Intracranial pressure monitoring in patients with acute brain injury in the intensive care unit (SYNAPSE-ICU): an international, prospective observational cohort study by Robba, C, Graziano, F, Rebora, P, Elli, F, Giussani, C, Oddo, M, Meyfroidt, G, Helbok, R, Taccone, FS, Prisco, L, Vincent, J-L, Suarez, JI, Stocchetti, N, Citerio, G