Chiari ii malformation

(also alled Arnold Chiari malformation), (Julius Arnold, 1835 - 1915, German pathologist; Hans Chiari, 1851 - 1916, German pathologist), hindbrain malformation first described by Chiari consisting of cerebellar herniation through the foramen magnum, elongation and downward displacement of the fourth ventricle, pons and medulla in to the upper cervical canal. These findings are thought to result from the development of the posterior fossa contents in a posterior fossa which is too small and has a low tentorial insertion. The displaced medulla may cause kinking of the junction between medulla and upper cervical cord in up 70% of patients. Other features include a wide tentorial notch, a beaked tectal plate, fenestration of the falx together with interdigitation of the falx in the occipital regions. Multiple small gyri, stenogyria, are seen on the medial occipital lobes. The massa intermedia is large. The ventricles have a dysplastic appearance with squared off frontal horns and colpocephaly. Almost all patients with Chiari II malformation are born with a myelomeningocele (Fig.1). Most patients (>80%) will develop hydrocephalus, commonly secondary to aqueductal stenosis. Other associated anomalies include callosal dysgenesis, neuronal migrational disorders, absent septum pellucidum and dorsal midline cyst. On skull radiographs of infants with Chiari II malformation the vault is dysplastic, demonstrating a lacunar appearance known as Luckenschadel skull. Chiari II malformation is frequently diagnosed antenatally. In addition to the presence of myelomeningocele, the cisterna magna will be effaced secondary to the downward displacement of the brain stem and cerebellum. The cerebellum is flattened producing a 'banana' shape. This may be evident from 14 weeks gestation. Associated abnormalities including hydrocephalus and colpocephaly and callosal dysgenesis may also be detected. Postnatal imaging of Chiari II malformation is best done with MRI.

Development of hydrocephalus can be monitored using ultrasound prior to closure of the anterior fontanelle and by CT or MRI after closure of the anterior fontanelle. It has been shown that antenatal repair of myelomeningocele causes the Chiari II malformation to resolve with return of the posterior fossa structures to their normal configuration. The distal cord is distorted and tethered at the level of the myelomeningocele. The tectum is often distorted in patients with a Chiari II malformation. The fourth ventricle may be isolated. This can occur when it herniates posteriorly and inferiorly behind the medulla or it may result from aqueductal scarring and narrowing. The isolated fourth ventricle may become "encysted".

Chiari III malformation is a very rare condition in which there is herniation of posterior fossa structures through a spina bifida at C1 - C2 level. Also, see Chiari malformation

EP