Abstract
Comorbidity is defined as “any distinct clinical entity that has existed or that may occur during a patient's clinical course who has the index disease under study.”
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It has become clear that the presence of comorbid disease can dramatically affect the treatment of the index disease. Specifically the presence of comorbid disease can often complicate, interfere with, or make the treatment of the index disease more difficult, making the prognosis worse.
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In addition, the presence of comorbid disease, because of its impact on the treatment of the index disease, can lead to spurious medical outcome data, especially if the comorbid disease is not classified, not analyzed, and its effect not controlled for.
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For these reasons, there has been significant psychiatric research interest on comorbidity, on different types of comorbidities present within psychiatric patients, the effects of comorbid disease on the index disease, and the effects of comorbid disease on treatment outcome.
Psychiatric comorbidities can be divided into five large categories. The first category is comorbidities between psychiatric disorders on Axis I of the
Diagnostic and Statistical Manual of Mental Disorders, 4th edition (DSM-IV).
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An example of this category of comorbidity is the presence of major depressive disorder in association with panic disorder. This first category of comorbidity is extremely common in the general population. For example, in a US psychiatric diagnosis lifetime prevalence study,
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it was found that of individuals with a lifetime psychiatric disorder, 79% had a comorbidity associated psychiatric disorder, with three or more disorders being common. The second major category of psychiatric comorbidity is that of comorbidities between Axis I psychiatric disorders and Axis II (personality disorders).
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An example of this type of comorbidity is the presence of depression (Axis I) in association with antisocial personality disorder (Axis II). This second major category of psychiatric comorbidity is also extremely common within psychiatric populations.
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In addition, there is significant evidence that the presence of a comorbid disorder on Axis II influences the symptoms and clinical course of the index Axis I disorder as well as the choice of therapy and treatment outcome.
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The third major category of psychiatric comorbidity is that between Axis I psychoactive substance use disorder diagnoses and other psychiatric disorders on Axis I. An example of this type of comorbidity is the presence of alcohol dependence in association with major depression. This category of comorbidity is extremely common within patients with psychiatric problems. Reports indicate that for those with any mental disorder, the lifetime prevalence for addictive disorders is about 29% (22% for alcohol, 15% other drugs).
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The fourth major category of psychiatric comorbidity is that of comorbidities within psychoactive drug use disorders only. An example of this type of comorbidity is the presence on Axis I of cocaine dependence in association with alcohol dependence. This category of comorbidity is also extremely common within the psychiatric population.
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The final category of psychiatric comorbidity is that between all psychiatric disorders (both on Axis I and Axis II) and between any nonpsychiatric disorder or physical illness (e.g., hypertension). It has become clear that this category of psychiatric comorbidity may be the most commonly encountered by the practicing physician because it appears that most forms of physical illness are associated with depression. Depression has been identified in 2% to 45% of patients with physical illness depending on the method used.
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In addition, research
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indicates that patients with physical illness are at greater risk for developing psychiatric illness. The lifetime prevalence rates for any psychiatric condition are far greater for patients with one or more medical conditions than those patients without medical conditions.
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Finally, psychiatric comorbidity on Axis I in association with medical illness can increase the incidence of sick leave.
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For a number of reasons, significant psychiatric comorbidity in chronic pain patients (CPPs) is expected. CPPs consider themselves to suffer from a physical illness for which physicians cannot seem to develop a cure
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; this physical illness is associated with significant impairment and disability that has tremendous impact on CPPs' lives
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; to control pain, CPPs are often placed on psychoactive substances, which have dependence and addiction potential
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; and because often no apparent tissue damage can be found to explain the cause of chronic benign nonmalignant pain, physicians frequently attribute CPPs' pain to underlying psychiatric illness. To address the issue of comorbidity between chronic pain and psychiatric disorders, research in this area is reviewed. This research is reviewed according to the major categories of psychiatric comorbidities described earlier.
