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Ocular Motility Anomalies in Developmental Misdirection of the Optic Chiasm
Vol. 113, No. 5
Correspondence
MATTHEW P. MADION, M.D. MICHAEL J. WILKINSON, M.D. Detroit, Michigan
References 1. Sergott, R. C, Savino, R. J., and Bosley, T. M.: Modified optic nerve sheath decompression provides long-term visual improvement for pseudotumor cerebri. Arch. Ophthalmol. 106:1384, 1988. 2. Corbett, J. J., Nerad, J. A., Tse, D. T., and An derson, R. L.: Results of optic nerve fenestration for pseudotumor cerebri. The lateral orbitotomy ap proach. Arch. Ophthalmol. 106:1391, 1988. 3. Davidson, S. I.: The surgical relief of papilloedema. In Cant, J. S. (ed.): The Optic Nerve, vol. 3. London, Henry Kimpton, 1972, pp. 174-179.
Ocular Motility Anomalies in Developmental Misdirection of the Optic Chiasm EDITOR: The article, "Ocular motility anomalies in developmental misdirection of the optic chi asm," by J. A. W. McCarty, J. L. Demer, L. A. Hovis, and M. R. Nuwer (Am. J. Ophthalmol. 113:86, January 1992), claims that a normally pigmented 35-year-old white man with con genital nystagmus had pattern reversal visualevoked potentials consistent with "a lack of normal decussation of nasal retinal fibers of the optic chiasm." The authors also suggest an association between this putative abnormal de cussation and reverse tracking in congenital nystagmus. Our view is that their conclusion is based on doubtful visual-evoked potential evi dence, and that their eye movement recordings can be interpreted differently. Their conclusion for the presence of an ab normal decussation is based solely on visualevoked potential data to stimulation of the temporal hemifield of the left eye. Responses could not be discerned to stimulation of the nasal field, or from either nasal or temporal hemifield stimulation of the right eye. We criti cize the visual-evoked potential aspects on two points:
601
1. Their interpretation of the responses to left hemifield stimulation of the left eye is highly questionable. The overall impression given by Figure 1 is of an atypical W-shaped waveform, which is about equal size over left and right hemispheres. The labels used to identify peaks have special connotations, which are not explained. In one place the text appears illogical: How can a positive peak at 135 milliseconds (that is, PI35) be "shown as a negative peak"? They provide no rationale for placing the P100 label on the first limb of the W waveform on the right (contralateral) scalp, and the PI35 label on the second limb of the left (ipsilateral) scalp visual-evoked po tential. There is greater justification in revers ing the labelling of the peaks (that is, PI00 ipsilateral and P135 contralateral) as this would agree with what is expected from hemi field stimulation studies in healthy controls (so-called paradoxical lateralization of P1001,2). P100 is predominantly generated by the macular pathway and is distributed on the side of the scalp ipsilateral to the stimulus hemifield. PI35 is produced by the pathway subserving paramacular areas of the visual field and is recorded over the contralateral scalp. It has been shown in healthy controls that the para macular PI35 component virtually disappears when a stimulus field is reduced to stimulate the macular area of the visual field only. Con versely, by experimentally obscuring the macu lar area, P100 becomes highly attenuated but PI35 remains unaltered, or may even enlarge. 3 Given that they detected responses from one hemifield only, McCarty and associates should have attempted these procedures to identify their peaks more reliably. 2. Their choice of stimulus mode was suboptimal. Many studies have documented that pat tern reversal stimulation produces poor visualevoked potentials when nystagmus is pres ent,4 6 so it is not unexpected that McCarty and associates' pattern responses were so poor. It is somewhat surprising that pattern-evoked potentials were detectable from the left hemi field of the left eye, given that no consistent responses were detected from the other three hemifields tested. A transiently presented pat tern stimulus (pattern onset) is superior to pat tern reversal because it produces larger re sponses and is more reliable in indicating an abnormal projection.4-6 To make their point convincingly, McCarty and associates would
602
May, 1992
AMERICAN JOURNAL OF OPHTHALMOLOGY
need to demonstrate pattern-evoked potentials with an opposite occipital distribution when comparing each eye (that is, a crossed asym metry). McCarty and associates also describe reverse tracking during smooth pursuit and optokinet ic nystagmus. So-called reverse optokinetic nystagmus is often seen in patients with con genital nystagmus and has been examined in detail by Halmagyi and associates. 7 They ar gued that during smooth pursuit or optokinetic tracking there is a shift in the null of the con genital nystagmus in the opposite direction of intended eye velocity. This results in a change in the nystagmus waveform even though the eyes may not deviate substantially from pri mary position. Thus, an optokinetic nystagmus stimulus moving to the right induces a null shift to the left so that the nystagmus wave form is similar to that normally associated with right gaze. The nystagmus beats appear, therefore, to be in the wrong direction when compared with normal optokinetic nystagmus; however, they are just the patient's congenital nystagmus with a shifted null rather than re versed optokinetic nystagmus. Even though the so-called reversed optokinetic response is similar in these two studies, McCarty and as sociates dismiss the argument made by Hal magyi and associates, 7 namely that it is stimu lus velocity (or possibly intended eye velocity) that shifts the null, not actual eye position. TONY KRISS, Ph.D. CHRIS HARRIS, Ph.D.
London, United Kingdom SCOTT R. LAMBERT, M.D.
Atlanta, Georgia
References 1. Barrett, G„ Blumhardt, L., Halliday, A. M., Halliday, E., and Kriss, A.: A paradox in the lateralisation of the visual evoked response. Nature 261:253, 1976. 2. Halliday, A. M.: Evoked Potentials in Clinical Testing. Edinburgh, Churchill Livingstone, 1982. 3. Blumhardt, L. D., Barrett, G., Halliday, A. M., and Kriss, A.: The effect of experimental scotomata on the ipsilateral and contralateral responses to pat tern reversal in one half-field. Electroenceph. Clin. Neurophysiol. 45:376, 1978. 4. Creel, D., Spekreijse, H., and Reits, D.: Evoked
potentials in albinos. Efficacy of pattern stimuli in detecting misrouted optic fibers. Electroenceph. Clin. Neurophysiol. 52:595, 1981. 5. Kriss, A., Timms, C , Elston, J., Taylor, D., and Gresty, M.: Visual evoked potentials in dissociated vertical deviation. A reappraisal. Br. J. Ophthalmol. 73:265, 1989. 6. Zubcov, A. A., Fendick, M. G., Gottlob, I., Wizov, S. S., and Reinecke, R. D.: Visual-evoked potentials in dissociated vertical deviation. Am. J. Ophthalmol. 112:714, 1991. 7. Halmagyi, G. M., Gresty, M. A., and Leech, J.: Reversed optokinetic nystagmus (OKN). Mechanism and clinical significance. Ann. Neurol. 7:429, 1980.
Reply EDITOR:
We appreciate the interest taken by Drs. Kriss, Harris, and Lambert in our report of ocular motility anomalies in a patient with evoked-potential evidence of misdirection of the optic chiasm. They suggested that tran siently presented patterns might have pro duced larger visual-evoked potentials. Our technique, which is standard for hemifield test ing,1 did produce, in two separate testing ses sions, adequate potentials to demonstrate the oppositely lateralized positivities at about 100 and 135 milliseconds, as may be seen in Figure 1. We followed standard hemifield procedures for scoring our wave peaks, 1 and we gave due consideration to paradoxic localization. Kriss, Harris, and Lambert have suggested that we reverse the labels P100 and P135, making the PI35 first to occur. We are not aware of any procedure for such a nonstandard maneuver. Since there is no anatomic necessity for chiasmal misrouting to be symmetrical either with respect to eyes or hemifields within eyes, we were not surprised to find that reversal of the side of stimulation did not produce a mirror image of the evoked-potential anomaly. We agree that greater amplitude waves would he advantageous. Transient stimulus presentation, when employed for full-field stimulation, has been shown to be superior to pattern reversal in demonstration of the anomalous visual pro jections of albinism, 2 although the basis for this advantage is unknown. Whether this su periority would extend to hemifield testing re mains to be demonstrated, as would relevance to other causes of nystagmus besides albinism. Drs. Kriss, Harris, and Lambert have reiterat-
Correspondence
MATTHEW P. MADION, M.D. MICHAEL J. WILKINSON, M.D. Detroit, Michigan
References 1. Sergott, R. C, Savino, R. J., and Bosley, T. M.: Modified optic nerve sheath decompression provides long-term visual improvement for pseudotumor cerebri. Arch. Ophthalmol. 106:1384, 1988. 2. Corbett, J. J., Nerad, J. A., Tse, D. T., and An derson, R. L.: Results of optic nerve fenestration for pseudotumor cerebri. The lateral orbitotomy ap proach. Arch. Ophthalmol. 106:1391, 1988. 3. Davidson, S. I.: The surgical relief of papilloedema. In Cant, J. S. (ed.): The Optic Nerve, vol. 3. London, Henry Kimpton, 1972, pp. 174-179.
Ocular Motility Anomalies in Developmental Misdirection of the Optic Chiasm EDITOR: The article, "Ocular motility anomalies in developmental misdirection of the optic chi asm," by J. A. W. McCarty, J. L. Demer, L. A. Hovis, and M. R. Nuwer (Am. J. Ophthalmol. 113:86, January 1992), claims that a normally pigmented 35-year-old white man with con genital nystagmus had pattern reversal visualevoked potentials consistent with "a lack of normal decussation of nasal retinal fibers of the optic chiasm." The authors also suggest an association between this putative abnormal de cussation and reverse tracking in congenital nystagmus. Our view is that their conclusion is based on doubtful visual-evoked potential evi dence, and that their eye movement recordings can be interpreted differently. Their conclusion for the presence of an ab normal decussation is based solely on visualevoked potential data to stimulation of the temporal hemifield of the left eye. Responses could not be discerned to stimulation of the nasal field, or from either nasal or temporal hemifield stimulation of the right eye. We criti cize the visual-evoked potential aspects on two points:
601
1. Their interpretation of the responses to left hemifield stimulation of the left eye is highly questionable. The overall impression given by Figure 1 is of an atypical W-shaped waveform, which is about equal size over left and right hemispheres. The labels used to identify peaks have special connotations, which are not explained. In one place the text appears illogical: How can a positive peak at 135 milliseconds (that is, PI35) be "shown as a negative peak"? They provide no rationale for placing the P100 label on the first limb of the W waveform on the right (contralateral) scalp, and the PI35 label on the second limb of the left (ipsilateral) scalp visual-evoked po tential. There is greater justification in revers ing the labelling of the peaks (that is, PI00 ipsilateral and P135 contralateral) as this would agree with what is expected from hemi field stimulation studies in healthy controls (so-called paradoxical lateralization of P1001,2). P100 is predominantly generated by the macular pathway and is distributed on the side of the scalp ipsilateral to the stimulus hemifield. PI35 is produced by the pathway subserving paramacular areas of the visual field and is recorded over the contralateral scalp. It has been shown in healthy controls that the para macular PI35 component virtually disappears when a stimulus field is reduced to stimulate the macular area of the visual field only. Con versely, by experimentally obscuring the macu lar area, P100 becomes highly attenuated but PI35 remains unaltered, or may even enlarge. 3 Given that they detected responses from one hemifield only, McCarty and associates should have attempted these procedures to identify their peaks more reliably. 2. Their choice of stimulus mode was suboptimal. Many studies have documented that pat tern reversal stimulation produces poor visualevoked potentials when nystagmus is pres ent,4 6 so it is not unexpected that McCarty and associates' pattern responses were so poor. It is somewhat surprising that pattern-evoked potentials were detectable from the left hemi field of the left eye, given that no consistent responses were detected from the other three hemifields tested. A transiently presented pat tern stimulus (pattern onset) is superior to pat tern reversal because it produces larger re sponses and is more reliable in indicating an abnormal projection.4-6 To make their point convincingly, McCarty and associates would
602
May, 1992
AMERICAN JOURNAL OF OPHTHALMOLOGY
need to demonstrate pattern-evoked potentials with an opposite occipital distribution when comparing each eye (that is, a crossed asym metry). McCarty and associates also describe reverse tracking during smooth pursuit and optokinet ic nystagmus. So-called reverse optokinetic nystagmus is often seen in patients with con genital nystagmus and has been examined in detail by Halmagyi and associates. 7 They ar gued that during smooth pursuit or optokinetic tracking there is a shift in the null of the con genital nystagmus in the opposite direction of intended eye velocity. This results in a change in the nystagmus waveform even though the eyes may not deviate substantially from pri mary position. Thus, an optokinetic nystagmus stimulus moving to the right induces a null shift to the left so that the nystagmus wave form is similar to that normally associated with right gaze. The nystagmus beats appear, therefore, to be in the wrong direction when compared with normal optokinetic nystagmus; however, they are just the patient's congenital nystagmus with a shifted null rather than re versed optokinetic nystagmus. Even though the so-called reversed optokinetic response is similar in these two studies, McCarty and as sociates dismiss the argument made by Hal magyi and associates, 7 namely that it is stimu lus velocity (or possibly intended eye velocity) that shifts the null, not actual eye position. TONY KRISS, Ph.D. CHRIS HARRIS, Ph.D.
London, United Kingdom SCOTT R. LAMBERT, M.D.
Atlanta, Georgia
References 1. Barrett, G„ Blumhardt, L., Halliday, A. M., Halliday, E., and Kriss, A.: A paradox in the lateralisation of the visual evoked response. Nature 261:253, 1976. 2. Halliday, A. M.: Evoked Potentials in Clinical Testing. Edinburgh, Churchill Livingstone, 1982. 3. Blumhardt, L. D., Barrett, G., Halliday, A. M., and Kriss, A.: The effect of experimental scotomata on the ipsilateral and contralateral responses to pat tern reversal in one half-field. Electroenceph. Clin. Neurophysiol. 45:376, 1978. 4. Creel, D., Spekreijse, H., and Reits, D.: Evoked
potentials in albinos. Efficacy of pattern stimuli in detecting misrouted optic fibers. Electroenceph. Clin. Neurophysiol. 52:595, 1981. 5. Kriss, A., Timms, C , Elston, J., Taylor, D., and Gresty, M.: Visual evoked potentials in dissociated vertical deviation. A reappraisal. Br. J. Ophthalmol. 73:265, 1989. 6. Zubcov, A. A., Fendick, M. G., Gottlob, I., Wizov, S. S., and Reinecke, R. D.: Visual-evoked potentials in dissociated vertical deviation. Am. J. Ophthalmol. 112:714, 1991. 7. Halmagyi, G. M., Gresty, M. A., and Leech, J.: Reversed optokinetic nystagmus (OKN). Mechanism and clinical significance. Ann. Neurol. 7:429, 1980.
Reply EDITOR:
We appreciate the interest taken by Drs. Kriss, Harris, and Lambert in our report of ocular motility anomalies in a patient with evoked-potential evidence of misdirection of the optic chiasm. They suggested that tran siently presented patterns might have pro duced larger visual-evoked potentials. Our technique, which is standard for hemifield test ing,1 did produce, in two separate testing ses sions, adequate potentials to demonstrate the oppositely lateralized positivities at about 100 and 135 milliseconds, as may be seen in Figure 1. We followed standard hemifield procedures for scoring our wave peaks, 1 and we gave due consideration to paradoxic localization. Kriss, Harris, and Lambert have suggested that we reverse the labels P100 and P135, making the PI35 first to occur. We are not aware of any procedure for such a nonstandard maneuver. Since there is no anatomic necessity for chiasmal misrouting to be symmetrical either with respect to eyes or hemifields within eyes, we were not surprised to find that reversal of the side of stimulation did not produce a mirror image of the evoked-potential anomaly. We agree that greater amplitude waves would he advantageous. Transient stimulus presentation, when employed for full-field stimulation, has been shown to be superior to pattern reversal in demonstration of the anomalous visual pro jections of albinism, 2 although the basis for this advantage is unknown. Whether this su periority would extend to hemifield testing re mains to be demonstrated, as would relevance to other causes of nystagmus besides albinism. Drs. Kriss, Harris, and Lambert have reiterat-