Functional Visual Loss

Nonphysiological or functional visual loss can be either subconscious (hysteria) or deliberate and willful (malingering). Commonly encountered neuro-ophthalmic complaints include visual impairment or complete loss of vision, visual field defects such as constricted fields, and monocular diplopia. When confronted with a patient whose complaints and examination do not seem to correlate (subjective/ objective mismatch) or whose visual deficit is nonphysiological, there are several clinical tools that help determine whether these findings are nonorganic.

The following clinical tests, when abnormal, are useful in a patient with suspected nonphysiological visual loss: (1) lack of a normal linear improvement of Snellen visual acuity with decreasing distance or increasing letter size (for example, a patient who correctly identifies a 20/100 letter at 20 feet should equally identify a 20/50 letter at 10 feet); (2) presence of normal color vision and stereoacuity despite severely affected Snellen acuity; (3) normal menace or threat reflex, in which an approaching examiner's hand or bright light directed into the eye of interest causes a blink response; (4) presence of optokinetic nystagmus using an optokinetic nystagmus (OKN) strip; (5) presence of fixation of a patient's eyes on his or her reflected image during the "swinging mirror" test; (6) suggestive Schmidt-Rimpler test result, which involves having the patient hold up his or her hand while instructed to look at it, which is a test of proprioception more so than vision. A patient with functional visual loss often looks everywhere but directly at the hand. Next, the patient is asked to touch the two index fingertips together, which, in nonorganic instances, is performed improperly; and (7) normal EEG and psychogalvanic skin responses to light. With an intact afferent visual system, light shone into a normal eye causes a dampening of the posterior dominant alpha rhythm on EEG. The psychogalvanic skin reflex uses an electrode placed on the skin to measure the sympathetic response when a bright light is shone into the eye of interest. A normal response produces a deflection; no response is expected from the blind eye. y VEPs may also be helpful in this regard, although abnormal responses can be intentionally generated.

Monocular visual loss in the absence of a relative afferent pupillary defect suggests a nonorganic etiology. Tests that require binocularity (unbeknownst to the patient) can be extremely helpful in these cases. Perception of nine of nine stereo dots in the Titmus test requires 20/20 vision in both eyes. y During the Worth's four-dot test the patient views red and green lights through a red glass over the right eye and a green one over the left. The right eye will see the red dots, and the left eye will see the green ones. It is impossible for an individual with one blind eye to see red and green dots simultaneously. In another maneuver, a prism is placed over the suspect eye while the patient reads the Snellen chart. Reading continues uninterrupted if the eye is blind. If vision is really normal in that eye, the patient pauses while refixating.y Perhaps the best stratagem is to fog the good eye secretively with a +10.00 diopter lens, then ask the patient to read the Snellen chart with both eyes. Any line read correctly must have been seen by the proported blind eye.

"Tunnel vision" is a classic nonphysiological visual field defect, and the patient usually complains of decreased peripheral vision. y Tangent screen testing reveals a lack of physiological expansion of the patient's perceived visual fields when the target size and distance from the screen are doubled. A patient's complaint of monocular hemianopia that upon formal visual field testing is present while

testing the involved eye, absent while testing the unaffected eye, then present again when testing under binocular conditions is also nonphysiological. Although there are identifiable causes of monocular diplopia, such as disturbances of the ocular media and some occipital lobe lesions, most are also considered nonphysiological, especially when they do not resolve with a pinhole.

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