A normal visual field is a cone: as the testing distance doubles, the linear diameter of the field at the tangent screen must also double. A field whose linear diameter is unchanged when the patient is moved from 1 metre to 2 metres is a tubular field, which is physically impossible in disease and is a positive sign of functional visual loss. Spiralling of the isopter, in which the field shrinks progressively as successive meridians are tested, reflects fatigue of effort rather than any anatomical defect and is the other classic perimetric sign.
The rest of the examination supports the same conclusion. Symmetric bilateral acuity loss to 20/200 with normally reactive pupils, no relative afferent pupillary defect, normal fundi and a normal retinal nerve fibre layer excludes clinically significant anterior visual pathway disease. Other bedside tests of functional visual loss include the fogging test, in which increasing plus lenses are placed before the better eye so that acuity measured binocularly must be coming from the supposedly blind eye, the mirror test and optokinetic nystagmus for total visual loss, and stereoscopic acuity testing, which cannot be normal if one eye truly sees nothing. A monocular hemianopia that does not change on binocular testing is another non-anatomical pattern.
These perimetric patterns are solid PGY-3 and PGY-4 neuro-ophthalmology, placing this item at the Intermediate level of RITE content. Remember that functional visual loss can coexist with genuine ocular disease, so a positive sign does not exclude a second pathology.
Incorrect Answers
- A. Retinitis pigmentosa constricts the field but the field still expands with testing distance and produces bone-spicule pigmentation with an abnormal electroretinogram.
- C. Bilateral occipital infarction produces congruous homonymous defects respecting the vertical meridian, usually with a sudden onset and abnormal imaging.
- D. Chiasmal compression gives bitemporal defects respecting the vertical midline, often with optic atrophy and a relative afferent pupillary defect.
- E. Glaucomatous loss follows the nerve fibre layer with arcuate defects and a nasal step, accompanied by disc cupping and raised intraocular pressure.
Testing Pearls
- Tubular fields do not expand with increasing test distance and are physically impossible in disease.
- Spiralling isopters on kinetic perimetry indicate fatigue of effort, not anatomical loss.
- Absence of a relative afferent pupillary defect with dense bilateral acuity loss is a key clue.
- The fogging test forces the supposedly blind eye to perform the binocular acuity task.
- Functional visual loss may coexist with genuine ocular disease.
References
- Jankovic J, Mazziotta JC, Newman NJ, Pomeroy SL, editors. Bradley and Daroff's Neurology in Clinical Practice. 8th ed. Philadelphia, PA: Elsevier; 2022. p. 2084.
- Stone J. Functional neurological disorders: the neurological assessment as treatment. Pract Neurol. 2016;16(1):7-17.
- Espay AJ, Aybek S, Carson A, Edwards MJ, Goldstein LH, Hallett M, et al. Current concepts in diagnosis and treatment of functional neurological disorders. JAMA Neurol. 2018;75(9):1132-1141.
