This paper reviews the varied presentations of Arnold-Chiari Malformation (ACM) in terms of anatomy, clinical presentation, diagnostic workup, and therapeutic considerations. Emphasis is placed on the Type I condition as it appears in the adult. ACM is a developmental anomaly in which the cerebellar tonsils and portions of the posterior fossa structures herniate through the foramen magnum. It is frequently accompanied by a broad spectrum of additional anatomical variations and can present clinically in a variety of ways. Two cases of ACM Type I in the adult are presented. Both patients were treated initially by conservative measures before MRI could be obtained to establish the diagnosis. Neither one of these patients showed improvement with such a therapeutic regimen. It is important for practitioners to be aware of the varied presentations of ACM in the adult.
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