Prognosis as the critical variable in classification of the functional psychoses.

Kraepelin used a disease entity concept to define the two major functional psychoses (i.e., distinctive patterns of onset, symptoms, and course of illness). There have been many subsequent studies using patient outcome or distribution of illness among relatives to test the validity of these nosological classes. Differences between chronic schizophrenia and (predominantly nonchronic) affective disorders are often reported, but it is difficult to discern whether these differences are associated with diagnostic symptomatology rather than premorbid prognostic status. The evidence suggesting a genetic link between good prognosis schizophrenia and affective disorders may be misinterpreted if premorbid prognostic processes are the distinguishing heritable components between chronic schizophrenia and nonchronic illnesses. Which components of severe psychiatric disorders are heritable is not yet clear. The developmental, psychological and neurological processes associated with premorbid and early morbid characteristics of illness appear good candidates. Considering such variables as prognostic (early morbid or premorbid) rather than diagnostic would permit examination of class assignment (e.g., schizophrenia, bipolar affective disorder) and premorbid development. The literature now suggests that chronicity in psychiatric illness breeds true and that symptom constellations breed true. Both of these views may be corrects, and the taxing study design required to simultaneously study these two attributes of disease entities is warranted. This may establish good prognosis schizophrenia as a "third psychosis," or may affirm its standing within traditional nosology.