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Chiari I malformation

Chiari I malformation is the caudal descent of the cerebellum and brainstem through the foramen magnum. Symptoms are proportional to the degree of descent. MRI is the imaging modality of choice. Treatment with posterior decompression is usually reserved for symptomatic patients or those with a syrinx


Chiari I malformations are more frequently encountered in females 2

Clinical presentation

Unlike Chiari II, III and IV malformations, Chiari I malformations often remain asymptomatic until adulthood.

The likelihood of becoming symptomatic is proportional to the degree of downward descent of the tonsils. All patients who have greater than 12 mm of descent were symptomatic, whereas approximately 30% of those whose descent measured between 5 and 10 mm were asymptomatic 1

Brainstem (medulla) compression and syringomyelia with associated symptoms and signs account for clinical presentation.


Although Chiari I malformations are often isolated abnormalities many associated anomalies have been described. 

A cervical cord syrinx is present in ~35% (range 20-56%) and hydrocephalus in up to 30% 1,3 of cases and both are thought to result from abnormal CSF flow dynamics through the central canal of the cord and around the medulla.

In ~35% (range 23-45%) of cases there are associated skeletal anomalies 1, 3:


Chiari I malformation is characterised by inferior herniation of the cerebellar tonsils through the foramen magnum, due essentially to a mismatch between size and content of the posterior fossa. Chiari I needs to be distinguished from tonsillar ectopia, which is an asymptomatic and incidental finding in normal individuals, whereby the tonsils protrude through the foramen magnum by no more than 3-5 mm 1-2.

Radiographic features

Although historically visible on myelography, cross sectional imaging (especially MRI) is needed to accurately diagnose and assess for Chiari I malformations. In either case the diagnosis is made by measuring how far the tonsils protrude below the margins of the foramen magnum. The distance is measured by drawing a line from the inner margins foramen magnum (ophisthion to basion), and measuring the inferior most part of the tonsils. As is to be expected, values used vary somewhat from author to author 2:

  • above foramen magnum: normal
  • <3 mm: also normal but the term benign tonsillar ectopia can be used
  • 3 to 6 mm: indeterminate, and needs to be correlated with symptoms and presence of syrnix, etc
  • >6 mm: Chiari 1 malformation

Some authors advocate a simpler rule 1

  • above foramen magnum: normal
  • <5 mm: also normal but the term benign tonsillar ectopia can be used 
  • >5 mm: Chiari 1 malformation

To make matters worse the 'normal' position of the cerebellar tonsils varies with age. In neonates the tonsils are located just below the foramen magnum and descend further during childhood, reaching their lowest point somewhere between 5 and 15 years of age. As the individual ages further the tonsils usually ascend coming to rest at the level of the foramen magnum 3. As such although 5mm descent in an adult should be viewed with suspicion, in a child it is most likely normal 3


With modern volumetric scanning, and high quality sagittal reformats relatively good views of the foramen magnum and tonsils can be achieved although the intrinsic lack of contrast (compared to MRI) makes accurate assessment difficult. More frequently the diagnosis is suspected on axial images where the medulla is embraced by the tonsils and little if any CSF is present. This is referred to as a crowded foramen magnum


MRI is the imaging modality of choice. On sagittal imaging, the best plane for assessing for the presence of Chiari I malformations, the tonsils are pointed, rather than rounded and referred to as peg-like. The sulci are vertically oriented, forming so-called sergeant stripes. Axial images through the the foramen show crowding of the medulla by the tonsils. A syrinx may be seen in spinal cord.

Treatment and prognosis

Chiari I malformations can be divided into three stages (although few use such nomenclature in day to day practice):

  •  I: asymptomatic
  • II: brain stem compression
  • III: syrinx

Treatment is usually reserved only for symptomatic patients or those with a syrinx. It consists of decompressing the posterior fossa, by removing part of the occipital bone, and posterior arch of C1 as well as performing a duroplasty.

History and etymology

It was first described in 1891 by Hans Chiari, Austrian pathologist (1851-1916), based on the autopsies of children.

Differential diagnosis

Imaging differential considerations include:

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