Are Psychiatric Disorders Related to Each Other?

side by side headshots of Terrie Moffitt and Avshalom Caspi
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Are Psychiatric Disorders Related to Each Other?

By Eugene Rubin MD, PhD and Charles Zorumski MD. -- Psychology Today

The way we think about individual psychiatric illnesses may be changing.

The current, criteria-based approach towards diagnosing psychiatric disorders evolved from research in the 1960s and early 1970s by faculty in the Department of Psychiatry at Washington University in St. Louis. Those investigators analyzed data from clinical observations, longitudinal follow-up of patients, and family history information to define diagnostic criteria for a group of psychiatric illnesses that they believed were well validated based on several defined metrics. Although this approach was not based on disease mechanisms, it did allow for reliable categorization of disorders—reliable meaning that different clinicians would likely agree on the same diagnosis for a given patient. Some of the illnesses included in the original 1972 publication from the Washington University group were schizophrenia, bipolar disorder, major depression, obsessive compulsive disorder, certain anxiety disorders, anorexia nervosa, and alcohol and drug dependence.

This approach to diagnosis was incorporated into the third edition of the DSM (the Diagnostic and Statistical Manual) in 1980. Subsequent DSMs increased the number of psychiatric disorders although some of this expansion lacked strong supporting data regarding validity of the additional diagnoses.

Over time, it has become increasingly evident that many patients fulfill criteria for several psychiatric disorders simultaneously. For instance, an individual with schizophrenia may also have symptoms that fulfill criteria for other disorders, including major depression, anxiety disorders, alcoholism, drug use disorders, and obsessive compulsive disorder. 

Over the last several years, data from genetic studies involving tens of thousands of patients seem to support the concept of a common psychopathology-related factor—called the “p factor”—that increases the risk of developing many psychiatric disorders. As highlighted in a recent paper by Avshalom Caspi and Terrie Moffitt in the American Journal of Psychiatry, the “dose” of this factor may determine the types of psychiatric illness an individual may develop. The concept of a “p factor” can be conceptualized as being akin to the concept of “g,” a general intelligence factor that may underlie human cognitive abilities.

In a paper recently published in the journal Science, the Brainstorm Consortium reported that they found a “high degree of genetic correlation among many of the psychiatric disorders” when they conducted meta-analyses of genetic studies involving hundreds of thousands of patients and controls. When the same genetic approaches were applied to neurological disorders, no common underlying factor was evident. In other words, dementias, Parkinson’s disease, epilepsies, and strokes do not seem to share a single factor increasing the risk of developing several of these illnesses. These observations suggest that the underlying mechanisms of these neurological disorders are more independent than the underlying mechanisms of psychiatric disorders. Additionally, neurological and psychiatric illnesses showed no genetic overlap, with the exception of migraine headaches.

Interestingly, the psychiatric disorder that is genetically related to the largest number of other psychiatric disorders is schizophrenia. This illness involves cognitive symptoms, positive psychotic symptoms such as delusions and hallucinations, and negative symptoms such as withdrawn behavior and lack of motivation. This observation suggests that schizophrenia broadly influences symptoms that are integral to what some might define as the “mind,” spanning cognitive, emotional, and motivational brain systems.

If further data support the concept of a p factor underlying the risk of a large variety of psychiatric disorders, what are the implications regarding diagnostic criteria for individual illnesses? It is possible that further studies may lead to a redefinition of certain psychiatric disorders that is more based on underlying mechanisms. It may also result in fewer major categories of disorders and fewer individual diagnoses. It is presently uncertain how many and which psychiatric disorders share genetic overlap. In the recent Science paper, post-traumatic stress disorder appeared to be an outlier unrelated to schizophrenia and the other disorders that were studied.

There are also significant implications for treatment. Different approaches to treatment may be developed that target common underlying biology. Such treatments may be effective in the treatment, or even prevention, of a variety of disorders. In fact, many of our current medications are relatively non-specific. For instance, current antidepressants are used to treat patients with depression, anxiety disorders, obsessive compulsive disorders, and some pain states, including migraine headaches. Perhaps this is a reflection of these drugs influencing some component of an underlying common (p) factor.

The science underlying psychiatry has advanced to the point that discoveries are leading to new treatments and to new ways of defining psychopathology. Whether the concept of a “p factor” proves useful remains to be determined, but the possibility of finding effective treatments that can alter the course of multiple illnesses is an intriguing idea.

Story originally posted on Psychology Today.

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