Management of infratentorial subdural hygroma complicating foramen magnum decompression: a report of three cases.

Decompression of the foramen magnum for symptomatic Chiari malformation attends a small but significant risk of infratentorial subdural extra-arachnoid hygroma when an arachnoid-sparing procedure is attempted. We present three cases whereby an arachnoid-sparing procedure was carried out and resulted in infratentorial subdural hygroma and hydrocephalus. The complication was managed by re-exploration of the posterior fossa and wide arachnoidotomy. In cases whereby the decision has been made to open the dura, we recommend routine arachnoidotomy in foramen magnum decompression, avoiding the risks of infratentorial subdural hygroma. In cases where arachnoid-sparing procedures have been attempted and subdural hygroma subsequently develops, we advocate re-exploration of the posterior fossa rather than cerebrospinal fluid diversion.