In a new publication in Schizophrenia Bulletin Swedish registers were used in the so far largest study set out to understand the relationship between OCD and psychosis and bipolar disorders. Clinically it is clear that psychotic delusions can sometimes be hard to tell apart from obsessions, especially when there is low insight.
In this population-based longitudinal and multigenerational family study, we examined the patterns of comorbidity, longitudinal risks, and shared familial risks between these disorders. Participants were individuals with a diagnosis of OCD (n = 19814), schizophrenia (n = 58336), bipolar disorder (n = 48180), and schizoaffective disorder (n = 14904) included in the Swedish Patient Register; their first-, second-, and third-degree relatives; and population-matched unaffected controls and their relatives. Individuals with OCD had a 12-fold increased risk of having a comorbid diagnosis of schizophrenia and a 13-fold increased risk of bipolar disorder and schizoaffective disorder. This still means that those disorders disorders will not affect most OCD patients. Longitudinal analyses showed that individuals first diagnosed with OCD had an increased risk for later diagnosis of all other disorders, and vice versa. OCD-unaffected first-, second-, and third-degree relatives of probands with OCD had a significantly increased risk for all 3 disorders; the magnitude of this risk decreased as the genetic distance increased. We conclude that OCD is etiologically related to both schizophrenia spectrum and bipolar disorders.
Rücklab - Psychiatry research at Karolinska Institutet 
Very interesting.
Psychiatric disorders have been known to have various comorbidities. Obsessive compulsive disorder (OCD) too is one such disorder which can coexist with other anxiety disorders, mood and psychotic disorders. Of these, depressive symptoms are the commonest comorbidity. Mania in OCD can occur either as an independent comorbidity or as a result of an antidepressant-induced switch in a patient on anti-OCD drugs. Whatever the cause may be, this comorbidity implies that there can be a host of differences in the presentation, course, treatment guidelines and prognosis.