Updated 2006-01-18 by Juliana Leite, MD

Original text 1999-0617 Philippe Jeanty, MD, PhD & Sandra R Silva, MD

Synonym: The nickname “banana sign” has been applied to the deformity of the cerebellum.

Definition: The Arnold-Chiari was first identified in 1883 by Cleland. It is characterized by the prolapse of hindbrain structures below the level of the foramen magnum. It can be associated with skeletal abnormalities and neurological dysfunction. There are three types of Arnold-Chiari malformation:

• Type I: just a lip of cerebellum is downwardly displaced with the tonsils, but the fourth ventricle remains in the posterior fossa. This is mainly an incidental CT discovery.
• Type II: usually involved in prenatal cases and is a congenital deformity characterized by displacement of cerebellar tonsils, parts of the cerebellum, fourth ventricle, pons, and medulla oblongata through the foramen magnum into the spinal canal. In 95% percent of the cases it is accompanied by hydrocephalus and myelomeningocele.
• Type III: more severe form, with large herniation of the posterior fossa content and myelomeningocele and hydrocephalus.

Figure 1: Schematic changes in Arnold-Chiari type II

Figure 2: The banana sign: the lobes of the cerebellum have lost their lumpy appearance and the vermian incisure is not as marked as normal.

Incidence: Commonly associated with spina bifida; rare outside of spina bifida. There is a higher preponderance of Arnold-Chiari type I in females versus males (3:2).

Etiology: Autosomal-recessive for some forms.

Pathogenesis:The abundance of theories regarding the pathogenesis of Chiari malformation reflects the present lack of consensus on the subject. An understanding of the pathogenesis of Chiari malformations must be aimed at examining the pathophysiology at the craniocervical junction. A defect during embryonic development causes underdevelopment of the posterior fossa and blockage of the normal outlets of the fourth ventricle (foramina of Magendie and Luschka), resulting in increased pressure and causing the cerebellar tonsils to descend into the foramen magnum. Accumulated CSF is then forced caudally through the entrance into the central canal of the spinal cord at the caudal-most portion of the fourth ventricle, because of a pressure gradient.
DIAGNOSIS: The displacement of the cerebellum deforms the lateral lobes (which lose their “round shape”) and the vermian notch to appear more continuous (thus the bananas sign). The diagnosis has been made as early as the first trimester. D"Addario et al have evaluated the angle between the clivus and supra occiput and found it a useful parameter to diagnose Chiari II malformation.

Genetic anomalies: Unknown.

Differential diagnosis: Nonobstructive hydrocephalus, such as:

• dysgenesis of the corpus callosum
• aqueductal stenosis
• borderline ventriculomegaly
• Dandy-Walker malformation
• porencephaly
• schizencephaly

Associated anomalies: Hydrocephaly from obstruction of the foramina of Magendie, syringomyelia, diastematomyelia.

Prognosis: Poor due to the central nervous system anomalies.

Managment: Termination of pregnancy can be offered before viability. For those affected by spina bifida, an experimental intrauterine repair done between 23 and 30 weeks’ gestation has been proposed in a few centers in the United States. Standard prenatal care is not altered when continuation of the pregnancy is chosen. Confirmation of diagnosis after birth is important for genetic counseling.

References:

1. Sicuranza GB, Steinberg, P, Figueroa R. Arnold-Chiari Malformation in a Pregnant Woman. Obstet Gynecol 2003;102(5, part2):1191-194.
2. Cheng JS, Nash JBS; Meyer, GA. Chiari Type I Malformation Revisited: Diagnosis and Treatment. Neurologist 2000;8(6):357-62
3. D’AddarioV, Pinto V, Del Bianco A et al. The clivus-supraocciput angle: a useful measurement to evaluate the shape and size of the fetal posterior fossa and to diagnose Chiari II malformation. Ultrasound Obstet Gynecol. 2001;18(2):146-9.