Functional (Non-Organic) Visual Loss: Tests That Help
Last updated: November 2026
Functional visual loss means reduced vision or field without an organic cause. It is a diagnosis made by demonstrating better function than claimed, not just by finding nothing wrong, and organic disease must be excluded carefully first.
When to suspect it
- Vision worse than expected from the examination; inconsistent results; normal pupils with no RAPD in claimed severe unilateral loss.
- Behaviour inconsistent with the claimed vision.
- Remember that functional and organic disease can coexist, and some organic conditions (early Stargardt, retrobulbar optic neuritis, cone dystrophy, occipital lesions) have a normal-looking fundus.
Bedside tests
| Test | How it works |
|---|---|
| Pupils | No RAPD in claimed severe unilateral loss is strong evidence of good function |
| Optokinetic drum or tape | Nystagmus in response implies at least about 6/60 to 3/60 vision |
| Mirror test | Tilting a large mirror in front of a patient claiming no light perception produces following eye movements |
| Fogging | Place a high plus lens in front of the good eye during binocular acuity testing |
| Base-out prism (4 Δ) | A refixation movement of the "bad" eye shows it sees |
| Stereopsis | Good stereoacuity requires good vision in both eyes |
| Visual fields | Tubular field that does not widen with distance (on a tangent screen at 1 m and 2 m); spiralling fields on kinetic perimetry |
Investigations and management
- OCT (RNFL and macula), and electrophysiology (pattern ERG, VEP, multifocal ERG) when organic disease is still possible.
- Explain positively: "your eyes are healthy and the vision is expected to recover", without accusing the patient.
- Children often recover with reassurance; look for stressors (school, family).
Where residents slip
- Making the diagnosis without positive evidence of better function.
- Missing early macular or optic nerve disease with a normal fundus.
- Confronting or accusing the patient.
Test yourself: flip cards
Related: Approach to Diplopia: Monocular vs Binocular and the Three-Step Test, ERG, EOG and VEP Made Easy: One Test per Level, Visual Field Reference Card: Reliability, Glaucoma Criteria and Staging.
Key sources
- Bruce BB, Newman NJ. Functional visual loss. Neurol Clin. 2010;28:789-802
- Lim SA, et al. Functional visual loss in adults and children. Ophthalmology. 2005;112:1821-1828
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