Posttraumatic intradiploic arachnoid cyst of the posterior fossa.

We report the case of a 5-year-old girl with an enlarging suboccipital mass, a posttraumatic intraosseous arachnoid cyst. Diagnostic work-up revealed that the lesion consisted of an intradiploic arachnoid cyst and an extra-axial occipital pouch that communicated by way of an osseous and dural defect. Surgical repair was undertaken with good results. A search of the current literature has shown only seven previous reports of leptomeningeal cysts situated at the occipital bones, most of them the result of an antecedent skull fracture. A pathogenetic hypothesis is presented comparing the growth of arachnoid intraosseous cysts and the development of meningocencephaloceles.