Byline: James Phelps, MD

Psychiatric diagnosis is currently based on a system of categories, much like the Linnaean classification in biology. However, a seemingly opposite diagnostic system is emerging, usually dubbed the "dimensional view," in which related psychiatric conditions such as major depressive disorder (MDD) and bipolar disorder (BD) are seen as end points of a spectrum.

With this expansion of the concept of BD to include intermediate forms (ie, manifestations between clearly unipolar and clearly bipolar cases), concerns have arisen about the potential for overdiagnosis of BD1 and the blurring of our understanding.2 However, diagnostic error thus far has been skewed in the direction of underdiagnosis.3

The concept of overdiagnosis or underdiagnosis presumes some "natural types" that we diagnosticians are correctly or incorrectly classifying. Indeed, the term "classify" implies that we are looking at native classes. This is, for example, the process used by an ornithologist to identify a particular bird: scientists use visible traits such as size, color, beak shape, wing bars, eye rings, and other field signs. They may also observe preferences such as habitat (ground or high trees), interactions (flocking or pairing), diet (seeds, worms, or fish), and even behavior over time (eg, migration or wintering-over).

The DSM system is like that of the ornithologist. The purpose of the diagnostic interview is to gather observations, for which the DSM serves as a "field guide," organizing the observed findings by category of illness. But just as in ornithology, over time an experienced clinician relies less and less on the specific DSM rules and progressively more on pattern recognition. A good bird watcher distinguishes a woodpecker from a hawk not so much by specific field signs but from an overall gestalt-rapid synthesis of a whole complex of findings (eg, location in the tree, position on the branch, size, shape, color, behavior, call). Likewise, an experienced clinician might strongly suspect that a patient has BD based on observations on an inpatient unit, even before an interview.

Unlike birding, however, experience gives clinicians the ability to recognize intermediate cases that blur the boundaries of the classification system. Indeed, this lack of discrete boundaries has been studied quantitatively in at least 2 research settings, both of which showed a continuum of symptoms in patients with DSM-diagnosed MDD and BD. Some patients with MDD have no symptoms suggestive of BD, others have a few such symptoms, and still others have many with no gaps in this progression that might serve as a natural point of cleavage between the conditions.4

The same result was obtained by Benazzi5 in a study designed specifically to look for such gaps. Similarly, Mackinnon and Pies6 recently presented an elegant model for rapid-cycling and mixed-state BD, demonstrating that mood, energy, and creativity/speed-of-thought symptoms fluctuating asynchronously can create a continuum of forms of the illness. Although this model has not yet been subjected to rigorous study, it accords well with clinical impressions, just as Jamison7 summarized over a decade ago: "The clinical reality of manic-depressive illness is far more...