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Show Journal of Neuro- Ophthalmology 16( 1): 21- 22, 1996. © 1996 Lippincott- Raven Publishers, Philadelphia EDITORIAL COMMENT This paper by Dr. Michaeli- Cohen and colleagues spurred a great deal of controversy not only among our own reviewers but among reviewers for other journals as well. With this in mind, prior to making an editorial decision for or against publication, I wrote to the " big guns," as my predecessor would have called them, including John Flynn, David Guyton, Gene Helveston, Art Jam-polsky, Marshall Parks, and Gunter von Noorden. The following is a series of excerpts from their responses to my query about the " micropsia" reported by this patient. " I read with interest the case report, and I agree . . . that it seems unlikely that this micropsia is related to convergence." " I had no good alternative explanation other than the one you propose; that is, a central nervous system defect that is operative only on conditions associated with binocular diplopia. It is feasible to speculate that under binocular conditions, the retinal rivalry response triggers a cortical mechanism that in turn causes minification of the image." " I think that the patient's story of a fluctuating weakness of abduction and micropsia is certainly suggestive of a convergent spasm mechanism, as is the disappearance of the micropsia after occluding either eye. Against it is the failure to observe miosis or a myopic shift in the retinoscopy when the esotropia is present. I don't understand the fact that a prism correction effectively relieves the micropsia, as that doesn't fit together with the picture." " Micropsia, in my understanding, is a disorder of ' efferenz- kopie' of von Hoist. Every act of accommodation produces a change in the local sign of the retinal elements toward which the image of an object falls, crowding them closer together to compensate for the enlarging retinal image size. The result is size constancy of the object as it nears the eye, within certain limits. Barring a functional or psychogenic origin to her symptoms, this woman has, in fact, a disorder of this mechanism. Since all known retinal causes have been pretty effectively ruled out, this disorder has to have a central origin, but precisely where is unknown. I would suspect that such sensory motor integration involving motion and defocus is most likely to occur over the parietal processing stream ( in the sense of Maunsell and VanEssen), rather than in the temporal stream of visual processing." " I read with interest the case report, and I agree with you that it seems unlikely that this micropsia is related to convergence." " I don't know the explanation of how a brain lesion causes micropsia; however, I do realize how macular pathology, retinal detachment, macular edema, and central serous choroidopathy can cause micropsia. The most common cause of micropsia I see is the psycho- optical effect caused by over- accommodating. . . . I submit that it is more reasonable to consider that the CNS lesion ( brain infarct) stimulated the synkinetic near reflex and the presbyopia prevented any change in the diop-teric lens power; hence, the vision remained unchanged, but the psycho- optical effect of the accommodative convergence produced the micropsia." " If one measures in normal people . . . prism-induced convergence and prism- induced divergence of the stationary near target, one will notice the change in apparent size as one does this. Try it- you may like it." " These phenomena have been reported elsewhere ( I did not check the references, but I suggest Duke- Elder or texts on physiologic objects or the optometry literature), . . . and the explanation usually goes as follows: If a small target of regard is held at the normal reading distance- and prisms are slowly introduced- within the convergence range, the object appears to get smaller and smaller. The story goes that, with convergence, you ' expect' the object to get larger, but it does not. Thus you interpret it as getting smaller. Take it or leave it." " I believe that the authors are entirely correct in their analysis. In other words, I suspect the patient indeed was overconverging in day- to- day affairs, possibly, as they suggest, to increase the angle of separation of the diplopic images in order to ignore the second image more easily. Whatever the case, inappropriately increased convergence can certainly cause micropsia, although I have only been aware of this happening under binocular conditions in my own experience. . . . I have continued to include an experiment in the physiological optic laboratory whereby the student overconverges when looking at two coins, or playing cards, on a tabletop. When the overconvergence is enough 22 COMMENTARY such that the student fuses the diplopic images, thereby seeing three coins, the center coin always appears smaller than the outer two. This is apparently a phenomenon that has been known for some time, but what is peculiar is that the fused central image of three images is smaller, whereas the peripheral image seen by each eye appears normal. In other words, only the foveally appreciated image is the one that appears smaller, and under the conditions of this experiment, it only occurs when fusing the central images with both foveas. " Having just dictated this, I felt compelled to repeat the experiment using only one coin on the table and converging to see double. To my surprise, when diplopic, the image to which I directed either fovea indeed appeared smaller than the peripherally seen other image. The peripherally seen image is somewhat out of focus because of the peripheral optic aberrations and, therefore, would be expected to be seen as somewhat larger. I cannot tell whether it appears larger only by this amount or by an additional amount that may indeed be related to the convergence I am using. " Whatever the case, I do not think that the condition that the patient is describing is a result of central neurological dysfunction. Rather, she is probably just particularly sensitive to the size changes brought about by increased convergence, or perhaps she is just appreciating the size change from the out- of- focus, peripherally seen image in her deviated eye and thereby interpreting the foveally seen image as smaller. I hope this makes sense. " It is now clear, I believe, that overconvergence produces micropsia. A similar sensory experience should occur when ' stereoing' Kodachromes in free space. The fused middle image is smaller!!! if you converge." Ronald M. Burde, M. D. Editor- in- Chief / Neuro- Ophthalmol, Vol. 16, No. 1, 1996 |
