Hindawi Publishing Corporation Case Reports in Medicine Volume 2010, Article ID 261657, 4 pages doi:10.1155/2010/261657 Case Report Cervicothoracic Intradural Arachnoid Cyst Misdiagnosed as Motor Neuron Disease P. G. Sa¨mann,1 H. Himmerich,1, 2 T. Merl,1 C. Ero¨s,3 M. B. M u¨ller,1 J. C. Tonn,3 and B. Buchwald1 1 Max Planck Institute of Psychiatry, Kraepelinstraße 2-10, 80804 Munich, Germany 2 Claussen-Simon-Endowed Professorship, Department of Psychiatry and Psychotherapy, University of Leipzig, Semmelweisstraße 10, 04103 Leipzig, Germany 3 Department of Neurosurgery, Klinikum Großhadern, Ludwig Maximilians University, Marchioninistraße 15, 81377 Munich, Germany Correspondence should be addressed to H. Himmerich, hubertus.himmerich@medizin.uni-leipzig.de Received 2 December 2009; Revised 4 March 2010; Accepted 21 April 2010 Academic Editor: Jonathan Cole Copyright © 2010 P. G. Sa¨mann et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Recognizing syndromes which mimic ALS is crucial both to avoid giving this diagnosis erroneously and since there may be appropriate treatments. We report a 63-year-old woman diagnosed with possible ALS ﬁve years ago based on upper and lower motor neuron signs with typical electrophysiology and normal cranial MRI. At reassessment, spinal MRI revealed a cervicothoracic cyst with cord compression that was successfully treated neurosurgically. Histopathology conﬁrmed an arachnoid origin as suspected from MRI. Spinal cysts may mimic ALS and need to be thoroughly excluded by appropriate imaging. 1. Introduction Amyotrophic lateral sclerosis (ALS) is a subset of motor neuron disease in adults and usually causes death within a few years [1]. Deﬁnite diagnosis requires the presence of both upper and lower motor neuron signs in the bulbar, arm and leg musculature with clear evidence of progression. At initial presentation symptoms are often more focal, and especially in the early phase making the diagnosis of ALS challenging even for experienced neurologists. Electrophysi- ological investigations are necessary to diﬀerentiate ALS from potentially treatable polyneuropathies and myopathies [1]. Structural cranial or spinal pathologies should be excluded by magnetic resonance imaging (MRI) [1]. The misdiagnosis of ALS has far-reaching implications, for patients and their families given such a diagnosis, and because potentially curative treatments exist for certain ALS mimic syndromes. Overall, the Escorial criteria are conservative and include a criterion “progressive spread of symptoms or signs within a region or to other regions” to give suﬃcient weight to longitudinal observations [2]. We report a 63-year- old woman with major depression after being diagnosed with ALS for ﬁve years. At reassessment, renewed MRI revealed a cervicothoracic cyst which was successfully treated neurosurgically. 2. Case Report A 63-year-old woman was admitted to the clinic of the Max Planck Institute of Psychiatry with major depression. Her onset of depressed mood had been gradual after being diagnosed with ALS ﬁve years ago. The patient had noted intermittent cramp-like sensations round her chest and a weak, numb neck pain. These symptoms had been ﬂuctuating but not clearly progressive, for about two years when she was referred to a neurologist. File review showed that clinical presentation at that time already comprised objective weakness, atrophy and fasciculations in the upper limb muscles, along with extremely brisk tendon reﬂexes. No unequivocal sensory deﬁcit such as hypesthesia or dysesthesia was documented at that time. Electromyography showed abnormal spontaneous activity in the right deltoid muscle, abducens pollicis brevis and palmar interossei