BRAIN CME ABBREVIATION KEY AIDS ϭ acquired immunodeficiency syndrome CCF ϭ carotid-cavernous sinus fistula GBM ϭ glioblastoma NCC ϭ neurocysticercosis In and Around the Optic Chiasm: TB ϭ tuberculosis A Pictorial Review of Neuroimaging Received November 1, 2017; accepted March 3, 2018. F. Zaccagna, V. Pizzuti, D.G. Barone, P. Siotto, L. Saba, E. Raz, T. Matys, and T.F. Massoud From the Departments of Radiology (F.Z., T.M.) and Clinical Neurosciences (D.G.B.), School of Clinical Medicine, University of Cambridge, Cambridge, United Kingdom, Department of Neurology and Psychiatry (V.P.), CME Credit Sapienza University of Rome, The American Society of Neuroradiology (ASNR) is accredited by the Accreditation Council for Continuing Medical Education Rome, Italy, Department of (ACCME) to provide continuing medical education for physicians. The ASNR designates this enduring material for a maximum of 1 AMA Radiology (P.S.), Azienda PRA Category 1 CreditTM. Physicians should claim only the credit commensurate with the extent of their participation in the activity. To Ospedaliera Brotzu, Cagliari, Italy, obtain Self-Assessment CME (SA-CME) credit for this activity, an online quiz must be successfully completed and submitted. ASNR Department of Radiology (L.S.), members may access this quiz at no charge by logging on to eCME at http://members.asnr.org. Nonmembers may pay a small fee to University of Cagliari, Azienda access the quiz and obtain credit via https://members.asnr.org/webcast/content/course_list.asp?srcNeurographics. Activity Release Ospedaliero Universitaria, Cagliari, Date: November 2018. Activity Termination Date: November 2021. Italy, Department of Radiology (E.R.), New York University School of Medicine, New York, New York, and Division of Neuroimaging and Neurointervention (T.F.M.), ABSTRACT Stanford University School of Medicine, Stanford, California. The optic chiasm is a key anatomic structure along the visual pathway, situated at the Previously presented as an crossroads between the optic nerves and tracts. A wide range of diseases can affect the educational poster at The optic chiasm and its surrounding strategic region at the base of the brain. Management Foundation of the American Society of Neuroradiology Annual strategies for optic chiasm abnormalities vary substantially, depending on the abnormalities Symposium, April 22–23, 2017, revealed on neuroimaging. Scant attention has been paid to date to the comprehensive Long Beach, California. classification of neuroimaging manifestations of optic chiasm abnormalities. We compre- Please address correspondence to Fulvio Zaccagna, MD, Department hensively reviewed and presented the imaging findings in a wide spectrum of pathologies of Radiology, School of Clinical that originate from or involve the optic chiasm. This review will aid in differentiating the Medicine, University of Cambridge, Box 218, Cambridge Biomedical many neuroimaging appearances of lesions in this region. Campus, Cambridge, CB2 0QQ, United Kingdom; e-mail: Learning Objective: List the lesions that involve the optic chiasm and classify them accord- [email protected]. ing to their etiology. http://dx.doi.org/10.3174/ng.1700068 Disclosures 1 Based on information received INTRODUCTION asm. The intracranial course of the optic from the authors, Neurographics The optic chiasm is a key anatomic struc- tracts arises from the posterior aspect of has determined that there are no Financial Disclosures or Conflicts of ture along the visual pathway, situated at the chiasm, with an angle between 15° and Interest to report. the crossroads between the optic nerves 45°. and tracts. As is the case for many ana- When looking at the regional anatomy tomic names, its structure defines its name: (Fig 1), the chiasm sits just below the hy- “chiasma” is indeed derived from the pothalamus, overlies the tuberculum sellae Greek ␹␫␣´ ␨␻, meaning “to mark with an of the sphenoid bone and the pituitary X” (after the Greek letter ␹), which is the gland, and lies within the wall of the third shape of the chiasm when looked at from ventricle; it is surrounded by CSF, with the above or below. The optic nerves come to- chiasmatic cistern being just anterior. The gether within the chiasma to allow the infundibulum of the pituitary lies immedi- crossing of fibers from the nasal retina to ately posteriorly, and the mammillary bod- the contralateral optic tract; this enables ies are behind this, medial to the 2 optic visual inputs from the nasal half of the eye tracts. The optic chiasm also lies at a cross- to be processed by the contralateral occip- roads of blood vessels; the internal carotid ital lobe. It is estimated that approximately arteries lie on either side, and the anterior 53% of fibers cross within the optic chi- communicating artery is just directly above Neurographics • www.neurographics.org ͉ 1 Fig 1. Anatomy of the optic chiasm. Coronal (A) and sagittal (B) views of the optic chiasm and the surrounding structures. (Reproduced with permission from Di Ieva A, Rotondo F, Syro LV, et al. Aggressive pituitary adenomas–diagnosis and emerging treatments. Nat Rev Endocrinol 2014;10:423–35.) the chiasm. The blood supply of the chiasm can be quite has shown that humans with albinism have significantly variable. In most cases, the feeder vessels are branches of the smaller chiasmatic widths, smaller optic nerves and tracts, anterior communicating artery, anterior cerebral, posterior and wider angles between nerves and tracts. The size and communicating, posterior cerebral, and basilar arteries. How- configuration of the optic chiasm are distinctly different ever, there are many collateral vessels, so infarction of the from those in healthy subjects.4 chiasm is extremely rare. In view of the complex anatomic and topographic landmarks adjacent to the chiasm, a wide range Nondecussating Retinal–Fugal Fiber Syndrome. Nondecussat- of diseases can affect this structure. Scant attention has been ing retinal–fugal fiber syndrome, also called achiasma, is paid to the comprehensive classification of the neuroimaging extremely rare and may occur with or without nystagmus. manifestations of optic chiasm abnormalities. In this review, The optic chiasm is absent, with normal appearances of the we addressed the neuroimaging findings of lesions that involve remaining structures of the visual pathway. In achiasma, the chiasm and classify them according to their etiology. there is a disruption of retinal-fugal projections, as well as diminished organization and function throughout the vi- CONGENITAL AND IATROGENIC LESIONS sual pathways. Because of the absence of the optic chiasm, all nasal fibers fail to decussate, thereby misprojecting and Intrinsic Lesions malaligning with projections of the temporal retina of the same eye. Monocular visual evoked potentials reveal “mir- Ocular Albinism. Albinism is a rare congenital disorder ror reversely” interocular ipsilateral asymmetry.5 Besides characterized by a defective synthesis of melanin from ty- nondecussating retinal–fugal fiber syndrome, achiasma or rosine or its complete absence, which results from the mu- hypochiasmia has been reported in patients with congenital tation of 1 or more associated genes. The consequence of anophthalmos and with midline anomalies, and in patients such mutations is a partial or complete absence of pigment with albinism.5 MR imaging shows a complete absence (or in the skin, hair, and eyes. Varying degrees of hypopigmen- partial absence in patients with hypochiasmia) of the optic tation occur, owing to different mutation types, which give chiasm, with the optic nerves transitioning into the optic rise to phenotypic heterogeneity.2 Ocular albinism, an X- tracts. Functional MR imaging confirms electrophysiologic chromosome-linked disorder, results in the reduction or observations of crossed asymmetry, with each ocular cortex absence of melanin only in the eyes, with the melanocytic receiving a complete but monocular visual field. Despite system offering normal pigmentation to the rest of the these abnormalities, vision is normally preserved because of body.3 It is associated with several visual defects, such as a reorganization of intracortical connections.5 decreased visual acuity, monocular vision and photopho- bia, foveal hypoplasia, and congenital misrouting of fibers Radiation Necrosis. Radiation necrosis of the optic chiasm within the chiasm. Quantitative testing of visual evoked is extremely rare; however, it may be caused by radiation potentials usually demonstrates this misrouting at the chi- therapy for pituitary adenomas or other regional malignant asmatic level. Patients with albinism show an asymmetry of lesions. It may involve normal tissues and may mimic recur- visual evoked potentials between the 2 eyes, secondary to rent disease on both the clinical presentation and follow-up misrouting of the optic pathways. Indeed, achiasma (see imaging studies.6 Visual loss is the main symptom and, Nondecussating Retinal–Fugal Fiber Syndrome) has rarely when it occurs in a patient who has undergone surgery for a been reported in patients with albinism.2 MR imaging has a sellar, parasellar, or skull base tumor with adjuvant radia- limited potential in assessing this misrouting; however, it tion therapy, the clinical presentation may be helpful for the 2 ͉ Neurographics • www.neurographics.org differential diagnosis. Visual loss owing to radiation-in- optic chiasm or tracts and the pituitary gland. This lesion is duced optic neuropathy, in fact, causes a rapid deteriora- most likely