Le concept de « continuum de la psychose » est ancien et fut documenté dès la première moitié du xix^e siècle par Griesinger sous l’appellation « Einheitspsychose ». En pratique, la traditionnelle dichotomie kraepelinienne reste une base de travail courante pour les cliniciens. Cependant, les récentes avancées de la recherche en neurosciences plaident en faveur d’une compréhension dimensionnelle de la psychose. Les données issues des études épidémiologiques nous apportent des arguments supplémentaires pour penser la survenue de la schizophrénie et du trouble bipolaire en termes de continuum, à la fois aux plans individuels et/ou familiaux. Reste aujourd’hui à déterminer si l’on possède des arguments suffisamment solides pour pouvoir répondre à cette délicate question : quels sont les facteurs pouvant déterminer, chez un même individu, la survenue d’une schizophrénie ou d’un trouble bipolaire ? En outre, l’étude des prodromes de la maladie n’est pas toujours discriminante pour l’une ou l’autre de ces grandes entités. De plus, les troubles cognitifs définis comme marqueurs-traits dans la schizophrénie pourraient constituer des marqueurs-états dans le trouble bipolaire. Notre revue a donc pour objectif de faire le point sur l’état des connaissances actuelles en mettant en perspective les aspects épidémiologiques, prodromiques et cognitifs communs à la schizophrénie et au trouble bipolaire.

Historical aspects of the dichotomy between manic-depressive disorders and schizophrenia raise the question of a continuum between the two entities. Griesinger (1817–1868) proposed a unitary concept of psychosis: “Einheitspsychose”, adaptations of which have survived until the present day. Although Kraepelin’s traditional dichotomy is still a common base for clinicians every day: diagnosis, prognosis and treatment of psychotic disorders, recent epidemiological and neurobiological data are congruent with a dimensional aspect of psychosis. Epidemiological data are consistent with the existence of an individual and a familial overlap between bipolar disorder and schizophrenia. Schizophrenia is probably the most debilitating psychological disorder. It was primarily considered as a behavioural disorder, characterized by socially inappropriate and bizarre behaviour, but much attention has been focussed nowadays on the cognitive component and the cognitive pathology underlying schizophrenia. On the other hand, bipolar, or manic depressive disorder has been primarily considered as a mood or affective disorder, characterized by excessive swings of emotion and motivation. Manic depression is more about recurrent dimensions. However, symptoms associated with the diagnosis of schizophrenia can be associated with psychotic mood disorders: hallucinations and delusions (50%), disorganised speech and behaviour (all patients with moderate to severe mania or mixed episode), negative symptoms (all patients with moderate to severe depression). The social and job dysfunction may be due to disturbances in the volitional system in patients with schizophrenia or severe bipolar disorder.

A considerable body of literature exists concerning the relationship between cognitive impairment in schizophrenia, but there is less data about cognition in bipolar disorder. However, there are some notable similarities between data observed in schizophrenia and bipolar disorder. Many domains of cognition are disrupted in schizophrenia with varying degrees of deficit. Concerning mood disorders, cognitive dysfunction could be considered as a state marker. Globally some studies indicate that, compared with schizophrenia, those with bipolar disorder display a similar but less severe neuropsychological pattern of impairment. However, it is only recently that cognitive dysfunction has been recognized as a primary and enduring core deficit in schizophrenia and further studies in bipolar disorder are needed.

In this way, it has been suggested that psychotic symptoms may be distributed along a continuum that extends from schizophrenia to psychotic mood disorders with increasing level of severity. An explicative theory has to explain the evolution and the similarities between those affections including genetic and environmental liability.

Some individuals, who are at high risk for psychosis, can even develop bipolar disorder or schizophrenia. Likewise, common factors can explain cognitive and social disorders in psychosis. So, there are various arguments for the dimensional approach of psychosis. These data are not completely in contradiction with Kraepelin: schizophrenia is a chronic affection and bipolar disorder is a cyclic pathology. However, common symptoms are not in favour of a strict categorization.