The concept of psychiatric comorbidity is misused and poorly understood

A recently published research study written by a team of researchers from the University of Copenhagen and the University of Melbourne encourages a re-evaluation of the approach to psychiatric comorbidity, the co-occurrence of two or more mental disorders. The article Psychiatric Comorbidity: A Concept in Need of a Theory was published in Psychological Medicine.

Although the notion of psychiatric comorbidity is relatively new, it has rapidly become a foundational concept in psychiatric clinical practice and research. However, this latest study highlights several challenges related to its inadequate definition, differential diagnostic problems, and the reification or treatment of abstract entities as if they were concrete mental disorders.

According to the researchers, these issues could have a significant impact on the effectiveness of diagnostic assessments in current clinical and psychiatric research practices.

Developing a robust theoretical framework for addressing psychiatric comorbidity could revolutionize the way we understand, diagnose and treat multiple co-occurring mental disorders, said Dr Julie Nordgaard of the University of Copenhagen, lead author of the study. We need to move from the broad-based approach currently employed to a more nuanced system that captures the complex reality of patients’ experiences.

The concept of comorbidity was originally devised by Alvan Feinstein in 1970 for use in general practice. In the 1980s, comorbidity was introduced into psychiatry and over time its use has become commonplace in clinical thinking and practice. This explosion of comorbidities presents practical and ethical problems for the field and the people it treats.

Comorbidity typically refers to the concurrent clinical presence of two or more disease entities. However, Feinstein’s original formulation included qualifying criteria, such as clear disease origins or distinct clinical psychopathology, which psychiatry often has difficulty meeting. This has led others to add different conditions for its use in psychiatry, but not without problems.

Julie Nordgaard and co-authors say:

The number of patients diagnosed with comorbid psychiatry has increased significantly, and so has empirical research on comorbid mental disorders. This development is also reflected in diagnostic manuals, where the term comorbidity appeared 0 times in DSM-III but more than 600 times in DSM-5.
Despite the popularity of the concept of psychiatric comorbidity, it has often been criticized. Indeed, there is a striking inconsistency between the widespread use of psychiatric comorbidity in clinical practice and empirical research and theoretical studies which generally place strong reservations towards psychiatric comorbidity.

This article examines the origins and development of the concept of comorbidity. The authors propose several difficulties with its use in psychiatry, illustrated by clinical examples. They conclude by suggesting their own methods to remedy these problems.

In Feinstein’s original definition of comorbidity, an additional distinct clinical entity was required. He used the example of lung cancer co-occurring with, for example, coronary heart disease, discovered independently of the cancer. This demonstrates what the authors call the necessary mutual independence of comorbid entities.

Due to Feinstein’s formulation that comorbidity requires a clear etiology or clearly demarcated pathology, difficulties arise when comorbidity is used in psychiatry. For example, most mental disorders have an unknown etiology, given the uncertainty about the exact development of many mental disorders in terms of genetics and environment.

In terms of demarcation, psychiatry also encounters problems. Many mental disorders have overlapping symptoms, which can make diagnosis difficult.

Traditionally, according to the authors, diagnostic hierarchies have been put in place, suggesting that the diagnosis of comorbid disorders should be avoided if a diagnostic category can explain all of a person’s symptoms. Specific categories were classified in terms of how the diagnosis should proceed, such as organic disorders and schizophrenia overcoming anxiety and personality disorders.

The authors say that hierarchical diagnoses like this have gradually eroded and given way to an outbreak of comorbid diagnoses due to a distrust of the diagnostic hierarchy.

Additionally, the researchers argued in favor of a more hierarchical diagnostic system with explicit exclusion rules. Such a system could simplify clinical practice and research by reducing information complexity and mitigating cases of unwarranted psychiatric comorbidity where multiple diagnoses may not accurately represent a patient’s mental health condition.

There are several problems associated with comorbid diagnoses, such as that people tend to experience adjustment difficulties with even one diagnosis as they adjust to a new understanding of themselves and make life changes to cope with the disorder.

Furthermore, comorbidity can often lead to inappropriate polypharmacy or the prescribing of multiple drugs to address different symptoms. Finally, there is the risk of compartmentalizing a person’s psyche:

thus not seeing the patient as a whole, unified human being, but instead as a person with a psyche made up of, say, one part schizophrenia, one part OCD, and one part ADHD, and where each of these parts may invite different specific treatments of the disorder.

An alternative proposition in the paper is to consider issues of trait versus state in pathology. Trait conditions can vary in severity but are present for longer periods. Trait conditions include autism spectrum disorder, ADHD, schizophrenia, or personality disorders. On the other hand, state conditions are typically short-term, such as affective problems such as major depressive disorder.

When diagnosing someone with both depression and personality issues, the authors argue that both should not initially be diagnosed. Instead, depression should be treated first to see if the personality problems subside once the depression goes away. Only then should the diagnosis of comorbidity be considered. This is an example of considering trait and state conditions in the diagnosis to avoid rampant comorbidities and potentially losing sight of the overall clinical picture.

The authors ask, for example, whether an anxiety disorder that occurs independently of schizophrenia is really the same thing as anxiety that occurs parallel to or as an aspect of schizophrenia.

Ultimately, however, the clinical judgment required for a clear diagnosis requires varied clinical experience, reading of the psychopathological literature, and ongoing discussions of psychopathological phenomena with experts.

Some researchers have suggested things like biomarkers, treatment response, and risk factors as ways to assess diagnostic validity, but the authors argue that without a clear etiology/understanding of the pathogenesis of the disorders, these aren’t strong diagnostic tools.

Instead, they advocate the trait/state distinction and some degree of hierarchical diagnosis, as well as clear exclusion criteria for the diagnosis. For example, if someone is diagnosed with category A, they may not be diagnosed with category B. Some exclusion criteria already exist, but the authors believe it’s unclear, relying on doctors to determine whether symptoms are better explained by one diagnosis over another without strong guidelines.

Several researchers now believe that a one-dimensional approach would solve many of the problems discussed by the authors, but the authors argue that a one-dimensional approach might actually be grounded in the kind of liberal assessment of psychiatric comorbidity they are arguing against.

In support of the authors’ argument about polypharmacy as related to comorbidity, existing research has found that polypharmacy is on the rise and poses significant dangers. With regards to network-based or size-based approaches, it appears that comorbidity is considered the rule rather than the exception, which should lead researchers and clinicians to consider the nuances of comorbidity as a clinical tool.


Nordgaard, J., Nielsen, KM, Rasmussen, AR and Henriksen, MG (2023). Psychiatric comorbidity: a concept that needs a theory.Psychological Medicine, 1-7. (Connection)

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