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.