Are Hygromas and Hydrocephalus After Decompressive Craniectomy Caused by Impaired Brain Pulsatility, Cerebrospinal Fluid Hydrodynamics, and Glymphatic Drainage? Literature Overview and Illustrative Cases.

Background: Poorly 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.

Methods: Representative cases, clinical management, and literature overview are presented.

Results: We 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.

Conclusion: Closure of the cranial defect with cranioplasty improves cerebral blood flow and CSF pulsatile circulation and is frequently sufficient to resolve the external hydrocephalus.