To determine rates of psychiatric comorbidity in a 17-year period, investigators cross-referenced Danish national databases on 5.9 million people (mean baseline age, 32) to identify new diagnoses across 10 broad diagnostic groups in the International Classification of Diseases–10 (organic disorders, substance use, schizophrenia and related disorders, mood disorders, eating disorders, personality disorders, intellectual disorders, developmental disorders, behavioral disorders, and neurotic disorders [including anxiety, somatoform, and stress-related disorders]).
In analyses of pairs of diagnostic groups, receiving a diagnosis of any disorder significantly increased the risk for a second one. The likelihood of a new additional diagnosis was highest during the first 6 months after the initial diagnosis but remained elevated for the entire study period. Comorbidity was the same regardless of which disorder came first.
Although all diagnostic categories had a degree of comorbidity, the association was stronger in some (e.g., mood disorders, substance use, schizophrenia, personality disorders, and developmental disorders). Algorithms estimated the time-dependent risk for a patient to develop a second disorder in another group. For example, 40% of men and 50% of women who were diagnosed with a mood disorder before age 20 would get diagnosed with a neurotic disorder in the next 15 years.
Diagnostic uncertainty might account for the second diagnosis, a hypothesis supported by the finding that comorbidity was highest soon after the initial diagnosis. This does not explain, however, why comorbidity is sustained. Because many psychiatric diagnoses have similar symptoms, clinicians may later make additional diagnoses when emphasising specific features.
An editorialist argues that multiple disorders have a common pathophysiology and suggests that dimensions of illness are clinically more relevant than descriptive diagnoses. In any event, while treating one disorder, we should be vigilant for commonly associated conditions so that we can identify and treat them sooner.