Acquired Chiari malformation Type I associated with a supratentorial arteriovenous malformation

Case report and review of the literature

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✓ Chiari malformation Type I (CM-I), a condition defined by caudal descent of the cerebellar tonsils through the foramen magnum, is generally considered a congenital lesion. Several authors, however, have described an acquired form that appears identical to the congenital lesion on neuroimages. The most commonly reported cause of an acquired CM-I is cerebrospinal fluid diversion through a lumboperitoneal shunt. In this paper, the authors report the case of a patient in whom an acquired CM-I developed in association with a supratentorial arteriovenous malformation (AVM) of the brain. Development of the acquired CM was documented on serial magnetic resonance images. Moreover, the CM was seen to originate and worsen in concert with the clinicoradiological progression of the AVM. The underlying mechanism responsible for the acquired CM in this case is thought to be a high-flow venopathy of the transverse and sigmoid sinuses causing occlusion on the right and redirection of venous outflow into posterior fossa veins, with consequent venous congestion and swelling of the posterior fossa structures.

Abbreviations used in this paper: AVM = arteriovenous malformation; CM-I = Chiari malformation Type I; MR = magnetic resonance; SDH = subdural hematoma.

Article Information

Address reprint requests to: H. Hunt Batjer, M.D., Department of Neurological Surgery, The Feinberg School of Medicine, Northwestern University, 676 North St. Clair Street, Suite 2210, Chicago, Illinois 60611. email:

© AANS, except where prohibited by US copyright law.



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    Initial MR images. Axial T2-weighted images of the supratentorial (upper left) and infratentorial (upper right) compartments revealing a large AVM in the right temporal, parietal, and occipital lobes. A sagittal T1-weighted image (lower left) demonstrating normal positioning of the cerebellar tonsils within the posterior fossa. An axial T2-weighted image obtained near the vertex (lower right) revealing a normal flow void in the posterior aspect of the superior sagittal sinus.

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    Follow-up MR images. Axial T2-weighted images of the supratentorial (upper left) and infratentorial (upper right) compartments revealing an interval increase in number and size of the tortuous AVM vessels. Venous recruitment is evident as well as prolongation of the T2 signal in the adjacent parietooccipital deep white matter. A sagittal T1-weighted image (lower left) revealing 10 mm of cerebellar tonsillar herniation below the foramen magnum. An axial T2-weighted image obtained near the vertex (lower right) demonstrating a poorly delineated flow void in the posterior aspect of the superior sagittal sinus, suggestive of reduced flow and acquired venopathy.

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    Cerebral angiograms. Anteroposterior (left) and lateral (center) projections after right carotid artery injection revealing the diffuse nature of the AVM with no angiographically identifiable central nidus. A lateral projection after right carotid artery injection (right) demonstrating a pseudophlebitic pattern consistent with intracranial venous hypertension.

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    Postoperative sagittal T1-weighted MR image demonstrating significant improvement in the tonsillar herniation.


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