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The monocular vertical prism dissociation test
The Monocular Vertical Prism Dissociation Test KARL C. GOLNIK, MD, ANDREW G. LEE, MD, AND ERIC R. EGGENBERGER, DO
● PURPOSE: To determine if the monocular vertical prism dissociation test can differentiate between organic and nonorganic visual loss. ● DESIGN: A prospective, single-masked observational study. ● METHODS: Three institutional neuro-ophthalmology practices. Group 1 consisted of 30 normal controls. Group 2 included 30 patients with known organic visual loss. Group 3 contained 35 patients with suspected nonorganic monocular visual loss. Participants were asked to describe what they saw while viewing a single Snellen letter when a 4-prism diopter base-down prism was placed in front of their better eye. Outcome was measured by whether the participant sees one or two letters with the prism in place. ● RESULTS: Vision-appropriate results were given by all members of Group 1 (two images) and by all members of Group 2 (one image). Two images were seen by 31 of 35 members of Group 3, indicating nonorganic visual loss. The other 4 subjects in Group 3 saw one image; each was subsequently found to have occult pathology. ● CONCLUSION: The vertical prism test quickly differentiates organic from nonorganic monocular visual acuity loss. (Am J Ophthalmol 2004;137:135–137. © 2004 by Elsevier Inc. All rights reserved.)
Slavin described the monocular vertical prism dissociation test as a helpful adjunct in uncovering nonorganic visual loss.1 A 4-prism diopter prism is placed base down in front of the good eye of a patient with monocular visual loss. If the subject has symmetric vision in both eyes, two images should be seen; one above the other. If the subject is only able to see the letter with the good eye, then only one image should be seen. We investigated the utility of the monocular vertical prism dissociation test in differentiating between organic and nonorganic visual loss. We describe the results of a singlemasked, observational study using the monocular vertical prism dissociation test in patients with organic and nonorganic monocular visual loss and in normal controls.
DESIGN A PROSPECTIVE, SINGLE-MASKED OBSERVATIONAL STUDY.
METHODS ● SETTING:
Three
institutional
neuro-ophthalmology
practices. ● PATIENTS:
P
ATIENTS WITH UNEXPLAINED VISUAL LOSS OFTEN
require a disproportionate amount of physician time. Costly and sometimes invasive testing is often obtained in attempts to identify an organic process. Some of these patients will have nonorganic visual loss. Early differentiation between patients with occult organic visual loss and patients with nonorganic visual loss would reduce both physician time and unnecessary testing.
Accepted for publication July 21, 2003. From the Department of Ophthalmology, Neurology, Neurosurgery, University of Cincinnati and The Cincinnati Eye Institute, Cincinnati, Ohio (K.C.G.); Departments of Ophthalmology, Neurology, and Neurosurgery, University of Iowa Hospitals and Clinics, Iowa City, Iowa (A.G.L.); Department of Neurology, Michigan State University, East Lansing, Michigan (E.R.E.). Supported in part by an unrestricted grant from Research to Prevent Blindness. Inquiries to Karl C. Golnik, MD, 10494 Montgomery Rd, Cincinnati, OH 45242; fax: (513) 984-4240; e-mail: [email protected] 0002-9394/04/$30.00 doi:10.1016/S0002-9394(03)00865-1
©
2004 BY
Group 1 consisted of 30 healthy volunteers with normal vision who were unfamiliar with the monocular vertical prism test. Thirty patients with known organic visual loss comprised Group 2. The 35 patients in Group 3 were each referred for probable nonorganic visual loss. All subjects in the study were required to be between 14 and 65 years of age. Patients in Groups 2 and 3 had to have at least 3 lines’ difference in best-corrected visual acuity. Subjects were excluded from the study if: (1) there was a history of amblyopia; (2) strabismus was present; or (3) they were not alert and oriented. Normal controls were recruited from the eye clinic technical and house staff. All controls had 20/20 vision in each eye. All patients enrolled at the University of Iowa gave informed consent to participate under institutional review board review and approval. Patients unwilling or unable to provide informed consent were excluded from the study. Each patient in Groups 2 and 3 had a full ophthalmic examination, including best-corrected visual acuity determi-
ELSEVIER INC. ALL
RIGHTS RESERVED.
135
FIGURE 1. If the patient has poor vision in one eye, then only one image will be seen. A second blurred image (as shown) may be seen depending on level of vision in the “bad” eye.
FIGURE 2. When the prism is placed vertically in front of one eye, two vertically separated images (as shown) will be seen if the patient has good vision in each eye.
nation, color vision testing (when appropriate), pupillary examination, and dilated funduscopy. Before dilation, a single Snellen letter was shown on the distance chart. The letter was 2 lines larger than the best corrected visual acuity of the better eye (for example, visual acuity ⫽ 20/20 in better eye, a 20/30 letter is used). The patient was asked to identify the letter with both eyes open. A 4 prism diopters prism was then held base down in front of the better eye with best correction in place and both eyes open. The patient was asked what he or she saw on the Snellen chart. If only one letter was seen, the test was over (Figure 1). If two letters were seen, the patient was asked what letters, how they are oriented, and if one was clearer than the other (Figure 2). Thus, Group 1 subjects should see two letters, Group 2 subjects should see one letter, and if the subjects in Group 3 truly have organic visual loss, they will also see one image. However, if the Group 3 subject saw two letters, then the visual loss is nonorganic.
TABLE 1. Results of the Monocular Vertical Prism Dissociation Test Number of Subjects Seeing:
Group 1 (n ⫽ 30) Group 2 (n ⫽ 30) Group 3 (n ⫽ 35)
0 30 4*
30 0 31
perception in the worse eye. All subjects in Group 2 had organic visual loss resulting from optic neuropathy. Results of the monocular vertical prism dissociation test are given in the Table 1. All subjects in Groups 1 and 2 gave vision-appropriate responses. Thirty-one of 35 subjects in Group 3 saw two letters, thus contradicting their claimed level of visual acuity. Four of the Group 3 subjects gave the appropriate response for monocular visual loss. Further
VISUAL ACUITY IN GROUPS 2 AND 3 SUBJECTS RANGED
from 20/20 to 20/60 in the better eye and 20/50 to no light AMERICAN JOURNAL
2 Images
*These four subjects were each found to have organic visual loss after further evaluation. Group 1 ⫽ normal controls. Group 2 ⫽ organic visual loss. Group 3 ⫽ suspected non-organic visual loss.
RESULTS
136
1 Image
OF
OPHTHALMOLOGY
JANUARY 2004
testing of these patients identified organic disease. One patient had an abnormal cone response on focal electroretinography (acuity 20/60, 20/200). Two patients had oil droplet cataract (acuity 20/30, 20/80 and 20/20, 20/50). One patient had irregular astigmatism (20/25, 20/70).
DISCUSSION THE TERM “NONORGANIC VISUAL LOSS” IS USED TO DE-
scribe patients who are: (1) hysterical (conversion reaction); (2) suggestible; or (3) malingering for secondary gain.2 Nonorganic visual loss is common and may constitute up to 5% of a comprehensive ophthalmologist’s practice.3,4 Patterns of nonorganic visual loss include monocular or binocular loss of visual acuity, visual field abnormality, and double vision. Although a patient with unexplained visual loss and a normal examination may have nonorganic visual loss, occult causes of organic visual loss do occur. Thus, to be sure the visual loss is nonorganic, the ophthalmologist must show that the patient can see better than his or her claimed level of vision. Additionally, nonorganic visual loss may be overlayed on organic visual loss.5 Therefore, to show that the visual loss is entirely nonorganic, one must demonstrate normal visual function. Numerous tests and tricks have been described to detect nonorganic monocular visual loss. Development of optokinetic nystagmus,3 the mirror test,6 stereopsis testing,7 and binocular visual field testing7 can be used if the visual loss is severe. These tests will not provide a measurement of actual visual acuity. Polarized lenses8 and the potential acuity meter9 can be helpful but may not be readily available. Gradual fogging of the good eye in the phoropter7 and persistent refraction are worthwhile but time consuming. Red/green glasses/duochrome chart testing6 works well, but this method and most of the methods mentioned previously raise suspicion because something is placed over the “bad” eye. Other tests described involve specialized equipment not generally available.10,11 The monocular vertical prism dissociation test rapidly differentiates organic from nonorganic monocular visual acuity loss and allows determination of visual acuity in the nonorganic bad eye. We tested three groups of subjects. Group 1 consisted of normal individuals with symmetric acuity. We excluded anyone with a history of strabismus because of possible suppression. Every Group 1 subject gave the appropriate response (two images). Group 2 subjects all had known organic monocular visual loss. They all gave the appropriate response (one image). One subject in Group 2 initially said he saw two letters. Further questioning revealed that he was assuming it was the same letter but really could not make it out. If the patient sees two images, he or she should be asked if the images are of equal clarity. It is possible that an individual might be able to fuse 4 vertical prism diopters, but this would be very unlikely. These caveats aside, VOL. 137, NO. 1
the monocular vertical prism dissociation test seems to work as one would expect. Group 3 subjects were referred with normal examinations and suspected nonorganic visual loss. Thirty-one of 35 saw two images, thus contradicting their claimed level of vision. Interestingly, four gave the vision-appropriate response (one image). Further evaluation showed two patients to have oil-droplet cataracts, one had irregular astigmatism, and one had abnormal foveal focal electroretinogram. This is a very important finding because patients are sometimes labeled as “nonorganic” when no obvious cause for visual loss is identified. With the advent of techniques such as multifocal electroretinography, patients previously thought to be malingering can now be shown to have organic visual loss. The prism dissociation test can be used in any patient with monocular acuity loss as a quick screening test. Further diagnostic evaluation is essential when the patient sees only one image. An important advantage of this test compared with others is that the prism is held before the good eye. The patient seems not to be suspicious, because nothing is being done to the bad eye. In fact, patients will often ask why the bad eye was not tested. We found the monocular vertical prism dissociation test to be a quick and efficient method of detecting nonorganic monocular visual acuity loss. This test also allows quantitation of the actual visual acuity in the “bad” eye. However, if nonorganic visual loss is suspected and the patient only sees one image, then further diagnostic evaluation is necessary to identify the cause of the organic visual loss.
REFERENCES 1. Slavin ML. The prism dissociation test in detecting unilateral functional visual loss. J Clin Neuro-ophthalmol 1990;10:127–130. 2. Thompson HS. Functional visual loss. Am J Ophthalmol 1985;209 –213. 3. Miller BW. A review of practical tests for ocular malingering and hysteria. Surv Ophthalmol 1973;17:241–246. 4. Kathol RG, Cox TA, Corbett JJ, et al. Functional visual loss: I. A true psychiatric disorder? Psychol Med 1983;13:307–314. 5. Keltner JL, May WN, Johnson CA, et al. The California syndrome. Functional visual complaints with potential economic impact. Ophthalmology 1985;92:427–435. 6. Kramer KK, La Piana FG, Appleton B. Ocular malingering and hysteria: diagnosis and management. Surv Ophthalmol 1979;24:89 –96. 7. Bose S, Kupersmith M. Presentations of functional visual disorders. Neurol Clin 1995;13:321–339. 8. Fahle M, Mohn G. Assessment of visual function in suspected ocular malingering. Br J Ophthalmol 1989;73:651–654. 9. Levi L, Feldman RM. Use of the potential acuity meter in suspected functional visual loss. Am J Ophthalmol 1992;114: 502–503. 10. Martin TJ. Threshold perimetry of each eye with both eyes open in patients with monocular functional (nonorganic) and organic vision loss. Am J Ophthalmol 1998;125:857–864. 11. Mojon DS, Flueckiger P. A new optotype chart for detection of nonorganic visual loss. Ophthalmology 2002;109:810 –815.
VERTICAL PRISM TEST
137
● PURPOSE: To determine if the monocular vertical prism dissociation test can differentiate between organic and nonorganic visual loss. ● DESIGN: A prospective, single-masked observational study. ● METHODS: Three institutional neuro-ophthalmology practices. Group 1 consisted of 30 normal controls. Group 2 included 30 patients with known organic visual loss. Group 3 contained 35 patients with suspected nonorganic monocular visual loss. Participants were asked to describe what they saw while viewing a single Snellen letter when a 4-prism diopter base-down prism was placed in front of their better eye. Outcome was measured by whether the participant sees one or two letters with the prism in place. ● RESULTS: Vision-appropriate results were given by all members of Group 1 (two images) and by all members of Group 2 (one image). Two images were seen by 31 of 35 members of Group 3, indicating nonorganic visual loss. The other 4 subjects in Group 3 saw one image; each was subsequently found to have occult pathology. ● CONCLUSION: The vertical prism test quickly differentiates organic from nonorganic monocular visual acuity loss. (Am J Ophthalmol 2004;137:135–137. © 2004 by Elsevier Inc. All rights reserved.)
Slavin described the monocular vertical prism dissociation test as a helpful adjunct in uncovering nonorganic visual loss.1 A 4-prism diopter prism is placed base down in front of the good eye of a patient with monocular visual loss. If the subject has symmetric vision in both eyes, two images should be seen; one above the other. If the subject is only able to see the letter with the good eye, then only one image should be seen. We investigated the utility of the monocular vertical prism dissociation test in differentiating between organic and nonorganic visual loss. We describe the results of a singlemasked, observational study using the monocular vertical prism dissociation test in patients with organic and nonorganic monocular visual loss and in normal controls.
DESIGN A PROSPECTIVE, SINGLE-MASKED OBSERVATIONAL STUDY.
METHODS ● SETTING:
Three
institutional
neuro-ophthalmology
practices. ● PATIENTS:
P
ATIENTS WITH UNEXPLAINED VISUAL LOSS OFTEN
require a disproportionate amount of physician time. Costly and sometimes invasive testing is often obtained in attempts to identify an organic process. Some of these patients will have nonorganic visual loss. Early differentiation between patients with occult organic visual loss and patients with nonorganic visual loss would reduce both physician time and unnecessary testing.
Accepted for publication July 21, 2003. From the Department of Ophthalmology, Neurology, Neurosurgery, University of Cincinnati and The Cincinnati Eye Institute, Cincinnati, Ohio (K.C.G.); Departments of Ophthalmology, Neurology, and Neurosurgery, University of Iowa Hospitals and Clinics, Iowa City, Iowa (A.G.L.); Department of Neurology, Michigan State University, East Lansing, Michigan (E.R.E.). Supported in part by an unrestricted grant from Research to Prevent Blindness. Inquiries to Karl C. Golnik, MD, 10494 Montgomery Rd, Cincinnati, OH 45242; fax: (513) 984-4240; e-mail: [email protected] 0002-9394/04/$30.00 doi:10.1016/S0002-9394(03)00865-1
©
2004 BY
Group 1 consisted of 30 healthy volunteers with normal vision who were unfamiliar with the monocular vertical prism test. Thirty patients with known organic visual loss comprised Group 2. The 35 patients in Group 3 were each referred for probable nonorganic visual loss. All subjects in the study were required to be between 14 and 65 years of age. Patients in Groups 2 and 3 had to have at least 3 lines’ difference in best-corrected visual acuity. Subjects were excluded from the study if: (1) there was a history of amblyopia; (2) strabismus was present; or (3) they were not alert and oriented. Normal controls were recruited from the eye clinic technical and house staff. All controls had 20/20 vision in each eye. All patients enrolled at the University of Iowa gave informed consent to participate under institutional review board review and approval. Patients unwilling or unable to provide informed consent were excluded from the study. Each patient in Groups 2 and 3 had a full ophthalmic examination, including best-corrected visual acuity determi-
ELSEVIER INC. ALL
RIGHTS RESERVED.
135
FIGURE 1. If the patient has poor vision in one eye, then only one image will be seen. A second blurred image (as shown) may be seen depending on level of vision in the “bad” eye.
FIGURE 2. When the prism is placed vertically in front of one eye, two vertically separated images (as shown) will be seen if the patient has good vision in each eye.
nation, color vision testing (when appropriate), pupillary examination, and dilated funduscopy. Before dilation, a single Snellen letter was shown on the distance chart. The letter was 2 lines larger than the best corrected visual acuity of the better eye (for example, visual acuity ⫽ 20/20 in better eye, a 20/30 letter is used). The patient was asked to identify the letter with both eyes open. A 4 prism diopters prism was then held base down in front of the better eye with best correction in place and both eyes open. The patient was asked what he or she saw on the Snellen chart. If only one letter was seen, the test was over (Figure 1). If two letters were seen, the patient was asked what letters, how they are oriented, and if one was clearer than the other (Figure 2). Thus, Group 1 subjects should see two letters, Group 2 subjects should see one letter, and if the subjects in Group 3 truly have organic visual loss, they will also see one image. However, if the Group 3 subject saw two letters, then the visual loss is nonorganic.
TABLE 1. Results of the Monocular Vertical Prism Dissociation Test Number of Subjects Seeing:
Group 1 (n ⫽ 30) Group 2 (n ⫽ 30) Group 3 (n ⫽ 35)
0 30 4*
30 0 31
perception in the worse eye. All subjects in Group 2 had organic visual loss resulting from optic neuropathy. Results of the monocular vertical prism dissociation test are given in the Table 1. All subjects in Groups 1 and 2 gave vision-appropriate responses. Thirty-one of 35 subjects in Group 3 saw two letters, thus contradicting their claimed level of visual acuity. Four of the Group 3 subjects gave the appropriate response for monocular visual loss. Further
VISUAL ACUITY IN GROUPS 2 AND 3 SUBJECTS RANGED
from 20/20 to 20/60 in the better eye and 20/50 to no light AMERICAN JOURNAL
2 Images
*These four subjects were each found to have organic visual loss after further evaluation. Group 1 ⫽ normal controls. Group 2 ⫽ organic visual loss. Group 3 ⫽ suspected non-organic visual loss.
RESULTS
136
1 Image
OF
OPHTHALMOLOGY
JANUARY 2004
testing of these patients identified organic disease. One patient had an abnormal cone response on focal electroretinography (acuity 20/60, 20/200). Two patients had oil droplet cataract (acuity 20/30, 20/80 and 20/20, 20/50). One patient had irregular astigmatism (20/25, 20/70).
DISCUSSION THE TERM “NONORGANIC VISUAL LOSS” IS USED TO DE-
scribe patients who are: (1) hysterical (conversion reaction); (2) suggestible; or (3) malingering for secondary gain.2 Nonorganic visual loss is common and may constitute up to 5% of a comprehensive ophthalmologist’s practice.3,4 Patterns of nonorganic visual loss include monocular or binocular loss of visual acuity, visual field abnormality, and double vision. Although a patient with unexplained visual loss and a normal examination may have nonorganic visual loss, occult causes of organic visual loss do occur. Thus, to be sure the visual loss is nonorganic, the ophthalmologist must show that the patient can see better than his or her claimed level of vision. Additionally, nonorganic visual loss may be overlayed on organic visual loss.5 Therefore, to show that the visual loss is entirely nonorganic, one must demonstrate normal visual function. Numerous tests and tricks have been described to detect nonorganic monocular visual loss. Development of optokinetic nystagmus,3 the mirror test,6 stereopsis testing,7 and binocular visual field testing7 can be used if the visual loss is severe. These tests will not provide a measurement of actual visual acuity. Polarized lenses8 and the potential acuity meter9 can be helpful but may not be readily available. Gradual fogging of the good eye in the phoropter7 and persistent refraction are worthwhile but time consuming. Red/green glasses/duochrome chart testing6 works well, but this method and most of the methods mentioned previously raise suspicion because something is placed over the “bad” eye. Other tests described involve specialized equipment not generally available.10,11 The monocular vertical prism dissociation test rapidly differentiates organic from nonorganic monocular visual acuity loss and allows determination of visual acuity in the nonorganic bad eye. We tested three groups of subjects. Group 1 consisted of normal individuals with symmetric acuity. We excluded anyone with a history of strabismus because of possible suppression. Every Group 1 subject gave the appropriate response (two images). Group 2 subjects all had known organic monocular visual loss. They all gave the appropriate response (one image). One subject in Group 2 initially said he saw two letters. Further questioning revealed that he was assuming it was the same letter but really could not make it out. If the patient sees two images, he or she should be asked if the images are of equal clarity. It is possible that an individual might be able to fuse 4 vertical prism diopters, but this would be very unlikely. These caveats aside, VOL. 137, NO. 1
the monocular vertical prism dissociation test seems to work as one would expect. Group 3 subjects were referred with normal examinations and suspected nonorganic visual loss. Thirty-one of 35 saw two images, thus contradicting their claimed level of vision. Interestingly, four gave the vision-appropriate response (one image). Further evaluation showed two patients to have oil-droplet cataracts, one had irregular astigmatism, and one had abnormal foveal focal electroretinogram. This is a very important finding because patients are sometimes labeled as “nonorganic” when no obvious cause for visual loss is identified. With the advent of techniques such as multifocal electroretinography, patients previously thought to be malingering can now be shown to have organic visual loss. The prism dissociation test can be used in any patient with monocular acuity loss as a quick screening test. Further diagnostic evaluation is essential when the patient sees only one image. An important advantage of this test compared with others is that the prism is held before the good eye. The patient seems not to be suspicious, because nothing is being done to the bad eye. In fact, patients will often ask why the bad eye was not tested. We found the monocular vertical prism dissociation test to be a quick and efficient method of detecting nonorganic monocular visual acuity loss. This test also allows quantitation of the actual visual acuity in the “bad” eye. However, if nonorganic visual loss is suspected and the patient only sees one image, then further diagnostic evaluation is necessary to identify the cause of the organic visual loss.
REFERENCES 1. Slavin ML. The prism dissociation test in detecting unilateral functional visual loss. J Clin Neuro-ophthalmol 1990;10:127–130. 2. Thompson HS. Functional visual loss. Am J Ophthalmol 1985;209 –213. 3. Miller BW. A review of practical tests for ocular malingering and hysteria. Surv Ophthalmol 1973;17:241–246. 4. Kathol RG, Cox TA, Corbett JJ, et al. Functional visual loss: I. A true psychiatric disorder? Psychol Med 1983;13:307–314. 5. Keltner JL, May WN, Johnson CA, et al. The California syndrome. Functional visual complaints with potential economic impact. Ophthalmology 1985;92:427–435. 6. Kramer KK, La Piana FG, Appleton B. Ocular malingering and hysteria: diagnosis and management. Surv Ophthalmol 1979;24:89 –96. 7. Bose S, Kupersmith M. Presentations of functional visual disorders. Neurol Clin 1995;13:321–339. 8. Fahle M, Mohn G. Assessment of visual function in suspected ocular malingering. Br J Ophthalmol 1989;73:651–654. 9. Levi L, Feldman RM. Use of the potential acuity meter in suspected functional visual loss. Am J Ophthalmol 1992;114: 502–503. 10. Martin TJ. Threshold perimetry of each eye with both eyes open in patients with monocular functional (nonorganic) and organic vision loss. Am J Ophthalmol 1998;125:857–864. 11. Mojon DS, Flueckiger P. A new optotype chart for detection of nonorganic visual loss. Ophthalmology 2002;109:810 –815.
VERTICAL PRISM TEST
137