In patients with neuromyelitis optica, treatment with eculizumab resulted in a decrease in CSF free C5 concentration of more than 55%, while CSF eculizumab concentrations were lower than in our study.9 One possible explanation why such a decrease in C5a concentration was not reached in our study is because other enzymes, such as thrombin and trypsin, have been found to generate biologically active C5a despite the presence of eculizumab.17 Since CSF thrombin levels are increased in aSAH patients,18 inhibition of C5 cleavage to C5a by eculizumab might be insufficient. This evidence concerns the gene C5 and neuromyelitis optica.