BACKGROUNDAn increasing number of elderly patients are treated for aneurysmal subarachnoid hemorrhage. Given that elderly age is associated with both poor outcome and an increased risk of hydrocephalus, we sought to investigate the interaction between age and hydrocephalus in outcome prediction.METHODSWe enrolled 933 consecutive patients treated for subarachnoid hemorrhage between 2002 and 2010 and followed them for 1 yr after intensive care unit discharge. We first performed stepwise analyses to determine the relationship among neurologic events, elderly age (60 or more yr old), and 1-yr poor outcome (defined as Rankin 4-6). Within the most parsimonious model, we then tested for interaction between admission hydrocephalus and elderly age. Finally, we tested the association between age as a stratified variable and 1-yr poor outcome for each subgroup of patients with neurologic events.RESULTS24.1% (n=225) of subarachnoid hemorrhage patients were 60 yr old or more and 19.3% (n=180) had 1-yr poor outcomes. In the most parsimonious model (area under the receiver operating characteristic curve, 0.84; 95% CI: 0.82 to 0.88; P<0.001), elderly age and admission hydrocephalus were two independent predictors for 1-yr outcome (P<0.001 and P=0.004, respectively). Including the significant interaction between age and hydrocephalus (P=0.04) improved the model's outcome prediction (P=0.03), but elderly age was no longer a significant predictor. Finally, stratified age was associated with 1-yr poor outcome for hydrocephalus patients (P=0.007), but not for patients without hydrocephalus (P=0.87).CONCLUSIONIn this observational study, elderly age and admission hydrocephalus predicted poor outcome, but elderly age without hydrocephalus did not. An external validation, however, will be needed to generalize this finding.