© Springer-Verlag Berlin Heidelberg 2015
Geert Dom and Franz Moggi (eds.)Co-occurring Addictive and Psychiatric Disorders10.1007/978-3-642-45375-5_11. Epidemiological Perspectives on Comorbidity Between Substance Use Disorders and Other Mental Disorders
(1)
Division of Clinical Psychology and Epidemiology, Department of Psychology, University of Basel, Basel, Switzerland
Abstract
Epidemiological studies all over the world could show impressive associations between different groups of substance use disorders and mental disorders, such as affective disorders or anxiety disorders. Most studies investigated cross-sectional associations. The Munich longitudinal population based Early Developmental Stages of Psychopathology (EDSP) Study reported predictive associations between alcohol and cannabis use disorders and nicotine dependence and other mental disorders. This study revealed associations in both directs: from substance use disorders to other mental disorders and vice versa. Explanation of comorbidity should take into account therefore both directions of causality.
1.1 Introduction
The goal of this chapter is to discuss the comorbidity between substance use disorders and other mental disorders from an epidemiological perspective. An extensive literature has documented worldwide a strong association of problematic substance use (use and use disorders) with other mental disorders (for review see, e.g., Lieb and Isensee 2007; Moore et al. 2007; Lev-Ran et al. 2014). A review of European findings has recently confirmed the evidence for Europe (European Monitoring Centre for Drugs and Drug Addiction 2013a). Therefore, the chapter will not repeat these worldwide comorbidity findings but will instead focus on selected findings from European epidemiological studies that have examined comorbidity between substance use disorders and other mental disorders in European community samples. The aim is to demonstrate how epidemiological research can help us understand the phenomenon of comorbidity.
1.2 What Is Comorbidity?
The term comorbidity was introduced by Feinstein (1970) to “refer to any distinct additional clinical entity that has existed or that may occur during the clinical course of a patient who has the index disease under study” (pp 456–457). Or in other words: When two or more clinical conditions occur simultaneously or sequentially in the same person, they are said to be “comorbid.”
But why is it interesting to study comorbidity? There are clinical concerns (i.e., diagnostic issues, treatment strategy) that have to be reconsidered in comorbid patients, but the phenomenon of comorbidity is also of scientific interest. Comorbidity of substance use disorders and other mental disorders may, for example, reveal gaps in current knowledge and become the basis for interesting hypotheses for future studies.
To explain comorbidity of substance use and other mental disorders, two basic scenarios can be used (assuming that the comorbidity findings cannot be explained by methodological shortcomings):
1.
The two (or more) comorbid disorders are causally linked; for the comorbidity of substance use and other mental disorders, this means
(a)
The substance use disorder can cause the temporally secondary other mental disorder (e.g., through biological processes introduced by substance use);
(b)
The other mental disorder can cause the temporally secondary substance use or disorder (e.g., as a means of self-medication).
2.
The substance use disorder(s) and the other mental disorder(s) share disease-related factors, for instance, risk factors, causal factors, triggers, or abnormalities in the same brain regions.
Comorbidity seems to be a very complex phenomenon that may help researchers detect etiological pathways to disorders or consequences of diseases. In evaluating the complexity of comorbidity, epidemiologists focus first on determining if there is an association (cross-sectional) between substance use disorders and other mental disorders. If such an association can be shown, the next question addresses the longitudinal or predictive association: Does one (the primary) disorder (i.e., the disorder with the earlier onset) prospectively increase the risk (in terms of incidence) for the other (secondary) disorder? If yes, the primary disorder is said to be a risk factor for the development of the secondary one (see Kraemer et al. 1997). Risk means here the probability of developing the secondary disorder but does not imply a deterministic association. For a factor to be called a “risk factor,” a statistically significant association between the predictor and outcome under consideration must be shown. In the search for causality of the association, such a risk association is one critical component but is not sufficient on its own. Other criteria for causal relationships have been proposed in the literature (e.g., Rothman and Greenland 2005) and must be reviewed for the each association (e.g., biological plausibility, coherence of existing empirical knowledge, dose–response relationship.
In the following, we focus on selected epidemiological studies that have addressed cross-sectional and longitudinal (predictive) associations between substance use/disorders and other mental disorders. We start with a brief look at the size of the problem, that is, on the prevalence of mental and substance use disorders in Europe.
1.3 Size of the Problem in Europe
The best and most valid information about the prevalence of mental and substance use disorders can be taken from a comprehensive review on the 12-month prevalence and disability burden estimate of a broad range of mental disorders in the European Union (EU) that was conducted by Wittchen et al. (2011). The authors systematically reviewed the existing literature, reanalyzed existing data and national surveys, and consulted experts. They included studies and data from all member states of the EU (EU-27) plus Switzerland, Iceland, and Norway. Using this method, they estimated that each year, 38.5 % of the adult EU population suffer from at least one mental disorder. Adjusted for age and comorbidity, this corresponds to 164.8 million people affected per year. Anxiety disorders (14.0 %; 61.4 million people affected) and major depression (6.9 %; 30.3 million people affected) are the most frequent mental disorders. Almost 15 million people (3.4 %) are affected by alcohol dependence. More than one million adult Europeans are affected by drug dependence (opioid or cannabis dependence; prevalence rates: 0.1–1.8 %). These findings underline impressively that neither mental nor substance use disorders affect “only” few people. Rather, they must be considered an important care challenge for the EU in the twenty-first century.
1.4 Comorbidity Findings from Selected European Population-Based Studies
1.4.1 Description of Comorbidity
Results of the German National Health and Examination Survey Mental Health Supplement (GHS-MHS) can be used to develop a preliminary description of comorbidity between substance use disorders and other mental disorders. The GHS-MHS was the first nationwide study to investigate the prevalence of a broad range of mental disorders as defined in the Diagnostic and Statistical Manual of Mental Disorders (American Psychiatric Association 1994) and their comorbidities and correlates in the community in Germany. The survey included a random sample of N = 4,181 people aged 18–65 years. Diagnoses of mental disorders were based on fully standardized computer-assisted interviews (the Munich-Composite International Diagnostic Interview, or M-CIDI; see Jacobi et al. 2004). This study presented rates for 12-month comorbidity between substance use disorders and other mental disorders as follows: Among people who fulfilled DSM-IV diagnostic criteria for alcohol abuse or alcohol dependence, about half (55.1 %) presented this as a “pure” disorder (i.e., did not fulfill criteria for any other mental disorder). More than 20 % fulfilled diagnostic criteria for one other mental disorder, 7.8 % the criteria for two other diagnoses, and 14.4 % the diagnostic criteria for three or even more additional diagnoses. Among people who fulfilled diagnostic criteria for drug abuse or drug dependence, even a higher proportion were comorbid (total 54.7 %). Here, 29.0 % fulfilled criteria for one additional diagnosis, 12.9 % for two additional diagnoses, and 12.9 % for three or more additional diagnoses (within the same 12-month interval).
Results additionally revealed that among mental disorders, substance use disorders are the less frequently treated disorders. Among people with substance abuse or dependence, only about a quarter (23.0 % for alcohol abuse/dependence to 25 % for illicit drug abuse/dependence) received at least a “minimal intervention” for their condition. In contrast to these low treatment rates, treatment was offered far more often to people suffering from other mental disorders (e.g., any affective disorder: 52.5 %; panic disorder: 75.4 %; any eating disorder: 47.0 %; see Jacobi et al. 2004). The study also found that across disorders people with comorbid disorders tended to report higher healthcare utilization rates than those with pure disorders.
Remarkably high comorbidity rates specifically for alcohol use disorders were found in the Copenhagen City Heart Study (Flensborg-Madsen et al. 2009). This study included 18,146 individuals randomly selected from the population. Among people who fulfilled study criteria for alcohol use disorders, more than the half (50.3 %) fulfilled criteria for another disorder. A closer inspection revealed that among these people, 18 % fulfilled criteria for an affective disorder, 24 % for personality disorder, 8 % for psychotic disorder, 7 % for anxiety disorder, and 16 % for drug abuse. Comparable findings were reported for nicotine dependence from the German Transitions in Alcohol Consumption and Smoking (TACOS) Study (Schumann et al. 2004). This population-based study (N = 4,075 18- to 64-year-olds) found that among people with DSM-IV nicotine dependence, 19.5 % fulfilled criteria for another substance use disorder (abuse/dependence). A comparable rate (19.2 %) fulfilled criteria for any affective disorder and almost one quarter the criteria for any DSM-IV anxiety disorder (24.4 %). Together, descriptive data from the three studies show that an impressive proportion of people affected by substance use disorders also fulfill diagnostic criteria for at least one other mental disorder within the same time interval (12 months).
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