Mutually exclusive versus co-occurring diagnostic categories: the challenge of diagnostic comorbidity.

Diagnostic comorbidity is the rule rather than the exception in both DSM-IV and ICD-10. Three types of comorbidity include true comorbidity (clinically distinct entities), artifactual comorbidity (a byproduct of the DSM/ICD strategy to split categorical diagnoses) and spurious comorbidity. DSM has established 'mutually exclusive' relationships between disorders to reduce such spurious comorbidity. Four types of mutually exclusive relationships are identified: (1) between disorders that are on different levels of the Kraepelinian-defined hierarchy (e.g., schizophrenia and major depressive disorder); (2) between categories with identical features that are split according to age or duration (schizophrenia and schizophreniform disorder); (3) between categories in which the defining features of one disorder are contained in the definition of more broadly-defined disorder (Asperger's disorder and autistic disorder), and (4) between categories in which the defining features of one disorder are an associated feature of another disorder (e.g., dysthymic disorder and schizophrenia). Although less comorbidity may be desirable to reduce diagnostic complexity, in the absence of knowledge about underlying pathophysiology, the trend in successive editions of the DSM has been to reduce diagnostic hierarchies and increase comorbidity (e.g., elimination of exclusion between panic disorder and major depression in DSM-III-R because of evidence of independence). It is important to understand that comorbidity in psychiatry does not imply the presence of multiple diseases or dysfunctions but rather reflects our current inability to apply Occam's razor (i.e., a single diagnosis to account for all symptoms).