External hydrocephalus in adult neurocritical patients

Abstract Background: External hydrocephalus (EH) in adults is an underrecognized clinical entity characterized by CSF circulatory failure and progressive subarachnoid space (SAS) dilation under high intracranial pressure (ICP). Data regarding its incidence, pathophysiology, and management in adult neurocritical care are sparse. Objective: to describe the incidence, risk factors and outcome of EH in adult neurocritical patients. Methods: prospective study of patients with moderate-to-severe traumatic brain injury (msTBI) or spontaneous subarachnoid hemorrhage (SAH) admitted to intensive care unit (ICU) from January 2024 to June 2025. EH was diagnosed when SAS enlargement was found between two successive cranial CT scans, in the context of intracranial hypertension. Results: A total of 128 patients were included (78 msTBI, 50 SAH). The overall incidence of EH was 7.8% (10.3% in msTBI vs. 4.0% in SAH). The median time to EH onset was 4.5 days (IQR 4.0–16.5). In the general population, EH development was significantly associated with younger age (p=0.013), refractory intracranial hypertension (p<0.001), surgical intervention (p=0.018), and decompressive craniectomy (p=0.007). External hydrocephalus did not resolve spontaneously in any of the cases, requiring treatment with external lumbar drainage in 70%. Patients with EH demonstrated significantly prolonged ICU lengths of stay (34.0 vs. 9.0 days; p<0.001) and hospital lengths of stay (46.5 vs. 17.5 days; p<0.001). Conclusions: the first prospective study on EH adult neurocritical patients is presented. EH presents as uncommon but critical and complex driver of secondary neurological deterioration and prolonged hospital resource utilization in adult neurocritical care. Diagnosis requires a high index of suspicion based on clinical and radiological findings. It is associated with refractory intracranial hypertension, generally requiring an external lumbar drainage.