Imaging description

Hans Chiari described Chiari malformations as a congenital condition in the late nineteenth century [1]. Inferior descent of at least one cerebellar tonsil more than 5mm below the inferior margin of the foramen magnum is considered diagnostic (Fig. 99.1) [2]. There is a slight female preponderance. With the advent of MRI, it is seen with increasing frequency, in up to 1% of people undergoing imaging [3]. However, more than 50% of the individuals who are diagnosed with Chiari I malformation are asymptomatic. Apart from cerebellar descent, peg-like configuration of cerebellar tonsils with somewhat oblique orientation of the vertical sulci and elongated fourth ventricle with normal location are important imaging features (Fig. 99.2A). As a result of abnormal flow of CSF at the foramen magnum, Chiari malformation type I can lead to spinal cord syrinx formation (Fig. 99.3A) [4]. Factors associated with syrinx formation include increased tonsillar descent of more than 10mm, basilar invagination, retroverted odontoid process (Fig. 99.3B), and hydrocephalus [5]. While cervical spine is more frequently affected (Fig. 99.3C), syrinx formation can also take place only in the lower segments [6]. Hence total spine imaging should be considered in a patient with Chiari I malformation. Phase-contrast magnetic resonance imaging is increasingly utilized to evaluate the effect of tonsillar ectopia on CSF flow. Reduced flow in posterior subarachnoid space (Figs. 99.1B, 99.3B) is generally considered a sign of an abnormal CSF flow, with increasing correlation between symptomatic Chiari I malformation and asymptomatic tonsillar ectopia [7]. Abnormal tonsillar pulsations are also associated with a pegged tonsillar morphology [5].