Pedigree and genetic studies
Pedigree and genetic studies have not found any familial relationship or shared etiology between OCD and schizophrenia in their pure forms. However, specific genotypes of polymorphisms of the same gene may differentially confer risk for the 2 disorders.
The catechol-O-methyltransferase (COMT) gene is a candidate gene for schizophrenia because of its role in the breakdown of dopamine(Drug information on dopamine) in the prefrontal cortex. Zinkstok and colleagues49 found that the COMT high activity Val allele is associated with more OCS in young patients with schizophrenia, whereas patients with the Met/Met genotype had the lowest Y-BOCS scores. These results support the hypothesis that the COMT Val-Met polymorphism may be a modifier gene for the symptoms of schizophrenia.49
Poyurovsky and colleagues50 examined familial aggregation of schizophrenia spectrum disorders and obsessive-compulsive–associated disorders in schizophrenia probands with and without OCD. They found that relatives of OCD-schizophrenia probands had significantly higher morbid risks for OCD-schizophrenia and obsessive-compulsive personality disorder (OCPD); they also found a trend toward higher morbid risk for OCD. When morbid risks for OCD, OCPD, and OCD-schizophrenia were pooled, the significant between-group differences became robust. These data suggest a genetic contribution to the expression of OCS in individuals with schizophrenia. In addition, they lend support for the validity of a putative schizo-obsessive diagnostic entity.50
Neurobiology of the schizo-obsessive subgroup
Considerable work has been done to reveal the neurobiological basis of both schizophrenia and OCD.9 This research has focused primarily on elucidating key neurotransmitter systems, structural and functional neuroanatomy, and neuropsychology. However, there is very limited published research that focuses specifically on neurobiological features unique to this putative schizo-obsessive subtype.
Neurotransmitter systems. There is a paucity of published research on unique neurotransmitter involvement in the schizo-obsessive subtype group.
However, serotonin and dopamine have most consistently emerged as the principal neurotransmitters of interest in both disorders.9 The dopamine hypothesis in schizophrenia has long been regarded as the fundamental neurochemical premise; however, the superior efficacy of the serotonin-dopamine receptor antagonists in the treatment of schizophrenia also supports the importance of the serotonergic system in the pathophysiology of this disorder and may reflect the modulation of dopaminergic systems by serotonin.51,52
Conversely, in OCD a somewhat opposing picture has emerged with respect to neurotransmitter involvement.9 The serotonin hypothesis of OCD is supported by successful treatment of the disorder with serotonin reuptake inhibitors, pharmacological challenge studies, and cerebrospinal fluid neurotransmitter metabolite studies.53 However, several lines of evidence suggest that serotonin is not the sole neurotransmitter involved in OCD. Considerable evidence supports the additional role of the dopaminergic system in this disorder.51,53-57 Preclinical evidence of dopamine’s reciprocal modulatory effects on the serotonin system and successful treatment of refractory OCD with adjunctive dopamine receptor antagonists and serotonin-dopamine receptor antagonists have provided support for the dopamine-serotonin hypothesis of OCD.51,53,56,57
There is a lack of neurotransmitter data in the overlap group. However, one study examined the differences in whole blood serotonin concentrations in healthy volunteers versus patients with OCD, in persons with schizophrenia with and without OCS, and in clozapine(Drug information on clozapine)-treated schizophrenia in patients with and without clozapine-induced OCS. This study found that the groups with OCD, schizophrenia with OCS, and clozapine-treated schizophrenia with OCS had significantly lower levels of whole blood serotonin than did the healthy volunteers and the schizophrenia-only groups.58
Neuroanatomy and neurocircuitry. In contrast to the abundance of neuroimaging studies investigating structural brain abnormalities in persons with OCD and schizophrenia separately, there is a dearth of such studies examining the comorbid subgroup. However, the considerable overlap in the neurocircuitry and specific anatomical structures implicated in each disorder may account for symptom coexpression in this subgroup of patients.52,59,60 The functional circuitry implicated in the pathophysiology of OCD is generally believed to involve a cortico-striatal-thalamic-cortical circuit.61 Specific structures implicated in this pathway include the basal ganglia, orbitofrontal cortex, and anterior cingulate cortex.62
In schizophrenia, the dorsolateral prefrontal cortex circuit contains anatomical substrates similar to those of the OCD orbitofrontal circuit.52 Thus, the specific neuroanatomical sites identified by structural and functional neuroimaging studies performed in each of these disorders independently show considerable over-lap in implicated structures, including the basal ganglia, thalamus, anterior cingulum, orbitofrontal cortex, and regions of the temporal cortex, although some of these findings are controversial.59,60
Neuroimaging studies
Neuroimaging studies suggest the presence of specific neuroanatomical abnormalities in the overlap group that may differ from what is observed in the individual disorders. One MRI study of patients with juvenile-onset schizophrenia with OCS found significantly smaller left hippocampi in this group than in schizophrenia-only and control groups.63 In addition, there was an inverse correlation between illness duration and frontal lobe size in the comorbid group but not in the schizophrenia-only group.
Another MRI study of patients with juvenile-onset schizophrenia demonstrated significant enlargement of the anterior horn of the lateral ventricle and the third ventricle in patients with OCS compared with patients who did not have OCS.64 In a different study of patients with schizophrenia and various degrees of OCS, functional MRI found that one subgroup exhibited a negative correlation between activation of the left dorsolateral prefrontal cortex and OCS severity.65
Taken together, these findings suggest greater neuroanatomical dysfunction in the comorbid subgroup. Whether these findings reflect a specific pattern of dysfunction unique to this comorbid subgroup or a more severe form of illness with greater brain dysfunction is thus far unclear. The question warrants further study.
Neuropsychological testing
Several studies have compared the profiles of neurocognitive deficits in patients with schizophrenia without OCS with those of the schizo-obsessive subgroup. Most, but not all, of these studies have revealed more severe neuropsychological impairments in the comorbid subgroup.66 Compared with their schizophrenia-only counterparts, the comorbid subgroup demonstrated greater impairment in nonverbal memory, cognitive shifting abilities, visuospatial skills, and executive function as measured by performance on the Wisconsin Card Sorting Test.30,37,38,67,68
Summary
Epidemiological and biological data strongly suggest an integral relationship between OCD and schizophrenia in the comorbid subgroup of patients. The epidemiological data strongly suggest a unique relationship between these 2 disorders, given the marked degree of comorbidity that has been consistently observed and which appears to represent more than just a spurious association. Although pedigree and genetic studies of this overlap group are limited, preliminary findings suggest intriguing genetic influences on comorbid symptom expression.
The neurobiological data on each disorder suggest the involvement of common brain regions and neurotransmitter systems. However, more neuroimaging studies in the overlap group are required to determine whether specific structural abnormalities unique to this putative subtype are present Neuropsychological testing has generally revealed more severe impairment among patients in this comorbid group, which suggests a specific and active interaction between these 2 disease processes, but it has not identified a unique pattern of impairment.9
Hence, despite the growing body of evidence that supports the existence of a specific epidemiological, genetic, and neurobiological relationship between these 2 disorders, the association remains poorly understood. The question of whether this overlap group represents a distinct diagnostic entity or comorbid disorders that result from a greater magnitude of brain involvement, common neurodevelopmental predisposing factors, or other confounding factors is impossible to answer definitively on the basis of existing knowledge.
Further neurobiological and genetic research that focuses specifically on this comorbid group is essential to clarify the nature of this proposed diagnostic entity. The current literature suggests that this comorbid subgroup probably carries a greater overall illness burden and that these patients have more distress and impairment and worse outcomes—including the possibility of higher suicide risk. These clinical issues highlight the importance of identification and treatment of OCS in schizophrenia (Table) while the biological underpinnings continue to be elucidated.
