Congenital Achiasma

In evaluating a 3-month-old boy with horizontal nystagmus, brain MRI revealed absence of the optic chiasm. The remainder of the brain was normal in appearance. Achiasma was confirmed with diffusion tensor imaging and best visualized with optimized probabilistic-based tractography.

Photo Essay Section Editor: Timothy J. McCulley, MD Congenital Achiasma Augustin Lecler, MD, MSc, Arnaud Attye, MD, Catherine Edelson, MD FIG. 1. Axial (A) and coronal (B) T2 MRI shows absence of the optic chiasm with intact appearance of the intracranial optic nerves (arrows) and optic tracts (arrowheads), respectively. Axial gradient echo scan (C) confirms absence of the chiasm with a thin remnant of tissue at the expected location of the chiasm. Abstract: In evaluating a 3-month-old boy with horizontal nystagmus, brain MRI revealed absence of the optic chiasm. The remainder of the brain was normal in appearance. Achiasma was confirmed with diffusion tensor Department of Radiology (AL), Fondation Ophtalmologique Rothschild, Paris, France; Department of Neuroradiology (AA), CHU Grenoble, Paris, France; and Department of Ophthalmology (CE), Fondation Ophtalmologique Rothschild, Paris, France. The authors report no conflicts of interest. Address correspondence to Augustin Lecler, MD, MSc, Department of Radiology, Fondation Ophtalmologique Adolphe de Rothschild, 25 rue Manin, 75019 Paris, France; E-mail: [e-mail] 344 imaging and best visualized with optimized probabilisticbased tractography. Journal of Neuro-Ophthalmology 2018;38:344-346 doi: 10.1097/WNO.0000000000000650 © 2018 by North American Neuro-Ophthalmology Society A 3-month-old boy was evaluated for nystagmus and abnormal head movements. The patient could follow small objects, his pupils reacted to light, anterior segments were normal, and ophthalmoscopy was unremarkable. His nystagmus pattern was horizontal and pendular in primary Lecler et al: J Neuro-Ophthalmol 2018; 38: 344-346 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. Photo Essay FIG. 2. Diffusion tensor tractography. Axial (A) and coronal (B) images demonstrate absence of decussating fibers with normal appearing optic nerves. gaze, became jerk in right and left gaze, and remained horizontal and pendular in up-gaze and down-gaze. The patient had continuous slow side-to-side head movements. Brain MRI revealed absence of the optic chiasm (Fig. 1). Diffusion tensor tractography confirmed this with absence of FIG. 3. Optimized probabilistic-based tractography. Axial view confirms absence of fiber decussation with an ipsilateral only projection of the anterior visual pathways. Lecler et al: J Neuro-Ophthalmol 2018; 38: 344-346 decussating fibers (Fig. 2). Optimized probabilistic-based tractography also demonstrated absence of visual fiber decussation with the visual pathways only projecting ipsilaterally (Fig. 3). The patient was diagnosed with congenital achiasma and infantile (congenital) nystagmus. At a follow-up visit at age of 5 years, the patient's acuity was 20/100 in the right eye and 20/125 in the left eye. The nystagmus pattern remained unchanged. In achiasma, MRI demonstrates absence of the chiasm or a thin band of presumed connective tissue in the expected position of the chiasm. Achiasma must be distinguished from congenital aplasia of the optic chiasm where the optic nerves and sometimes the eyes and orbits also are abnormal. Achiasma may be associated with other congenital abnormalities such as septo-optic dysplasia (1), esophageal atresia (2), VACTERL syndrome (3), or agenesis of the corpus callosum (3). Diffusion MRI data provide an estimate of white matter fiber orientations noninvasively with diffusion tensor imaging (DTI), or with constrained spherical deconvolution (CSD), a higher-order model shown in optic pathways to be robust in detecting crossing fibers (4-6). The CSD-based method known as optimized probabilistic-based tractography can identify 2 appropriately oriented fiber populations in a chiasmal voxel containing 2 fiber populations. The DTI-based method not only fails to represent the number of fiber populations within each voxel, but also does not provide an orientation estimate that corresponds to either of the fiber populations (7). We used CSD estimation of the fiber orientation distribution to perform probabilistic tractography of the optic chiasm. This technique does 345 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. Photo Essay have limitations including reporter dependency, especially when laying a "seed" to initiate the fiber tracking and postprocessing errors. STATEMENT OF AUTHORSHIP Category 1: a. Conception and design: A. Lecler, C. Edelson, and A. Attye; b. Acquisition of data: A. Lecler, C. Edelson, and A. Attye; c. Analysis and interpretation of data: A. Lecler, C. Edelson, and A. Attye. Category 2: a. Drafting the manuscript: A. Lecler, C. Edelson, and A. Attye; b. Revising it for intellectual content: A. Lecler, C. Edelson, and A. Attye. Category 3: a. Final approval of the completed manuscript: A. Lecler, C. Edelson, and A. Attye. REFERENCES 1. Davis GV, Shock JP. Septo-optic dysplasia associated with seesaw nystagmus. Arch Ophthalmol. 1975;93:137-139. 346 2. Pensiero S, Cecchini P, Michieletto P, Pelizzo G, Madonia M, Parentin F. Congenital aplasia of the optic chiasm and esophageal atresia: a case report. J Med Case Rep. 2011;5:335. 3. Prakash S, Dumoulin SO, Fischbein N, Wandell BA, Liao YJ. Congenital achiasma and see-saw nystagmus in VACTERL syndrome. J Neuroophthalmol. 2010;30:45-48. 4. Lilja Y, Ljungberg M, Starck G, Malmgren K, Rydenhag B, Nilsson DT. Visualizing Meyer's loop: a comparison of deterministic and probabilitstic tractegraphy. Epilepsy Res. 2014;109:481-490. 5. Tournier JD, Calamante F, Connelly A. Robust determination of the fibre orientation distribution in diffusion MRI: non-negativity constrained super-resolved spherical deconvolution. Neuroimage. 2007;35:1459-1472. 6. Calamante F. Track-weighted methods: extracting information from a streamlines tractogram. MAGMA. 2017;30:317-335. 7. Farquharson S, Tournier JD, Calamante F, Fabinyi G, SchneiderKolsky M, Jackson GD, Connelly A. White matter fiber tractography: why we need to move beyond DTI. J Neurosurg. 2013;118:1367-1377. Lecler et al: J Neuro-Ophthalmol 2018; 38: 344-346 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited.