BDNF and delirium: APOE, apolipoprotein E; BDNF, brain-derived neurotrophic factor; DSD, delirium superimposed on dementia; HMG-CoA, hydroxymethylglutaryl-coenzyme A; PMID, PubMed Identification; PMCID, PubMed Central Identification; SSRI, selective serotonin reuptake inhibitors