eMedicine Specialties > Radiology > Pediatrics

Chiari I Malformation: Imaging

Author: Nasir H Siddiqi, MD, Assistant Professor of Radiology, Washington University; Attending Radiologist, Vascular and Inerventional Radiology, Mallinckrodt Institute of Radiology, Washington University
Coauthor(s): Lakshmana Das Narla, MD, Director of Pediatric Radiology, Associate Professor, Department of Radiology, Medical College of Virginia
Contributor Information and Disclosures

Updated: May 4, 2009

Radiography

Findings

Skeletal anomalies are easily delineated with radiography. Osseous anomalies of the skull base and skeletal system are observed in 25-50% of patients with Chiari I malformation and include the following, with the frequency of association 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%)
• Scoliosis (42%)
• Kyphosis
• Increased cervical lordosis
• Cervical ribs
• Fused thoracic ribs

Other associated skeletal abnormalities include a reduced height of the supraocciput and clivus.

Degree of Confidence

Radiography is reliable in the demonstration of skeletal abnormalities.

Computed Tomography

Findings

On CT scans, the following findings may be observed:

• Obliterated cisterna magna
• Hydrocephalus
• Flattened spinal cord
• Tonsillar ectopia
• Peglike cerebellar tonsils
• Normally positioned fourth ventricle

Rarely, spinal CT may show syringomyelia. CT can be used to assess associated bony abnormalities of the skull base and vertebral column (see Radiograph section above).

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 may be of value in patients in whom MRI is absolutely contraindicated.

Degree of Confidence

CT is reliable in detecting osseous abnormalities.

Magnetic Resonance Imaging

Findings

MRI has revolutionized the diagnostic evaluation for CMI. It can be used to detect Chiari I malformation (CMI) 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.^10,11,12,13

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 PCF
• Elongation of the fourth ventricle, which remains in the normal position
• Hindbrain abnormalities
• Obstructive hydrocephalus
• Associated abnormalities such as syringomyelia and skeletal abnormalities

Tonsillar ectopia

The degree of tonsillar ectopia is expressed as the number of millimeters that the tonsillar tips extend below a line connecting the basion with the opisthion. 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. Tonsillar tips that extend less than 3 mm below the landmark are normal.

Tonsillar herniation should be primary and not secondary to an intracranial mass lesion (eg, brain tumor, cerebral edema) to meet the criteria for congenital CMI. 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.

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 CMI. These other features include a syrinx, 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.

Other findings

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 PCF 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 C4-6 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 CMI. 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 PCF (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 CMI remains uncertain. Currently, to the author's knowledge, guidelines for the prediction of serious complications based on abnormalities seen on MRI flow studies have not been established.

Incidental CMI is more common than previously recognized. A number of patients who underwent imaging for reasons unrelated to CMI were found to have CMI. 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.

Degree of Confidence

Tonsillar ectopia of 5 mm is 100% specific and 92% sensitive for CMI.

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