Chiari I malformation: clinical presentation and management.

Chiari malformations are cerebellar anomalies. The four types of Chiari malformations, as described more than 100 years ago by Dr. Hans Chiari, have neither anatomic nor embryologic correlation. Their only commonality is that they all involve the cerebellum. Chiari I malformation consists of herniation of the cerebellar tonsils into the foramen magnum, thus crowding the craniocervical junction. Chiari II malformation is almost exclusively associated with myelomeningocele and hydrocephalus. It consists of herniation of not only the tonsils but also all the contents of the posterior fossa into the foramen magnum. This herniation involves the brainstem, fourth ventricle, and cerebellar vermis. Chiari III and IV malformations are rare. Chiari III represents an encephalocele (external sac containing brainstem and posterior fossa contents); thus, the cerebellum and brainstem are descending not only into the spine, but also into an external sac. Chiari IV consists of cerebellar hypoplasia. The Chiari I malformation has the latest mean age of clinical presentation. A Chiari type I anomaly presenting in adulthood is the focus of this article. Surgery is indicated with neurological dysfunction, symptomatic syrinx, or hydrocephalus. Of all Chiari I patients, 15%-20% will have hydrocephalus. For some of them, the hydrocephalus will resolve with ventriculoperitoneal shunting, alleviating the need for a Chiari decompression. Long-term prognosis for patients with symptomatic Chiari type I malformations who undergo surgical treatment is variable, based on the patients' presenting symptoms and spinal cord cyst response.