The co-occurrence of obsessive-compulsive symptoms (OCS) and psychotic illness has been a challenge for clinicians and investigators for more than a century.1 Over the past decade, interest in this area has burgeoned because of recognition of higher-than-chance comorbidity rates of schizophrenia and obsessive-compulsive disorder (OCD), and observations of appearance or exacerbation of OCS during treatment of schizophrenia with atypical antipsychotics.2-6 Emerging neurobiological and genetic evidence suggests that persons with comorbid OCD and schizophrenia may represent a special category of the schizophrenic population.
The evidence for a putative schizo-obsessive disorder is examined and practical treatment suggestions for this subgroup of patients are outlined in this article.7-9
Comorbidity between OCD and schizophrenia
The lifetime prevalence for schizophrenia is 1% and for OCD it is 2% to 3%.10 Comorbidity rates for OCD in the schizophrenia population are substantially higher than what would be expected to occur randomly. In the schizophrenic population, the reported prevalence of clinically significant OCS and of OCD ranges from 10% to 52% and from 7.8% to 26%, respectively.11-23
The higher-than-expected comorbidity rate for OCD and schizophrenia suggests a nonrandom association and possibly an integral relation between these 2 conditions.9 The question is whether this comorbid group with schizo-obsessive disorder represents a more severely ill group with greater brain dysfunction that could, in part, be caused by common neurodevelopmental predisposing factors, or whether the 2 conditions are part of a more complex syndrome that represents a distinct diagnostic entity. The answer could be clarified in part if neurobiological studies were to demonstrate a distinct neuroanatomical substrate in this comorbid group rather than the summation or superimposition of neurobiological lesions observed in the separate disorders.9
Clinical and research challenges
Recent studies have aimed to reduce bias and 
Notwithstanding these efforts to enhance diagnostic clarity and study validity, the distinction between obsessions and delusions is often difficult to discern.9 Paradoxically, DSM-IV allows for the OCD specifier “with poor insight.” This stands in contrast to the definition of an obsession as being recognized by the individual as foreign to him or her (ie, ego-dystonic), and implies the presence of insight. Insel and Akiskal24 proposed that “OCD represents a psychopathological spectrum varying along a continuum of insight,” and that this “obsessional delusion” does not signify a schizophrenia diagnosis. Complicating the matter further is the observation of perceptual disturbances that mimic various types of hallucinations or pseudohallucinations in some persons with OCD.25
Whether obsessions can be accurately detected in the presence of psychosis remains a matter of debate.9 To date, there is no universally accepted method of detecting OCD in the presence of schizophrenia, although most contemporary study designs have used the Structured Clinical Interview for DSM-IV Axis I psychiatric disorders and the Yale-Brown Obsessive-Compulsive Scale (Y-BOCS).26,27 In attempts to ascertain the reliability and validity of the Y-BOCS in this comorbid subgroup, de Haan and colleagues28 examined the properties of this psychometric tool in patients with recent-onset schizophrenia and comorbid OCS. These investigations found good internal consistency and interrater reliability in this population. However, their findings concerning the divergent validity against depressive and negative symptoms were inconsistent.
Although the phenomenological delineations between obsessions and delusions often remain unclear, there is substantial evidence that OCS in schizophrenia represents more than just an expression of enduring psychosis.9 This evidence includes observations that conventional antipsychotic medications appear to be of limited use in the treatment of OCS in schizophrenia, the persistence of OCS even after successful treatment of the psychotic symptoms, and the effectiveness of serotonin reuptake inhibitors in the treatment of OCS in patients with schizophrenia.29-33
Clinical relevance of OCS in schizophrenia
Early investigators concluded that the presence of OCS confers protection against cognitive deficits, functional impairment, and negative symptoms associated with schizophrenia.34,35 Psychodynamic theories postulated that obsessions constitute a defense against psychosis and prevent progression of the disease. However, more recent studies that used rigorous methods have not tended to replicate these earlier findings.36 Instead, recent studies have found that this comorbid group is burdened by a greater magnitude of cognitive deficits, negative and positive symptoms, neurological soft signs, distress, dysfunction, hopelessness, depression, suicidal ideation, and suicide attempts. A few studies have not replicated some of these findings.19,20,37-48
