Chiari malformations, types I-IV, refer to a spectrum of congenital hindbrain abnormalities affecting the structural relationships between the cerebellum, brainstem, the upper cervical cord, and the bony cranial base.
(See the images below.)
Although Cleland described the first cases of Chiari malformation in 1883, the disorder is named after Hans Chiari, an Austrian pathologist, who classified Chiari malformations into types I through III in 1891. Chiari’s colleague, Julius Arnold, made additional contributions to the definition of Chiari II malformation.
In his honor, students of Dr. Arnold later named the type II malformation Arnold-Chiari malformation. Other investigators later added the type IV malformation.
It is not at all clear that the 4 types of Chiari malformation represent a disease continuum corresponding to a single disorder. The 4 types (particularly types III and IV) are increasingly believed to have different pathogenesis and share little in common other than their names. Chiari type I malformation is the most common and the least severe of the spectrum, often diagnosed in adulthood. Chiari type II malformation is less common and more severe, almost invariably associated with myelomeningocele. Chiari type III and IV malformations are exceedingly rare and generally incompatible with life and are, therefore, of scant clinical significance.
Classification is based on the morphology of the malformations
Chiari I: >5mm descent of the caudal tip of cerebellar tonsils past the foramen magnum.
Chiari II: brainstem, fourth ventricle, and >5 mm descent of the caudal tip of cerebellar tonsils past the foramen magnum with spina bifida.
Chiari III: herniation of the cerebellum with or without the brainstem through a posterior encephalocele.
Chiari IV: Cerebellar hypoplasia or aplasia with normal posterior fossa and no hindbrain herniation.
This article discusses Chiari type I and II malformations with emphasis on the more common Chiari I malformation.
MRI is the most useful and most widely used imaging study for diagnosing Chiari malformation. CSF flow analysis through foramen magnum with phase-contrast cine MRI helps distinguish symptomatic Chiari I from asymptomatic cerebellar ectopia
and helps predict response to surgical decompression.
Other potentially useful tests include myelography as an alternative in patients in who cannot undergo MRI, and CT or radiographs of the neck and head, which may help reveal common associated bony defects, particularly of the craniocervical junction.
Patients with Chiari I malformations who have minimal or equivocal symptoms without syringomyelia can be treated conservatively. Mild neck pain and headaches can be treated with analgesics, muscle relaxants, and occasional use of a soft collar. Frankly symptomatic patients should be offered surgical treatment. The goals of surgical treatment are decompression of cervicomedullary junction and restoration of normal CSF flow in the region of foramen magnum.
The most common complications of Chiari decompression are pseudomeningocele formation and CSF leakage.
Chiari I patients may have an increased risk of concussion and postconcussion syndrome.
T2 hyperintense region on MRI (arrow) depicting edema in central cord region of a patient with Chiari I malformation. Left untreated, this patient is likely to develop cavitation of the edematous central cord, resulting in syringomyelia.
CSF hypotension syndrome: Postcontrast MRI before (A) and after (B) treatment with lumbar epidural blood patch. Notice the thick meningeal enhancement (arrows), the relative paucity of CSF in front of the brainstem and behind the cerebellar tonsils, and the engorgement of the pituitary gland before treatment (A). Notice reversal of these abnormalities and ascent of the cerebellar tonsils after treatment (B). In this case, an acquired Chiari malformation was not present, but in some cases it is.