Acquired Chiari malformation Type I associated with a supratentorial arteriovenous malformation. Case report and review of the literature.

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.