Comorbid psychiatric illnesses

Comorbidity among psychiatric illnesses is common, as is comorbidity between psychiatric and physical illnesses. Current knowledge of psychiatric comorbidity points to several possible underlying factors, notably an overlap in their definitions and symptoms; unidirectional and bidirectional causation; disordered sleep; and a range of shared risk factors. Psychiatric illness may lead to poorer self-care and sleep problems, whereas being physically ill may impact upon an individual’s psychological wellbeing. An integration of the various causal models that have been proposed to explain the comorbidities is discussed, incorporating different socio-psychological and biological factors to explain the development of depression and anxiety. These issues are detailed in the following chapter with a focus on depression. Implications for treatment are also discussed.