Developmental Anomalies to Watch Out for: Chiari and Dandy-Walker Syndromes

Chiari and Dandy-Walker syndromes represent 2 distinct spectrums of developmental hindbrain anomalies. MRI images of Chiari (left, fetal MRI) and Dandy-Walker syndromes (right) are shown. In Chiari spectrum (grades I-IV), the posterior fossa is small and its contents are displaced inferiorly (orange arrow). By contrast, in Dandy-Walker spectrum (Dandy-Walker malformation, Dandy-Walker variant, and mega cisterna magna), the posterior fossa may be enlarged (blue arrow).

Chiari I is the mildest form of Chiari malformation and the most common presentation. The hallmark of Chiari I is descent of the cerebellar tonsils (T) below the level of foramen magnum (white line). The tonsils generally have a peg-like appearance and are impacted in the small cistern (asterisk) behind the medulla and upper spinal cord, crowding and compressing the latter structures.

Chiari I symptoms are from 3 sources: impaired cerebrospinal fluid (CSF) flow across the foramen magnum (eg, exertional headaches); compression of the spinal cord, medulla, and cerebellum; and central cord symptoms due to incipient or overt syringomyelia. This axial MRI image across the foramen magnum in a Chiari I patient demonstrates crowding of the foramen magnum by the low-lying cerebellar tonsils (T), obliteration of the cisterna magna, and compression of the lower medulla (M).

Cystic cavitation of the spinal cord known as syringomyelia (asterisk) is a common finding in Chiari I malformation. In all cases of newly diagnosed syringomyelia, Chiari malformation must be excluded. Syringomyelia frequently resolves after surgical decompression of Chiari malformation (right), obviating the need for its direct treatment by shunting. Also notice the improvement of CSF flow across foramen magnum after decompression (arrows).

An intraoperative photograph shows cerebellar tonsils that extend well below the level of foramen magnum in a Chiari I case. Dissecting tools (separating the 2 tonsils in this photo) are being used to release arachnoid adhesions between the tonsils and ensure free flow of CSF through the obex. A pericranial graft is then used to perform a duraplasty, thus increasing the diameter of the dural ring at foramen magnum to relieve compression of the cerebellar tonsils against the brainstem and spinal cord.

Beware of acquired pseudo-Chiari in cases of CSF hypotension syndrome. Overcrowding of the foramen magnum and descent of the cerebellar tonsils can occur due to sagging of the posterior fossa contents when there is a spinal CSF leak. The clinical presentation of postural headaches and prominent meningeal enhancement (arrows) distinguish CSF hypotension syndrome. Cerebellar sagging and meningeal enhancement typically resolve after treatment of the spinal CSF leak with an epidural blood patch.

Chiari II malformation is the second most common entity in the Chiari spectrum. It occurs almost invariably in association with a myelomeningocele (orange arrow) and is, therefore, usually diagnosed at a very young age. Both the cerebellum and the brainstem are displaced inferiorly (white arrow), resulting in traction on the lower cranial nerves.

Many classic MRI findings have been described in Chiari II, although not all findings may be present. MRI findings include (1) colpocephaly; (2) beaked tectum; (3) cascade of an inferiorly displaced vermis behind the medulla; (4) elongated, tube-like fourth ventricle; (5) low-lying torcular herophili; (6) cerebellar hemispheres wrapping around the brainstem anteriorly; (7) concave clivus; (8) medullary spur; and (9) medullary kink.

Tectal beaking is one of the main radiographic hallmarks of Chiari II malformation. Here the midbrain tectum has a pointed appearance, resembling a bird's beak (arrows). Hydrocephalus and disproportionate dilatation of the occipital horns and atria (known as colpocephaly and often associated with agenesis of corpus callosum, arrowheads) are also very common in Chiari II. The vast majority of Chiari II patients require ventricular shunting. When brainstem and cranial nerve symptoms develop in a previously well-compensated Chiari II patient, shunt malfunction must first be excluded.

Both Chiari I and Chiari II may be associated with craniocervical skeletal abnormalities including occipitalization of atlas (arrow), basilar invagination (present in example), bifida of the C1 posterior arch, and foramen magnum variations. The spinous processes of the cervical vertebral bodies have been labeled.

Chiari III is a rare and usually fatal malformation, consisting of herniation of the cerebellum and brainstem into a high cervical myelomeningocele. A mild variant of Chiari III with an occipital meningocele (arrow) is shown without significant herniation of posterior fossa contents. Chiari IV is exceedingly rare and results from cerebellar agenesis, leaving a small CSF-filled posterior fossa. Image courtesy of Radiopaedia.com.

The Dandy-Walker complex refers to a spectrum of congenital developmental anomalies of the fourth ventricle and cerebellum including Dandy-Walker malformation, Dandy-Walker variant, and mega cisterna magna. Congenital obstruction of CSF outflow dilates the fourth ventricle, causing hypoplasia of the cerebellar vermis (red arrow) and brainstem (b), and dilation of the posterior fossa (yellow arrow) with thinning of the occipital squama (arrowheads).

Dandy-Walker malformation is the most severe presentation of the complex. The typical findings are an enlarged posterior fossa, absent/hypoplastic cerebellar vermis, and hemispheres (c), hydrocephalus (red arrows), cystic dilation of the fourth ventricle (yellow arrow), and thinning of the occipital bone cortex (blue arrows). Dandy-Walker malformation is commonly associated with central nervous system (CNS) abnormalities, including corpus callosum dysgenesis, holoprosencephaly, and cingulate gyrus dysplasia, as well as non-CNS abnormalities, including cleft palate, cardiac and urinary tract anomalies, and polysyndactyly. Treatment options include shunting of the lateral ventricles or posterior fossa cyst, or endoscopic fenestration.

Giant arachnoid cysts may mimic Dandy-Walker malformation. Retrocerebellar arachnoid cysts (yellow arrow) may displace the fourth ventricle (red arrow) and cerebellum anteriorly, dilate the posterior fossa, and cause hydrocephalus. However, there will not be associated CNS malformations and typically the cyst will displace numerous tiny vessels that would normally be present in the fourth ventricle. The distinction from Dandy-Walker malformation is important as arachnoid cysts may be treated with resection or marsupialization.

Mega cisterna magna is the mildest entity in the Dandy-Walker spectrum. The posterior fossa is enlarged because of an enlarged cisterna magna (arrow), but the cerebellum and fourth ventricle are morphologically normal.

Dandy-Walker variant, also termed hypoplastic vermis with rotation, is less severe than the classic Dandy-Walker malformation. There is typically less dramatic dilatation of the fourth ventricle (yellow arrow) and hypoplasia of the cerebellum without enlargement of the posterior fossa (red arrow). Agenesis of the corpus callosum is also seen.

Salient features of the malformations in Chiari and Dandy-Walker spectrums are organized according to severity of the pathoanatomic findings.

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For more information, see the following resources:

• Medscape Reference: Chiari Malformation
• Medscape Reference: Hydrocephalus
• Medscape Reference: Imaging in Chiari II Malformation
• Medscape Reference: Imaging in Dandy-Walker Malformation

Contributor Information

Author
Peyman Pakzaban, MD
Consulting Neurosurgeon
Houston MicroNeurosurgery
Chairman
Department of Surgery
Bayshore Medical Center
Pasadena, Texas

Peyman Pakzaban, MD, has disclosed the following relevant financial relationships:
Received consulting fee from: Aesculap Implant Systems

Editor
Lars Grimm, MD, MHS
House Staff
Department of Diagnostic Radiology
Duke University Medical Center
Durham, North Carolina

Disclosure: Lars Grimm, MD, MHS, has disclosed no relevant financial relationships.

Reviewers
Charles M. Glasier, MD
Professor
Departments of Radiology and Pediatrics
University of Arkansas for Medical Sciences
Chief
Magnetic Resonance Imaging Vice-Chief
Pediatric Radiology
Arkansas Children's Hospital
Little Rock, Arkansas

Disclosure: Charles M. Glasier, MD, has disclosed no relevant financial relationships.

Justin A. Siegal, MD
MRI Section Head
Department of Radiology
Virginia Mason Medical Center
Seattle, Washington

Disclosure: Justin A. Siegal, MD, has disclosed no relevant financial relationships.