Epidemiology

Methodological Issues

There are several methodological issues to consider when evaluating the literature on the epidemiology of dual diagnosis. First, data come from both epidemiological and clinical studies. Several large-scale epidemiological studies examining rates of dual diagnosis in general population samples have been carried out since the mid-1980s. These studies provide representative information on rates of mental illness and substance use disorders, use structured diagnostic interviews, and generate results that are reliable and relevant to the population as a whole. Most of the information on rates of dual diagnosis comes from studies of clinical populations. Although such studies are not representative of the general population, they provide valuable information on the types of problems that are faced by individuals in treatment, as well as on the links between dual diagnosis, service utilization, impact on illness, and treatment outcome. Importantly, individuals with multiple disorders are more likely to seek treatment, a condition known as "Berkson's fallacy" (Berkson, 1949), so that estimates of the prevalence of comorbid disorders will be higher in clinical samples. Relatedly, factors such as inpatient or outpatient status and chronicity of illness may affect rates of dual diagnosis. For example, research on patients with schizophrenia has found that more severely impaired inpatients are less likely to abuse substances than patients who are less ill (Mueser et al., 1990). Dual diagnosis rates have also been found to differ by setting, with hospital emergency rooms reflecting higher estimates than other settings (Barbee, Clark, Crapanzano, Heintz, & Kehoe, 1989; Galanter, Castaneda, & Ferman, 1988).

Second, definitions of what constitutes dual diagnosis are far from uniform. Studies of dual diagnosis often employ differing definitions of substance use disorders, making prevalence rates diverse and difficult to compare. For example, definitions of substance abuse vary, ranging from problem use of a substance, to abuse or dependence based on DSM criteria. This is a particularly important issue in terms of diagnostic criteria for both mental and substance use disorders. The publication of DSM-IV (American Psychiatric Association, 1994) and the changes in that system from its predecessor may affect dual diagnosis prevalence rates in both community and clinical samples. These changes include a greater focus on determining that a mental disorder is independent from a substance use disorder by determining that the mental disorder either predated the substance use disorder or persists for at least four weeks following cessation of alcohol or drug use. Others do not specify the exact nature of the substance use they are assessing. Among studies that define the type of substance use they are assessing, different diagnostic criteria are often used, making interpretation and comparison difficult. In addition, the methods used to determine psychiatric and substance use diagnoses can influence findings. The types of diagnostic measures used include structured research interviews, nonstructured clinical interviews, self-report ratings, and reviews of medical records. Although structured interviews are the most reliable method of diagnosis (Mueser, Bellack, & Blanchard, 1992), research with clinical samples will often employ less well-standardized assessments. Relatedly, studies measure different substances in their assessments of dual diagnosis, typically including alcohol, cocaine, heroin, hallucinogens, stimulants, and marijuana. Importantly, some substances are not typically considered in assessments of dual diagnosis. For example, nicotine is usually not considered a substance of abuse in dual diagnosis research, despite the high rates of use among individuals with both mental illness (Lasser et al., 2000) and substance abuse (Bien & Burge, 1990), as well as a growing literature that suggests that nicotine dependence has links, perhaps biological in nature, to both major depression (Quattrocki, Baird, &Yurgelun-Todd, 2000) and schizophrenia (Dalack & Meador-Woodruff, 1996; Ziedonis & George, 1997). Others have found elevated rates of psychiatric and substance use disorders in smokers (Keuthen et al., 2000). Taken together, factors such as the type of problematic substance use assessed, the measures that are used, and the specific substances that are included in an assessment all contribute to varying meanings of the term dual diagnosis.

A final methodological issue involves the split between the mental health treatment system and the substance abuse treatment system, and the impact that this separation has on dual diagnosis research. The literature on dual diagnosis really includes two largely separate areas of investigation: research on substance abuse in individuals with mental illness, as well as research on mental illness in primary substance abusers. In order to get an accurate picture of dual diagnosis and its full impact on clinical functioning and research in psychopathology, both aspects of this literature must be examined.

Findings FroM MAjor Epidemiological Studies

Over the last 25 years, there have been a number of large-scale epidemiological studies of mental illness that examine rates of dual diagnosis, including the Epide-miologic Catchment Area Study (ECA; Regier et al., 1990), the National Comorbidity Survey (NCS; Kessler et al., 1994), the National Comorbidity Survey Replication (NCS-R; Kessler & Merikangas, 2004), and the National Longitudinal Alcohol Epidemiology Study (NLAES; Grant et al., 1994). Although each study differs somewhat from the others in methodology, inclusion/exclusion criteria, and diagnostic categories assessed (see Table 2.1 for a brief description of methods for these studies), there are a number of points that we can take from this literature that can contribute to our thinking about and understanding of dual diagnosis.

Dual Diagnosis Is Highly Prevalent in Community Samples First, epidemiological studies consistently show that dual diagnosis is highly prevalent in community samples. Each of these studies finds that people with mental illness are at greatly increased risk of having a co-occurring substance use disorder, and people with a substance use disorder are likewise much more likely to meet criteria for an Axis I mental disorder. For example, the Epidemiologic Catchment Area Study (ECA; Regier et al., 1990) was the first large-scale study of comorbidity of psychiatric and substance use disorders in the general population, and documented high rates of dual diagnosis among both individuals with primary mental disorders and those with primary substance use disorders. Overall, individuals with a lifetime history of a mental illness had an odds ratio of 2.3 for a lifetime history of alcohol use disorder and 4.5 for drug use disorder, a clear illustration of how those with mental illness are at substantially increased risk of having a comorbid substance use diagnosis. When examined by type of disorder, antisocial personality disorder (ASP) showed the highest comorbidity rate (83.6%), followed by bipolar disorder (60.7%), schizophrenia (47.0%), panic disorder (35.8%), obsessive-compulsive disorder (32.8%), and

Table 2.1

Methods of Several Major Epidemiological Studies on Dual Diagnosis

Study Years Methods

Table 2.1

Methods of Several Major Epidemiological Studies on Dual Diagnosis

Study Years Methods

ECA (Regier et al., 1990)

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