Department of Pathology, University of Oklahoma Health Sciences Center

NeuroTest Question #12 Next question Previous question

Answer: D (Chiari type II malformation) Level of difficulty: 3 NeuroTest NeuroLearn NeuroHelp

Discussion: 

• Lesion in question: This is a case of Chiari type II malformation. In addition to displacement of the vermis (black arrow), there is also malformation of the brain stem (whtie arrow).
• Tonsillar herniation due to a posterior fossa tumor: In this situation, the tonsils tends to bulge out of foramen magnum which is a feature that is lacking here. In addition, the herniated tonsils is usually dusky in appearance and edematous because of the increase in intracranial pressure; this feature is also absent here. 
• Dandy-Walker syndrome: The three essential features of Dandy-Walker syndrome include complete or partial agenesis of the vermis, cystic dilatation of the fourth ventricle and enlargement of the posterior fossa. The vermis is present in this cases and is free of hydrocephalic changes. Hydrocephalus is a frequent but inconstant finding. Other CNS findings include elevation of the tentorium cerebelli and lateral, transverse sinuses and torcula (torcular Herophilli), and lack of patency of the foramina of Magendie and Luschka. Other cerebral and visceral anomalies are present. It is the presene or absence of other cerebral and viseral abnormalies that determines the prognosis.
• Chiari type I: Conical elongations of the cerebellar tonsils and neighboring parts of the cerebellar hemispheres that extend into the vertebral canal (i.e., below the foramen of magnum). The protruded cerebellar tissue could be histologically normal, infracted or sclerosed. The medulla is either unaffected or flattened by the cerebellar tongues. Often associated with syringomyelia, hydromyelia, syringomyelia, and less commonly hydrocephalus.
• Chiari type II: Displacement of the cerebellar vermis combined with deformities of the medulla and tectal plate. Often associated with syringomyelia, hydromyelia, spinal bifida, meningocele, and hydrocephalus. It can also associate with other malformation of the brain, cranium and meninges, cardiovascular, gastrointestinal and genitourinary systems. Most, if not all, Chiari type II malformations are associated with with neural tube defects.
• Chiari type III: Encephalocele formed by herniation of the structures of the posterior fossa, including the cerebellum, through an occipitocervical or high cervical bony defect. There may also be beaking of of the tectum, elongation and kinging of the brain astem and lumbar spina bifida.

Comment: KarMing-Fung@ouhsc.edu