Chiari III malformation Varieties of MRI appearances in two patients S. Cakirer* Department of Radiology, Istanbul Sisli Etfal Hospital, 80220 Istanbul, Turkey Received 30 March 2002; accepted 15 April 2002 Abstract Chiari III malformation is an extremely rare anomaly, which is characterized by the herniation of posterior fossa contents, i.e., cerebellum, brain stem and fourth ventricle, and in some cases upper cervical spinal cord through a low occipital and/or upper cervical osseous defect. We report different presentations of Chiari III malformation in two unusual cases to show the diversity of imaging findings of this rare malformation. D 2002 Elsevier Science Inc. All rights reserved. Keywords: Chiari III malformation; Variations; Magnetic resonance imaging 1. Introduction In his initial report of 1891, Chiari described a single case of caudal medullary displacement and cerebellar herniation, initially through the foramen magnum, then dorsally through a cervical spina bifida, resulting in a cervical encephalocele [1,2]. Since that time, the definition of Chiari III malforma- tion has been expanded by some authors to include cases of hindbrain herniation into cephaloceles, which encompass both the low occipital and upper cervical regions [3–6]. We present magnetic resonance imaging (MRI) pictures of two male patients at the ages of 16 and 2 months with Chiari III malformation. They reveal different posterior fossa findings related to this extremely rare malformation that was reported in only 22 patients in the literature. 2. Case reports 2.1. Case 1 A male baby was born at full term as the product of a spontaneous delivery, as the third child of healthy non- consanguineous parents. He had a prominent hydroceph- alus, and a lack of occipital and upper cervical bony firmness associated with a mild swelling at the back of his neck at birth. An urgent ventriculoperitoneal shunt operation had been performed for his hydrocephalus in a regional hospital. When he was 16 months old, he was referred to our hospital, since the swelling at the back of his neck still persisted and somewhat increased in size. He had moderate degree of quadriparesia. A cranial MRI study was requested, which was per- formed on a 1.0-T MR scanner (1.0 T Siemens Magnetom, Erlangen, Germany) with spin-echo (SE) T1 (TR = 450 ms, TE = 15 ms, flip angle = 90°), fast spin-echo (FSE) T2 (TR = 3500 ms, TE = 90 ms, flip angle = 90°) sequences on three orthogonal planes. The slice thickness/interval values were 5 cm/1 cm. The volume of posterior fossa was small with low-lying transverse sinuses. Tentorium cerebelli was found to be hypoplastic with a heart-shaped incisura. The cerebellar hemispheres were creeping around the brain stem. The anterior border of brain stem was almost ironed out with dilated neighboring anterior cisternal spaces. The deformed mesencephalon revealed mild tectal beaking posteriorly. The fourth ventricle was inferiorly located, but somewhat dilated in its inferior portion. The cerebellovermian formations, as well as the fourth ventricle, and some parts of medulla oblongata located almost totally below basion–opisthion line revealed a posterior herniation through an osseous defect at the level of inferior occipital bone and posterior elements 0899-7071/03/$ – see front matter D 2002 Elsevier Science Inc. All rights reserved. PII:S0899-7071(02)00498-9 * Sinan Cakirer, 67 Ada, Kardelen 4/2, Daire 37, 81120 Atasehir- Istanbul, Turkey. Tel.: +9-216-4553522. E-mail address: scakirer@yahoo.com (S. Cakirer). Journal of Clinical Imaging 27 (2003) 1 – 4