CSF overdrainage in shunted intracranial arachnoid cysts: a series and review

Cerebrospinal fluid (CSF) overdrainage in hydrocephalus is well recognized. Overshunting following cysto-peritoneal (CP) drainage in patients with arachnoid cysts (AC) is rarely documented. We report five patients with acquired Chiari malformation I and three with posterior fossa overcrowding due to excessive CSF drainage in shunted intracranial ACs. We review our observations and discuss the current knowledge on the pathogenesis and management of this complication. The medical records of the eight patients were analyzed in regard to clinical manifestation, cyst and shunt characteristics, management, and outcomes. Mean age of the patients was 5.5 years. After an average interval of 5 years, five patients developed symptoms related to hindbrain herniation and three to severe shunt overdrainage following CP shunting. Several management modalities were utilized that achieved a good result in seven instances. Some shunted ACs may evolve with overdrainage syndromes. Posterior fossa overcrowding and tonsillar herniation constitute their most severe forms. CSF hypotension, bone changes, venous engorgement, and probably cerebral chronic edema at the posterior fossa constitute the main factors involved in the pathogenesis of this entity. We also review previous instances of acquired Chiari malformation originating after AC shunting. Posterior fossa overcrowding and acquired Chiari I malformation can develop after excessive CSF drainage of intracranial ACs. Overshunting manifestations require prompt recognition and management. Preventive measures consist of making a stringent selection of cases being considered for surgery, avoiding CP drainage, and placing of a programmable valve as initial treatment of intracranial ACs if shunting is considered.