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The infratentorial compartment represents the second most common location of arachnoid malformations. Ten arachnoid cysts of the posterior fossa, operated on between 1970 and 1983, are reviewed. These lesions, although congenital and developmental in nature, may present at any age, and males are more frequently affected. A high rate of birth-related trauma (50% in this series) is conceivably due to fetal macrocranium, and the enlarged head and psychomotor retardation prevail in infancy and childhood. In arachnoid cysts occurring during adulthood, symptoms and signs more clearly indicate a dysfunction of the posterior fossa. Besides computerized tomography, pneumoencephalography and metrizamide techniques are recommended to rule out a Dandy-Walker syndrome in doubtful cases, and to obtain information about the cerebrospinal fluid (CSF) circulation. It is particularly important to establish the presence and type of communication of cysts with the CSF pathways. Although infratentorial cysts often communicate, they can be space-occupying masses because of increasing CSF retention, which may be due to a ball-valve mechanism or to inadequate communication. The frequently associated hydrocephalus (seven of the 10 cases in this series had hydrocephalus) seemed to be dependent mainly upon mechanical factors. The authors discuss the indications for intracranial surgery versus shunting procedures and report the results achieved by direct cyst excision.

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