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Background: Cerebrospinal fluid (CSF) overdrainage in hydrocephalus is well recognized. Overshunting following cysto-peritoneal (CP) drainage in patients with arachnoid cysts (AC) is rarely documented., Aim: 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., Patients and Methods: The medical records of the eight patients were analyzed in regard to clinical manifestation, cyst and shunt characteristics, management, and outcomes., Results: 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., Discussion: 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., Conclusions: 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.

Introduction: Several emerging clinical and neuroimaging conditions due to CSF intracranial hypotension have been documented. Recently, a few instances of a severe form of cervical myelopathy due to CSF overshunting have also been reported. Patients with this type of cervical myelopathy usually evolve with marked clinical manifestations of spinal cord involvement., Materials and Methods: In this article, we report a 20-year-old girl, previously given a ventriculoperitoneal shunt at age 6 years, who presented with neck pain after a whiplash injury., Results and Discussion: Magnetic resonance imaging revealed a cervical extramedullary mass that mimicked a cervical extradural hematoma. Further neuroimaging studies demonstrated that it corresponded to an engorged cervical epidural venous plexus that we attributed to chronic CSF overdrainage. After expectant management, the patient experienced a total recovery of her symptoms. To our knowledge, this is the first report of this condition occurring in an asymptomatic patient. Our aim in reporting this patient is twofold: (1) to warn the clinician about these neuroimaging findings that may mimic a cervical extradural hematoma and (2) to illustrate that engorged cervical epidural plexus may occur in patients in the absence of clinical manifestations of myelopathy. Recognition of this entity is important to distinguish it from a mass lesion in which a surgical intervention might be indicated.