CLINICAL IMAGES Chiari type I a~d II malformations S Andronikou MB BCh, FCRad(D), FRCR (Lond) C Weiman MBChB E Kader MBChB Department of Paediatric Radiology University of Cape Town and Institute of Child Health, Red Cross War Memorial Children's Hospital, Cape Town Despite the same name, Chiari types I and II are unrelated entities. Chiari II malformation is initially encountered in children, occurring in virtually all those who have myelomeningocoele. Chiari type II malformation This is also known as the Arnold- Chiari malformation. Once a myelomeningocoele has been closed surgically after birth, most patients develop hydrocephalus. This is the commonest indication for radiological evaluation at which time the features of Chiari type II will be visualised. This anomaly is formed when the posterior neuropore fails to close resulting in failure of the ventri- cles to expand and so create a normal size posterior fossa with subsequent failing in separation of the thalami.' Intra-uterine surgical closure of the myelomeningocoele (http://www.fetalsurgery.com) pre- vents development of this malforma- tion. The imaging features result from Fig. 1. Chiari type " malformation. Sagittal T1 weighted MRI image shows the small posterior fossa, tonsillar vermian herniation through foramen magnum (1), low-lying venous confluence with a steep (vertically orientated) straight sinus (2), low- lying 4th ventricle with a narrow AP diameter (3), tectal (quadrigeminal plate) beaking (4), scalloped clivus (5) and corpus callosum hypogenesis (6). Many of the supratentorial features (not shown here) are best seen on axial images. squeezing of the growing cerebellum out of a small posterior fossa. The radiological features are best demon- strated on a sagittal Tl weighted MR! image. Chiari type I This is usually an isolated abnor- mality but can be seen with condi- tions that have a small posterior fossa. The subgroups of this entity relate to various craniocervical junction abnormalities. Caudal cerebellar ton- sillar ectopia is the primary abnormal- ity. In adults and children less than 5 years of age, projection of the cere- bellar tonsils below the foramen mag- num of greater than 5 mm is abnor- mal. Between the ages of 5 and 15 45 SA JOURNAL OF RADIOLOGY • March 2002 Fig. 2. Axial CT scan through the high parietal region shows marked cerebral interdigItation (arrows) through a fenestrated falx cerebri. Note the parallel lateral ventricles which are prominent pos- teriorly. Fig. 3. Chiarl type I malfomation. Sagittal Tt weight- ed MRI image shows the tonsillar ectopia (long arrow). Associated syringohydromyelia (short arrows) can be found in 20 - 25% of these patients. years tonsillar ectopia is only patho- logical when there is more than 6 mm projecting through the foramen mag- num. Reference I. Barkovich AJ. Paediatric Neuroimaging. 2nd ed. Philadelphia: Lippincott-Raven, 1996: 238-247, 455-488.