Selection of the appropriate tests of the afferent visual pathway will depend on whether the dysfunction appears to be visual acuity loss (complete or partial, bilateral or monocular) or a visual field defect.
Bilateral no light perception
Several tests, visual and nonvisual, are used when a patient reports complete bilateral blindness (ie, no light perception). A patient’s inability to perform nonvisual tasks may provide evidence of a nonorganic component to that patient’s report of total blindness. Proprioceptive testing, such as failure to sign a paper or adequately perform a finger-to-nose test, in the absence of systemic neurologic diseases, should alert the clinician to problems (conscious or unconscious) with patient cooperation.
In this test, the patient is asked to touch the fingertips of each hand together. A truly blind person can easily touch the fingertips together, because it requires proprioception, not vision (Fig 13-1). Therefore, a patient who claims to have no vision but cannot touch the fingertips of each hand together is responding in a nonphysiologic fashion.
The presence of normal pupillary reactions suggests that the anterior visual pathways are intact. However, it does not prove the loss is nonorganic; there could be involvement of the bilateral anterior visual pathways or the pathways posterior to the branching of axons to the pretectal nuclei (eg, lateral geniculate nucleus, optic radiations, or occipital cortex). An aversive light reaction by a patient who claims blindness establishes at least some level of afferent input.
Optokinetic nystagmus drum
Perhaps the easiest test to perform is to slowly rotate an optokinetic nystagmus (OKN) drum in front of the patient. If the patient claims to see nothing but the patient’s eyes move with the drum, a nonorganic component has been established. It is possible for malingerers to purposely minimize or prevent the response by looking around or focusing past the drum.
Figure 13-1 Finger-touching test. The patient is asked to touch the tips of the index finger of each hand together. A, A truly blind patient can easily perform this task. B, A patient with nonorganic visual loss may demonstrate the inability to touch the fingers together.
(Courtesy of M. Tariq Bhatti, MD, and Mays A. El-Dairi, MD.)
In this test, a large mirror is slowly rotated side to side in front of the patient. As in the drum test, if the patient claims to see nothing but the examiner notes eye movement with the mirror, then a subjective–objective mismatch has been documented.
Flash and pattern-reversal visual evoked potentials (VEPs) can play a role in assessing a possible nonorganic disorder. Although both false-positive and false-negative results are possible, the results can be used to confirm organic disease, for example a VEP with increased latency and decreased amplitude. Normal VEP results for a patient who reports severe monocular or binocular vision loss and who has normal clinical examination findings support a diagnosis of nonorganic disturbance of vision. An abnormal pattern-reversal visual evoked response in a patient who has normal findings on neuro-ophthalmic examination should not, by itself, lead to a diagnosis of an organic defect. A patient can use a variety of techniques (eg, inattention, lack of concentration, meditation, defocusing) to willfully suppress a visual evoked response (see the section “Visual evoked potential” in Chapter 3).
Monocular no light perception
All of the tests described for bilateral no light perception may be performed unilaterally. The clinician may also employ tests that involve ocular viewing confusion or tests that require binocularity.
Relative afferent pupillary defect
In patients who claim complete vision loss in 1 eye only, the absence of a relative afferent pupillary defect (RAPD) substantially increases, but does not confirm, the likelihood of nonorganic visual loss when eye examination findings are normal (see Chapter 3).
Base-out prism test
For this test, the clinician places a 4–6 prism diopters (D) prism base-out in front of 1 eye while the patient keeps both eyes open; normally, this procedure elicits an inward shift of that eye (either as a conjugate saccade followed by a convergent movement of the opposite eye or by a convergent movement alone). Movement that occurs when the prism is placed over the eye with worse vision (“bad eye”) indicates vision in that eye.
Vertical prism dissociation test
A 4 Δ prism is placed base-down in front of the eye with better vision (“good eye”) of a patient who reports monocular vision loss. If the subject has symmetric vision in both eyes, 2 images should be seen, 1 above the other. If the subject is able to see the letters only with the “good” eye, then only 1 image should be seen (Fig 13-2).
Golnik KC, Lee AG, Eggenberger ER. The monocular vertical prism dissociation test. Am J Ophthalmol. 2004;137(1):135–137.
For confusion tests to be successful, the patient must be unaware of which eye is actually being tested. Testing must appear to be simply a normal part of the examination. Any suspicion on the part of the patient will be detrimental to the test interpretation.
In this test, the clinician employs a trial frame with plus and minus cylinder lenses (6 diopters [D]), which have their axes aligned; these are placed in front of the patient’s “good” eye. The patient is asked to read the Snellen chart while one of the cylinder lenses is rotated. The rotation will severely blur vision as the two axes are rotated out of alignment. If the patient continues to read, he or she is doing so with the “bad” eye (Fig 13-3).
The examiner places duochrome (Worth 4-dot) glasses over the patient’s eyes, with the red lens over the “bad” eye. The red-green filter is then placed over the Snellen chart, presenting the letters simultaneously either on a green or a red background. The green lens will prevent the good eye from seeing the red background letters. If the red letters are read, the patient is reading with the “nonseeing” eye (Fig 13-4).
Polarized Lens Test
The patient wears the polarized lenses that are used in the stereoacuity test while reading a chart specially projected with corresponding polarized filters. The clinician can ask the patient to read specific letters or lines to determine whether the patient is reading with the “bad” eye.
Figure 13-2 Monocular vertical prism dissociation test. A, With nonorganic vision loss, the patient will describe seeing 2 images, 1 over the other. B, True organic vision loss will render the patient unable to see the second image or to see only a very blurred second image.
(Courtesy of Lanning Kline, MD.)
Figure 13-3 Fogging technique demonstrating the use of paired cylinders. A, A trial frame with +6D and –6D cylinder lenses are placed at parallel axes in front of the “good” eye. B, As the patient reads the Snellen chart, a small turn of the cylinder axis for 1 lens makes the axes no longer parallel and blurs the image seen by the “good” eye, so that the patient is now reading with the “bad” eye.
(Illustration by Mark Miller.)
Figure 13-4 Duochrome test. A, A patient with nonorganic visual loss wears red-green glasses (red over the left eye) and views a duochrome chart, unknowingly reading both sides of the chart. B, A patient with organic visual loss of the left eye is unable to read the left side of the chart.
Stereopsis requires binocular vision. Patients may be tested for stereopsis with the standard stereoacuity test (using their appropriate near correction). Any evidence of stereopsis demonstrates that vision is present in the “bad” eye. It should be noted that patients with vision in only 1 eye may be able to detect asymmetries in the first several circles on the basis of monocular clues. However, if the circle is “standing out from the page,” the patient has binocularity.
Monocular reduced vision
Patients who report symptoms of reduced vision are more challenging. The clinician must convincingly demonstrate that the patient has better visual acuity than initially claimed. Many of the tests described for patients with binocular or monocular no light perception can also be used for patients with monocular (and binocular) reduced vision.
The confusion tests described in the section “Monocular no light perception” are useful if the patient reports substantial reduction in visual acuity in 1 eye. The fogging test, duochrome test, and polarized lens test may help the clinician obtain a quantitative visual acuity measurement if the patient is cooperative enough to continue reading. Unlike the assessment of a patient who claims monocular blindness, in this case, simply demonstrating that the patient can read is not sufficient; the clinician must determine the actual visual acuity.
As previously stated, the presence of stereopsis indicates that the patient has at least some vision in both eyes. Although attempts have been made to equate visual acuity and quantitative stereopsis, this relationship suggests, but does not establish, nonorganic vision loss (Table 13-2).
Table 13-2 Comparison of Visual Acuity to Stereopsis
Binocular reduced vision
Binocular reduced vision is the most difficult symptom to address in patients with non-organic vision loss. Proving a nonorganic disorder requires time and patience from the examiner to induce the patient to admit to seeing better than initially claimed.
This examination begins with a visual acuity determination on the smallest line on the Snellen chart (20/10). If the patient cannot see these letters, the examiner announces the use of a “larger line” and then switches to the 20/15 line and several different 20/20 lines. The examiner continually expresses disbelief that such “large” letters cannot be identified. If the patient still denies being able to read the letters, he or she is asked to determine the number of characters present and whether the characters are round, square, and so on. The examiner might suggest that the characters are letters and that the first one is easier to identify than the others. By the time the “very large letters” (ie, 20/50) are reached, the patient often can read optotypes much smaller than initially identified.
Alternately, small (⅛ D) plus and minus lenses or small cylinders are added and subtracted while the patient is asked how many letters are visible and what shape they are. It is sometimes possible to gradually improve the best-recorded visual acuity with this method.
The examiner can have the patient wear trial frames with 4 lenses equaling the correct prescription but suggest that they are special magnifying lenses that might enable improved vision. The potential acuity meter can also be presented as a means of “by-passing the visual block.” Improvement in either case suggests a nonorganic component.
Use of alternative charts
Patients may be persuaded to see substantially better by a switch in optotypes. For example, a patient who refuses to read type smaller than 20/200 using standard optotypes might read much better from a tumbling E chart or a chart with numbers.
Specialty charts with the top line in 50 optotype instead of 400 optotype are available. Patients who say they can read only the “top line” immediately improve their resolution by 4 lines. Alternatively, the standard chart may be moved farther away.
Visual field defect
Although less common than visual acuity loss, patients do occasionally report difficulty seeing on 1 side. The problem may be binocular but is more commonly monocular with nonspecific constrictions.
Automated perimetry testing
In general, this type of testing is not helpful in distinguishing organic from nonorganic visual field loss. The machines are quite easy to fool; if motivated, an observer can reproduce homonymous defects, altitudinal defects, or even arcuate and central scotomata. Nonetheless, central scotomata are extremely rare in nonorganic vision loss and warrant further evaluation. There are no characteristic changes in automated perimetry testing results that would confirm the suspicion of a nonorganic deficit. However, one unusual situation in which automated perimetry testing might be useful would be for a patient with a monocular defect that appears to respect the vertical midline. If repeating the field testing with both eyes open produces a similar, or even incomplete or “missing-half,” defect, then a nonorganic component is present (Fig 13-5).
Frisen L. Identification of functional visual field loss by automated static perimetry. Acta Ophthalmol. 2014;92(8):805–809.
Sitko KR, Peragallo JH, Bidot S, Biousse V, Newman NJ, Bruce BB. Pitfalls in the use of stereoacuity in the diagnosis of nonorganic visual loss. Ophthalmology. 2016;123(1):198–202.
This test is useful in the case of a dense visual field defect. The area where the patient “can’t see” is carefully identified. Later, the patient is tested for “motility.” As part of this examination, stimuli are placed in various areas of the patient’s peripheral field, including those areas where the patient “couldn’t see.” Accurate saccades to these nonauditory targets indicate an intact visual field.
Figure 13-5 “Missing-half” field defect. A 33-year-old man complained of decreased vision temporally in the right eye after being in a motor vehicle accident. Automated perimetry testing demonstrates a normal field in the left eye (A) and a temporal defect in the right eye (B). Visual field testing performed with both eyes open (C) demonstrates persistence of the visual field defect, indicating a nonorganic basis for the visual complaint.
(Courtesy of Karl C. Golnik, MD.)
In some cases, confrontation testing (“silent visual fields”) might initially appear to confirm a dense visual field defect. The patient is subsequently asked to count fingers in the “nonseeing” field and is instructed to report “none” when no fingers are seen. As the test progresses, the examiner begins showing fingers without saying anything. A response of “none” each time the fingers are put up confirms vision in that area.
Kinetic perimetry testing
In manual kinetic perimetry (Goldmann) testing, the visual field is tested continuously in a clockwise or counterclockwise direction starting with the I4e stimulus. A common nonorganic response shows a spiraling isopter getting closer and closer to fixation as testing continues. As larger stimuli (III4e and V4e) are employed, there is often further constriction, resulting in overlapping isopters (Fig 13-6). The latter can also be detected with automated kinetic perimetry. It is important to make sure there is no step across the vertical or nasal horizontal midline; a step across the midline may indicate that at least some physiologic component is present in the field abnormality.
Tangent screen testing
In tangent screen testing, the patient is tested with a 9-mm white stimulus at 1 m. The areas of patient response are marked and the patient is moved back to 2 m. The test is then repeated using an 18-mm white stimulus. The field should expand to twice the original size. Failure to expand (tubular or gun-barrel field) is nonphysiologic and indicates a nonorganic component to the field constriction (Fig 13-7).
Figure 13-6 A 59-year-old woman complained of 1 year of headaches. A, B, Automated static perimetry using a V-sized test object demonstrated severe constriction. C, D, Subsequent kinetic perimetry visual field testing revealed crossing and spiraling isopters, indicating non-physiologic constriction of the visual fields.
(Courtesy of Steven A. Newman, MD.)
Figure 13-7 The tangent visual field test at 1 and 2 meters (m). The visual field normally expands on increasing the testing distance from 1 to 2 m. This is due to the increase in the visual angle from X° to 2X°. In a patient with nonorganic vision loss, the normal expansion of the visual field is absent and remains contracted despite the increase in testing distance.
(Based on an illustration by Swaraj Bose, MD.)
Excerpted from BCSC 2020-2021 series: Section 5 - Neuro-Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.