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Show resonance imaging revealed no abnormalities along the visual pathways but did demonstrate "ovoid" globes (Fig. 3). Optical coherence tomography and B-scan ultra-sonography confirmed nasal staphylomata in both eyes and showed the eyes to be longer from front to back nasally (29.06 mm, right eye, and 31.40 mm, left eye) than tem-porally (27.74 mm, right eye, and 29.04 mm, left eye) The diagnosis of refractive bitemporal visual field defects due to staphylomata was confirmed by repeating the examination with the patient wearing her distance refraction, which abolished the field defects (Fig. 2B). A posterior staphyloma is a protrusion of the posterior wall of the eye (2), commonly associated with high myopia (3), and is known to give rise to visual field defects (4,5). One possible explanation is that nerve fibres are under mechanical tension in the area of the staphyloma which reduces axonal flow impairing optic nerve function (5). An alternate explanation is that the field defects are due to a differing refractive error in the location of the staph-yloma caused by increased axial length (6). Our findings support the latter possibility; we repeated visual field testing with a higher power of the correcting lens, compensating for the staphylomatous refractive error. Light previously defo-cussed over the staphyloma was now in focus and therefore perceived by the patient (Fig. 4). Although a rare cause, bilateral nasal staphylomata should be included in the differential diagnosis of bitem-poral hemianopia. Unnecessary imaging to rule out com-pressive lesions of the optic chiasm could have been avoided by retesting fields with a different refractive correction. Clues to diagnosis were bilateral nasal retinal pallor, visual field defects crossing the vertical midline, and the presence of full fields to confrontation. Anjali Gupta, MBChB Victoria Eye Unit, Wye Valley NHS Trust, County Hospital, Hereford, United Kingdom J. M. Alaric Smith, MBChB, MRCP, FRCOphth Victoria Eye Unit, Wye Valley NHS Trust, County Hospital, Hereford, United Kingdom The authors report no conflicts of interest. ACKNOWLEDGMENTS The authors thank Dr Raveesh Hanasoge, MD, DNB, FRCR, for help with MRI images. REFERENCES 1. Wang X, Neely AJ, McIlwaine GG, Tahtali M, Lillicrap TP, Lueck CJ. Finite element modelling of optic chiasmal compression. J Neuroophthalmol. 2014;34:324-330. 2. Ohno-Matsui K. Proposed classification of posterior staphylomas based on analyses of eye shape by three-dimensional magnetic resonance imaging and wide-field fundus imaging. Ophthalmology. 2014;121:1798-1809. 3. Chang L, Pan C-W, Ohno-Matsui K, et al. Myopia-related fundus changes in Singapore adults with high myopia. Am J Ophthalmol. 2013;155:991-999. 4. Tsatos M, Eke T. Staphyloma causing visual field defect, illustrating the risk of globe perforation with sharp-needle anaesthesia. Eye. 2007;21:857-858. 5. Ohno-Matsui K, Shimada N, Yasumzumi K, et al. Long-term development of significant visual field defects in highly myopic eyes. Am J Ophthalmol. 2011;152:256-265. 6. Elkington AR, Frank HJ, Greaney MJ. Refraction by the eye and optics of ametropia. In: Clinical Optics, 3rd edition. Oxford, MS: Blackwell Science Ltd, 1999:99-121. Reduced Apparent Diffusion Coefficient in Neuromyelitis Optica-Associated Optic Neuropathy In their excellent review of neuromyelitis optica (NMO), Morrow and Wingerchuk (1) discuss the criteria that distinguish this condition from multiple sclerosis. Here, we describe diffusion-weighted magnetic resonance imaging (DWI) of the optic nerve in a patient with NMO-associated optic neuropathy. A 21-year-old man developed vision loss in the right eye that was diagnosed as optic neuritis. After a delay of 5 months, he was referred for neuro-ophthalmology evaluation. Visual acuity was 20/400 in the right eye and 20/20 in the left eye. Visual field testing showed severe depression in the right eye and normal function in the left eye. The right optic disc was pale and the left optic disc appeared normal. Magnetic resonance imaging (MRI) revealed slight atrophy of the right optic nerve, but no demyelinating plaques. A year later, the patient experienced rapidly progressive vision loss in the left eye. The acuity was 20/400 in the right eye and 20/800 in the left eye, with generalized visual field loss in the left eye. The right optic disc was pale and the left optic disc was edematous. The patient was treated intravenously with methylprednisolone, followed by oral prednisone. Serum and cerebrospinal fluid were positive for anti-AQP4 immunoglobulin G, as determined by a cell-based fluorescence assay (2). Three weeks later, acuity in the left eye was 20/80, with improvement in the visual field. MRI of the orbits 4 weeks after the onset of vision loss showed enhancement of the left optic nerve (Fig. 1A), and DWI revealed increased signal along the course of the left optic nerve (Fig. 1B). The apparent diffusion coefficient (ADC) map demonstrated hypointense signal within the left optic nerve (Fig. 1C). The ADC value along a representative 45.6 mm2 segment of the left optic nerve measured 0.875 · 1023 mm2/s and along a 48.4 mm2 segment of the right optic nerve was 1.63 · 1023 mm2/s. DWI obtained a year earlier, Letters to the Editor: J Neuro-Ophthalmol 2015; 35: 94-105 101 Letters to the Editor Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. when visual function in the left eye was normal, yielded an ADC in the left optic nerve of 1.03 · 1023 mm2/s. In patients with NMO, immunohistochemical analysis of lesions has shown local depletion of aquaporin-4 channels (3,4). Loss of AQP4 channels in the optic nerve may con-tribute to the reduction of water movement. It remains to be determined if our finding of restricted diffusion will be rep-licated in other patients with NMO-optic neuropathy. A broader issue is whether DWI will prove useful in differen-tiating different types of optic neuropathy. It has been pro-posed that DWI may disentangle ischemic optic neuropathy from optic neuritis, which can have overlapping clinical pro-files (5). In a recent study, DWI was abnormal in 5/5 patients with acute ischemic optic neuropathy, but only in 2/25 patients with acute optic neuritis (6). However, other studies have concluded that impaired diffusion is a common feature in optic neuritis (7,8). Further MR studies are needed to clarify the prevalence, time course, and correlation with severity of findings of DWI abnormalities in ischemic optic neuropathy, optic neuritis, and NMO-optic neuropathy. Jonathan C. Horton, MD, PhD Departments of Neurology and Ophthalmology University of California San Francisco, California Vanja C. Douglas, MD Department of Neurology University of California San Francisco, California Soonmee Cha, MD Department of Radiology University of California San Francisco, California Supported by National Eye Institute and Research to Prevent Blindness. The authors report no conflicts of interest. REFERENCES 1. Morrow MJ, Wingerchuk D. Neuromyelitis optica. J Neuroophthalmol. 2012;32:154-166. 2. Fryer JP, Lennon VA, Pittock SJ, Jenkins SM, Fallier-Becker P, Clardy SL, Horta E, Jedynak EA, Lucchinetti CF, Shuster EA, Weinshenker BG, Wingerchuk DM, McKeon A. AQP4 autoantibody assay performance in clinical lab service. Neurol Neuroimmunol Neuroinflamm. 2014;1:e11. 3. Misu T, Fujihara K, Kakita A, Konno H, Nakamura M, Watanabe S, Takahashi T, Nakashima I, Takahashi H, Itoyama Y. Loss of aquaporin 4 in lesions of neuromyelitis optica: distinction from multiple sclerosis. Brain. 2007;130:1224-1234. 4. Roemer SF, Parisi JE, Lennon VA, Benarroch EE, Lassmann H, Bruck W, Mandler RN, Weinshenker BG, Pittock SJ, Wingerchuk DM, Lucchinetti CF. Pattern-specific loss of aquaporin-4 immunoreactivity distinguishes neuromyelitis optica from multiple sclerosis. Brain. 2007;130:1194-1205. 5. He M, Cestari D, Cunnane MB, Rizzo JF III. The use of diffusion MRI in ischemic optic neuropathy and optic neuritis. Semin Ophthalmol. 2010;25:225-232. 6. Bender B, Heine C, Danz S, Bischof F, Reimann K, Bender M, Nagele T, Ernemann U, Korn A. Diffusion restriction of the optic nerve in patients with acute visual deficit. J Magn Reson Imaging. 2014;40:334-340. 7. Fatima Z, Motosugi U, Muhi A, Hori M, Ishigame K, Araki T. Diffusion-weighted imaging in optic neuritis. Can Assoc Radiol J. 2013;64:51-55. 8. van der Walt A, Kolbe SC, Wang YE, Klistorner A, Shuey N, Ahmadi G, Paine M, Marriott M, Mitchell P, Egan GF, Butzkueven H, Kilpatrick TJ. Optic nerve diffusion tensor imaging after acute optic neuritis predicts axonal and visual outcomes. PLoS One. 2013;8:e83825. FIG. 1. A. Postcontrast T1 axial 3-T magnetic resonance imaging with fat saturation demonstrates enhancement of the left optic nerve (arrow). B. Diffusion-weighted image shows bright signal in the left optic nerve, corresponding to the area of contrast enhancement. C. Apparent diffusion coefficient map reveals dark signal (arrow) in the left optic nerve consistent with restricted diffusion. 102 Letters to the Editor: J Neuro-Ophthalmol 2015; 35: 94-105 Letters to the Editor Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. |
