Background
Historically, Hans von Chiari described certain hindbrain abnormalities as postmortem findings in infants; these came to be known as Chiari malformations. Four types of Chiari malformations are described in the literature: types I, II, III, and IV. 1,2(Chiari malformation types II, III, and IV are distinct from type I and are not discussed in this article). Chiari I malformation is the most common, having been estimated to occur in 1 in 1000 births. 1
Arnold Chiari type I malformation (Type-I ACM) is characterized by cerebellar tonsils herniation and downwardly displaced below the level of the foramen magnum. Typically, tonsils lying 5 mm or more (normally 3 mm) below the foramen magnum on neuroimaging are consistent with an ACM. However, there is no known direct correlation between clinical severity and the tonsils position.
The actual nature of CM-I has not been fully understood.2 In most cases, CM-I does not become symptomatic until adolescence or adulthood 3-5 and the mean age at presentation is approximately 18 years. 6 Also, symptom onset is often insidious. Usually, Symptoms of Type-I ACM develop as a result of the following mechanisms; cerebellum, medulla, and upper spinal cord compression, and disruption of CSF flow through the foramen magnum. Medulla and spinal cord compression may result in spinal cord injury and lower cranial nerve and nuclear dysfunction. Also, the cerebellum compression may result in various neurological symptoms such as dysequilibrium, dysmetria, ataxia, and nystagmus. Disruption of the Cerebral Spinal Fluid (CSF) flow through foramen magnum probably leads to the most common symptom, which is pain.