Abstract
Background
Comorbidity within mental disorders is common – individuals with one type of mental disorder are at increased risk of subsequently developing other types of disorders. Previous studies are usually restricted to temporally-ordered pairs of disorders. While more complex patterns of comorbidity have been described (e.g. internalizing and externalizing disorders), there is a lack of detailed information on the nature of the different sets of comorbid mental disorders. Additionally, mental disorders are associated with premature mortality, and people with two or more types of mental disorders have a shorter life expectancy compared to those with exactly one type of mental disorder. The aims of this study were to: (a) describe the prevalence and demographic correlates of combinations of mental disorders; and (b) estimate the excess mortality for each of these combinations.
Methods
We conducted a population-based cohort study including all 7,505,576 persons living in Denmark in 1995–2016. Information on mental disorders and mortality was obtained from national registers. First, we described the most common combinations of mental disorders defined by the ICD-10 F-subchapters (substance use disorders, schizophrenia spectrum disorder, mood disorders, neurotic disorders, etc.). Then, we investigated excess mortality using mortality rate ratios (MRRs) and differences in life expectancy after disease diagnosis compared to the general population of same sex and age.
Results
At the end of the 22-year observation, 6.2% individuals were diagnosed with exactly one type of disorder, 2.7% with exactly two, 1.1% with exactly three, and 0.5% with four or more types. The most prevalent mental disorders were neurotic disorders (4.6%) and mood disorders (3.8%), even when looking particularly at persons with a specific number of disorders (exactly one type, exactly two types, etc.). We observed 616 out of 1,024 possible sets of disorders, but the 52 most common sets (with at least 1,000 individuals each) represented 92.8% of all persons with diagnosed mental disorders. Mood and/or neurotic disorders, alone or in combination with other disorders, were present in 64.8% of individuals diagnosed with mental disorders. People with all combinations of mental disorders had higher mortality rates than those without any mental disorder diagnosis, with MRRs ranging from 1.10 (95% CI 0.67 – 1.84) for the two-disorder set of developmental-behavioral disorders to 5.97 (95% CI 5.52 – 6.45) for the three-disorder set of schizophrenia-neurotic-substance use disorders. Additionally, any combination of mental disorders was associated with shorter life expectancies compared to the general population, with estimates ranging from 5.06 years [95% CI 5.01 – 5.11] for the one-disorder set of organic disorders to 17.46 years [95% CI 16.86 – 18.03] for the three-disorder set of schizophrenia-personality-substance use disorders.
Discussion
Within those with mental disorders, approximately 2 out of 5 had two more types of mental disorders. Our study provides prevalence estimates of the most common sets of mental disorders – mood disorders (e.g. depression) and neurotic disorders (e.g. anxiety) commonly co-occur, and contribute to many different sets of comorbid mental disorders. The association between mental disorders comorbidity and mortality-related estimates revealed the prominent role of substance use disorders with respect to both elevated mortality rates and reduced life expectancies. Substance use disorders are relatively common, and these disorders often feature in sets of mental disorders. In light of the substantial contribution to premature mortality, efforts related to the ‘primary prevention of secondary comorbidity’ warrant added scrutiny.
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