Crossing Isopters In Functional Visual Loss

ERG electroretinogram, VEP visually evoked potentials, OKN optokinetic nystagmus 206

ERG electroretinogram, VEP visually evoked potentials, OKN optokinetic nystagmus 206

Feigned Binocular Blindness

Observations of Behavior and Triggering of Reflexes

For those complaining of complete blindness, behavioral observation provides telling information. When greeted for the first time, does the patient reach for your hand, bump into obstacles, or have trouble finding the examination chair? On passage down a flight of stairs, those with functional visual loss have a problem with each step, whereas those with organic blindness will indeed experience some uncertainty with the first step, but will be able to negotiate the rest of the flight without hesitation.

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In those with functional blindness, intactness of the following reflexes can be demonstrated: the blink reflex response to rapidly approaching objects, refixation movements in response to abrupt prism placement before an eye, and optokinetic nystagmus responses to a train of moving objects (a qualitative measure of reflex intactness).

A particularly effective test is the oculomotor response to movement of a mirror that is large enough to fill most of the field of view (■ Fig. 15.1). A large bathroom mirror (or similar type) that can be held in front of the eye(s) to be tested is slowly turned back and forth on a vertical or horizontal axis. Gazing into the moving mirror, the subject will see both distant and near objects appearing to move with equal angular velocity, which will elicit ocular pursuit movements when vision is intact. The stimulus to eye movement in the mirror test is especially strong and cannot be reliably suppressed.

Similarly, the fixation target or star figure in a direct ophthalmoscope, when projected onto the fovea, presents a stimulus to fixation that is very difficult to suppress.

Complete blindness caused by disease in the anterior (pregeniculate) afferent pathways will destroy the pupillary light reflexes, both direct and consensual. Accommodative convergence (the patient's own hand can be used as a proprioceptive stimulus) or squeezing attempts at lid closure against the examiners forcible grasp to hold the eyes open will elicit pupillary constriction, regardless of visual sensory intactness. Retrogeniculate disease will usually leave the pupillary light reflexes intact during clinical testing (Chap. 5).

Deception

Tasks of coordination that require no visual input, such as pointing to one's own body parts or bringing the index fingers together, will frequently be failed by those with functional blindness, while the organically blind have a robust sense of body position based on proprioception, and can perform these tasks with no effort. Another test is to provoke saccades first verbally ("left," "right"), and then only by index finger movements.

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Fig. 15.1. Mirror-induced ocular pursuit movements. The examiner turns a large mirror slowly back and forth in the horizontal direction, while the subject gazes at the reflected image. Both distant and near objects will appear to move with the same angular velocity, eliciting eye movements that the examiner can easily see. It is almost impossible for the subject with intact vision to suppress the movements, unless he/she can fix his/her gaze on some stationary object; hence, the use of a large mirror that fills the field of view as widely as possible. The examiner must take care to note that the subject is not looking intently at some stationary object outside the margin of the reflected image

Fig. 15.1. Mirror-induced ocular pursuit movements. The examiner turns a large mirror slowly back and forth in the horizontal direction, while the subject gazes at the reflected image. Both distant and near objects will appear to move with the same angular velocity, eliciting eye movements that the examiner can easily see. It is almost impossible for the subject with intact vision to suppress the movements, unless he/she can fix his/her gaze on some stationary object; hence, the use of a large mirror that fills the field of view as widely as possible. The examiner must take care to note that the subject is not looking intently at some stationary object outside the margin of the reflected image

Objective Testing Methods for When Bilateral Blindness Is Alleged

The ERG and VEP tests are available as objective tests, when the above mentioned methods are not conclusive. Despite their objectivity, however, these tests are also subject to at least occasional false-negative results (see below).

Differential Diagnosis of Cortical Blindness

Cortical blindness or the loss of function in the primary visual cortex (area 17, Vb or striate cortex), is marked by the complete absence of conscious visual perception, e.g., complete loss of the blink reflex. Optokinetic nystagmus cannot be elicited in affected adults. The pupillary reactions to light and accommodative convergence are retained; fundus appearance and ocular motility are normal. The VEP is not always clearly diagnostic, and its reliability in this setting is controversial.

The existence of blindsight, an extrastriate, unconscious visual perception, will be mentioned here only as a potential, selective, residual perception under particular conditions (e.g., motion perception), and not as a proof for testing claims of blindness. Additional neurological symptoms in company with cortical blindness may or may not be present (e.g., hemiplegia, somatosensory loss, aphasia, disorientation). Many of these patients tend to ignore their visual loss, a form of visual agnosia, also called Anton's syndrome. Lesions of extrastriate visual cortex (areas 18, 19, and higher cortical levels) do not produce blindness (meaning that visual reflexes can be elicited), but are associated with disturbances of visual recognition (e.g., loss of visual memory, alexia, or prosopagnosia), while visual perception remains intact. (Agnosias are covered in Chap. 13).

Alleged Monocular Blindness and Monocular Visual Impairment

Testing Visual Reflexes

Pupillary reflexes. With the swinging flashlight test (see Chap. 2), asymmetry in the afferent limb of the pupillary light responses can be demonstrated. A relative defect is present when the direct response falls below the consensual response. In cases of total unilateral visual loss, an am-aurotic pupil shows no direct response to light, but retains an intact consensual response. The relative afferent pupillary defect (RAPD) is particularly important when dealing with monocular or highly asymmetric visual loss. Unilateral optic neuropathies, optic tract disease, chiasmal damage, and pregeniculate lesions are the usual causes of an RAPD. For diseases not affecting the afferent visual pathways, the swinging flashlight test is not usually helpful. This is true for example in cases of amblyopia. The swinging flashlight test will yield a normal result when obscuration of the refractive media is the cause of visual loss, except in cases of very dense vitreous hemorrhages.

When pupillary reflexes cannot be tested because of pharmacologic paresis, surgical distortion or inflammatory disease, binocular tests (see below) can be used.

Fixation reflexes. During cover testing, interrupting fixation of the healthy eye with an occluder will stimulate a refixation movement of the "bad" eye toward the object of regard.

Pearl

In cases of alleged monocular blindness, one can patch the "good eye" and then use the tests for bilateral blindness, as described above.

Binocular Tests

These are especially helpful, since one can often establish the actual visual acuity of the bad eye. There are the so-called confusion tests, because the subject being tested is not easily aware of which eye is contributing to the test results. More properly, truly binocular tests require the simultaneous contributions of both eyes, as in tests of stereoacuity and tests of responses to monocular prism introduction during binocular viewing. The relationship between stereoacuity and monocular acuity remains somewhat in dispute, but failure to detect stereopsis is in any event not very helpful. For example, subjects with poor development of stereopsis during childhood can have normal acuity in each eye, symmetrically alternating fixation, and no stereo vision at all.

Of the numerous confusion tests, only a few are given here. One can use polarizing filters, red-green glasses, or obscuration of the better eye, e.g., by progressive introduction of plus lenses during acuity testing, or when testing near vision with a plus lens before the better eye (only), sudden switching to reading distant optotypes with the "bad" eye. This will often yield a more accurate measure of acuity in the affected eye.

In the tests described by Fahle et al., computer-controlled presentation of optotypes with very short display times can be given in rapid binocular alternation, causing the test subject to lose track of which eye is being tested. However, such tests are likely to be used in referral centers only, and are not available in most practices.

Pearl

If during near tests of reading one quickly introduces a vertical opaque ruler over the nose, the nasal halves of each visual field will be obscured. In this case, both eyes are seeing monocularly, and no impairment of reading fluency will be demonstrated in subjects who have normal vision in both eyes. In patients with monocular organic disease, the ruler will disrupt their ability to read.

Alleged Binocular Loss of Visual Acuity

Proof of deception is most difficult to establish in this instance, since the partner eye cannot be used as a basis for comparison, and the remaining vision in each eye is usually good enough to prevent useful testing of reflex responses in binocular comparisons. Testing methods for this scenario rely on four basic principles.

Confusion of the Sizes of Optotypes Used in Acuity Testing

Size comparison of projected or charted optotypes that the patient is familiar with can be made more difficult when the examiner begins with the smallest available characters, or uses single optotypes in a random sequence of sizes. Instruments like the Freiburg acuity test, which determines spatial resolution thresholds on a monitor screen, allow the use of testing environments with which the patient is not likely to be familiar.

When measuring near acuity, one can use magnifying lenses with varying degrees of dioptric power: With increasingly plus lenses and with closer viewing distances, the higher the acuity should be, while with the addition of minus lenses and with increases in reading distance, the lower the acuity should be.

The Mojon chart displays optotypes, the contours of which are based on a shift between two lines (Vernier acuity). The visibility of the optotype is independent of its size, when the frequency of the lines is constant.

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By testing with a projected optotype of constant size at varying distances rather than varying sizes at a constant distance, the patient can be confused as to the angular size of the characters being used.

Probability of Seeing

When testing malingerers intent on deception, Graf has used a series of Landolt rings with four possible openings (top, bottom, left, and right) and records 32 responses as to the number openings seen. Based on probability alone, guessing should yield eight characters correctly identified out of the 32 presentations. If only two or less presentations are correctly identified, it can be inferred that the patient is being willfully deceptive.

Testing of Central Vision by Other Methods

Testing of central vision by methods other than the usual varying angular sizes of optotypes presents the experienced test subject with an unfamiliar setting, which can yield informative results. Examples include the following:

■ Central luminance threshold at the perimeter: Both the threshold of luminance increment perception and visual acuity fall in synchrony with increasing distances from the center of the visual field. The two measures remain closely related, independent of eccentricity. At a visual acuity of 20/20 (1.0), the central threshold of luminance increment perception will be 0.32 cd/m2 (■ Fig. 15.2).

■ Determination of grating acuity (preferential looking method)

■ Testing of fixation maintenance with the target star of the direct ophthalmoscope or with a scanning laser ophthalmoscope. With the latter instrument one can use not only fixation targets of varying size, but the patient's fixation behavior can also be documented by video recordings.

■ Determination of acuity by laser interferometry: One can measure the spatial resolution of the retina with an interferometer's stripe pattern, which is a completely unfamiliar method to most patients and which offers

Visual Acuity Testing Documentation
Fig. 15.2. Threshold luminance levels and visual acuity as a function of retinal location, as measured by the Tübingen manual perimeter (modified according to Aulhorn)

no basis for comparison with settings that familiar.

are more

Objective Testing Methods for Alleged Bilateral Loss of Acuity

An objective determination of acuity is possible with use of a VEP method in which one measures the smallest angular subtense of a flickered checkerboard pattern that can produce a recordable cortical response. This can then be compared to a normative curve of acuities recorded by using normal subjects. This method is not entirely reliable, as the recorded potentials can be suppressed by the test subject (see below).

Optokinetic nystagmus can also be used as a quantitative method of acuity determination, either by recording the spatial frequency of the striped pattern that induces the nystagmus, or by determination of the angular size of a stationary fixation object needed to inhibit the nystagmus. The latter method has been improved with the use of an infrared nystagmography instrument that (except in the case of amblyopia) produces a good correlation between the resolution of the inhibiting fixation object and the level of spatial acuity.

When using objective methods, one should take care that the measurement is not influenced by poor cooperation, deficient fixation, or intentional accommodation.

Another objective method is the psychogalvanic response, a so-called lie detector. A few authors have used this technique to differentiate between malingering and

Malingering Isopter

Fig. 15.3. The differential diagnosis of concentric constriction of the visual field. Top Typical configurations of the visual field in feigned loss of peripheral vision. a Symmetrical constriction of the more peripheral isopters (tunneling of the visual field). b Spiraling of isopters. c Crossing of adjacent isopters. Bottom Constriction of the visual field in cases of organic disease. d Symmetrical narrow

Fig. 15.3. The differential diagnosis of concentric constriction of the visual field. Top Typical configurations of the visual field in feigned loss of peripheral vision. a Symmetrical constriction of the more peripheral isopters (tunneling of the visual field). b Spiraling of isopters. c Crossing of adjacent isopters. Bottom Constriction of the visual field in cases of organic disease. d Symmetrical narrow ing predominantly of the central isopters, caused by loss of media clarity. e Asymmetric constriction of the peripheral isopters in heredofamilial retinal degenerations. f Sharp discontinuities of isopters at the vertical midline and macular sparing in visual field loss caused by bilateral occipital lobe disease functional loss of vision. Only among malingerers was the dermal galvanic resistance pathological, while patients with functional visual loss could not be distinguished from normal subjects.

Testing Strategies for Alleged Visual Field Defects

Alleged Concentric Constriction of the Visual Field

Nonphysiologic constriction of the visual field is usually marked by a symmetrical narrowing of the peripheral isop-ters in particular, so that they lie very close to the central isopters (■ Fig. 15.3 a). While this sort of finding is suspicious, proof of the falsity of the responses is needed, and as much as possible, determination of the actual isopter positions is necessary. The latter is especially important in cases of exaggeration with the intent to qualify for financial aid based on a visually disabled status, because such a status is usually defined by the isopter position for a particular peripheral test object, such as the Goldmann III4e.

A relatively symmetrical narrowing or depression of the predominantly central isopters (inside 30° of eccentricity) is usually attributable to media opacities or an age effect (■ Fig. 15.3 d). Tapetoretinal degenerations in their earliest phases often present with asymmetrical constriction of the more peripheral isopters (■ Fig.15.3 e). Marked constriction of all isopters with preservation of only 10 to 20° of visual field is often found in the later stages of the tapetoretinal degenerations. This presentation would be difficult to confuse with functional visual loss, since there is usually a prominent abnormality of the fundus appearance and the ERG will be extinguished. A central island of vision can often be found in the advanced stages of glaucomatous disc cupping. Less frequent cases of bilateral, sequential occipital cortical infarction can spare the macular representation of the visual fields, but this should be detectable by the presence of small discontinuities in some isop-ters at the vertical meridian, both above and below fixation, due to small asymmetries in the extent of damage between the two occipital lobes (■ Fig. 15.3 f).

I Pearl

Strange configurations of constricted isopters, such as spiral shapes (■ Fig. 15.3 b), the crossing of isopters (■ Fig. 15.3 c), or other bizarre forms indicate a functional disturbance.

Testing Methods for When Concentric Constriction of the Visual Fields Is Suspected of Being Functional (■ Table 15.4)

Presence of Bizarre Alterations of the Visual Field

A normal visual field and one constricted by organic disease will have isopters that are evenly spaced, more or less, outlining the form of a cone with its peak at the center. On the contrary, functional disturbances usually cause the isopters to congregate closely with one another. This is variously referred to as tunnel or tubular constriction, since the cylindrical shape implied is fundamentally nonphysio-logic. This is most conveniently investigated by old-fashioned tangent screen testing, done at distances of 1 and 2 m (■ Fig. 15.4). The position of an isopter is first marked on the black felt with white pins (making the response locations visible is helpful). Then the distance between the subject and the screen is doubled (from 1 to 2 m), and to maintain a constant angular size of the stimulus, it too is doubled in diameter. Testing will then show an identical result, rather than the physiologic expansion one would normally expect (the diameter of the isopter should double in size), thus marking the result as functional. This test is easily modified and can be adapted to a number of different testing environments.

T| Pearl

For monocular concentric visual field constriction binocular testing at the perimeter can be helpful. A genuine constriction to the level of 10° or less in one eye should preserve and allow demonstration of the physiologic blind spot of the other, unaffected eye.

Table 15.4. Test methods for when alleged visual field defects are suspect

Concentric constriction

Hemianopic visual field loss

One and two meter tangent screen testing

Binocular perimetry

Varying strategies of isopter testing

Tests of reading ability

Comparison of various perimetric methods (kinetic, static, automated threshold determination, video, microperimetry)

Hemifield stimulation

Fusional vergence responses to prism-induced strabismus

Testing Tangent Screen Malingering

Fig. 15.4. Angular magnification of the visual field with increasing distance: With a doubling of the distance between the eye and the testing plane (usually a tangent screen), the diameter of an isopter will also double

Various Strategies for Isopter Testing

When testing bilateral concentric constriction of the visual fields, one can use a variety of strategies during isopter mapping with manual perimetric methods (e.g., with the conventional Goldmann or Tübingen perimeters):

■ Repetition of stimulus presentations tests the reproducibility of response

■ Moving the test objects from seeing to nonseeing areas (e.g., centrifugal motion from the center to the periphery), the subject can be asked to indicate the point of disappearance. Physiologically, the isopters thus recorded should lie only marginally outside the previously recorded locations.

■ Changes in the sequence of test object presentation: reversing the conventional method of testing by using objects of lower stimulus values prior to using the larger or brighter test objects

■ Determination of the true isopter location can be successfully confirmed with the following strategy, though only when the patient is not yet familiar with it: Initially, the patient is asked to fix attention on the fixation target. Later, the subject is told that to make the test easier, looking directly at the moving test object will be allowed. By observing fixation behavior, the spontaneous eye movements toward the test object can then allow a more accurate estimate of the isopter's true location, confirming that vision is intact peripheral to the isopter recorded by conventional testing.

General Principles of Perimetry

Testing with alternate strategies, other than those described above, are very difficult for patients to compare. As general rule, when a visual defect appears on routine testing, it is usually helpful to test with another method. If the same result is obtained, the defect is much more likely to be a truly pathological defect. Other than conventional kinetic perimetry with the Goldmann or Tübingen perimeters, one can use threshold static perimetry (manual or as an automated grid), video screen devices or microperimetry using a scanning laser ophthalmoscope.

Testing Principles for Alleged Hemianopia

Binocular Perimetry

The most important method for testing alleged hemianopia is binocular perimetry:

■ A true homonymous hemianopia will produce blindness to one half of visual space, and the physiologic blind spot will be detectable in the temporal seeing hemifield.

■ True bitemporal hemianopia will show absence of the temporal half of the visual field in both eyes, i.e., the fields consist only of the nasal halves of the field; therefore the physiologic blind spots will not be detectable.

■ True binasal hemianopias are very uncommon, but due to the absence of any overlapping binocular field, both physiologic blind spots and the peripheral temporal borders of the fields of both eyes will be detectable.

Additional Considerations Reading ability. Homonymous hemianopias produce disturbances of reading, fluency, particularly when the field defects lie close to the center (see Chap. 24). Bitemporal and binasal hemianopias will have the same effect, but only in monocular viewing. Standardized reading texts can be used for testing.

The hemifield slide phenomenon. Binocular viewing in bitemporal or binasal hemianopias has no corresponding points of binocular vision, and all locations in the visual field are seen monocularly. This destroys the afferent pathway for fusional vergence reflexes. Since there is no motor control of binocular alignment, underlying heterophorias become manifest, e.g., a patient who had an antecedent exophoria will manifest an exotropia. Since the two visible hemifields are no longer linked to one another, separation, vertical displacement, and/or horizontal displacement of the remaining hemifields will frequently occur:

■ Reading ability is impaired, and is particularly bad for reading tables of numbers.

■ Vertical misalignment of the two hemifields will cause a vertical discontinuity during binocular viewing with one half of an object seen as higher or lower than the other.

■ Exodeviation causes overlap of the nasal parts of the visual field with resulting diplopia. Esodeviation leads to a "gap" (see Chap. 2, ■ Fig. 2.3).

■ Bitemporal hemianopias result in a postfixational blindness. When viewing objects that are relatively close, there will be an area of blindness that starts and expands beyond the object of regard.

■ For alleged binasal or bitemporal hemianopias, placement of a prism before one eye will produce overlapping and doubling of images. Truly pathological defects will not allow any corrective movements, but those with functional disease will show intactness of fusional vergence movements to correct the prism-induced strabismus.

Pupillary deficits. Disease of the optic tract will always, and chiasmal disease will frequently be accompanied by a relative afferent pupillary defect.

Hemifield stimulation in functional homonymous hemifield loss. An isolated focus of disease in one side of visual cortex will show intact optokinetic nystagmus when the entire visual field is stimulated. Hemifield stimulation during optokinetic nystagmus or VEP testing will show detectable differences between the left and right hemifields when the loss is organic but not when it is functional.

Steps after Finishing the Malingering Tests

Once the malingering tests have been completed, the diagnosis of alleged visual loss should be clear, and the claimed loss of function should be either confirmed or denied.

Functional disturbances of vision occur most commonly in situations of conflict, inadequate support, excessive demands, or among those with suggestible or neurotic personality disorders. In a conflict situation the visual complaint will develop as a kind of appeal for help with symbolic content (can no longer see something or someone). The alleged loss has the effect of providing a compensatory gain: attention, care, considerate treatment. If the conflict is not resolved, the symptom may become permanently fixed or transferred to another organ (symptom shift, ■ Fig. 15.5). In this situation, the ophthalmologist is vested with an important responsibility, since he/she can differentiate clearly

Malingering Vision Loss
Fig. 15.5. The pathogenesis and psychodynamics of functional loss of vision (modified according to Trauzettel-Klosinski S, Klosinski G [1997] Psychogene Augenerkrankungen. In: Deter HC [ed] Angewandte Psychosomatik.Thieme, Stuttgart, pp 407-421)

between a functional visual loss and an organic disease. If sufficient time passes following the onset of functional visual loss, and it is shifted to a complaint of headache or abdominal pain, the physician then tasked with the patient's care will find it significantly more difficult, and often not definitively possible, to prove that the problem is a nonor-ganic disturbance.

On the other hand, a malingerer will have a specific desire or request: financial gain or protection from imprisonment, or the military draft. The individual's status as an opponent to, rather than as a partner with, the physician will become evident during the examination.

Steps to Be Taken When Malingering Is Proven

1. The malingerer should be confronted with the contradictions of his/her claims.

2. He/She should be made aware of the disadvantageous consequences of pursuing his claims, such as loss of insurability or license to drive.

3. To avoid a repetition of the claims, other physicians participating in his/her care should (consistent with proper confidentiality of the patient's records) be advised of the conclusions. In this regard, care should be taken that information about the patient's evaluation not be passed on without the patient's written consent. It is recommended that the malingering nature of the claim be clearly described, e.g., "contradictory claims" or "discrepancy between morphological and functional findings."

4. If the patient agrees to perform the tests properly and is in real need, a legally sanctioned method for the patient to receive assistance can often be found.

Steps to Be Taken When Functional Loss of Vision Is Proven

1. One should inform the patient that "fortunately," no organic disease is present.

2. One should try to discover the underlying causes through careful questioning.

3. For potentially solvable problems, such as excessive burdens experienced by students, one can offer practical assistance in having his/her work load reduced.

4. For problems whose sources are more difficult to identify, one can offer help with referral to a psychiatrist, to determine whether psychotherapy, medications, special testing, and/or crisis intervention are needed. This should be done with the purpose of avoiding fixation of the problem or transfer of the symptoms to another organ system.

5. Knowing the nature of the problem, other physicians involved in the patient's care can be spared the expenditure of time and expensive tests, as well as the risk of surgical intervention.

6. In every case, the patient must be given a face-saving pathway to retreat from the symptom complex. This can be as simple as an optimistic suggestion that a spontaneous recovery of function is just ahead, since there are no signs of disease. The suggestion can often be helped by the use of harmless but complex and time-consuming treatments, such as instillation of artificial tears according to precise directions as to the number of drops and the time of day at which they should be instilled. In addition, one can speak of the "beneficial effect" of a flash VEP.

Conclusion

With the methods described here, one can usually be successful in proving the functional nature of a visual complaint. The unambiguous diagnosis allows the institution of appropriate measures to help patients with functional loss of vision, and exposes the deception behind claims of visual loss in malingerers.

Further Reading

Aulhorn E, Harms H (1972) Visual perimetry. In: Jameson D, Hurvich LM (eds) Visual psychophysics, handbook of sensory physiology, vol. 7/4. Springer, Berlin Heidelberg New York Bach M (1995) Der Freiburger Visustest. Automatisierte Sehschärfenbestimmung. Ophthalmologe 92: 174-178 Fahle M, Barth V, Henke-Fahle S, Mohn G (1989) Zur Einschätzung der Sehschärfe bei Simulation und Aggravation. Klin Monatsbl Augen-heilkd 195: 356-362 Gräf M (1999) Information from false statements concerning visual acuity and visual field in cases of psychogenic visual impairment. Graefes Arch Clin Exp Ophthalmol 237: 16-20 Gräf M, Dettmar T, Kaufmann H (1996) Objektive Sehschärfenbestimmung. Weiterentwicklung einer infrarotnystagmographischen Methode und Vergleich mit dem Muster-VEP. Ophthalmologe 93: 396-403

Hahn GA, Penka D, Gehrlich C, Messias A, Weismann M, Hyvärinen L, Leinonen M, Feely M, Rubin G, Dauxerre C, Vital-Durand F, Feather-ston, Dietz K, Trauzettel-Klosinski S (2006) New standardised texts for assessing reading performance in four European languages. Br J Ophthalmol 90: 480-484 Hajek A, Zrenner E (1988) Verbesserte objektive Visusprüfung mit visuell evozierten corticalen Potentialen durch schnelle Reizmustersequenzen unterschiedlicher Raumfrequenz. Fortschr Ophthal-mol 85: 550-554

Hess CW, Meienberg O, Ludin HP (2002) Visuell evozierte Potentiale bei akuter kortikaler Erblindung. In: Struppler A (ed) Elektrophysiolo-gische Diagnostik in der Neurologie. Thieme, Stuttgart Miller NR, Keane JR (1998) Neuro-ophthalmologic manifestations of nonorganic disease. In: Miller NR, Newman NJ (eds) Clinical neuro-ophthalmology, 5th edn., vol. 1. Williams and Wilkins, Baltimore, pp 1765-1786

Mojon DS, Flückiger P (2002) A new optotype chart for detection of nonorganic visual loss. Ophthalmology 109: 810-815 Trauzettel-Klosinski S (1989) Various stages of optic neuritis assessed by subjective brightness of flicker. Arch Ophthalmol 107: 63-68 Trauzettel-Klosinski S, Klosinski G (1997a) Psychogene Augenerkrankungen. In: Deter HC (ed) Angewandte Psychosomatik. Thieme, Stuttgart Trauzettel-Klosinski S (1997b) Untersuchungsstrategien bei Simulation und funktionellen Sehstörungen. Klin Monatsbl Augenheilkd 211: 73-83

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