ΔMS is one of the most common reasons you get paged in the hospital. First, you must know 4 categories (decreasing order of frequencies). If you suspect delirium, use 3D-CAM (Table 12.2) and etiologies as acronym DELIRIUM (Table 12.3). Differential diagnosis between serotonin syndrome (SS) and neuroleptic malignant syndrome (NMS) is critical (Table 12.5). Use Hunter’s decision rule for SS (Table 12.6). One of the most challenging conditions is antibody-mediated encephalitis and once diagnosed, use NEOS score for prognosis (Table 12.10). Occasionally, in acute mental status change, you puzzle to find elevated ammonia level among patients with no history of liver disease. Table 12.11 for nonhepatic causes of hyperammonia.