Erethism, also known as erethism mercurialis, mad hatter disease, or mad hatter syndrome, is a neurological disorder which affects the whole central nervous system, as well as a symptom complex, derived from mercury poisoning. Erethism is characterized by behavioral changes such as irritability, low self-confidence, depression, apathy, shyness[1][2] and timidity, and in some extreme cases with prolonged exposure to mercury vapors, delirium, personality changes and memory loss.