• Giuseppe Di Perna, Gianluca Piatelli, Andrea Rossi, Alessandro Consales, Pietro Fiaschi, Margherita Castaldo, and Marco Pavanello.
• Department of Neuroscience "Rita Levi Montalcini", Neurosurgery Unit, University of Turin, Turin, Italy; Division of Neurosurgery, IRCCS Istituto Giannina Gaslini, Genova, Italy. Electronic address: dr.giuseppediperna@gmail.com.
• World Neurosurg. 2021 Apr 1; 148: 44-53.

IntroductionArachnoid cysts are benign cerebrospinal fluid collection within a duplication of arachnoid membrane and, when found in the retrocerebellar site, they may be associated with tonsils herniation. This rare situation of coexisting retrocerebellar arachnoid cyst (AC) and Chiari malformation type 1 (CM-1) have been previously reported in few cases (10 patients) with syringomyelia and hydrocephalus described to be the most relevant issues. The aim of this paper is to describe 3 pediatric cases of this condition with a systematic review of the literature, underlining the importance of surgical management tailored to the pathogenic mechanism.MethodsA restrospective analysis of patients treated for coexisting CM-1 and ACs at the authors' institution has been carried out.ResultsA case of a 10-month-old baby with coexisting AC and CM-1 with tri-ventricular hydrocephalus treated with endoscopic third ventriculostomy, a case of a 1-year-old child with a huge retrocerebellar AC and CM-1 treated with a cysto-peritoneal shunt, and a case of a 15-year-old child with retrocerebellar AC causing symptomatic CM-1 treated with C0-C2 decompression, AC fenestration and duraplasty are described. A long-term follow-up is reported.ConclusionsSurgical management of coexisting ACs and CM-1 should not aim at the complete resolution of the cyst or of tonsil herniation, especially when pediatric patients are treated. Rather, the purpose of the neurosurgeon should be to understand the underlying pathogenic mechanism, and then restoring both the cerebrospinal fluid flow in the posterior fossa and the dynamic equilibrium between ventricles, cyst, and subarachnoid space.Copyright © 2020 Elsevier Inc. All rights reserved.

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