Arachnoidal cysts of posterior fossa

ARACHNOIDAL CYSTS have been reported as occurring at spina1,l cerebral,2 and cerebella6 levels, b u t they have a particular predilection for the midline of the posterior fossa. As relatively uncommon lesions, they are rarely considered in differential diagnosis. Usually the diagnosis is made when the lesion is exposed a t surgery. Prior to surgery, if a ventriculogram has been done, a posterior fossa space-occupying mass may be diagnosed, bu t ordinarily nothing specific enough to make a more positive diagnosis can be seen. Recently diagnosis in a patient who harbored such a lesion was made preoperatively on the basis of a pneumoencephalogram.