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.