NEURO-IMAGES Posterior fossa arachnoid cyst herniation through craniocervical junction Serkan Arıbal 1 • Emre Zorlu 2 • Ferhat Cuce 3 • Hakan Mutlu 4 Received: 25 March 2016 / Accepted: 12 May 2016 Ó Belgian Neurological Society 2016 Keywords Chiari malformation Á Arachnoid cyst Á Craniocervical junction A 36-year-old female patient presented with a history of intermittent headache. Magnetic resonance imaging (MRI) showed a well-defined, extraaxial cystic mass lesion loca- ted in the left retrocerebellar part of the posterior fossa which was compitable with an arachnoid cyst (Fig. 1a, b). Although arachnoid cysts are a not infrequent occurence in daily neuroradiological practice, this arachnoid cyst demonstrated partial herniation through the foramen mag- num. To the best of our knowledge, it is an unusual feature of this lesion (Fig. 2a, b). As a matter of fact, we did not encounter any case report in the medical literature describing a herniation of cranial arachnoid cyst through the foramen magnum into the cervical spinal canal, as it was seen in our patient. Furthermore, the images also showed a Chiari Type 1 malformation (Fig. 2c). Most of Chiari type 1 malfor- mations are congenital. In rare cases, Chiari malforma- tions can be acquired during life due to the space- occupying conditions, especially within the posterior fossa (mass lesions, hematomas, etc) [1, 2]. Because the arachnoid cyst in our patient is located in the left side of the posterior fossa, while the tonsillar herniation is nearly symmetric on both the sides (Fig. 2d), we consider that the arachnoid cyst and the tonsillar herniation are coex- istence in our case. Because we believe the patient’s headache to be related to both the herniated arachnoid cyst and the Chiari & Serkan Arıbal serkanaribal@gmail.com Emre Zorlu ezorlu@gmail.com Ferhat Cuce ferhatcuce@hotmail.com Hakan Mutlu hakanmutlu1@yahoo.com 1 Radiology Service, Aksaz Military Hospital, Marmaris 48750, Mug ˘la, Turkey 2 Department of Neurosurgery, GATA Haydarpas ¸a Teaching Hospital, U ¨ sku ¨dar, I ˙ stanbul, Turkey 3 Radiology Service, Mevki Military Hospital, Ankara, Turkey 4 Department of Radiology, GATA Haydarpas ¸a Teaching Hospital, U ¨ sku ¨dar, I ˙ stanbul, Turkey 123 Acta Neurol Belg DOI 10.1007/s13760-016-0651-0