• Paul T Akins and Kern H Guppy.
• Department of Neurosurgery, The Permanente Medical Group, Kaiser Sacramento Medical Center, Sacramento, California, USA. Electronic address: paul.t.akins@kp.org.
• World Neurosurg. 2019 Oct 1; 130: e941-e952.

BackgroundPoorly understood cranial fluid accumulations are frequently observed after decompressive craniectomy and often termed "external hydrocephalus." These findings are difficult to explain using traditional models of hydrocephalus.MethodsRepresentative cases, clinical management, and literature overview are presented.ResultsWe present a hypothesis that abnormal cranial fluid accumulations develop after decompressive craniectomy in a vulnerable subset of patients as a result of 1) the large compliant cranial defect with durotomy causing reduced internal brain expansion, ventricular squeezing, and pulsatile cerebrospinal fluid (CSF) circulation; 2) impaired pulsatile CSF flow along major cerebral arteries and the adjoining perivascular spaces (Virchow-Robin spaces); 3) reduced clearance of interstitial fluid by the glymphatic system; and 4) redistribution of CSF from the subarachnoid space into the subdural and subgaleal compartments and the ventricles.ConclusionClosure of the cranial defect with cranioplasty improves cerebral blood flow and CSF pulsatile circulation and is frequently sufficient to resolve the external hydrocephalus.Copyright © 2019 Elsevier Inc. All rights reserved.

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