A recently published research study, penned by a team of researchers at the University of Copenhagen and the University of Melbourne, encourages a reevaluation 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.
Though the notion of psychiatric comorbidity is relatively new, it has rapidly become a fundamental concept in clinical practice and psychiatric research. However, this latest study highlights numerous challenges related to its inadequate definition, differential diagnostic problems, and the reification—or treating of abstract entities as if they were concrete—of mental disorders.
According to the researchers, these issues could significantly impact the efficacy of diagnostic assessments in current clinical and psychiatric research practices.
“Developing a robust theoretical framework to approach psychiatric comorbidity could revolutionize how we understand, diagnose, and treat multiple concurrent mental disorders,” said Dr. Julie Nordgaard from the University of Copenhagen, the study’s lead author. “We need to shift from the broad-brush approach currently employed to a more nuanced system that captures the complex reality of patient experiences.”
The concept of comorbidity was originally devised by Alvan Feinstein in 1970 for use in general medicine. In the 1980s, comorbidity was brought into psychiatry, and over time its use has become common in clinical thinking and practice. This explosion of comorbidity presents practical and ethical issues to the field and the people it treats.
Comorbidity typically refers to the co-occurring clinical presence of two or more “disease entities.” However, Feinstein’s original formulation included qualifying criteria, such as clear origins of the disease or distinct clinical psychopathology, that psychiatry often has difficulty meeting. This has led others to add different conditions for its use within psychiatry, but not without problems.
Julie Nordgaard and co-authors state:
“The number of patients diagnosed with psychiatric comorbidity has increased significantly, and empirical research on comorbid mental disorders has similarly grown. This development is also reflected in the diagnostic manuals, where the term ‘comorbid’ 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. In fact, there is a striking incongruence between the widespread use of psychiatric comorbidity in clinical practice and empirical research and theoretical studies that generally pose strong reservations toward psychiatric comorbidity.”
The current 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 for remedying these issues.
In Feinstein’s original definition of comorbidity, a “distinct additional clinical entity” was required. He used the example of lung cancer co-occurring with, for example, coronary artery disease, discovered independently of the cancer. This demonstrates what the authors call the necessary mutual independence of comorbid entities.
Because of Feinstein’s formulation, where comorbidity requires either clear etiology or clearly demarcated pathology, difficulties arise when comorbidity is used in psychiatry. For example, “most mental disorders have…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 runs into problems. Many mental disorders have overlapping symptoms, which can make diagnosis tricky.
Traditionally, according to the authors, diagnostic hierarchies were put in place, suggesting that diagnosing comorbid disorders should be avoided if one diagnostic category can account for all of a person’s symptoms. Specific categories were ranked in terms of how diagnosis should proceed, such as “organic disorders” and “schizophrenia” outranking anxiety and personality disorders.
The authors state that hierarchical diagnosis like this has “gradually eroded” and given way to the outbreak of comorbid diagnoses because of a distrust in the diagnostic hierarchy.
Furthermore, the researchers argued in favor of a more hierarchical diagnostic system with explicit exclusionary rules. Such a system could streamline clinical practice and research by reducing informational complexity and mitigating unwarranted psychiatric comorbidity—instances where multiple diagnoses might not accurately represent a patient’s mental health condition.
There are several problems associated with comorbid diagnoses, such as the fact that people tend to experience adjustment difficulties with even one diagnosis as they adapt to a new understanding of themselves and make life changes to cope with the disorder.
Additionally, comorbidity can often lead to inappropriate polypharmacy or the prescription of multiple pharmaceutical drugs to deal with different symptoms. Finally, there is a risk of compartmentalizing a person’s psyche:
“thereby not seeing the patient as a whole and unified human being but instead as a person with a psyche composed of, say, one part schizophrenia, one part OCD, and one part ADHD, and where each of these parts may invite different disorder-specific treatments.”
One alternative proposal in the paper is considering trait versus state issues in pathology. Trait conditions “can fluctuate in severity” but are present for more extended periods. Trait conditions include “autism spectrum disorder, ADHD, schizophrenia, or personality disorders.” On the other hand, state conditions are typically shorter-term, like affective problems such as major depressive disorder.
When diagnosing someone who presents with both depression and personality struggles, the authors argue that both should not initially be diagnosed. Instead, the depression should be treated first to see if the personality issues recede once the depression goes away. Only then should the comorbid diagnosis be considered. This is one example of considering trait and state conditions in diagnosis to avoid rampant comorbidity and potentially losing sight of the overall clinical picture.
The authors ask, for example, whether an anxiety disorder occurring independently of schizophrenia is really the same thing as anxiety occurring alongside or as an aspect of schizophrenia.
Ultimately, however, the “clinical judgment” required for clear diagnosis requires “varied clinical experience, reading psychopathological literature, and ongoing discussions about 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 clear etiology/understanding of the pathogenesis of disorders, these are not strong diagnostic tools.
They argue instead for the trait/state distinction and some degree of hierarchical diagnosis, as well as clear exclusionary criteria for diagnosis. For example, if someone is diagnosed with category A, then they cannot be diagnosed with category B. Some exclusion criteria already exist, but the authors believe that it is unclear, relying on clinicians to determine whether symptoms are best explained by one diagnosis over another without strong guidelines.
Several researchers now believe that a dimensional approach would solve many of the problems the authors discuss, but the authors argue that a dimensional approach may actually be grounded in the kind of “liberal assessment of psychiatric comorbidity” they are arguing against.
Supporting the authors’ argument about polypharmacy as linked to comorbidity, existing research has found that polypharmacy is on the rise and poses significant dangers. As for network or dimensional based approaches, it does seem 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, K. M., Rasmussen, A. R., & Henriksen, M. G. (2023). Psychiatric comorbidity: A concept in need of a theory. Psychological Medicine, 1-7. (Link)