A disorder reflects a set of symptoms (mental or physical) that causes significant impairment or distress. Comorbidity is the simultaneous presence of two or more disorders within a person. For example, a person diagnosed with generalized anxiety disorder and major depressive  disorder  may  experience  symptoms  of the former condition, such as chronic, uncontrollable worry, as well as symptoms of the latter condition, such as negative mood or thoughts of suicide. Comorbidity is common among people seeking mental health services, and it increases the risk for more severe symptom development if left untreated. Accordingly, an understanding of the causes and consequences of comorbidity is recognized by mental health clinicians and researchers as critical to the classification, development, diagnosis, and treatment of mental disorders.

Comorbidity can often be a product of how disorders are classified. For example, a classification system that splits symptoms into a greater number of diagnostic groups is likely to produce increased comorbidity estimates, whereas a classification system that lumps symptoms into a fewer number of diagnostic groups is likely to produce decreased comorbidity estimates (but as a result may miss important distinctions). Furthermore, the more symptoms two disorders have in common, the more likely it is that both disorders will be diagnosed.

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When comorbidity is present in an individual, researchers and clinicians distinguish between primary disorders and secondary disorders. The primary disorder is the disorder that is most severe and debilitating, and the other disorders are secondary comorbid disorders. Therefore, the rate of comorbidity between two disorders depends on which disorder is considered to be primary. This distinction is important because it may form the basis for treatment recommendations. For example, an individual who has a primary anxiety disorder  and  secondary  major  depressive  disorder may benefit more from treatments that reduce anxiety symptoms than those that reduce symptoms of depression. Finally, in some classification systems, such as the Diagnostic and Statistical Manual for Mental Disorders (DSM), a diagnosis cannot be assigned when symptoms do not meet criteria for a given disorder. It has been argued that much potentially useful information about symptom comorbidity is lost by disregarding additional symptoms that do not meet criteria for a comorbid disorder. However, the current system helps ensure that diagnoses are only assigned when sufficient symptoms are present to indicate a disorder.

The study of comorbidity may also provide information about processes that give rise to different mental disorders. This is the case with certain disorders that are associated with increased risk for the development of other disorders. Disorders A and B may be highly comorbid, such that people with disorder A are at increased risk for the development of disorder B, and vice versa. This pattern of comorbidity may reflect a shared genetic or psychological vulnerability between the disorders. However, many alternative explanations would need to be ruled out before one could conclude that a single vulnerability causes disorders A and B. For example, the comorbidity may be due solely to a greater tendency in individuals with both disorders to seek treatment than in individuals with only one of the two disorders.

Finally, comorbidity is an essential concept in treatment.  Compared  with  individuals  who  have only one disorder, individuals with comorbidity have poorer overall functioning and poorer response to a variety of treatments. As a result, comorbidity is an important consideration when clinicians are planning treatment. Because most available treatments are designed to treat a single disorder, clinicians may focus on treating the symptoms of the primary disorder while addressing comorbid disorders indirectly. For example, clinicians may try to improve treatment  response  by  targeting  symptoms  common  to  both disorders, which may share a common cause.

Much remains to be learned about comorbidity. A better understanding of the specific ways in which different approaches to classification, such as splitting or lumping symptoms into diagnostic categories, influence comorbidity will be crucial to future revisions of widely used diagnostic manuals. In addition, research is needed to clarify the complex relationships between basic developmental processes and comorbidity. Finally, the study of comorbidity may inform the development of new treatments that directly target the occurrence of multiple disorders.


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