The complexity of psychiatric comorbidity: A conceptual and methodologicaldiscussion

Comorbidity is widely used in psychiatry, although few studies have conside red the conceptual and methodological problems deriving from the transposit ion of this term from medicine to psychiatry. Comorbidity should be defined as two or more diseases, with distinct aetiopathogenesis (or, if the aetio logy is unknown, with distinct pathophysiology of organ or system), that ar e present in the same individual in a defined period of time. In psychiatry , comorbidity is often an artefact for several reasons: (a) different asses sment methods; (b) improper utilisation of the term comorbidity to indicate the association of symptoms instead of diseases; (c) number and characteri stics of hierarchical exclusion rules used in classification systems; (d) n osologic classification in disorders (a generic term) instead of syndromes (a more precise concept, that allows clinicians to consider the hierarchy a nd the qualitative specificity of symptoms); (e) excessive splitting of cla ssical syndromes into small disorders with inappropriate and overlapping bo undaries; (f) too frequent revision of the diagnostic criteria, that change s diagnostic threshold; (g) number of clinical entities considered. Biologi cal and psychological hypotheses that investigate the complexity of comorbi dity findings are here presented; it is underlined that comorbidity should be the epidemiological descriptive starting point to build hypotheses that must be clear and rigorously defined, with specified usefulness and limits. Finally, the hypotheses should be tested with specific methodologies. Copy right (C) 2000 S. Karger AG, Basel.