Psychiatric illnesses do not generally occur in isolation. Many people with a psychiatric disorder have one or more other psychiatric disorders. Termed comorbid disorders, co-occurring disorders, or dual diagnosis, people with psychiatric illnesses have a variety of other conditions. In addition, a variety of medical and psychosocial problems are more common in people with psychiatric illnesses. The risk of a substance use disorder in people with another psychiatric illness is very high. This increased risk seems to be a general feature of most, if not all, psychiatric illnesses. However, the prevalence is particularly high in bipolar disorder and schizophrenia. Although dual diagnosis patients may be the patients most often receiving psychiatric treatment, diagnostic criteria, treatment, and some treatment models treat disorders as separate and isolated conditions. Furthermore, due to the complicated nature of multiple psychiatric disorders and symptoms, dual diagnosis patients are often excluded from research.
While the magnitude of the problem may be clear, the etiology is much less so. Many psychiatric symptoms, including psychosis, depression, mania and anxiety, could potentially lead to the use of substances. The etiology could be self-medication, or a variety of other factors including trait markers of some illness such as impaired judgment and insight or increased impulsivity. Unstable housing and other environmental factors may also lead to greater exposure to substances of abuse. On the other hand, substances of abuse can also lead to the development of psychiatric disorders or cause symptoms resulting in misdiagnosis of occurring disorders that actually represent symptoms of substance use or withdrawal. This conundrum, combined with the fact that substance use and many other psychiatric illnesses tend to develop in late adolescence or early adulthood, makes causality difficult to determine.
Whatever the etiology, substance use disorders in people with major psychiatric illnesses are generally associated with a variety of adverse consequences. Psychiatric symptom exacerbations and hospitalization are common. Treatment non-adherence is also common and likely, in part, underlies some of the other consequences. Furthermore, many psychiatric disorders are associated with an increased risk of suicide. The presence of a substance use disorder, which in itself is a suicide risk factor, may further increase suicide risk in dual diagnosis patients.
Despite the frequency of dual diagnosis and the poor outcomes that can be associated with it, relatively little data are available on its treatment. This paucity of data likely reflects the challenges in designing interventions studies in complicated dual diagnosis populations and the bias toward clinical trials in narrowly defined populations with few comorbidities. Many of the largest clinical trials are conducted by the pharmaceutical industry in order to obtain FDA approval for a medication. These studies generally exclude participants with active or recent substance use. Thus, there is a disconnect between the patients studied in major research studies and those presenting for clinical care.
The current issues of the American Journal of Drug and Alcohol Abuse is devoted entirely to the topic of dual diagnosis. The definition of dual diagnosis used is relatively broad in scope. The papers included cover a variety of topics within the field. However, the issue is not intended to be comprehensive in scope as in a textbook. Instead, each paper focuses on a topic with dual diagnosis that is within the area of expertise and research of the author. The papers, nonetheless, cover many important topics in the field and are from experienced researchers. The pace of research on dual diagnosis has increased in recent years, and the reviews reflect this increased interest in the topic and suggest that the field is moving forward at a much more rapid pace than in the past.