International prospective observational study on intracranial pressure in intensive care (ICU): the SYNAPSE-ICU study protocol by Citerio, G, Prisco, L, Oddo, M, Meyfroidt, G, Helbok, R, Stocchetti, N, Taccone, F, Vincent, J-L, Robba, C, Elli, F, Sala, E, Vargiolu, A, Lingsma, H