Is mental illness categorical or dimensional?

In my last post I discussed the politics of psychiatric nosology and the revision of the Diagnostic and Statistical Manual of Mental Disorders (DSM).  While the machinations behind specific disorders are fascinating, it is easy to miss the forest for the trees.  The basic idea of dividing mental distress and disability into diagnostic categories is itself controversial.

The DSM takes the stance that there are discrete “bins” (disorders) that individual patients can be sorted into.  For over 25 years there has been discomfort in the professional community about this, particularly in the case of personality disorders (Axis II of the DSM).  One of the architects of Axis II, Theodore Millon, PhD, objected to purely categorical personality diagnosis.  His website says:

It is Millon’s view that there are few pure variants of any personality prototype. Rather, most persons evidence a mixed picture, that is, a personality that tends to blend a major variant with one or more subsidiary or secondary variants.

Statistics on Axis II disorders seem to bear this out.  Many studies show great overlap between categories, and many patients fit into more than one.  Clinical experience concurs: The patients I see are rarely “classic” cases of anything; everyone is unique.  Even everyday life experience suggests that personality features exist on a continuum:  One person is a bit detail-oriented, another is mildly obsessive, and another has serious problems with obsessiveness.  Yet the current DSM-IV-TR only defines “obsessive-compulsive personality disorder” (and allows mention of “obsessive compulsive personality traits,” but this doesn’t count as a disorder).  No nuance, no matter-of-degree.  Why do we have such ham-handed tools to describe psychiatric problems?

One reason is that psychiatry is a branch of allopathic Western medicine.  Unlike, say, traditional Eastern healing, we think in terms of disease categories.  And this fits fairly well for the most severe Axis I psychiatric disorders, such as schizophrenia.  The German psychiatrist Emil Kraepelin founded contemporary scientific psychiatry in the late 1800s by distinguishing the pattern of symptoms in schizophrenia (“dementia praecox”) from that in manic depression.  In the early-to-mid 20th century, Freudian theory deflected such categorization, but it re-emerged as the dominant paradigm since the publication of DSM-III in 1980.  The 1990s “Decade of the Brain” heralded intensive research efforts to understand mental disorders from a medical perspective.  Today the vast majority of published psychiatric research is biomedical in nature, facilitated by the DSM’s categorical framework.

In contrast to the medical psychiatric tradition, psychology has long pursued dimensional features of personality using empirical data.  Perhaps best known is the Five Factor Model.  Such models capture the variability and nuance of personality, but do not make sharp normal-versus-abnormal distinctions.  Each person exhibits one factor to a certain degree, the next factor to another degree, and so forth.  The combinations are nearly infinite.

The good news is that organized psychiatry is waking up to the value of dimensional assessment.  Position papers (e.g., here, here, and here) have long argued for this with respect to Axis II.  As working groups now meet to plan DSM-V, dimensional adjuncts are being considered for all traditional categorical diagnoses.

Is mental illness categorical or dimensional?  The “real” answer, I suspect, is that some psychiatric disorders will eventually be understood to have biological origins.  Schizophrenia will likely go the way of general paresis (syphilis) and Alzheimer’s Disease:  When the medical cause of a psychiatric condition is finally understood, it is no longer considered a psychiatric condition.  It becomes the province of neurology, infectious disease, or another branch of medicine.  Psychiatry is left with conditions that defy medical explanation.  This is why I feel that, ultimately, dimensional factors are an irreducible feature of psychiatry.  Whether understood using Freudian psychodynamics, learning theory, or another psychological paradigm, human emotions and behavior will always be more subtle and nuanced than a categorical nosology can describe.

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