REFERENCE · DIAGNOSTIC TECHNIQUE

Visual Acuity Comparisons and Fogging

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

Assess a defined complaint of reduced visual acuity using selected optical comparisons within a complete eye examination.

Visual acuity: The ability to distinguish fine detail, commonly measured with letters or symbols on a chart.

Distance comparison and fogging address different questions. Fogging is chiefly useful when assessing marked loss in one eye with useful vision in the other. It is not a general test for photophobia or every form of double vision.

Anatomy and Physiology

Changing chart distance changes the size of its image at the eye. A nearer letter subtends a larger angle; record distance rather than treating chart labels as unchanged. Fogging uses a lens to blur the better-seeing eye while both eyes remain open. [2]

Refraction: Measurement of the lens correction needed to focus an image. Binocular: Using both eyes.

These are optical comparisons. They do not directly measure a proposed brain mechanism for FND.

Preparation and Safety

Agree the question, explain the lenses and obtain consent. Check that the person can use the selected chart and understands the response required. Use suitable correction, accessible symbols or communication support, safe seating and breaks. These are teaching safeguards, not a validated scoring system.

Do not remove the person’s usual supports outside the supervised comparison. Stop for discomfort, distress or withdrawn consent. A difficult or unfinished test is not evidence of deception.

Performing the Assessment

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]

Record the chart, correction, starting distance and measured acuity. Repeat at a measured nearer distance using the appropriate chart interpretation. Compare the result with the optical change expected, considering comprehension and reliability. Do not assign a diagnosis from an isolated unchanged answer. [1]

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]

The specialist establishes each eye’s baseline, then uses plus lenses over the better-seeing eye during binocular chart reading. Lens power is adjusted and its effect verified so the examiner can interpret which eye supports reading. Record lenses, correction and responses; no fixed home lens recipe is supplied. [2]

Interpreting Findings

Supportive comparison: Document the specific visual capacity demonstrated and why the optical setup supports that conclusion. Keep the conclusion restricted to the tested task.

No supportive discrepancy: This comparison has not demonstrated positive evidence; it does not independently exclude FND.

Indeterminate: If the lens effect, correction, chart access or reliability is uncertain, the result cannot answer the question. Reconsider the setup and differential rather than increasing pressure to perform.

Stereopsis and other binocular methods have separate limitations; see the inventory. Reading several letters does not establish reading endurance, safe travel or independence. Ask about the daily task that remains difficult.

Explaining the Result

Authored explanation example, not a patient quotation:

“The lenses let us compare how your eyes contribute to this reading task. I can explain the particular response we observed and what it tells us. It does not mean that the difficulty you experience is imaginary or that every aspect of your vision has been tested.”

If the setup was inconclusive, say so explicitly and explain the next assessment step.

Evidence and Limitations

The cited clinical reviews describe these methods; this educational sequence is not a validated battery and has no assigned sensitivity, specificity or severity score. Do not convert a positive comparison into a judgment about intention. The full review of each historical validation study remains pending. [1][2]

Record what the finding leaves unanswered, including coexisting disease and sustained visual use. See the overview.

Media and Accessibility

A useful static illustration would label the two eyes, lens position and chart distance. Show what the equipment does without staging an abnormal patient response. Caption any demonstration, provide a transcript and describe the visual layout in words. Filming requires consent and specialist review. 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.