does not exclude coexisting ocular disease. [1]

Stereopsis

Depth-perception testing can establish a minimum level of binocular function. It may contradict a particular degree of reported loss, but estimates of each eye’s acuity have limits. It does not establish that all visual symptoms are functional. [1]

Bagolini lens testing

Striated lenses produce a binocular pattern that requires input from both eyes. Seeing the expected pattern can demonstrate preserved binocular function. This is a specialist clinical method, with limited standalone diagnostic-accuracy evidence for FND. [1]

Colour or polarizing dissociation

Selected targets visible separately to each eye can reveal visual capacity not accessible during ordinary testing. These are described ophthalmic comparisons, not interchangeable validated tests; optical setup and coexisting disease matter. [2]

OCT, retinal testing and visual evoked potentials

Retinal imaging and electrical-response tests address structural or physiological differential diagnoses. They are not positive FND tests; normal results alone do not establish functional visual loss, and coexisting eye disease is common. [1][3]

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Functional visual symptoms may affect sharpness, the visual field or the sense that vision is available. The symptoms are real. Diagnosis depends on showing that some visual function is preserved in a way that does not match the reported difficulty, while also completing the examination needed to look for eye, optic-nerve and brain conditions.

One possible observation is optokinetic nystagmus: a striped target moving across the visual field can produce small tracking and resetting eye movements. That response may demonstrate that enough of the visual pathway is working to detect the moving pattern. It does not measure full visual acuity and cannot, by itself, diagnose functional visual symptoms. [1]

The clinician may choose other methods, such as fogging one eye or using a mirror, depending on the symptom. These are specialist examination tools, not tricks and not tests to repeat at home.

Visual symptoms may be episodic or prolonged

Functional visual loss, blur, field change or visual access may come and go, fluctuate with the task or environment, or remain troublesome for long periods. Busy visual scenes, fatigue, migraine, light or sensory load may intensify symptoms for some people. A symptom being absent in a quiet examination room does not show that it was not disabling elsewhere.

When a familiar visual episode starts, stop driving, cooking, using tools, navigating stairs or another activity that depends on reliable vision. Sit or move to a safe position, use the visual aid and lighting strategy already recommended, and use only a previously trained visual or attention cue. Sudden new or painful visual loss, flashes/floaters, a curtain-like shadow or visual change with acute neurological symptoms needs urgent assessment.

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Do not wave objects toward the person’s eyes, set up obstacles or surprise them to see whether they avoid something. Those demonstrations can cause falls, distress or eye injury and are not a responsible diagnostic assessment. Support the person’s current visual-safety plan and report changes to the appropriate clinician.

When vision suddenly becomes less usable

An episode may settle quickly or last much longer. Help stop visually hazardous activity, provide the person’s usual mobility or visual support and reduce unnecessary visual complexity if that is part of their plan. Do not secretly test what they can see or remove sunglasses, lenses or another aid to make them practise.

Once safe, note what the person was doing, the type of visual change, whether one or both eyes seemed affected, duration, associated headache or neurological symptoms and recovery.

New, sudden or painful visual loss still needs urgent assessment according to local medical advice, even if the person has had functional visual symptoms before.

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Episodic and prolonged presentations

Clarify whether visual symptoms are continuous, intermittent or situational; document duration, frequency, visual environment, migraine or sensory context, associated neurological symptoms, functional risk and recovery. A normal interval examination does not itself establish a functional cause, and symptom fluctuation does not replace the positive visual examination and appropriate exclusion of ocular, optic-nerve and neurological disease.

For established episodic functional visual symptoms, give an onset plan that starts with driving/fall/injury safety and adapts a previously demonstrated preserved visual response, graded visual task or environmental strategy. Do not deliberately reproduce a visually provocative episode merely to confirm the history.

Technique outline: optokinetic response within a positive visual examination

Define the complaint precisely and complete the indicated assessment of acuity, pupils, ocular movements, anterior and posterior segments, fields and neurological function before interpreting a functional sign.
Explain that the moving striped target helps demonstrate one preserved visual response and does not quantify all aspects of sight.
Seat the patient safely and present an optokinetic drum, strip or validated digital target at an appropriate distance and field size.
Move the pattern smoothly in one direction while observing both eyes for a slow following phase and quick resetting phase. Repeat in the opposite direction if clinically useful.
Relate the response only to the level of visual function it can support. Do not convert its presence into a precise acuity estimate or assume it excludes coexisting disease.
Select other positive methods—such as mirror tracking, fogging or binocular visual-field comparison—according to the presentation and the examiner’s training.
Explain the preserved function respectfully and connect it to a treatment or rehabilitation plan. Arrange reassessment when the pattern changes or the evidence is incomplete.

Media contributor brief

Use a clinician and healthy volunteer or a specifically consenting patient. Record:

A wide view showing safe seating, clinician and striped target.
The smooth movement of the target, without rapid flashing or an abrupt start.
A close-up of both eyes showing the tracking and resetting response.
A simple diagram of the slow and fast eye-movement phases.
A closing statement: “This response demonstrates some preserved visual processing. It does not measure all vision or diagnose the cause alone.”

Provide a static diagram as the default media for readers who are sensitive to moving patterns. Do not film obstacle avoidance, menace responses or any demonstration that risks startling or shaming the participant.

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Research and Sources

Evidence notes

The 2024 clinical article recommends making a positive diagnosis by demonstrating preserved vision, explaining the finding constructively and assessing possible comorbid eye or neurological disease. [1]

Citation table

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.
[3]	Scott JA, Egan RA. Prevalence of organic neuro-ophthalmologic disease in patients with functional visual loss. American journal of ophthalmology. 2003;135(5):670-675. DOI. PMID: 12719075. FND-CIT-0180.

Technique outline created: August 24, 2026 · Ophthalmology and orthoptics review pending

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