Chronic hydrocephalus after aneurysmal subarachnoid hemorrhage: a brain–CSF interface framework for mechanisms, conceptual states, and surgical management

Chronic hydrocephalus following aneurysmal subarachnoid hemorrhage (aSAH) may be more usefully conceptualized as the cumulative failure of several interacting brain–CSF boundaries than as a single obstructed conduit. A purely mechanical model cannot account for inflammatory CSF secretion, delayed absorptive failure, impaired clearance, altered tissue compliance, or the heterogeneous need for permanent shunting, so we use a brain–CSF interface and compartment framework to organize these observations. Hemorrhagic CSF may constitute a shared upstream exposure, distributing clot, hemoglobin, heme, iron, oxidative species, inflammatory mediators, complement, coagulation products, and profibrotic signals to the choroid plexus, ventricular ependyma, aqueductal outlets, cistern–arachnoid surfaces, glymphatic and meningeal lymphatic pathways, and periventricular tissue with anatomically variable effects. When CSF production, ventricular and cisternal passage, glymphatic and meningeal lymphatic clearance, and periventricular compliance fail to recover after the initial insult, chronic or shunt-dependent hydrocephalus may ensue—although this temporal association does not establish causality. We propose interface-dominant conceptual states—obstructive/ventricular-clot, hypersecretory-inflammatory, fibrotic-absorptive, clearance-failure, compliance-related, and mixed—as a hypothesis-generating tool to connect mechanism, state assignment, risk prediction, and procedural choice across acute and rehabilitative care while keeping the limits of existing evidence explicit. In this framework, external ventricular drainage addresses urgent ventricular pressure and outflow failure, lumbar drainage facilitates craniospinal clearance in selected patients with patent pathways, selected clot-directed procedures may reduce local clot burden but have not proved prevention of chronic hydrocephalus, and permanent shunts bypass persistent decompensation.