In 1891, 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 characterized by herniation of the cerebellar tonsils through the foramen magnum into the cervical spinal canal (see the images below). The cerebellar tonsils often are elongated and peglike. Mild caudal displacement and flattening or kinking of the medulla may be present. The vermis cerebelli and the fourth ventricle are normal or only minimally deformed.
Chiari I malformation is not directly associated with other congenital brain malformations, specifically myelomeningocele, which is a feature of Chiari II malformation. However, craniovertebral malformations are common in patients with Chiari I malformation. In addition, distention with cerebrospinal fluid (CSF) of the central canal of spinal cord (ie, hydromyelia) or paracentral cavities (ie, syringomyelia) is present in approximately 25% of patients with Chiari I malformation. The cervical cord is the most common site of syringohydromyelia.
The true incidence of Chiari I malformation is not known; before the availability of magnetic resonance imaging (MRI), this condition was rarely diagnosed. In one study, a rate 0.6% was reported in all age groups, and a rate of 0.9% was reported in a study of only pediatric patients. Therefore, this condition is more common in both the adult and pediatric populations than was recognized previously. A female predominance has been reported in some large case series, with a male-to-female ratio of 2:3.
Although the association of Chiari I malformation with cranial and vertebral anomalies is well known, this condition is not directly associated with other neuroectodermal abnormalities. When present, neural abnormalities are attributable to the effects of tonsillar herniation. Acquired Chiari I malformation is reported to develop after lumboperitoneal or ventriculoperitoneal shunt placement, in addition to other conditions. Radiologic findings in patients with acquired Chiari I malformation cannot be distinguished from those in congenital form of the malformation.
Although syringohydromyelia is probably secondary to pathologic CSF dynamics in which the exaggerated pulsatile systolic wave in the spinal subarachnoid space drives the CSF through anatomically continuous perivascular and interstitial spaces into the central canal of the spinal cord, more severe obstruction occasionally can cause hydrocephalus.
In a pediatric case series, syringomyelia (development of a fluid-filled cavity or syrinx within the spinal cord) was observed in 14% of patients; because spinal MRI results were available in only a minority of patients, this figure may represent an underestimation of the presence of syringomyelia. In addition, sufficient time may not have elapsed for syringomyelia to develop.
The anatomic landmarks of this disorder are depicted in the image below. A discussion of the precise anatomy may be found in the sections about osteology and neuroanatomy in Gray's Anatomy. 
MRI is the imaging modality of choice except in patients in whom MRI is contraindicated. MRI demonstrates the abnormal CSF flow and configuration and position of the brain and spinal cord. Brainstem auditory evoked potentials is widely used during posterior fossa decompression to assess functional integrity of the central auditory system during surgery. Improvement has been shown to occur primarily during bony decompression.  MRIs may not reliably demonstrate abnormal findings of the skeleton associated with Chiari malformations. [5, 6, 7, 8] Radiography-based imaging modalities such as conventional radiography or computed tomography (CT) scanning are preferred for this indication.
Radiologic findings in patients with acquired Chiari I malformation cannot be distinguished from those in congenital form of the malformation.
Skeletal anomalies of Chiari I malformation are easily delineated with radiography, including a reduced height of the supraocciput and clivus. Osseous anomalies of the skull base and skeletal system are observed in 25-50% of patients with this condition and include the following (the frequency of association is in parentheses):
Platybasia, basilar invagination (25-50%)
Atlantooccipital assimilation (1-5%)
Klippel-Feil syndrome (5-10%)
Incomplete ossification of C1 ring (5%)
Proatlantal remnant spina bifida at the C1 level
Retroflexed odontoid process (26%)
Increased cervical lordosis
Fused thoracic ribs
CT scanning is reliable in detecting osseous abnormalities. On CT scans of patients with Chiari I malformation, the following findings may be observed:
Obliterated cisterna magna
Flattened spinal cord
Tonsillar ectopia (see the image below)Axial computed tomography scan obtained at the C1 level after cervical myelography. The arrows mark cerebellar tonsils that are abnormally low.
Peglike cerebellar tonsils
Normally positioned fourth ventricle
Rarely, spinal CT scans may show syringomyelia. This modality can also be used to assess associated bony abnormalities of the skull base and vertebral column (see Radiograph).
In the past, CT cisternography and/or myelography, supplemented by image reconstruction in nonaxial planes, was used to assess tonsillar position and configuration. CT myelograms do not demonstrate the lower brainstem and bulbomedullary junction in sufficient detail. Associated syringomyelia is often missed.
CT scanning may be of value in patients in whom MRI is absolutely contraindicated.
Magnetic Resonance Imaging
MRI has revolutionized the diagnostic evaluation for Chiari I malformation, as this modality can be used to detect Chiari I malformation that previously remained unrecognized or was misdiagnosed. Tonsillar position, tonsillar configuration, and many associated abnormalities are depicted on sagittal and axial T1- and T2-weighted MRIs. [5, 6, 7, 8, 9, 10, 11, 12]
On MRIs, the following findings may be observed:
Displacement of cerebellar tonsils below the level of the foramen magnum
Pointed and/or peglike tonsils
Narrow posterior cranial fossa
Elongation of the fourth ventricle, which remains in the normal position
Associated abnormalities such as syringomyelia and skeletal abnormalities
The degree of tonsillar ectopia in Chiari I malformation is expressed as the number of millimeters that the tonsillar tips extend below a line connecting the basion with the opisthion (see the first image below). Perform all measurements by using sagittal T1-weighted images. Use the signal intensity of the cortical bone, not that of the marrow, to define the anatomic landmarks (see the second image below). Tonsillar tips that extend less than 3 mm below the landmark are normal. Tonsillar ectopia of 5 mm is 100% specific and 92% sensitive for Chiari I malformation.
Tonsillar herniation should be primary and not secondary to an intracranial mass lesion (eg, brain tumor, cerebral edema) to meet the criteria for congenital Chiari I malformation. The most reliable criterion is herniation of at least 1 cerebellar tonsil that is 5 mm or more below the plane of the foramen magnum, as defined above. Asymmetric tonsillar herniation may be observed. See the images below.
Tonsillar herniation of less than 5 mm does not exclude the diagnosis. Herniation of both tonsils that are 3-5 mm below the foramen magnum, accompanied by certain other features, may suggest Chiari I malformation. These other features include a syrinx (see the image below), cervicomedullary kinking, elongation of the fourth ventricle, and a pointed or peglike appearance of the tonsils.
Cerebellar tonsils ascend with age. Some authorities suggest the following criteria for tonsillar ectopia: (1) herniation of 6 mm in those aged 0-10 years, (2) herniation of 5 mm in those aged 10-30 years, (3) herniation of 4 mm in those aged 30-80 years, and (4) herniation of 3 mm in those aged 80-90 years.
Narrowing or obliteration of the retrocerebellar CSF spaces is observed in association with a meniscus sign at the lower pole of the cerebellar tonsils. The height of supraocciput is reduced, and the slope of tentorium is increased. The posterior cranial fossa volume, in absolute terms and expressed as a ratio of supratentorial volume (posterior fossa ratio), is significantly smaller; however, mean brain volumes did not differ in patients and control subjects.
The cervical subarachnoid space below the level of the C2-3 disks is markedly narrowed in patients with syringomyelia as a result of spinal cord expansion. The posterior subarachnoid space below the tip of the cerebellar tonsils may be completely obliterated.
Other findings include anterior displacement of the cerebellum, kinking of the medulla, compression of the fourth ventricle, hydrocephalus (mild or moderate), and an empty sella. The cerebral aqueduct is frequently elongated and narrowed; however, no significant descent of the latter structure or the brainstem is observed.
Syringohydromyelia is most commonly observed between the C4 and C6 levels. Holocord hydromyelic cavities may be present. Cervical/upper thoracic and bulbar/cervical syringes also are observed. Isolated thoracic syringes are not described. The level of widest syrinx diameter most frequently occurs at the C2-3 level. Asymmetric or multiple axial syringes are described.
CSF flow abnormalities
Several investigators have studied CSF flow abnormalities in Chiari I malformation. All patients had narrowing of the CSF pathways at the foramen magnum, at the C2-3 disk level, and in the posterior subarachnoid space below the tip of the cerebellar tonsils. The cardiac cycle and respiration-related CSF flow pulsatility are altered. These effects are detectable with motion-sensitive MRI sequences, and they can be gated to the cardiac cycle.
A prolongation of CSF systole is observed in the area above the foramen magnum. In the anterior subarachnoid space below the foramen magnum and in the posterior subarachnoid space immediately below the tips of the cerebellar tonsils, systolic velocities are reduced, and the duration of CSF systole and the ratio of systolic-to-diastolic CSF displacement are decreased. These findings indicate impaired CSF systolic (craniocaudal) pulsations. Diastolic flow is unimpaired.
A reduction of CSF flow can be observed in the subarachnoid space of the posterior cranial fossa (cisterna magna; retrocerebellar, premedullary, and prepontine cisterns), along with a compensatory pulsatile downward motion of the cerebellar tonsils. These flow abnormalities have been shown to revert to normal levels after cranial decompression.
A relationship exists between CSF flow abnormalities detected on MRIs and syringomyelia. However, CSF flow abnormalities are not correlated with the degree of tonsillar ectopia or the presence of clinical symptoms or their severity.
Cine MRI may be helpful in demonstrating a disturbance of CSF velocity and/or flow at the foramen magnum in patients with tonsillar ectopia of less than 5 mm.
The usefulness of MRI flow studies in the management of Chiari I malformation remains uncertain. To the author's knowledge, current guidelines for the prediction of serious complications based on abnormalities seen on MRI flow studies have not been established.
Incidental Chiari I malformation is more common than previously recognized. A number of patients who underwent imaging for reasons unrelated to Chiari I malformation were found to have this condition. In the absence of a syrinx or clinical symptoms and signs, some authors consider follow-up imaging unjustifiable. Careful clinical assessment remains the cornerstone of proper diagnosis and management.