Radiography
Findings
Plain radiography has been used most frequently for examining bone anomalies, scoliosis, and ventriculoperitoneal shunt malfunction. Plain images can show enlargement of the cervical spinal canal and posterior midline fusion defects, as well as anterior bony abnormalities, such as C1 and C2 dislocation.
Routine radiography can demonstrate l ü ckenschãdel in neonates with myelodysplasia. Additional bony changes that can be seen include scalloping of the posterior surface of the petrous pyramids and the clivus.
Degree of Confidence
Plain radiographs have diagnostic importance in the evaluation of cranial and vertebral bony abnormalities and in the evaluation of ventriculoperitoneal shunt disconnection or displacement of the shunt catheter.
Computed Tomography
Findings
CT scans are used most commonly in patients with Chiari II malformation, for the diagnosis of hydrocephalus, and for the evaluation of suspected shunt malfunction.
Lückenschãdel can be demonstrated on CT scans (see Image 8). Abnormalities are composed of areas of thinning or pits within the calvarial bone. The areas of thinning typically occur on the inner surface of the skull, but they have been seen on both the inner and outer aspects. Within each of the depressions on the inner aspect of the skull, brain tissue can be seen bulging into the bony defect.
CT scans may demonstrate an abnormally large foramen magnum, the flat floor of the posterior fossa, and scalloping of the petrous pyramids and clivus (see Image 2). CT scanning also demonstrates tectal beaking, cerebellar tissue wrapping around the brainstem, fenestrations of the falx, manifestations of hydrocephalus, and shunt malfunction.
CT myelography is rarely performed in patients with Chiari II malformation because of the advent of MRI. High-resolution CT scanning of the spine may be essential for surgical planning, particularly in patients with severe scoliosis and associated complex segmentation anomalies.
Three-dimensional (3-D) CT scans and CT angiograms can be used for a precise analysis of complex osseous malformations, of the relationships between vascular and bony structures, and of postoperative changes in the bone (see Image below and Images 12-13 in Multimedia).
Postoperative 3-dimensional computed tomography scan. This image shows osseous decompression with a large opening of the foramen magnum and resection of the posterior arch of the atlas (arrows). Courtesy of Duffau et al.
Degree of Confidence
CT scanning is the routine diagnostic method for following up patients with a Chiari II malformation and associated hydrocephalus. An encysted fourth ventricle is one of the causes of shunt malfunction. Because the internal regulation of the shunt valve is usually affected by intracranial pressure changes, the pressure dynamics between infratentorial and supratentorial compartments are important.
CT scanning also is helpful for follow-up to assess the dynamic changes in the infratentorial and supratentorial ventricular systems.
Chiari II malformation associated with subtle gyral malformations and spinal cord abnormalities, such as a syrinx, are often missed on routine CT images. Thin-section sagittal and/or coronal reformatted CT images may be useful for assessing spinal anomalies. The most common cause of a syrinx of the spinal cord in patients with a Chiari II malformation is ventriculoperitoneal shunt malfunction. All patients with a Chiari II malformation and syrinx (as detected with spinal MRI) should undergo cranial CT scanning and a radiographic shunt series to exclude shunt malfunction as a cause for appearance or worsening of the syrinx.
False Positives/Negatives
When posterior fossa and cervical CT scans are evaluated individually, the craniocaudal elongation of cerebellum in a Chiari II malformation can be confused with cervical spinal tumors and cerebellar tumors that ascend through foramen magnum or cause cerebellar herniation.
Magnetic Resonance Imaging
Findings
Antenatal magnetic resonance image shows a Chiari II malformation in a fetus. Courtesy of Umit Aksoy, MD, Uludag University, Bursa, Turkey.
T1-weighted axial magnetic resonance image. This image demonstrates heart-shaped incisura and a petrous ridge in a patient with a Chiari II malformation.
MRI provides studies of the altered anatomy and excellent images in all projections (sagittal, axial, coronal). The sagittal view is one of the most useful (see Image below Images 1 and 9-10 in Multimedia).
Sagittal T2-weighted magnetic resonance in a patient with a Chiari II malformation (same patient in Images 9-11 in Multimedia). This image shows a thoracic-level myelomeningocele (arrows). The spinal cord, in addition to the thoracic placode, also extends distally and is further tethered at the sacral level.
Spin-echo T1-weighted sagittal and coronal images and T2-weighted sagittal images, including those of the cervical region, should be obtained. In the authors' department, sagittal images are obtained with 3- or 4-mm thickness and 1-mm gap, and axial and coronal scans are obtained with 5-mm thickness and 1- or 2-mm gap. Craniocervical magnetic resonance (MR) examination is performed in children with myelomeningocele, whereas spinal MR examination is performed in patients with a Chiari II malformation.
Hindbrain anomalies, hydrocephalus, and syrinx cavities in the spinal cord are well demonstrated on T1-weighted images. The extent of the S-shaped medullary kink can be easily evaluated by using sagittal MRI in patients with a Chiari II malformation (see Images 1 and 3). MRI of the head is easily performed to assess the size and position of the ventricles, and it provides important additional information (see Images 1 and 6).
Enlargement of the massa intermedia, occlusion of the cerebral aqueduct, and beaking of the tectum are best evaluated by using a sagittal projection on MRI (see Images 1 and 3). Gyral interdigitation, cerebral gyral anomalies, an engulfed brainstem, and the level of the medullary kink can easily be evaluated by using axial MRI.
MRI enables accurate, objective, and detailed identification of spinal cord and vertebral abnormalities, including syringomyelia and diastematomyelia, both preoperatively and postoperatively (see Image below and Images 9-11 in Multimedia). MRI has also been used as a diagnostic tool during the antenatal period, allowing the intrauterine diagnosis, and even treatment, of spina bifida.
Sagittal sonogram in a patient with a Chiari II malformation (same patient as in Images 9-11). This image shows the thoracic spinal cord (arrowheads) and myelomeningocele (arrows). Note the tethering of the placode at the site of the dysraphic defect.
Degree of Confidence
MRI is reliable and the best diagnostic method for distinguishing the differential diagnosis between Chiari II malformation and other craniocervical pathologies (see Differentials and Other Problems to Be Considered).
Although the soft tissue can be evaluated easily by using MRI, bone anomalies that can be depicted easily on plain radiographs and CT scans may be missed on MRI.
Ultrasonography
Findings
Original reports discuss the limited use of US in patients with hydrocephalus and congenital cerebral anomalies, but current applications greatly expand the role of US in the evaluation of the developing central nervous system.8,9 Many of the typical findings described by using cranial CT scans and MRIs also can be demonstrated on the cranial sonograms.
Commercially available equipment is used with a 3- to 7.5-MHz transducer, depending on the size of the patient's head. In newborns, 5- to 7.5-MHz transducers are used, and 3- to 5-MHz transducers are used in older infants. Scans obtained with the transfontanel approach or with the transducer placed directly over the cervical region can demonstrate the downward displacement of the vermis and the medullary kink.
US is suited especially for evaluating the ventricular system in patients with a Chiari II malformation. The lateral ventricles often take on a characteristic pointed appearance that is best demonstrated in the coronal projection. In addition, the lateral ventricles are frequently asymmetric, and they may appear colpocephalic, such that the cerebral mantle appears thinnest over the occipital horns (see Image 4). The choroid plexus of the lateral ventricle can be demonstrated best by using sagittal US sections; it is unusually prominent and has been described as having a drumstick configuration or dangling choroid.
The characteristic prominence of the massa intermedia and beaked tectum that are often associated with the Chiari II malformation are also visible (see Image 4). The enlarged massa intermedia may appear to fill the third ventricle. A prominent anterior commissure, herniation of the third ventricle into the suprasellar cistern, and an enlarged suprapineal recess are often seen on midline sagittal sections. Dysgenesis of the hindbrain with downward displacement and elongation of the fourth ventricle, medulla, and the cerebellum results in a relatively small-appearing posterior fossa (see Image 4).
In the neonatal period, spinal US can be performed by using areas of dysraphism as an acoustic window. In this way, spinal abnormalities in patients with Chiari II malformation can be detected (see Image 11).
US is routinely used during the antenatal period as a screening method. With careful examination, cranial and spinal pathologic changes related to Chiari II malformations in the fetus can be seen. Reportedly, a lacunar skull can be detected by using US in the fetus.
Intraoperative US is used for surgical purposes, especially lateral ventricular shunt tube placement in infants, and for aiding in the intraoperative placement of catheters into cavities in the syrinx.
Degree of Confidence
US is heavily operator dependent. Findings associated with the Chiari II malformation, such as pachygyria, polymicrogyria, heterotopias, and dural abnormalities, may be missed with the use of US.
Angiography
Findings
Digital subtraction angiography (DSA) can be helpful in excluding abnormal cases of Chiari II malformation that involve the vertebral artery before surgery. This procedure can be performed noninvasively by using MR angiography or CT angiography. MR angiography can be preferred, especially in infants and children, because of the associated radiation exposure with CT angiography.
More on Chiari II Malformation |
| Overview: Chiari II Malformation |
 Imaging: Chiari II Malformation |
| Follow-up: Chiari II Malformation |
| Multimedia: Chiari II Malformation |
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Further Reading
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Keywords
Chiari II malformation, neural tube defects, Arnold-Chiari malformation, meningomyelocele, hydrocephalus, Cruveilhier-Cleland-Chiari malformation, Chiari I malformation, Chiari III malformation, Chiari IV malformation, hindbrain anomaly, posterior fossa, myelomeningocele
