External hydrocephalus in the adult population with acute brain injury. A narrative review.

External hydrocephalus is a frequent and poorly recognized condition in the adult population with acute brain injury. It refers to cerebrospinal fluid flow impairment, with enlargement of the subarachnoid space in the context of raised intracranial pressure. This manuscript reviews the current evidence on epidemiology, pathophysiology, neuroimaging, diagnostic criteria and treatment strategies of acute external hydrocephalus in adult patients with acute brain injury. A total of 15 studies were included, mainly retrospective and case series, with a limited methodological quality. Intracranial bleeding, tearing of the arachnoid membrane and excess of cerebrospinal fluid are the factors described in its pathogenesis. Neuroimaging is important in differentiating between external hydrocephalus and subdural hygroma. The key observation is the progressive enlargement of the subarachnoid space. The cornerstone of the management of acute external hydrocephalus is raised intracranial pressure control. An external lumbar drainage could be an effective measure in this scenario. Finally, an algorithm to manage acute external hydrocephalus with raised intracranial pressure is presented.