Oneiroid syndrome (OS) is a condition involving dream-like disturbances of one's consciousness by vivid scenic hallucinations, accompanied by catatonic symptoms (either catatonic stupor or excitement), delusions, or psychopathological experiences of a kaleidoscopic nature. The term is from Ancient Greek "ὄνειρος" (óneiros, meaning "dream") and "εἶδος" (eîdos, meaning "form, likeness"; literally dream-like / oneiric or oniric, sometimes called "nightmare-like"). It is a common complication of catatonic schizophrenia, although it can also be caused by other mental disorders. The dream-like experiences are vivid enough to seem real to the patient. OS is distinguished from delirium by the fact that the imaginative experiences of patients always have an internal projection. This syndrome is hardly mentioned in standard psychiatric textbooks, possibly because it is not listed in DSM. History The German physician Wilhelm Mayer-Gross first described oneiroid states in 1924. Mayer-Gross's 1924 dissertation "Self-descriptions of Confusional States: the Oneiroid Form of Experience" () is considered to be the first monograph discussing oneiroid states. It is the psychopathological method (known to German psychiatrists as the "phenomenological method" – phänomenologische Methode). Use of term The term "oneiroid syndrome", while generally known to European and Russian psychiatrists, remains largely unfamiliar in the United States. ICD-9 adapted for the Soviet Union Included in the 9th edition of Manual of the International Statistical Classification of Diseases, Injuries, and Causes of Death, adapted for the USSR (ICD-9, 1983), were two diagnoses of oneiroid states in section (catatonic schizophrenia): ICD-9 code 295.24: oneiroid catatonia as a variant of shiftlike progressive schizophrenia (); ICD-9 code 295.25: oneiroid catatonia as a variant of recurrent schizophrenia (). According to Soviet psychiatric research, oneiroid syndrome occurs alongside catatonic schizophrenia in the great majority of cases. The catatonic phenomena in catatonic schizophrenia (code ) may be combined with oneiroid syndrome, as it is written in the current version of the ICD-10. Clinical characteristics Oneiroid syndrome is characterized by the extraordinary and fantastical nature of its psychotic experiences. Characteristic to the condition are mixed feelings, conflicted thoughts, contradictory experiences and actions, a sense of dramatic changes in the world, and simultaneous feelings of triumph and catastrophe. Oneiroid syndrome is often accompanied by frequent hallucinations and pseudohallucinations, as well as visual illusions. Patients do not identify the perceived phenomena as belonging to the real world, but rather as belonging to other realms or spheres, which cannot be observed or accessed by ordinary people. Patients often participate mentally in