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Show Confusion Between Bitemporal Hemianopia and Cecocentral Scotoma Osaguona et al (1) reported a case of a 72-year-old woman with 20/200 visual acuity in both eyes from ethambutol toxicity. They interpreted automated 24-2 visual field studies as showing a bitemporal hemi-anopia. Magnetic resonance images of the optic chiasm were described as showing hyperintense signal on T2-weighted images. As Glaser pointed out, "On occasion, bilateral ceco-central sctomas may mimic the bitemporal depression of chiasmal interference" (2). In this patient, a cecocen-tral scotoma is present, by definition, because the acuity is only 20/200, and there is confluent field loss between the blind spot and central fixation. In a true temporal hemianopia, visual acuity is relatively preserved and the vertical midline is respected. Finally, the magnetic resonance images appear com-pletely normal. Jonathan C. Horton, MD, PhD Departments of Ophthalmology, Neurology, and Physiology, University of California, San Francisco, San Francisco, California Supported by National Eye Institute and Research to Prevent Blindness. The author reports no conflicts of interest. REFERENCES 1. Osaguona VB, Sharpe JA, Awaji SA, Farb RI, Sundaram ANE. Optic chiasm involvement on MRI with ethambutol-induced bitemporal hemianopia. J Neuroophthalmol. 2014;34:155-158. 2. Glaser JS. Neuro-ophthalmology, 2nd edition. Philadelphia, PA: Lippincott Williams & Wilkins, 1990:156. Confusion Between Bitemporal Hemianopia and Cecocentral Scotoma: Reply We appreciate Dr. Horton's comments. In addition to the numerous cases of ethambutol-induced bitempo-ral hemianopia reported in the past, animal studies have also revealed chiasmal damage affecting both the crossing and the noncrossing axons in experimentally induced ethambutol tox-icity (1). Kho et al (2) characterized ethambutol-induced bitemporal hemianopia in a series of patients, and this study has shown the existence of bitemporal visual field defects with or without superimposed central/cecocentral scotomas, and in those without coexisting central or ceco-central scotomas, the visual field defects were highly sug-gestive of chiasmal injury. Although a cecocentral scotoma can occasionally be confused with bitemporal hemianopia, the bitemporal visual field defect in our case plausibly aligned along the vertical midline, and it is not confined to just the central portion of the temporal fields. We agree that the poor visual acuity in our patient cannot be explained by the chiasmal lesion alone, and there may also be additional involvement of the adjacent parts of the optic nerves as described previously in a histopathological study (1). But, this was not evident radiographically. Magnetic resonance images (MRI) from our article were reviewed again by our neuroradiologist and also in a "masked" fashion by 3 other neuroradiologists who had not seen the imaging and were not aware of the case. These neuroradiol-ogists readily and unanimously concluded that 1) the original MRI was clearly abnormal with increased T2 signal within the chiasm and was also mildly swollen, and 2) the signal intensity within the chiasm normalized on the follow-up study. This imaging finding correlates with our patient's bitemporal hemianopia. Arun N. E. Sundaram, MD, MSc, FRCPC Richard I. Farb, MD, FRCPC Vivian B. Osaguona, BSc, MBCHB, FWACS Toronto Western Hospital, Toronto, Ontario, Canada The authors report no conflicts of interest. REFERENCES 1. Lessell S. Histopathology of experimental ethambutol intoxication. Invest Ophthalmol Vis Sci. 1976;15:765-769. 2. Kho RC, Al-Obailan M, Arnold AC. Bitemporal visual field defects in ethambutol-induced optic neuropathy. J Neuroophthalmol. 2011;31:121-126. 428 Letters to the Editor: J Neuro-Ophthalmol 2014; 34: 422-428 Letters to the Editor Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. |
